Acoustical Materials and Acoustics

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This article describes sound-absorbent materials that might be used in the home. For a more general description of how sounds are transmitted, controlled, and measured, see Acoustics.

All materials will absorb at least some of the sound energy that impinges upon them. It may be as little as a sheet of glass, which will absorb about 2 1/2 %; the rest of the sound energy is either reflected toward the source or transmitted through the glass. At the other extreme is an open window that will, as far as a person inside a room is concerned, “absorb” almost 100 % of the sound energy “striking” it; an open window is a near-perfect sound absorber.

Between these extremes lies everything else. Ordinary building materials such as wood, brick, stone, plaster, and so on, absorb something less than 5 % of the sound energy striking them. To be counted as an acoustical material, a substance must absorb more than 50 % of the impinging sound energy. To do this, the material must have a porous, fibrous structure—that is, it must have a great many convoluted, interconnected passageways exposed to the surface that run deep into the material; and the thicker the material, the better a sound absorber it will be. This is because the air molecules that carry the sound energy can migrate deeply into the interior of the material, where, as they repeatedly strike and bounce off the surrounding fibers, their rapid, vibratory motion is converted by friction into heat.

Mere light-weightess counts for little or nothing as far as the ability to absorb sound is concerned. Cork and foamed plastics, for example, are very lightweight materials. Their structures consist of innumerable cells. However, almost none of these cells lie exposed on the surface nor are they interconnected with each other. Whatever sound-absorptive properties these materials may have is due to the fact that any impinging sounds may be reflected from them in a diffuse fashion because of the irregularity of their surfaces.

Interestingly enough, thin, hard materials like a sheet of metal or plywood will also absorb impinging sound vibrations, as long as they are stiff enough. They do this by vibrating sympathetically with the frequency of the sounds that strike them. The energy of the sound waves is thus dissipated by the vibratory motion they induce in the panels. Panels of this kind absorb mainly low-frequency sounds. High-frequency sounds are largely reflected from or transmitted through the panels.

Noise Reduction Coefficients

Every building material, and each kind of wall and floor/ceiling construction, will absorb (or transmit) sounds according to their frequencies. It depends on whether, and at what frequencies, sympathetic vibrations can be induced in the material or construction. Each material or construction will be most efficient at absorbing sounds having particular frequencies. Materials and constructions, therefore, are tested at a range of frequencies to determine their overall sound-absorptive properties, these frequencies being 125, 250, 500, 1000, 2000, and 4000 cycles per second. The % of impinging sound absorbed at each frequency is called the sound-absorption coefficient for that frequency. Tables of the sound-absorption coefficients for a wide range of materials and constructions are published for use by architects, acoustical engineers, and others. For an example, see Acoustics (Table 2).

As a practical matter, knowing how a particular acoustical material reacts to a range of impinging sounds is too complicated for a builder or the ordinary homeowner. Instead, a simplified rating system is used in which each material is assigned a noise reduction coefficient (NRC). For any given acoustical material, its NRC is obtained by averaging the sound-absorption coefficients obtained at 125, 250, 500, 1000, and 2000 cycles. The resulting average is the material’s NRC. NRC ratings are widely used, by manufacturers of sound-absorption materials, and most builders and homeowners tend to take these NRC ratings at face value. It should be remembered, however, that these are average figures. Any particular material may absorb either more or less sound at a given frequency than is indicated by its NRC and , therefore, it may be more or less suitable for a particular application than other materials having a similar NRC.

What Acoustical Materials Can and can't Do

Although it's true that acoustical materials can reduce the apparent loudness of sounds within a room, it's important to understand how this is accomplished. Otherwise, it will be impossible to understand when acoustical materials should be installed and when not.

When a sound originates within a room, the sound waves reach an occupant of that room in two ways. First, a small proportion of the sound waves travel directly from their source to the occupant. Second, most of the sound waves bounce repeatedly off the walls, ceiling, and floor of the room before they reach the occupant. There is a very slight time lag between the moment when the direct sound waves reach the occupant and when these reflected sound waves reach him. Furthermore, because these reflected sound waves reach the occupant at slightly different times, depending on the size and proportions of the room and on the relative positions of the origin and occupant in the room, the reflected sound waves give a reverberant, echo-like quality to the original sound. it's this reverberant sound that is responsible for the warm, rich quality we associate with indoor sounds.

What effect does the installation of an acoustical material have on the sound within the room? Assume the ceiling has been covered with acoustic tiles. As a consequence, the occupant will have the distinct impression that the room is now much quieter than it was before. The acoustic tiles will not have any effect at all on the intensity of the sound waves traveling directly from the source to the occupant. This should be obvious. The original sound will be just as loud as it was before. As for the reverberant sound waves, however, they will decrease in intensity much more quickly because every time a sound wave bounces off the ceiling, a proportion of its sound energy will be absorbed by the acoustic tiles. How much of the sound energy will be absorbed will depend on the material’s NRC.

