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What you need to know about Room Acoustics

In the Southeast Asia region especially, acoustic properties of residential buildings are often neglected by designers, developers, contractors, and even home buyers. Noises from both internal and external environments affects occupants’ daily lives, causing nuisance which can strongly deteriorate one’s living quality as a long-term effect. In this article, we will investigate building/room acoustics, and the actions that can be undertaken to improve the acoustical environment inside a building.

Room acoustics

In general, the acoustics of rooms can be divided into two groups: low frequency and high frequency. Sound in rooms can be highly affected by the reflective properties of the surfaces in the room. This is because multiple reflections may occur if the room surfaces are highly reflective, which then leads to a reverberant field in addition to the direct field from the source especially at higher frequency range. Therefore, at any point in the room, the overall sound pressure level is influenced by the energy contained in both the direct and reverberant fields (Crocker, 2007).

Sound transmissions in buildings

Sound can be transmitted within a building by transmitting through air in the spaces bounded by walls or roofs/ceilings, known as airborne transmission. Another way would be through structural transmission through the structural assemblies of the building, or impacts.

Airborne sound originates from a source that radiates sound waves into the air, which would then impinge on the building surfaces. A good example of airborne sound will be speech, or music from a television or loudspeaker. On the other hand, impact sound is being generated when an object strikes the surface of a building. The commonly heard impact sounds that we can hear in buildings are footsteps, furniture-dragging sounds, cleaning, and other equipment that is used directly on the floor surfaces. To overcome these noises, good sound isolation should be considered for all the possible paths for sound and the junctions between walls and floors, not just at the direct path through common wall or floor.

Sound insulation – airborne and impact

It is imperative to consider the control of airborne and impact sound transmission through the building elements like walls, ceilings, or floors, as stated above. In this case, sound insulation methods will be crucial. Different methods can be implemented for airborne, impact and flanking sounds (Crocker, 2007).

For airborne sound, insulation can be applied at any building element. This is because when sound hits on a surface, a very small fraction of the incident energy will be radiated from the other side. The sound transmission loss (TL), which is the ratio of the incident sound energy relative to the transmitted sound energy is typically measured. TL can be expressed in decibels (dB), and it is sometimes known as sound reduction index (R) in European and ISO standards. The elements to be used in buildings for sound insulation are measured in accordance with standards, where the commonly seen method would be the two-room method. A test specimen would be mounted between a reverberant source room, and a receiver room such that the only significant path for sound to transmit through is the specimen, and other possible transmission paths would be suppressed. As such, it will be useful to determine the TL of the building elements/materials so that one can estimate the airborne sound insulation performance inside the building space.

As for impact sound which typically radiates from a floor into rooms below or horizontally, insulation can be done via floor coverings or floor slabs. This is because the applications of these items can reduce the impact sound pressure levels that travels into the receiver room. The typical methods of insulation are adding soft floor coverings on concrete slab, increasing the thickness of concrete floors, or implementing floating floors.

Single number ratings

To know the acoustic information of an insulation element, the standard method would be to refer to the single number ratings of that element. These ratings would be assigned to building materials based on their sound transmission spectra by the means of reference curves or weighted summation procedures.

The most used single-number rating for airborne sound insulation is the Sound Transmission Class (STC), which is in accordance with the American Society for Testing and Materials (ASTM) E413. There is another equivalent number called the Weighted Sound Reduction Index (Rw), which is based on the International Organization for Standardization (ISO) standard ISO 717.

The figure above shows an example of STC contour fitted to a concrete slab’s data. The differences between data points below the contour line and the value of contour are called the “deficiencies”. According to ASTM E413, the sum of deficiency should not be greater than 32 dB, and each individual deficiency should not exceed 8 dB (also known as the 8-dB rule). The reference contour for ASTM covers the frequency range from 125 Hz to 4000 Hz. The Rw contour from the ISO 717 has the same shape, except that it covers a broader frequency range of 100 Hz to 3150 Hz. Also, there is no 8-dB rule in ISO 717. Comparing both standards, the numbers from both ratings are usually close. However, the weighted summation method developed in ISO 717 accounts for the higher importance of low frequencies in traffic noise and modern music systems. As such, this method allows corrections/spectrum adaptation terms to be produced that can be used in conjunction with the Rw rating.

As for impact sound insulation, the sound pressure levels are often collected using a standard tapping machine and normalised, which will then be used with a reference curve to calculate its rating, typically the Impact Insulation Class (IIC), or the weighted index Ln,w. In fact, these ratings are commonly used in building codes. Again, the rating curves are identical in each standard, but there are some differences among them still. For instance, the ASTM IIC method does not allow any unfavourable deviation to exceed 8 dB. An increasing IIC rating would indicate that the impact sound insulation improves. Conversely, the Ln,w rating would decrease as the impact sound insulation gets better. We can take the relationship between both ratings as follow (assuming that the 8-dB rule is not invoked):

However, there is debate regarding the usefulness of ISO tapping machine data obtained on different types of floors. Therefore, the latest version of ISO 717-2 proposed the use of C1, a spectrum adaptation term to consider low-frequency noise that is normally generated below a lightweight joist floor.  is the unweighted sum of energy in the one-third octave bands (50 or 100 Hz – 2500 Hz) minus 15 dB. According to the standard, this rating is expected to have a better correlation with the subjective evaluation of noise coming below floors, especially for low frequency ones.

The single rating numbers mentioned above are all useful when it comes to determining the level of acoustic insulation a material can provide. With the explanation above about room acoustics and the insulation measures that can be implemented, it will give a better idea on how one should tackle and handle the room acoustics in a building.

References

Crocker, M. J. (2007). Chapter 103: Room Acoustics. In C. H. Hansen, & M. J. Crocker (Ed.), Handbook of Noise and Vibration Control (pp. 1240-1246). Adelaide, South Australia, Australia: John Wiley & Sons, Inc. doi:ISBN 978-0-471-39599-7

Crocker, M. J. (2007). Chapter 105: Sound Insulation—Airborne and Impact. In A. C. Warnock, & M. J. Crocker (Ed.), Handbook of Noise and Vibration Control (pp. 1257-1266). Ottawa, Ontario, Canada: John Wiley & Sons, Inc. doi:ISBN 978-0-471-39599-7

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