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Questions and Answers:

1. Does adding sound-absorbing material to the surface of a wall or a floor significantly increase the sound insulation?

The addition of a typical sound-absorbing material to the surface of a wall may change the high frequency transmission losses. This is where common sound-absorbing materials are most effective. However, the sound transmission class does not usually change, or only changes by one or two decibels. This is because the low frequency transmission is not changed by the addition of typical absorptive materials on walls. The addition of a carpet to a floor, will certainly increase impact insulation ratings, but usually does not change the sound transmission class. Many people believe that adding sound-absorptive material to walls or doors will improve sound isolation between apartments. On the receiving room side, if enough absorptive material is added the room will be less reverberant and intrusive noise levels will be slightly reduced; the reduction can be as much as 3 dB if the amount of absorption in the room is doubled. The room will also feel 'dead' acoustically, so there is a perception of some improvement. In a normally 'dead' room like a bedroom, it is very difficult to make significant changes to the amount of absorption because it is already so absorptive. Careful measurements have shown that adding absorption to reduce sound transmission is not the most effective way of dealing with this problem.

2. Is there any information on how much extra It costs to provide good sound insulation and low background noise in a building?

Most cost studies have looked at the incremental cost of materials needed to improve the STC ratings of walls. In general, the cost of the extra materials to improve a specific type of wall is relatively small. It may involve only the addition of some extra wallboard, some sound-absorbing material or the use of a larger airspace.

Other costs, which have not been studied, are associated with the inspection of drawings, site inspections and measurements by a qualified acoustical consultant. One consultant estimated that these costs amounted to about $1000 per apartment unit, a small fraction of the total cost.

3. What can be done to reduce sound transmission by way of the plenum space in an office building?

In typical office buildings the sound travels through the ceiling material, through any gaps in the ceiling and into the plenum. It then travels freely in the plenum and down by similar paths into adjacent offices. Typical ceiling board materials provide limited sound attenuation, especially if there are ventilation and other openings. To improve the situation, one tries to attenuate the sound passing through the ceiling or in the plenum path as much as possible. Closing off the gap above the partition with blocking panels of materials such as gypsum wallboard or mass loaded vinyl, will give significant improvements. Good results have been obtained recently by stuffing the gap with a pile of glass fiber batts about 400 mm wide. All leaks should be reduced as much as possible, especially those at the junction of the partition and the ceiling. Since the plenum is usually used for ventilation, any air flow paths that might increase sound transmission should have adequate sound attenuation, for example, lined ducts should be used.

4. If apartment doors have improved sound Insulation, as you suggest, does this not mean that fire alarms are likely to be less audible?

Yes, if the alarms are in the corridor. However, even in apartments where doors are not particularly good, fire alarms may not be audible if there are too many doors between the alarm and the occupants. It is preferable to have fire alarms inside apartments, with electrical connections so that all alarms are triggered by one. When selecting the location of fire alarms in a home or an apartment, it is necessary to know the sound power of the device, so that reasonably accurate predictions of its effectiveness can be made.

5. What is involved in making a measurement of sound transmission loss in a building?

A loudspeaker and amplifier are used to generate loud random noise in a room on one side of the partition under test. For at least 16 frequencies, starting at 125 Hz and ending at 4000 Hz, the average sound pressure level is measured in this source room and the room on the other side of the partition. The difference between these two sets of numbers is called the noise reduction. The larger the area of the partition separating the two rooms, the more sound energy will pass through it. The more absorptive the receiving room, the lower the sound pressure level in that room will be. The noise reduction values are therefore adjusted to compensate for these two factors to give the sound transmission loss; this is a measure of the sound power transmitted per unit area and is independent of the area of the partition and the receiving room properties. The transmission losses are used to calculate the sound transmission class.

ASTM E597 describes a quicker procedure, which gives a number very close to the field sound transmission class. It provides a convenient way of rapidly testing several or all of the party walls and floors in an apartment without the expense of the full test. Ratings from E597 are usually within about 2 dB of the FSTC rating for the partition.