It is possible to have too much of a good thing, to overdo the amount of acoustical material installed in a room. If, for example, a room not only has acoustic tiles on the ceiling but has wall-to-wall carpeting and walls covered with drapes as well, sounds become deadened. Voices lose their warm, rich quality. Instead, they sound thin, flat, and distant, as if the speakers were standing in an open field some distance from each other. If the room is large enough, it's even possible for voices to become lost altogether. it's necessary, therefore, to strike a balance between excessive noise levels within a room and excessive sound absorption.

Although acoustical materials can do a great deal to reduce the intensity of sounds that originate in a room, they are unable to do much with sounds that originate outside a room and are transmitted into it. (For the reasons, see ACOUSTICS.)

It will suffice here to note that outside airborne sounds enter a house by inducing a vibratory motion in the windows, doors, and walls. These Structures then transmit these sounds into the house. Acoustical materials can't do anything at all to pre vent this mode of sound transmission. They simply lack the mass and solidity to dampen sound vibrations.

The reader might object that if an acoustical material can absorb sounds originating within a room, why can’t it absorb sounds originating outside the room? One part of the answer is that the sounds “absorbed” by an acoustical material consist not only of the sounds that are truly absorbed but also of the sounds that pass completely through the material and are not, therefore, reflected back to the occupant. and a large proportion of the sound that is “absorbed” by an acoustical material is, in fact, transmitted completely through the material because a lightweight material (which an acoustical material is) is inherently incapable of preventing the transmission of sounds through it. (For an explanation, see ACOUSTICS.)

Another part of the answer is that acoustical materials absorb mainly high-frequency sounds. An acoustical material absorbs very little of the low-frequency sounds that strike it and most of the irritating outdoor sounds that homeowners object to are such low-frequency sounds as traffic and aircraft noises.

Furthermore, sounds are absorbed within a room only after the sound waves have struck the acoustical material a great many times. When an outside sound enters a room, it passes through the acoustical material only once, with a negligible effect on its intensity. But once an outdoor sound has entered a room its intensity will be reduced quickly by repeated reflection off the wall, floor, and ceiling surfaces. But the original sounds passing through the wall directly to an occupant will be unaffected by this attenuation of the reflected sounds.

Finally, acoustical materials are not the effective sound absorbers that one is led to expect by manufacturers’ literature. Innumerable tests by a great many laboratories have shown that the maximum possible amount of sound reduction that can be achieved by the installation of an acoustical material is something like 9 to 10 decibels, with most materials achieving a sound reduction of about 5 decibels. (For a definition of decibels, dB, for short, see ACOUSTICS.) Thus, although it's perfectly feasible to install acoustical materials in a room and achieve a reduction in the overall intensity of the sound, one should also remember that these sound levels are comparatively low to begin with. it's quite another kettle of fish to try and reduce the extremely high sound levels originating inside a boiler factory, say, by installing acoustical materials. It can’t be done. and if the reader happens to live next to an airport runway where the noise levels may reach an intensity of 100 dB, all that the installation of acoustical materials will do for his peace of mind is reduce the intensity of the noise within his house to 90 or 95 dB, which is trivial.

Acoustics

Acoustics is the science of sound. Insofar as this article is concerned, acoustics is the science of controlling or limiting or suppressing unwanted sounds. That is, we will take acoustics to mean acoustical noise control. The subject is more encompassing than might at first appear for it includes: (1) the general design and layout of a house to prevent or limit the transmission of unwanted sounds from one part of a house to another, or from outside the house to inside, (2) methods of construction that will prevent or limit the transmission of unwanted sounds from one part of a house to another part, or from outside the house to inside, and (3) the use of sound- absorbent materials on the walls and ceiling of a room to reduce the level of sound within the room. it's this last item that most people understand to mean by “sound control” or “soundproofing” or “sound insulation.” This is, however, a comparatively minor aspect of acoustics.

Acoustics has become much more important since World War II because we are living in an increasingly noisy world. Traffic noises and the roar of jet aircraft outside the house have made life within more uncomfortable. Inside the house there is radio, television, and hi-fi equipment, innumerable electrical gadgets that whine, grind, or throb, and the constant, enervating, low-frequency noise of heating equipment in winter and air conditioners in summer (though, in truth, we seldom com plain about the noises we ourselves make). Compounding the problem has been the development of new lightweight materials that, whatever structural and economic advantages they may confer, vibrate easily in response to impinging sounds and thus both amplify and pass all these sounds along.

In this article, then, we shall describe how the intensity of sounds is measured, how sounds are transmitted throughout a house (or from outside the house to inside), how unwanted sounds can be prevented from spreading (that is, how the sounds are isolated), and how a room can be insulated against unwanted sounds. Note the distinction between the isolation of sounds and insulation against sounds. The distinction is important and should be kept in mind.

SOUND INTENSITY and DECIBELS

The faintest possible sound that a person can hear has about one-trillionth (1/1,000,000,000,000) watt per sq meter of energy. A confidential sotto-voce conversation will have about 10,000 times more energy than this. An ordinary office or dinner-table conversation will have 100,000 times more energy, and so on, to the loudest sound one can hear—a nearby jet engine operating at full power, say—which would be about a trillion times more intense than the faintest possible sound we could hear, and quite painful.

