Interior Building Acoustics

Kanwall Inc.

Introduction

Everybody knows about acoustics – it is part of our daily lives. We are surrounded by sound. It is essential for communication and to provide audible warnings. If we like what we hear it can give pleasure but as noise (unwanted sound), it can be disturbing, stressful and result in a loss of efficiency.

However, not everyone understands in detail how sound behaves within and around buildings. For the design team, and for contractors and suppliers, misunderstandings can lead to the wrong and inappropriate selection and use of acoustic products and materials, often with unfortunate results. Therefore the theme of this article is to explain the most important principles of building acoustics, describe how practical solutions can be found, and clarify the myriad of different units and criteria which are used.

CBD-10: Noise Transmission in Buildings

The old-fashioned building, in which massive walls supported massive floors, provided among its hidden virtues considerable protection against the transmission of noise. In decided contrast is the modern building, in which lightweight construction is the aim, and in which the walls, divested of their load-bearing responsibilities, have become merely curtains. The result is that in the modern building sound insulation must be dealt with explicitly, as a basic requirement.

It is proposed to discuss here the chief ways in which noise considerations affect the design of buildings, illustrating with three common structures: the apartment building, the office building, and the industrial plant. The analysis will be made as quantitative as this brief treatment will permit. A few special construction details will be discussed in the final section.

The most important point to be made is that the successful design is one in which noise is a consideration at every stage. A noisy site, for example, will strongly affect the over-all design of a building, and may necessitate costly noise control measures. Similarly within the building the spatial separation of quiet and noisy regions will simplify the problem of achieving adequate noise insulation between them. Finally, noise insulation design must be carried through to the last detail: there is no merit, for example, in specifying a good partition and then allowing it to become punctured with service outlets, or badly fitted doors.

Apartment Buildings

The problem to be considered in apartment buildings and other multiple-dwelling structures is the transmission of noise from one dwelling unit to another (on the assumption that the occupants of an individual dwelling can settle their local noise problems among themselves).

Noises produced in apartments vary with the occupants and with their activities of the moment. Similarly one's tolerance of extraneous noise varies with one's own activities; in fact if all the occupants of a building were always doing the same things at the same time there would be no noise problem. More commonly, however, one occupant will be trying to sleep while his neighbor watches a late television show; or a day worker will be living below someone who works in the evening and dines at 3 a.m. The successful building is one that can accommodate the wide variety of tenants and tenant activities amicably. A completely "soundproof" building is not practical; the modest objective in the following discussion is to satisfy perhaps 75% of apartment dwellers 90% of the time.

An apartment building can usually be laid out in plan so that the most critical rooms (bedrooms, living rooms) are protected from adjoining apartments by a buffer zone of non-critical areas such as bathrooms, kitchens, closets and hallways. For separating such non-critical areas a party wall having an average sound transmission loss of 45 decibels * is adequate. The next best arrangement is to place quiet rooms such as bedrooms on the two sides of the party wall; in this case the separation should be a 50-db wall. The worst arrangement is to place a critical region of one apartment adjacent to a noisy region (such as a bathroom or kitchen) of the adjacent unit; even a 50db wall is inadequate in this case.

Similar considerations apply to floors separating dwelling units. For separating critical areas an airborne sound transmission loss of 50-db is necessary. Since in the conventional building pattern it is not easy to vary the floor construction from critical to non-critical areas, it is usually the 50-db requirement that governs. An arrangement with noisy regions over critical regions should be avoided. 1 An additional problem of special importance in floors is impact sound (e.g. footsteps), originating as a vibration in the separating structure itself. Floors separating apartments should provide adequate insulation against impact sounds. Since a satisfactory floor design is the most costly noise control measure it is worth noting that floor transmission is the commonest and most disturbing noise problem in existing apartment buildings.

The floor problem does not arise, of course, in row dwellings. It is possible to minimize it also in apartment buildings by designing two story apartments; if they are planned so that the bedrooms of individual apartments are beneath their own living rooms the impact problem largely disappears. Airborne noise transmission is still important, but this is more readily dealt with.

Office Buildings

Noise insulation requirements for an office building are not as stringent as those for an apartment building. Offices are usually occupied for only about 8 of the 24 hours, a moderate amount of business noise is usually acceptable, and interference with sleep is rarely a concern. The main requirement for partitions between tenants in an office building is speech privacy: The speech originating in one tenancy should not be intelligible in an adjoining tenancy. An exact specification depends upon the ambient noise level in the listening "room", but generally an average transmission loss of 35 to 40 db is adequate. Office building floors usually provide adequate insulation for airborne sound, and impact noise is of minor importance except when heavy machines or other sources of vibration or impact are involved.

