Blocking exterior noise sources

New England Conservatory Jordan Hall_Acentech_photo credit Robyn Ivy
All photos © Robyn Ivy

by Terence Tyson, PE, CDT
Whether at work or home, indoor acoustic comfort requires freedom from unwanted sound. Exposure to elevated noise levels may disrupt sleep and interfere with activities requiring concentration. Left uncontrolled, long-term exposure to noise levels not otherwise considered hazardous can lead
to stress.

Often, high levels of indoor noise are caused by exterior sound sources, such as vehicular traffic, rail, and aircraft operation, mechanical equipment, crowd noise, and amplified sound systems as well as sources associated with construction activity. This article examines the effect common sources of exterior noise have on noise levels experienced inside commercial, residential, and institutional buildings, and discusses classification of those receiver locations in terms of relative sensitivity to noise.

This author also reviews the relevant associated guidelines, regulations, and metrics, including thresholds of acceptability and thresholds of change, and statistical sound measurements. Finally, options for mitigation to reduce indoor noise exposure caused by exterior sources will be examined.

Noise metrics
Sound is a complex phenomenon that occurs across a broad range of frequencies, from very low to very high. A complete description of a particular sound at a given instant in time would require denoting the loudness of each individual frequency across the entire audible spectrum. To avoid this, single-number simplifications of sound spectra have been developed and are used routinely to describe and regulate sound levels.

The first such measure, dBA, is a sort of corrected sound level that compensates for the human ear’s relatively poor sensitivity to low frequency sound. ‘A-weighted’ sound levels are easy to measure, and tend to correlate with the annoyance factor of many environmental noise sources. It is therefore the metric most frequently referenced in codes and ordinances.

Since sound is not necessarily constant over time, reporting a sound level as a single number average of some kind requires accounting for this time-based fluctuation. One such metric is denoted Leq, which represents the energy average of all sound measured during a particular measurement period. It is defined as the sound pressure level that, if constant for the duration of the measurement, would contain the same total sound energy as the actual time-varying sound recorded during that period. This metric is commonly used to represent the overall average sound pressure level recorded for a given measurement.

New England Conservatory #1_Acentech_photo credit Robyn Ivy
The New England Conservatory’s Jordan Hall has a capacity of 1051 and is a National Historic Landmark. Its practice and concert spaces are particularly ‘sound-critical,’ so the building’s location near outside noise sources involved overcoming certain challenges.

Additionally, statistical measures accounting for the fluctuation of sound over time are also useful. L10 for example, denotes a sound level that is exceeded 10 percent of the time during a particular measurement, and takes into account sporadic or intermittent noise peaks. It has been found to correlate well with the disturbance people feel when close to busy roads. Similarly, L90, which describes the sound levels exceeded 90 percent of the time, is generally considered to represent the residual background noise level recorded in outdoor measurements.

The day-night equivalent sound level (Ldn) is another important metric for assessing outdoor noise sources. It is defined as a 24-hour continuous Leq with a 10-dB penalty added to all noise levels recorded between 10 p.m. and 7 a.m. The penalty accounts for the extra sensitivity people have to noise during sleeping hours.

Many design/construction professionals are familiar with sound transmission class (STC). It is a single-number rating of the airborne sound transmission loss performance of a partition, door, or window assembly. The higher the STC value, the more effective an assembly is at blocking sound. However, it is important to note STC is weighted toward the measurement of isolation of speech frequencies. It is not a reliable measure of performance at low frequency.

To account for low frequencies associated with exterior noise sources, outdoor−indoor transmission class (OITC) is used to rate sound transmission across a building envelope. It is similar to STC, but is weighted to account for lower frequencies, and as such is the performance metric considered in this article.

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