This is an enormous range of energy. That our ears can hear this range of sound energy is a tribute to their design and construction. How do the ears do it? By compressing the range of sound intensity. it's a peculiarity of the way we hear that we don't hear equal increments of sound energy as equal increments of “loudness.” That is, if sound A is 10 times more intense than sound B, as measured objectively by a sound- level meter, we would hear the difference in intensity not as 10 times “louder” but only as twice as “loud.” and “loud” is in quotation marks because loudness is a very subjective quality that differs among individuals.


ill. 1. The relationship between actual sound intensities and their subjective loudness is shown in the diagram. E measure subjective loudness.

Our hearing system compresses enormously the range of sound intensities that strike our ears, and the more intense the sound, the more drastically is the actual intensity of the sound compressed. In short, we hear differences in the intensity of sounds logarithmically. This is shown in ill. 1, which com pares the logarithmic system of measuring sounds with the decimal system we are all familiar with. Note that the logarithms measure the ratios of sound energy, not the actual values. That is, a 100 % change in the logarithmic scale is equal to a 10-point change on the decimal scale. A 1000 % change on the logarithmic scale is equal to a 20-point change on the decimal scale. A 10,000 % change on the logarithmic scale is equal to a 30-point change on the decimal scale. and so on, between any two points on this chart within its entire range. What all this means is that if sound A is 100 times more intense than sound B, as measured by a sound-level meter, we will think sound B is only twice as loud. and if sound A is 1000 times more intense than sound B, we will think sound B is only three times as loud, and so on. This relationship between the subjective loudness of sounds and their actual sound intensities is true over the entire range of our hearing.

In acoustics, the relationship between actual sound intensities and their subjective loudness is measured by the decibel scale (abbreviated dB), named in honor of Alexander Graham Bell. Thus, on the decibel scale, a 10-point change in the apparent loudness of a sound means that the sound seems to us to become twice as loud (or one-half as loud) as it was before. In fact, of course, the actual intensity of the sound will be 100 times as great as (or less than) it was before. One of the things that the decibel scale does, therefore, is allow us to rate the intensity of sound subjectively. In Table 1, for example, a whisper is shown to be about 10 dB louder than the rustle of leaves; that is, the whisper will sound about twice as loud to us as the leaves do. In fact, of course, the noise made by the whisper is 100 times as loud as the noise made by the leaves. In the same way, Table 1 shows that there is about a 10-dB difference in apparent loudness between the noise of a loud horn in traffic and the sound made by an express train passing close to us at high speed. Again, the actual difference in sound intensity between traffic noises and an express train is enormously greater than the difference between a whisper and the sound of rustling leaves. Nevertheless, the subjective reality is that the train will sound only twice as loud as traffic and rustling leaves will sound only twice as loud as a whisper.

Table 1. The Decibel Scale of Sound Intensities

SOUND ISOLATION

With this understanding of what decibels represent firmly in our minds, we can now consider how sounds travel from one room to another within a house and what can be done to isolate the sound—to prevent its traveling from one room to another.

Imagine two rooms separated from each other by a solid partition. The construction of the partition is irrelevant for the moment. A sound originating in one of these rooms will travel through the air as alternately compressed and rarefied molecules of air. The force with which these air molecules strike the partition, though extremely weak, is still sufficiently strong to cause the partition to vibrate sympathetically with the original sound. and because this apparently solid partition vibrates, no matter how imperceptibly, the original sound will be transmitted into the second room.

The loudness with which the sound is heard in the second room will be weaker than the original sound for several reasons. First, all sound waves attenuate in strength as they travel away from the source of the sound. For example, whatever the strength of the original sound (and neglecting for the moment the effects of reverberant sound within a room), 10 ft from the source the sound waves will be about one-half as energetic as they were originally; 20 ft from the source they will be about one-quarter as energetic; 30 ft away they will be about one-eighth as energetic, and so on, until the sound becomes too weak to be heard. This conforms to our ordinary experience of how sounds travel. In short, the force with which the original sound impinges on a partition will depend on the distance between the origin of the sound and the partition.

Second, a certain %age of the impinging sound waves will be reflected back from the partition into the room. The actual % reflected will depend on the nature of the surface. A smooth, hard plaster surface, for example, reflects more sound than heavy cloth drapes do.

Third, some of the sound will be absorbed by the wall, depending, again, on the nature of the wall material. Materials like plaster, glass, concrete, and plywood absorb very little sound. A soft, porous material like fiberboard absorbs a great deal. The type of porosity in the material is important. A material like fiberboard that has a fibrous structure in which a great many interconnecting internal passageways are exposed on the surface will trap a large %age of impinging air molecules. The molecules will dissipate their energy in the form of heat as they repeatedly strike the sides of these passageways. The structure of another type of porous material consists of self-contained cells that are not exposed to the air. Cork and Styrofoam are examples. These materials are relatively poor sound absorbers.