Unfortunately, with the current fashion in office buildings, even the modest requirement of speech privacy is not always met. Typically large floor areas are finished without partitions and subdivided to meet tenants' space requirements with prefabricated office partitions. A suspended acoustical ceiling is commonly used to provide an unbroken surface masking miscellaneous pipes, ducts, and electrical services. However satisfying they may be visually, such partitions and suspended ceilings are frequently almost transparent acoustically.

Progress is being made in the development of partitions, and some excellent individual panels are now available. But in typical installations they are joined together by flimsy, leaky cover plates and filler strips resulting in an assembly that has a transmission loss of less than 25 db. Frequently the partitions meet an exterior curtain wall with only a narrow window mullion on which a satisfactory joint must be attempted. Other common features of the curtain wall, such as a continuous perimeter heating strip, introduce sound channels that nullify the value of a good partition.

Finally there is the problem of noise transmission over the partition above the suspended ceiling. This may be prevented (1) by using ceiling panels backed by a heavy impervious layer that reduces sound penetration through the ceiling, or (2) by building adequate partitions in the space above the ceiling.

The whole problem might be simplified by restricting the location of major partitions to modular intervals, perhaps coinciding with the structural module. At these intervals adequate partitions could be provided above the ceiling and suitable joint details could be incorporated in the curtain wall.

Executive offices and conference rooms constitute the most critical noise control problem in office buildings. Speech privacy is usually a requirement, necessitating the same considerations as above even for walls within a tenancy. An additional concern is to provide conditions quiet enough for comfortable speech over ranges of perhaps 10 to 20 ft. Meeting the speech privacy requirements will take care of most business noises except possibly business machine noise. Commonly the remaining problem is the noise produced by mechanical equipment such as ventilators. Quiet ventilator design is primarily the responsibility of the equipment manufacturer and the heating engineer. A suitable performance specification is to require that the equipment should not raise the noise level above the appropriate Noise Criterion given below. These specify noise levels, as a function of frequency that has been found acceptable for the applications indicated. (For further details see tables given below)

NC-30- Executive offices, conference rooms seating 50 people. NC-35- Small offices, semi-private offices, conference rooms seating 20 people. NC-40- General offices, in which speech and telephone communication are important. NC-45- Large general offices, drafting rooms. Normal communications at 3 to 6 ft. NC-55- Business machine rooms, communication in raised voice at 3 to 6 ft.

2 dB Building Element 20 Solid or hollow core door without seals 28 1-3/4” (44mm) solid core door with seals 30 1/4” (6mm) sealed glazing 35 ½” (12mm) sealed glazing 40 ½” (12mm) plasterboard either side of metal stud work with mineral wool in cavity 45 6” (150mm) high density block work wall 50 10” (250mm) cavity block wall 55 10” (250mm) solid in-situ concrete slab

The large general office is a compromise between sound insulation and such factors as space economy, lighting and ventilation. The main noise sources are telephone conversations, and small machines such as typewriters. A partial solution is to use sound absorbing hoods or partial partitions around the principal offenders. More mechanized equipment such as card-punching and sorting machines, and reproduction equipment should be placed in a separate room where possible. Heavy machines should be mounted on properly designed vibration-isolating bases.

Industrial Noise

The typical industrial plant comprises offices, factory areas, storage space, and other occupancies. The special noise control problem is the factory area. No general solution can be prescribed for factory noise since the intensity and character of industrial noises vary widely. Apart from airborne noise there is often a vibration problem, which can usually be solved by providing special foundations or vibration-isolating mountings.

Sometimes the machine manufacturer is prepared to offer guidance; otherwise a qualified acoustical consultant should be retained. The principal considerations will nevertheless be sketched below.

The requirements are: (1) To prevent hearing impairment among machine operators, (2) To facilitate necessary speech communication among operators, and (3) To prevent the transmission of excessive noise into other parts of the building or into adjacent buildings.

The building designers' share in minimizing the first two problems is to provide sound-absorbing surfaces or space absorbers within the factory space. Sound-absorbing hoods or partial enclosures around the principal noisemakers are also sometimes feasible and useful.