6. What is the difference between noise isolation class and field sound transmission class?

If noise reduction values are treated as though they were transmission loss values, and the sound transmission loss contour is fitted in the usual fashion, then the number so derived is called the noise isolation class. No adjustments are made for partition area or room absorption and no steps are taken to eliminate flanking paths. The noise isolation class therefore characterizes the sound transmission between two rooms, taking into account all flanking paths and the physical conditions at the time of measurement.

7. How does the field sound transmission class relate to the value measured in the laboratory?

There is some controversy over this issue. If there is significant flanking transmission in a building, the apparent sound transmission class will be lowered. One can expect to get ratings as much as 5 dB lower in the field than those obtained in the laboratory, but with good construction practice, values almost as high as those in the laboratory can be achieved.

8. If the sound insulation were measured for a large number of party walls in an apartment, would the FSTC ratings vary significantly?

This is an area where not enough is known. A certain amount of variation is associated with the test procedure itself, but this is small relative to the amount of variation associated with the building. This will be determined by the degree of care used to make sure that partitions and floors are identical and constructed properly and that there is no flanking transmission. However, if a field test is performed and the field sound transmission class is found to be 5 dB lower than the expected value, then something is wrong.

9. Does plastic piping Increase or reduce plumbing noise?

The type of piping used in plumbing systems is not the major factor controlling the noise generated in a building. There are differences between the various materials and in general, the heavier the pipe, the less sound is radiated. The noise radiated from pipes is usually negligible compared to that radiated from the partitions and the structure. It is much more important to ensure that all resilient supports for the plumbing system are properly installed and that quiet fixtures are used where possible.

10. What kinds of resilient materials are used below floating floors?

A floating floor system is designed to have a fundamental resonance far enough below the frequencies of interest that vibration transmission is significantly reduced for frequencies higher than this resonance. The mass of the floating layer and the stiffness of there silent supports determine this frequency. Materials used as resilient supports include rubber or neoprene pads, cork pads, some special composite materials such as glass fiber/neoprene and, in some critical applications, steel coil springs. In some cases, the whole sub floor is covered with a blanket of mineral wool, which provides sound absorption as well as resilient support.

11. Can suspended absorbing panels be used to control reverberation in spaces such as a school gymnasium and how should they be designed and installed for maximum effect?

Suspended absorbing panels or unit absorbers are an effective and practical solution to controlling reverberation in spaces such as industrial locations and school gymnasia. Although manufactured unit absorbers of various shapes are available, one can construct units from standard 0.61 by 1.22 m (2 by 4 ft) rigid absorbing panels such as those used in some suspended ceilings. Placing two 2.5 cm thick panels back to back in a suitable frame will produce a unit absorber with more than double the sound absorption expected from tests on larger panels of the same total thickness of material (5 cm) measured against a rigid backing. This is due to the small size of the panel and to the fact that both sides of the panel are exposed to the sound field. The low frequency performance of the unit can be increased by including an air space between the two absorbing layers, or by using thicker layers of absorbing material.

It is generally more satisfactory to hang panels vertically so that both sides of the panel are fully exposed to the sound field. The panels should be distributed evenly throughout the room. When panels are hung horizontally, there will be decreased absorption at medium and higher frequencies as the panel is moved closer to the ceiling. With horizontal panels, increased absorption can be produced over particular narrow low frequency ranges, but these increases are not large and will be associated with a loss of absorption at medium and high frequencies. When panels are installed too close together, they tend to shadow each other and the absorption per panel will decrease. Results from one set of tests for 0.61 by 1.22 m panels suggest that this effect becomes significant at densities of greater than about one panel per three square metrics of ceiling. Of course, as more panels are added there will always be an increase in the total sound absorption, but the average absorption per panel may decrease.

12. Can carpeting be used on walls as a physically rugged, but acoustically effective sound-absorbing material?

This is probably not the most cost-effective solution and in some cases may not be permitted by fire regulations. In general, carpets are only good sound absorbers at high frequencies, and not very effective at lower frequencies, due to their not being very thick. Porous sound-absorbing materials must approach wavelengths in thickness to be optimum absorbers. At a frequency of 100 Hz, typical of many bass sounds in music, the wavelength of sound is 3.4m.Thus it is nearly impossible to have an optimum porous absorber at this frequency. However at higher frequencies, where the wave length is shorter, thin porous layers such as carpets are more effective sound absorbers. The actual mid-frequency absorption of carpets can vary considerably. Where the carpet is porous to airflow, then thicker and denser carpets will tend to be better absorbers. In some cases, carpets that are rubber-backed or that have a sealed backing on a foam pad can have greater mid-frequency absorption because of membrane absorber effects.