The proportion of impinging sound energy that is absorbed by any given material is measured by its absorption coefficient. This is a number that will lie somewhere between 0 (for complete reflection of the sound) to 1.0 (for complete absorption). An open window, for example, has an absorption coefficient of 1.0, since it can reflect nothing, while a sheet of glass has an absorption coefficient of 0.027, which means it reflects all but 2.7 % of the sound energy striking it. Elaborate tests have been conducted on a wide range of construction materials to determine their absorption coefficients at different frequencies. Table 2 shows a representative sample of materials and their absorption coefficients.

Table 2. Absorption Coefficients of CommonBuilding Materials*. Complete tables of coefficients of the various materials that normally constitute the interior finish of rooms may be found in the various books on architectural acoustics. The following short list will be useful in making simple calculations of the reverberation in rooms. *This table is reprinted in abridged form through the courtesy of the Acoustical Materials Association. The values reported here are reviewed from time to time as more

information becomes available from researchers in the field of acoustics. The reader is referred to the annual bulletin of the AMA for future changes.

When one subtracts the sound that is either reflected or absorbed, what remains is the sound energy that actually passes through the partition. The difference, in decibels, between the original amount of sound energy on opposite sides of the partition is the partition’s transmission loss. A partition that is very efficient in preventing the passage of sound has a high transmission loss; it's an excellent sound barrier. On-the other hand, a wall that has a low transmission loss is a poor sound barrier.

Innumerable tests of different wall constructions under standardized Conditions have been made by a number of manufacturers and testing organizations such as the American Society for Testing and Materials (ASTM), to determine the ability of particular constructions in preventing the transmission of sound. According to ASTM standards E 90 and C 423, these tests are conducted at 18 standard frequencies within the range of 100 to 5000 cycles per second. Table 3 gives the transmission-loss values, in decibels, for common types of wall and floor constructions. The effectiveness of any particular construction shown in Table 3 as a sound barrier may be judged by comparing its transmission loss with the following figures:

  • 25 to 30 dB loss—A conversation conducted in normal tones will be heard through the partition.
  • 30 to 35 dB loss—A loud conversation can be heard through the partition, but much of the conversation will be unintelligible.
  • 35 to 40 dB loss— Normal conversation is inaudible. Loud conversations can be heard, but the words are muffled and unintelligible.
  • 40 to 45 dB loss—Even a loud conversation is inaudible, but singing, musical instruments (especially brass instruments and the piano), and a radio or television set turned up loud can be heard distinctly enough to be annoying.
  • 45 dB loss and up—Even loud sounds will be muffled.

In addition, for single-family dwellings, it's considered that the partition between a bedroom and adjoining rooms should have a transmission loss of 40 to 45 dB; the partitions between a bathroom and adjacent rooms (except between a bedroom and its connecting bathroom) should have a transmission loss of 40 to 45 d B; and the partition between any other two rooms, and between any room and the outdoors, should have a transmission loss of 35 to 40 dB. The party wall of a two-family dwelling separating one dwelling from the other should have a transmission loss of better than 45 dB.

These figures are merely a rough guide to the qualities of any particular construction as a sound barrier. Many other things have to be considered as well. For example, once sounds pass through a partition, the actual intensity of the sounds that reach the ear may be less because drapes and furnishings installed in the room will absorb a portion of the sounds. The effect of the drapes and furnishings will be to increase the actual transmission loss. In addition, other noises may be present that help to mask, or muffle, the sounds coming from the next room. This masking noise gives the illusion that the transmission loss is greater than it actually is. Background noise is always present inside most houses, whether the occupants are aware of it or not, and its presence does help to increase the effective transmission loss of any particular wall construction. The following levels of background noise are typical:

  • Under 25 dB—A low background-noise level usually found in quiet suburban or residential communities. Traffic noises are absent, and so are the sounds of mechanical equipment.
  • 25 to 35 dB—Average background-noise levels, in which one can hear light or distant traffic noises and perhaps low- level mechanical equipment.
  • 35 dB and up— Prominent Street and traffic noises, noisy central air-conditioning equipment and noisy window- mounted air-conditioner units.

In any room in which extreme quiet is required, these back ground noise levels may themselves become a nuisance that must be dealt with rather than ignored.

Finally, there is the frequency of an offending noise to be considered. The decibel ratings given above for average conditions in which high-frequency and low-frequency sounds are jumbled together. In general, however, low-frequency sounds are less easy to suppress than high-frequency sounds. In a general way, the sound resistance of a typical wall construction is reduced by about 5 dB for every halving of the frequency. That is, if a wall construction is rated as having a 40 to 45 dB transmission loss at a frequency of 2000 cycles, the same wall will have a transmission loss of only 25 to 30 dB for sounds having a frequency of 250 cycles. The higher the sound frequencies, the easier they are to suppress, which is why such low-frequency sounds as traffic noises, jet aircraft, fan rumble, and air-conditioning noises are so difficult to suppress. Fortunately, our sense of hearing makes up for this characteristic of sounds, in part, by being much less sensitive to low-frequency sounds.

ill. 2. The relationship between the mass of a construction and its ability to prevent the transmission of sounds.