The third problem is dealt with as in the preceding examples, except that the frequency content of the noise should be considered in conjunction with the frequency versus transmission loss characteristics of the walls and floors used to enclose it.

Building Materials and Finishes

When sound strikes the boundaries of an enclosed space, e.g. when it is incident on the walls, ceiling or other surfaces, some of the sound energy will be reflected back into the room, some will be transmitted through the boundary into adjacent separate areas, and the remainder will be absorbed within the structure of the material or at its surface.

The sound that is reflected back off the boundaries will affect the levels and quality of sound generated within the room and, not surprisingly, this process is referred to as room acoustics. The sound that is transmitted through the room boundaries will be of most significance as to its effect on adjacent areas, and this relationship is known variously as sound insulation, sound attenuation or sound reduction.

The level and frequency response of reflected sounds will be strongly influenced by the type and absorptive characteristics of the surface materials and how they are mounted. Porous absorbers, typified by fibrous materials such as carpets, fabric covered upholstery, glass and rock fiber quilts and slabs, open cell plastic foams and the like, will provide maximum sound absorption at the middle and high frequencies (> 500 Hz). Alternatively, panel absorbers such

3 as thin rigid sheets of plywood, MDF, plasterboard and sheet metals etc, are more efficient at absorbing the lower frequencies (< 250 Hz). Figure 1 shows the typical absorption performance for these two main groups of sound absorbers. Of course, some common building products are able to provide sound absorption which combines the performance of both panel and porous absorbers and therefore the sound absorbent coefficients for these products is spread over a broader range of frequencies than for the materials of just one type. Figure 2 gives the typical measured values for dense mineral fiber suspended ceiling tiles which demonstrates this point.

Figure: 1

Figure: 2

4 The way in which a material is installed will also affect the amount of sound it absorbs. The performance of porous materials located against a solid surface can be improved significantly if the material is installed with airspace behind. The bigger the airspace, the better will be the improvement, particularly at the lower frequencies, but 50mm cavity depth should be the minimum considered. In the case of panel materials, airspace behind them is essential because if panels are fixed directly to a solid surface they will not be able to vibrate when sound strikes them and so low frequency sound absorption will be minimal.

The absorption efficiency of a material is described by its “Sound Absorption Coefficient” which is a number that can vary between 0 (no absorption or total reflection) and 1.0 (100% absorption or no reflection) and, as can be seen from figures 1 and 2, this generally varies with frequency. The values of random incidence sound absorption coefficients (which are of most relevance within practical building acoustics) are derived from tests undertaken in a reverberation chamber of an acoustics laboratory. The relevant standard in the North America is ASTM C 423, BS EN 20354:1993. (UK) ISO 354:1987 (International) and DIN 52 212 (German) are all very similar test procedures and therefore the results for materials produced and tested in different countries can all be used with equal confidence. There is another more simplified (and cheaper) method of obtaining sound absorbent coefficients using a Standing Wave Tube or Apparatus, (defined in ASTM C 384-90a) but this method only produces values measured at 90° incidence to the material and therefore these results should only be considered as indicative and must not be used for direct comparison with those obtained in a reverberation chamber. It should be noted that due to the way that some materials are installed when tested, the sound absorption coefficient values derived are sometimes reported as greater than 1.0, i.e. more than 100% sound absorption! In practice this cannot of course happen as no building material is capable of absorbing more sound energy than is incident upon it. Therefore product literature and data which claim values in excess of 1.0 should be ignored for practical purposes and in assessments, specifications and calculations all such optimistic (and potentially misleading) figures should be rounded down to 1.0.

When assessing or specifying the sound absorption of different materials it is sometimes cumbersome and over complicated to consider the values at each third octave or octave band. This is where the NRC or Noise Reduction Coefficient (identified in ASTM C 423-90a) can be helpful. NRC is a single value obtained by taking the arithmetic average, to the nearest 0.05, of the four sound absorption values centered on 250, 500, 1000 and 2000Hz, 0.55 at 1000Hz and 0.63 at 2000Hz would have an NRC of 0.5, (not 0.52).

An alternative to NRC, as a single figure descriptor, is the “Weighted Sound Absorption Coefficient” (α w). This has been recently introduced by the publication of prEN 11654.