Other absorbing materials can be protected by covering them with wooden slats, perforated metal, expanded metal, or other such constructions. One should leave at least fifty percent of the porous absorbing material exposed to sound, to avoid reducing its effectiveness. When the percentage of exposed surface is much less than this, resonant tuned cavity absorption effects can occur; increased absorption will be found at particular frequencies, while overall absorption will decrease.

13. How should one select a duct silencer?

One must consider three factors: the acoustical insertion loss or attenuation, the self noise or flow noise produced by the silencer, and the static pressure drop induced by the silencer. When a muffler is found with a suitable static pressure drop, the insertion loss in decibels must be large enough to reduce the fan noise levels below specified criteria for the rooms that the fan is supplying. Muffler insertion loss is usually specified in octave band frequency ranges. Typically, silencers are much less effective at low frequencies and adequate performance in this range is particularly important. One should obtain the manufacturers insertion loss data measured at approximately the same flow velocity as will be used in the planned installation, because insertion loss values tend to decrease with increased air flow velocity.

It is also important to verify that self noise of the silencer (the noise produced by air flow through the silencer) is not substantially greater than the attenuated fan noise. The self noise will vary considerably with air flow velocity, so the noise generated at the velocity planned for the installation must be known. If the self noise exceeds the attenuated fan noise, then an optimum silencer has not been chosen. Another silencer should be selected, with a little less insertion loss and less self noise. For example, many mufflers splitters (absorbent structures in the middle of the muffler); these tend to increase the insertion loss but also to increase the self noise and the static pressure drop of the muffler. Thus, where the maximum possible insertion loss is not required, there are choices to optimize the effectiveness of the silencer.

14. Does painting a ceiling tile reduce Its sound absorption?

Ceiling tiles absorb sound because they are porous to the movement of air associated with sound. Painting a ceiling tile could completely seal the surface and drastically reduce its effectiveness as a sound absorber. It may be possible to add one very thin coat of non-bridging latex paint without severe degradation of the absorption of the tiles. Tiles that have rough textured surfaces will be less severely degraded by such a coat of paint, as the surface is less likely to be sealed. The method of application is also of some importance. Spray applications will drive the paint into the pores of the tiles and vigorous brushing could have the same effect. Light application with a roller should give the best results.

15. Auditorium acoustics seems to be more an art, or perhaps a black art, than a science. Are there newer, more scientific, approaches to assessing the acoustical problems of auditoria?

The question of auditorium acoustics is a little beyond the scope of these seminars, but there certainly have been considerable developments in this field. Reverberation time is no longer the sole objective measure for assessing a hall. A number of newer acoustical measures relate to subjective assessments of halls. NRC has developed efficient methods for measuring these quantities, and has assisted clients by evaluating various auditoria.

16. Does the stud spacing in a wall affect the sound transmission class?

From theoretical considerations, we expect that the STC may be slightly reduced by reducing the spacing between studs or by decreasing the space between the nails or screws fastening the gypsum board to the studs. Only a few laboratory tests of these effects are available, and no clear conclusions can be drawn from them. For typical spacing of studs and fasteners, the STC is unlikely to vary by as much as 2 dB.

17. Are all resilient channels and all lightweight steel studs the same?

How much vibrational energy is transferred through resilient steel channels or steel studs depends on their stiffness. This depends not only on the material (usually 24 gauge sheet steel) but also on how they are formed. Independent test data for reliable comparison of the effectiveness of various products are not available.

18. Is there a significant difference between solid wood floor joists, truss joists, or wooden trusses? Is it better to use separate joists to support the ceiling in a floor system?

For acoustical purposes, there is little difference among common joist types. They must be quite stiff to support the floor, and hence effectively transmit vibrations from the floor above to the ceiling below (or vice versa). Using resilient channels to support the ceiling reduces this problem, and using separate joists to support the ceiling is even better. In actual buildings, eliminating this path for vibration transfer may give little benefit because of vibration transfer between the floor and the supporting walls.