Table 3: Transmission-Loss Values for Common Types of Wail and Floor Constructions

Building Construction

Single walls

2-in, solid gypsum sand-aggregate plaster (18 lb per sq ft)

6-in, hollow-core cinder block, painted both sides (33 lb per sq ft)

6-in, hollow-core cinder block, sand-aggregate plaster both sides (43 lb per sq ft)

4 ½ -in solid brick, plastered both sides (45 lb per sq ft)

7-in. stone-aggregate concrete, plastered both sides (90 lb per sq ft)

2 x 4 wood studs, ½-in. gypsum board both sides (6 lb per sq ft)

2 X 4 wood studs, V. sand-aggregate plaster on in. gypsum lath both sides (16 lb per sq ft)

Double walls

Two wythes of plastered 3-in, dense Concrete, 3-in. airspace between (bridging in airspace and at edges) (85 lb per sq ft)

Two wythes of plastered 4 solid brick, 2-in. airspace between (sound-absorbing material in airspace—bridging at edges only) (90 lb per sq ft)

Two wythes of plastered 4’/a-in. solid brick, 12-in. airspace between (wythes completely isolated) (90 lb per sq ft)

Number is not a transmission-loss value but a room-to-room noise reduction value adjusted for a receiving room with a 0.5-sec reverberation time at the listed frequency. The actual transmission-loss value should be within ±2dB of the listed noise reduction value.

Floor-ceilings

Typical residential floor-ceiling wood finish; and subfloors on wood joists, gypsum lath and plaster below (about 15 lb per sq ft)

Concrete floor slab, %-in. plaster finish coat below (about 45 lb per sq ft)

Doors

1 3/8-in, hollow-core wood door, normally hung

1 3/8-in, solid wood door, normally hung

1 3/8-in, solid wood door, fully gasketed

Specially constructed 2 3/8-in, wood door, full double gasketing

Soundproof Wall Construction

There are two surefire methods of building a partition having a high transmission loss: (1) make the wall heavy, (2) build the wall in two completely separate halves with an air space between.

The heavier a partition—that is, the more mass it has per unit volume—the more effective a sound barrier it will be. A solid 6-in.-thick concrete wall or an 8-in.-thick brick wall make much more effective sound barriers than the usual 2 x 4 in, stud wall in which ½ in.-thick gypsum-board is nailed to both sides of the wall. The reason is that the impinging air molecules find it much more difficult to induce a vibratory motion in a massive wall than in a lightweight wall. Weight makes the difference. However, there is a limit to how much one can increase the weight of a wall, and a limit to the effectiveness of the weight increase, as is shown in ill. 2.

There is a second method, however, of obtaining a high transmission loss in a wall—.-building an air cavity within the partition. This is shown in ill. 3. To be effective, however, the two halves of the wall must not have any interconnection between them whatsoever, since any such interconnection will transmit sounds across the cavity.

ill. 3. The wider an air gap within a wall construction, the more effectively will the construction prevent the transmission of sounds through it.

It may appear, by analogy with heat insulation, that stuffing the wall with an insulating material such as glass fiber or rock wool will increase the transmission loss of the wall. Experience shows, however, that an insulating material increases the transmission loss by 2 or 3 dB at most. Insulation materials don’t work for the same reason that acoustical materials don’t—they are too light and porous. it's one of the great fallacies of modern home construction that stuffing an acoustical material in a partition will help prevent the transmission of sound through it. In addition, trying to increase the transmission loss of a wall, or a ceiling, by attaching acoustical tiles, insulation board, drapes, or a similar lightweight, porous material against one of the external surfaces is also a waste of money because the lightweight and porous nature of these materials makes them inherently incapable of preventing the transmission of sounds through themselves, much less through a wall or ceiling. However, it should be added that attaching an insulation material to one of the interior surfaces of a wall partition does help increase the transmission loss through the partition, if the installation is done correctly.

An alternative to an air cavity is to support the two sides of a partition on flexible mountings, which are in turn attached solidly to the partition studs. The walls are thus free to vibrate, and to dampen the vibrations of impinging sound waves. Flexible mounts of this type, of which there are several patented designs on the market, are as effective as an air space in increasing the partition’s transmission loss.

An even simpler but equally effective method of obtaining a high transmission loss (about 45 to 50 dB) at small expense is to nail fiberboard to both sides of the studs—making sure the nails are driven well below the surface of the fiberboard—and then to attach 1/2-in.-thick gypsum-board panels to the fiber board with adhesive. For a description of this method, see DRYWALL CONSTRUCTION. The fiberboard has a springy quality to it that enables it to absorb sound vibrations just as flexible mounts do, and the gypsum-board adds the mass that is necessary if the impinging sound vibrations are to be dampened. If one wants a plaster wall instead of a gypsum-board wall, then one can attach gypsum lath to flexible mountings, over which a standard plaster coating is applied. Such a plaster wall will have a transmission loss of about 50 dB. (For procedure, see PLASTERING.)

