Acoustical Design Performance

In multifamily buildings, sound transmission between dwelling units is understandably a significant design issue. In an ideal setting, the only sound that we would hear inside buildings and spaces would be the intentional, desirable ones. But, we all know that is not automatically true since unwanted sound or background noise can readily penetrate between walls or floor/ceiling assemblies. Such sound intrusion can create a lack of privacy or disrupt the normal activities of the people inhabiting the dwelling units. A certain amount of background noise is certainly commonplace and sometimes preferable depending on the setting. However, excessive background noise can seriously degrade the ability to understand speech, listen to music, or most notably, to sleep. Based on all of this, architects and others have routinely sought out effective methods, materials, and assemblies that can reduce or control noise intrusions into dwelling units. As already noted, MgO panels offer a means to provide enhanced acoustical performance and improved living conditions.

Acoustics Principles

Sound is typically characterized both by loudness and frequency. Loudness is measured in decibels (dB) such that the higher the dB rating, the louder the sound. The commonly referenced range is 0 dB (threshold of hearing) to around 130 dB (threshold of pain) with human speech being in the middle around 50 to 65 dB. Sound frequency, meaning the time frequency of radiating waves of sound, is measured in Hertz (Hz). One Hz is equal to a frequency of one cycle per second. The human ear can typically hear frequencies or sound “pitch” between about 20 Hz (very low pitched, bass sounds) up to around 20,000 Hz (very high-pitched, piercing sounds). Sound exists above and below this range, but our ears do not hear it, unlike some animals such as dogs or bats that do indeed hear very high-pitched sounds (over 20,000 Hz) or elephants that can hear lower sounds (below 20 Hz).

To limit or reduce the amount of sound entering or leaving a space, the different materials and assemblies that enclose that space must be addressed. The goal is to control or attenuate the amount of sound passing through walls, ceilings, floors, etc. such that an acceptable level of background noise is achieved. Conversely, it may be equally important to contain the sound in a given space so it does not spill out to other areas. Either way, there are some common and useful means to measure sound transfer and attenuation.

• Transmission loss (TL) is a fundamental measurement of the ability of a material or building assembly to block or reduce sound. It is measured in decibels at different frequencies to determine how much sound transmission is lost at each measured frequency. Generally speaking, a TL of 10 means that the sound is 10 dB quieter on the listening side compared to the sound originating side.
• Sound transmission class (STC) is determined by ASTM E90: Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements. The results of this test determine the STC rating of the tested materials or assemblies. A higher STC rating (50–60 or above) means that more sound is abated while a lower STC rating (35 or less) means that more sound is transmitted through. The STC number is derived from values tested at sixteen different standard frequencies ranging from 125 Hz to 4000 Hz. Acoustical engineers fit these values to the appropriate Transmission Loss curve to determine a final STC rating. There is also an option to determine STC levels based on field measurements of a space, which may be appropriate for testing existing buildings. Note that the STC measurement is accurate for speech sounds but less so for amplified music, mechanical equipment noise, transportation noise or any sound with substantial low-frequency energy below 125 Hz.
• Impact insulation class (IIC) is a measure of the ability of a floor-ceiling assembly to absorb or deflect sound from impacts (such as people walking or objects dropping) and keep it from being transmitted to the space below. As such, IIC is a measure of structure borne sound, not a measure of airborne sound. It is determined by ASTM E492: Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine. This test method prescribes a uniform procedure for reporting laboratory test data, that is, the normalized one-third octave band sound pressure levels transmitted by the floor-ceiling assembly created by a standard tapping machine. The results are based on the measured impact sounds in the space below and result in a numeric rating. A higher IIC rating means that more sound that is absorbed while a lower number means more structure borne sound is transmitted through.

Shown are magnesium oxide (MgO) subfloor panels being used on a multifamily construction project.

Building Codes and Sound

Building codes and other standards have long recognized the significance of acoustical control, particularly in dwelling units. Historically, Chapter 12 of the IBC dealing with interior environments has included a section on sound transmission. In the 2015 version of the code, these requirements were expanded to apply not only to dwelling units but to any sleeping rooms as well. That means that not only multifamily buildings, but other residential occupancies such as hotels, motels, dormitories, boarding houses, etc., all need to address the requirements of Section 1207 on sound transmission (changed to Section 1206 in the 2018 IBC).

The background for the development and updates to the code language can be found in a publication by the International Code Council (ICC) titled ICC G2 2010 – Guideline for Acoustics. Cited there is a 1995 British report titled “Building Regulation and Health” that, among other things, looked at the number of extremely severe health risks that are due to noise. Also cited is a Canadian study of over 600 families living in multifamily housing that concluded “Noise from neighbors in multi-unit buildings is a serious problem that degrades the quality of life of the residents.”

Based in part on this research, the IBC has consistently set the minimum threshold for airborne sound transfer in common interior walls, partitions, and floor/ceiling assemblies as no less than an STC rating of 50 for laboratory determined levels and 45 if field tested. Similarly, the IBC calls for a minimum structure borne sound transfer in floor/ceiling assemblies as no less than an IIC of 50 for laboratory-determined levels, 45 if field tested. The research cited above, however, suggests that these code minimums might fall short of satisfying many people when it comes to acoustical control in their spaces.

Based on all of the above, the ICC Guideline for Acoustics actually recommends acoustical performance beyond the current code minimum. While this higher performance is not mandated by the IBC, it does point to the common evidence-based need to exceed the code minimums to satisfy the greatest number of people and protect their health and welfare.

Achieving Acoustical Performance

To reach the STC and IIC levels that are either minimally required, acceptable, or preferred, especially in wood framed construction, attention to detail and material choices is required. In conventional terms, that can mean adding sound-deadening material, adding more sound reflective material, offsetting components, or using other extraordinary means to achieve the intended results.

One alternative to some of these conventional approaches is to consider the use of MgO panels in the floor/ceiling assembly as a primary or secondary subflooring material. There may be a misperception by some designers that adding a material such as MgO to a floor assembly will add mass and some sound deflection but not really increase STC or IIC values. In fact, MgO has been used as a subfloor material in a variety of different assembly types and has been tested according to the code requirements and guidelines. The results show that ratings of STC-60 are indeed readily achievable using MgO underlayment along with rather conventional methods and materials in the floor/ceiling assembly. Similarly, ratings of IIC 53–58 have been demonstrated in the same manner, thus exceeding code minimums in these cases.

MgO can be used in different flooring assemblies to help improve the acoustical performance of those assemblies.

Overall, MgO can be a significant acoustical contributor to floor/ceiling assemblies in multifamily buildings or other building types that contain sleeping rooms. It can help designs meet or exceed the code minimum or the preferred levels of sound control that is best suited for human health and wellness.