With this method the measured values of random incidence of sound absorption coefficients obtained in accordance with ASTM C 423-90a or BS EN 20354 are converted into octave bands at 250, 500, 1000, 2000 and 4000Hz and are plotted on a graph. A standard reference curve is then shifted towards the measured values until a “best fit” is obtained. The derived value of α w will vary between 0.00 and 1.00 but is only expressed in multiples of 0.05 e.g. α w = 0.65.

If the measured sound absorption coefficient at one or more frequencies is considerably higher than the values of the shifted reference curve then a “shape indicator” is applied to the weighted sound absorption coefficient. If the excess occurs at 250Hz, the notation L is used. For excesses at 500Hz or 1000Hz M is used and for 2000Hz H is used. So for example, a weighted sound absorption with an excess at 500Hz could be described as α w = 0.65 (M).

It should be noted that the weighted sound absorption coefficient is only intended to be used for routine applications in normal offices, corridors, classrooms, hospitals etc. It is not appropriate for specialist environments requiring expert acoustic design. Also, as the lower limit for the reference curve is 250 Hz, the rating method is not suitable for products which exhibit low frequency sound absorption below this frequency, e.g. panel absorbers.

Room to Room Sound Reduction

In modern commercial developments when large open areas are subdivided into separate cellular rooms, the overall level of sound reduction that is achieved between spaces is often very disappointing. This is generally because there are potentially many sound transmission paths between adjacent spaces and not all of them are always adequately considered and the appropriate level of sound reduction specified.

Possible sound transmission paths that can occur between rooms are:

• Inadequate intermediate partition(s) and whether doors and / or glazing are present. • The suspended ceiling and void if partitions are not erected through the ceiling to the underside of the structural soffit or if effective acoustic cavity barriers are installed. • Recessed air handling luminaries to a common ceiling plenum. • Recessed continuous lighting troughs within a suspended ceiling.

5 • Recessed continuous linear air diffusers within a suspended ceiling. • Ventilation grilles or diffusers connected via a common ceiling or raised floor plenum or sharing common ductwork. • The continuous raised platform floor and common void if partitions are not erected from the structural slab or effective acoustic cavity barriers installed. • Continuous perimeter piped heating or power/data cable conduit systems. • Continuous perimeter curtain walling and internal lining systems. • Doors to common corridors particularly if the doors to adjacent rooms are close to each other and do not have good acoustic frame and threshold seals.

It can be seen that it is not just the obvious paths between rooms, such as partitions or ceilings, which must be considered. Unless all the elements that can conspire to lower the required sound reduction are evaluated at the design stage and the necessary sound accentuating details implemented, acoustically weak paths may result. In other words, increasing the sound reduction of the partition or suspended ceiling to compensate for poor room to room sound insulation will be a waste of time and money if other significant flanking sound transmission paths remain present and are not dealt with.

The sound reduction of specific building elements may be accurately determined by measurement of controlled test samples in an acoustics laboratory in accordance with ASTM E 90, BS 2750 or ISO 140. The section entitled “Applicable Standards” at the end of the text indicates the various parts of the standard which may be used depending upon the element under test. Unlike sound absorption coefficients, which can only be obtained on laboratory test samples and not under site conditions, sound insulation can also be measured for site installations. However it is very important to realize that it is generally not possible to determine, with any degree of accuracy, the site sound reduction of individual elements or components. Only the combined effect of all the elements contributing to the room to room sound insulation can easily be obtained. If the performance of an individual component is required then laboratory tests will be necessary.

Building Details

Transmission Loss of Walls and Floors

Airborne sound transmission losses of representative walls are given in Table I. It will be noted that a high transmission loss may be achieved either with a heavy wall or with a complicated one composed of several relatively independent layers. In either case it is essential that the wall be as air-tight as possible. Cracks or holes at joints or around service outlets can spoil an otherwise excellent construction.

Table I. Airborne Sound Transmission Loss of Typical Walls Transmission loss 50 db or more. (Recommended between critical areas of adjoining A dwellings.) 1 Single masonry wall weighing at least 80 lb per sq ft including plaster if any. Masonry cavity wall - 2 leaves of masonry spaced at least 2 in. apart, each leaf weighing at 2 least 20 lb per sq ft. leaves tied together with butterfly ties at 2-ft2 centers. Composite wall - basic wall masonry weighing at least 22 lb per sq ft; on one side of basic wall 3 an additional leaf consisting of M-in. gypsum lath mounted with resilient clips, %-in. sanded gypsum plaster. Stud wall - 2- by 4-in. studs; on each face ½-in. gypsum lath mounted with resilient clips, ½-in. 4 sanded gypsum plaster; paper-wrapped mineral or glass wool batts between studs. Staggered stud walls - 2- by 3-in. studs at 16-in. centers on common 2- by 6-in. plate; on each 5 face 9-in. gypsum lath, ½-in. sanded gypsum plaster; paper-wrapped mineral or glass wool batts between one set of studs.