In an existing house, it would be difficult, if not impossible, to tear down a partition in order to erect a more sound-resistant partition in its place, not to mention the expense of doing so. There is, however, a simple and quite efficacious method by which the transmission loss of an ordinary 2 x 4 in. stud wall can be vastly improved—the construction of a false wall immediately next to the stud wall with a cavity between them. The construction is shown in ill. 4. Under no circumstances should the false wall touch the real wall at any point, not even for the installation of electrical outlet boxes, wiring, insulation, or whatever. The two walls must remain completely isolated. If pipes or any other equipment must run through the wall, then care should also be taken to isolate them from the walls to prevent their transmitting vibrations from one wall to the other. A construction of this kind is capable of achieving an STC rating as high as 50 dB (see below).

ill. 4. Building a false wall next to an existing wall greatly increases the transmission loss between two rooms. The false wall should be spaced at least 1 in. from the real wall.

STC Ratings

ill. 5 shows a variety of possible wall constructions. Each type of construction has been given an “STC” rating, the “STC” standing for sound transmission class. The STC ratings are an attempt to summarize, in the form of a single numerical value, the results of the transmission-loss tests mentioned above.

The STC rating for a given construction is obtained as follows. First, sound transmission class contours are drawn on a graph, as shown in ill. 6. These contours indicate that subjective response of the ear to differences in apparent loudness. Note that the contours slope upward and to the right as frequency increases, which indicates that high-pitched sounds are subjectively louder to the ear and are, therefore, more disturbing than low-pitched sounds having the same sound intensity. The transmission-loss test results for the construction are now plotted on the graph. Most constructions and individual wall materials vary in their ability to suppress impinging sound vibrations, depending on the frequencies of the vibrations. ill. 6 shows that this particular construction is least capable of preventing the passage of sounds having a frequency of about 1000 cycles. At this frequency, the transmission-loss curve drops to the 30-dB contour. As this is the point at which the construction tests most badly, it's assigned an STC rating of 30 dB.

ill. 5. SIC ratings show how effectively these single and double wall constructions prevent the transmission of sounds (U.S.Forest Service).

Doors, Windows, and Cracks

For the sake of simplifying the discussion of how sounds travel from one room to another, we have assumed that all partitions are solid and without openings. This is obviously artificial, since all rooms have doors and most rooms have windows as well. The presence of doors and windows can alter radically the efficiency with which a partition acts as a sound barrier.

And so can cracks. A thin crack 2 or 3 ft long in an otherwise solid brick wall can reduce the transmission loss of the wall by 50 %. A long crack of this kind (the bottom of every door has one) transmits sounds of all frequencies: a small hole—a keyhole, say—transmits mainly high-frequency sounds. A very small opening can let in a remarkable amount of sound. The author remembers hearing of a New York brownstone that was converted into apartments. When the house was built 1/4-in.- diam. brass speaking tubes had been installed in the walls. They led from the upstairs bedrooms to the kitchen, which was located on the ground floor. These tubes were left in place when the building was converted, which gave the tenant occupying the new apartment in the former kitchen an unparalleled opportunity to listen in on the conversations of the other tenants in the building. and all these sounds came through 1/4-in, holes located in the wall.

ill. 6. Determining the STC rating of a particular wall construction. In this case the construction has an STC rating of 30 dB.

An ordinary hollow-core door with side panels 1/8-in. thick, which is the cheapest and most widely installed door in present-day dwellings (see DOOR, WOOD), has a transmission loss of about 15 dB, a figure that includes the cracks that usually surround the sides and bottom of the door. As we have noted, one can hear a conversation through a partition rated at 25 to 30 dB, so having a poorly mounted hollow-core door connecting two rooms is little better than having no door at all. Installing a rubber seal around the edges of the door will increase the transmission loss to about 20 dB, which will still allow an ordinary conversation to be overheard. Even a solidly constructed, well-hung door with a rubber seal properly installed around the frame will have a maximum transmission loss of 30 dB.

The larger a wall, the less effect a door has on the overall transmission loss. When a door (or window) occupies 10 % or more of a wall area, its STC rating will control the STC rating of the entire wall. This being the case, it's obviously a waste of money to build a partition with a transmission loss greater than of a door in that wall.

The transmission loss of a doorway can be increased considerably by installing double doors. A double door is usually installed, for example, between a doctor’s waiting room and consulting room. One of the doors will be used as an ordinary door with the second door usually being swung back against the wall where it's out of the way. Both doors are closed at the same time only when the conversation between doctor and patient is confidential.

When double doors are installed, the air space between the doors should be as wide as possible, and at least 4 in. wide in any case. The heavier and more solid the door construction the greater its transmission loss, of course, and installing rubber seals around the edges of both doors will increase the transmission loss even more. A double-door installation of this type can have an overall transmission loss of 45 dB and more.