19. How much sound-insulating material is optimum in a wall or a floor? Are the fit of the insulation in the cavity, and the density or absorption coefficient of the sound-insulating material significant?

The highest STC should be obtained when the cavity is almost full, but the difference between two-thirds full and completely full is unlikely to change the STC of typical partitions by more than one decibel. The absorptive material must not be packed too tightly into the cavity, as this can provide an additional path for vibration transfer. A porous, acoustically absorbing material is needed, but available test data have not clearly established strong dependence of the transmission loss on the absorption coefficient or density.

20. Is there a preferred way of attaching multiple layers of wallboard together? Is glue better than screws? Is there any benefit to having unbalanced construction (i.e., more sheets of wall board on one side than the other) in walls or floors?

Multiple layers should be attached together so that the intervening airspace is as small as possible (Preferably much less than 1 mm) to avoid problems with the mass-air-mass resonance. The difference between using screws or spots of glue appears to be small, and with these methods of fastening, the distribution of sheets of wallboard between the two faces has little effect on sound transmission. There is a slight benefit from using layers with different coincidence frequencies, but this generally affects only high frequency performance, and may not change the STC. Different coincidence frequencies may be obtained by using gypsum board of two appreciably different thicknesses (for example 13 mm and 25 mm) or by fastening the layers on one face of the partition with widely spaced screws or spots of glue, and on the other face with glue spread over the whole surface.

21. If I have the choice of doubling the airspace in a cavity wall or of doubling the number of layers of wallboard on each side, which should I choose? Is there a practical upper limit to the airspace that can be used in cavity walls?

The relative merits of doubling the airspace or doubling the surface mass depend on the specific details. Some of the acoustical effects depend on the mass only, some on the spacing, and some on both factors. For intermediate spacing (from 25 to 100 mm) the effect of doubling either factor is similar, but for very large or small spacing the gain from doubling the airspace may be slight. For large air spaces (above 150 mm), increasing the airspace provides some additional gain, but is less effective than increasing the mass.

22. Fastening a layer of gypsum board to the inner face of the studs In a double stud wall reduces the STC; is there a negative effect if a layer of rigid glass fibre or wood-fibre board Is used, or If the sheets of gypsum board are just leaned in place between the two rows of studs with some gaps?

Acoustically, it is not a good idea to put solid layers in the middle of a cavity wall or floor. This holds for gypsum board or wood-fiber board. If the glass fiber were porous enough, it would not cause unwanted resonances but there is no acoustical advantage to using this kind of material in preference to balls. Gypsum board or wood-fiber board placed loosely in the space between the two rows of studs might not reduce the transmission loss as much as having the material rigidly attached but would give little improvement in the system sound transmission loss; using this material on the exterior surface of the partition is more effective.

23. In relatively warm climates, like Vancouver, it is difficult to justify economically the use of double glazing on the basis of energy savings. Is it possible to get high sound insulation without double glazing?

Sealed single glazing gives sound transmission class ratings of about 30 to 35, depending on glass thickness and other details. With laminated glass the STC is somewhat higher, but quite thick glass is required to exceed STC 40. Whether a double window or laminated glass is less expensive would depend on local prices and the sound insulation required.

24. When using laminated glass, Is 6 + 6 mm laminate preferable to four layers of 3 mm glass?

The advantage of laminated glass is largely due to the energy losses in the layers binding the glass together. How much the transmission loss is improved depends not just on the number of layers, but also on their characteristics. With suitable interlays, the number of layers should not be significant.

25. For windows with double glazing, what are the benefits of using two different thicknesses of glass, or slanting the glass on one side so the airspace is not uniform?

Slanting the glass has negligible effect on sound transmission through the window (although it may be useful for avoiding problems with optical reflections). Using two different thicknesses of glass gives a small improvement in STC. This is discussed in JD. Quirt, Sound Transmission Through Windows, CBD 240, Division of Building Research, National Research Council Canada, Ottawa,1985.

This article was published as part of the technical documentation produced for Building Science Insight '85, "Noise Control in Buildings," a series of seminars presented in major cities across Canada in 1985.