Table Contd. to next page

6 Transmission loss 45 to 49 db. (Recommended between non-critical areas of adjacent B dwellings.) 1 Single masonry wall weighing more than 36 lb per sq ft including plaster if any*. 2 Composite masonry – as in A-3 except gypsum lath supported on furring. Staggered stud dry wall - 2 sets of 2- by 3-in. studs at 16-in. centers on common 2- by 4-in. plate; on each face 2 layers of %-in. gypsum wallboard, the first layer nailed, the second 3 cemented; joints staggered and both sets sealed; mineral or glass wool blanket or batts in the interspace. C Transmission loss 40 to 44 db. 1 Single masonry wall weighing at least 22 lb per sq ft including plaster if any*. D Transmission loss 35 to 40 db. 1 Stud wall – 2- by 3-in. or 2- by 4-in. studs 3/8-in. gypsum lath & ½-in. sanded gypsum plaster. Stud wall – 2- by 3-in. or 2- by 4-in. studs, 2 layers of 3/8-in. plasterboard, the first layer nailed, 2 the other cemented, joints staggered. * If porous blocks are used one face of each block section must be sealed with plaster or heavy paint.

Similar considerations apply to floors, with the additional problem of impact noise. Table II lists a few typical floor constructions. The impact ratings shown are based on a nonstandard test, and are for comparison only. For separating dwelling units an impact rating of better than 20 db (in this scale) is recommended. Although a satisfactory impact rating may be obtained with a suspended ceiling arrangement (sec Table II), a floating floor is generally more successful since it prevents impact vibrations from getting into the main structure and thence into adjoining parts of the building.

Table II. Sound Transmission Loss of Typical Floors Impact Rating (db) A Airborne transmission loss 50 db or more. 4-in. solid concrete or equivalent slab weighing at least 50 lb per sq ft; ceiling side 1 30 bare or plastered directly on slab; floor side wood furring, rough and finish floors. As in (1) except floor side 1-in. foamed plastic or paper-covered glass fiber quilt, 2 30 supporting 2-in. concrete. As in (1) except floor side parquet or linoleum; ceiling side wood furring, ½-in. gypsum 3 5* lath, ½-in. sanded gypsum plaster. As in (3) but ceiling side ½-in. gypsum lath suspended on resilient clips, ½-in. sanded 4 20 gypsum plaster. 5 As in (3) but ceiling mounted on separate joints supported at waIls. 25 Open steel joints or similar structure; on floor side form-work, paper-covered glass 6 fiber quilt or foamed plastic, 2-in. concrete; ceiling side ½-in. gypsum lath on resilient 30 clips, ½-in. sanded gypsum plaster. B Airborne transmission loss 45 - 49 db. 1 4-in. solid concrete or equivalent slab construction weighing 50 lb per sq ft. 22 2 As above but floor side finished in linoleum or wood parquet. 5* 3 As in (1) but floor side finished with carpet and underlay. 10* * Impact rating not adequate for separating dwelling units.

7 Doors - Doors should be avoided in partitions for which a high sound insulation is required. Where they must be employed in such walls they should be of solid wood and should be carefully fitted to minimize cracks around them. A refrigerator-type gasket is useful around top and sides; either a sill or a special drop-strip may be used at the floor.

Plumbing fixtures - Plumbing fixtures are troublesome noise producers, and the service pipes may provide an efficient direct path between occupancies. To minimize such problems quiet fixtures should be used; service pipes should be resiliently supported and should not be fastened to critical walls. Doors leading to public washrooms should not open directly into quiet areas and should be well fitted. Ventilating louvers, if any, should incorporate noise filters. In apartment planning it is good practice to group bathrooms and kitchens of adjacent units together, and to locate the services (with due precautions against air leaks) in the party wall. Each set then acts as a buffer against noise from the adjoining apartment.

* The decibel (db) is a convenient acoustical unit used here for specifying the transmission loss of partitions. In this article the values of airborne sound transmission loss are averages based on measurements at 9 standard test frequencies, according to ASTM Recommended Practice E90-55.