What to do about the windows, if anything, depends on the amount of outside noise, the total area of the windows in the wall, and the amount of sound isolation desired. As with doors, there is no point in constructing a wall having a high transmission loss if sounds are going to come through the windows anyway. Under laboratory conditions, an ordinary 1/4-in.-thick pane of glass has a transmission loss of about 25 to 30 dB. But when this glass is installed in a wood or metal sash that can be opened and closed, the transmission loss drops considerably.

A simple and inexpensive method of increasing the transmission loss of a window installation is to permanently install storm sash. That is, the storm-sash frame should be caulked to the window frame to prevent any sounds leaking past the edges of the sash. A more expensive method of increasing the transmission loss is to install a double window having an air space at least 4 in. wide between the panes. The transmission loss of such a construction in a brick wall will be between 42 and 47 dB.

When windows are permanently sealed, however, another way of ventilating the room must be found. It then becomes necessary to install a forced-air ventilating system or an air- conditioning system of some kind. Whatever method of ventilation is used, the outside air should never be obtained from an inlet installed in the same wall as the windows. This would negate sealing the windows in the first place.

Impact Sounds

The sounds we have discussed so far travel through the air, and the soundproofing techniques we have discussed have dealt with methods of preventing the transmission of these airborne sounds through solid barriers.

There is, however, another class of sounds that is perhaps more irritating and insidious than airborne sound. These are the impact sounds that travel through the solid structure of a house, and especially through the floor-ceiling construction. Impact sounds include, for example, the sound of footsteps transmitted directly through a floor-ceiling to the room below, the sounds transmitted by a piano directly to the floor via its legs, water hammering transmitted throughout a house by the water-supply pipes, and fan and compressor noises transmitted directly from an air-conditioning unit to the window ledge on which it's resting, and from there to the rest of the building structure.

Airborne sounds and impact sounds can't be compared directly, and the STC ratings we have described are of no use in measuring the sound levels of impact noises transmitted through a construction, or the effect of these impact noises upon the occupants of a dwelling. ill. 7, for example, shows several floor-ceiling constructions. Most of these constructions have enough mass and rigidity to suppress effectively any airborne sounds impinging on them, and this is reflected in their high STC ratings; it's rare to hear conversations or other airborne sounds in a room directly above or below the room in which these sounds originate. ill. 7 also shows the impact noise ratings (INR) of these constructions. The two methods of measurement have nothing to do with each other.

ill. 7. A comparison of STC ratings and Impact Noise Ratings (INR) of different ceiling/floor constructions (U.S.Forest Service).

The assignment of an INR to a floor-ceiling construction is an attempt to measure how effectively the construction suppresses impinging impact noises. The ratings are based on the ability of a standard construction to suppress various types of standardized impact sounds. it's assumed that this standard construction will suppress these impact noises by a reasonable amount. This standard construction has been given an INR of zero. The impact noise resistance of other floor-ceiling constructions are now compared to this standard construction and their performances are rated accordingly. If, for example, a particular floor-ceiling construction suppresses impact noises more effectively than the standard construction, it's given a plus (+) rating, the value of which depends on the amount of improvement. If, on the other hand, the construction sup presses impact noises less effectively than the standard, it's given a minus (-) rating, the value of which depends on the amount of deterioration. Typical INRs are shown in ill. 7.

In an existing house, the only thing that can be done to reduce impact sounds is to install heavy carpeting and carpet pads on the floors. Carpeting is very effective at dampening or muffling the sounds of footsteps and similar impact noises. It will, in fact, improve the INR by +20 to +30 points. (As far as the STC rating of a floor-ceiling construction is concerned, carpeting will have no effect whatsoever on the STC rating.) Pianos and hi-fi equipment should rest on flexible mountings of some kind to reduce the amount of impact noise.

Suspended Ceilings

It is sometimes suggested that a suspended acoustical-tile ceiling of the type found in offices and commercial establishments will absorb impact sounds transmitted from a floor above. In this type of construction, a light metal framework is suspended from the ceiling by wires or metal rods and acoustical tiles are then set in place on this framework. This type of ceiling will absorb impinging sounds to some extent, and it will hide exposed pipes and ducts as well, but it will have almost no effect whatsoever on suppressing any airborne sounds or impact sounds that may be transmitted through the ceiling from the floor above, mainly because of the lightness of the overall construction and also because the ceiling, since it's suspended by rigid wires or rods, will merely transmit the sounds unchanged.

Another type of suspended ceiling often installed to suppress unwanted sounds is a false ceiling suspended below the actual ceiling by wood furring strips. This false ceiling is constructed by analogy with a false wall. This construction suffers from the same basic defect as any light construction—it simply hasn’t the mass to absorb the impinging sound vibrations. and whereas a false wall is completely separate from the actual wall, a false ceiling must of necessity be supported by the actual ceiling. Such a false ceiling does have some slight effect in reducing airborne sounds when it's suspended by flexible mounts, but it will not reduce impact sounds.

A more fundamental difficulty with suspended ceilings is that, even when they do reduce sound levels, the impact sounds that are suppressed are only a small proportion of the sounds that continue to be transmitted to the room below via the wall studs and floor joists. it's very difficult, if not impossible, to achieve any degree of sound reduction from a suspended ceiling unless an effort is made at the same time to prevent the passage of impact sounds along the solid pathways afforded by the flooring, joists, and studs. In particular, the wall studs must be isolated from the flooring, and the wall construction must also be heavy and solid enough to prevent the inducement of sympathetic sound vibrations. This is very difficult to achieve in a 2 X 4 in. stud wall.

SOUND INSULATION

We come at last to the subject of sound absorption, that aspect of acoustics most people think of when they hear the words “sound control,” “soundproofing,” or “sound conditioning.”

By sound absorption, we mean the ability of the wall and ceiling surfaces of a room to absorb impinging sound waves and prevent their being reflected back into the room. This sound absorption is usually achieved by the application of acoustical materials of some kind to the walls and ceiling. These materials are capable of altering the general quality of sound within the room, as well as reducing the overall sound level.

Sound Propagation in a Room

Imagine an ordinary room. Within this room, a short, sharp sound of some kind originates. As the sound waves spread away from the source, they strike the walls, ceiling, and floor and are reflected from these surfaces. If you are sitting in this room and have an extraordinarily acute sense of hearing, you will hear first the original sound and then an intensification of that sound as all these reflections strike your ears. After the original sound has died away, the reflected sounds will continue to bounce about for a brief period; then they, too, will fade away.

These reflected sounds produce what is called reverberation—a multiplicity of very weak echoes that impinge too rapidly upon the ear to be distinguished individually but which, in sum, give the original sound a rich, warm quality that it would otherwise lack. One has only to contrast the sound of a voice heard indoors with the same voice heard outdoors where there is no reverberant effect. Outdoors, the voice sounds much smaller and thinner.

The reverberation, however, can't last too long. If it does, the individual sounds will bounce back and forth an excessive number of times and interfere with each other. The sound acquires an echo-like, booming quality that is unnatural and may make speech unintelligible.

What one does when one applies an acoustical material to the walls and ceiling of a room is alter the reverberant characteristics of that room; and , also, reduce the overall sound levels a slight amount. We shall not concern ourselves with the manipulation of this reverberation. This is a very complex subject and has more to do with maximizing the quality of the music heard at home and in the concert hall than with general noise and sound levels in the home.

Most people who consider installing an acoustical material in their home are usually influenced by their experience with these materials in an office or commercial establishment. In the typical large office, for example, there is the constant clatter of typewriters. Copiers and other kinds of office equipment make their noises. Telephones ring continually. There is the constant buzz of conversation. Yet all these sounds seem to have a subdued quality to them that the observer infers, correctly, is due to the use of an acoustical material on the ceiling. and , indeed, if it weren’t for this material, the apparent noise level in most offices would be much higher than it's , and the offices would be much more enervating places to work than they are.

But the analogy between noises in the home and in the office is inexact. In the first place, we can control or limit the noise level within our home to suit our convenience. If a television set or radio is too loud, we can easily turn the volume down to a more comfortable level. If the children are noisy, we can send them outdoors to play or tell them to be quieter. If we are operating an appliance or a piece of electrical equipment, the noise level is rarely loud enough to be irritating. Besides, as we have noted, the noises we make ourselves are rarely objectionable to us, whatever others may think of them.

In the second place, and more important, the noises that are objectionable in a home usually originate outside the home, or outside the room in which we happen to be working or resting. and as we have emphasized several times, acoustical materials are almost useless in preventing the transmission of these outside noises into the room—they simply lack the weight, the solidity, and the mass that are required to dampen or suppress these sound vibrations.

What acoustical materials can do is absorb some of the sounds that originate within the room itself. The amount of sound absorbed is limited. Innumerable tests have shown that the most that acoustical materials can achieve in the way of reducing sound levels is 5 to 10 dB, with 5 dB being the usual maximum, If the typical noisy office has a sound level of 50 dB, say, then installing a sound-absorbent material will reduce the apparent sound level to 40 or 45 dB. In addition, the material will also reduce the reverberant effect produced by the sounds bouncing off the walls, ceiling, and furnishings. it's this loss of reverberation that makes the typical office sound much less noisy than a sound-level meter might indicate.

But in a home it's this reverberant quality to a room that gives warmth and color to the human voice. When a sound- absorbent material is used to excess in a room, the voice acquires a flat, thin quality that is unnatural. There is a curiously distant quality to a conversation, the voices seem to come from a distance, and the sound patterns of the words may fall off so quickly that comprehension becomes difficult. it's , for example, the silent, mortuary-like atmosphere that one sometimes encounters in the executive offices of a large corporation, where the ceilings are covered with acoustical tile, the windows with heavy drapes, and the floors with thick carpeting. Silence is golden.

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