Controlling mechanical system noise

by Aaron Bétit
A typical urban and suburban environment has numerous sources contributing to the exterior ambient noise. Among these are the environmental sounds from a building’s heating, ventilating, and air-conditioning equipment. How can design professionals help mitigate the distraction caused by HVAC?

In addition to specific air exchanges and heating/cooling requirements, mechanical systems must comply with the local noise ordinances and operate without disturbing adjacent properties. The proliferation of mixed-use developments can increase the concerns of mechanical system noise due to the blend of commercial, cultural, residential, and industrial uses in directly adjacent properties. For development to succeed, it is important to design buildings that control noise appropriately.

Environmental noise
Noise is unwanted or objectionable sound as translated from minute air pressure fluctuations by our ears. The fundamental measure of sound amplitude is the decibel, which is abbreviated to dB; as people can hear a large change in sound pressure, the decibel was created as a logarithmic metric.

Human reaction to a sound varies depending on the frequency content, the duration, the amplitude of other noise sources, and the time the noise level
is generated. Some examples:

• 
frequency content—the noise generated by a window fan can be significantly less distracting compared to the sound of a garbage truck backing up, even when compared at the same amplitude;
• 
duration—a short report of a warning siren once
a day at noon is considerably less disturbing than the sound of a neighbor practicing on a drum kit in an adjacent garage for hours on end; and
• 
time of day—if an air-cooled chiller is operating at a consistent amplitude directly outside an apartment façade, it will typically be less distracting in daytime hours when the receptor is active than at night, when a receptor is attempting to sleep. The noise may seem louder at night than during the day.

The ear does not respond equally to high- and low-pitch noises. Starting in the 1930s, scientists developed response characteristics to represent the sensitivity of a typical ear. Adjustments to amplitude based on frequency called ‘A-weighting’ were developed to allow for a metric that more closely follows the human reaction to noise. A-weighted decibels—dBA—are modeled after the sensitivity of the ear at sound levels commonly found in the environment. In most cases, the A-weighting decibel is the standard metric used to evaluate exterior noise.

The ambient noise level’s duration is another important aspect of human response to noise. Numerous metrics have been developed to help determine the impact of ambient noise levels over a period. In the most basic form of evaluation, sound pressure is averaged over a specific length of time. This is referred to as the Equivalent Sound Level (Leq).

While the Leq metric is appropriate for evaluating continuous noise sources, there are times when the noise of concern is not continuous—some mechanical equipment has a cyclical noise, or only occurs in short durations. To evaluate these conditions, the Maximum Sound Level (Lmax) can be used to document the highest sound pressure level from an activity. However, it is important to note Lmax can be compromised by anomalies in the data or by extraneous sound sources. In part for that reason, percentile noise metrics (Ln, where n is the percentile of interest) are often used to characterize environmental noise.

The 90^th percentile noise level (L90) is defined as the noise level exceeded 90 percent of the time during the measurement period; it is a reasonable proxy for ‘ambient’ levels used in some local and state regulations. The 10^th percentile noise level (L10)
is sometimes used to characterize transient noise. Similarly, the median noise level (L50) is used in lieu of the mean or Equivalent Noise Level (Leq)
by some local ordinances.

Twenty-four-hour metrics document an ambient noise environment over a typical day and are often used when evaluating community noise. The Day-Night Sound Level (Ldn) is measured in dBA and describes the receptor’s cumulative noise exposure from all noise events during a 24-hour period. The events between 10 p.m. and 7 a.m. are increased by 10 dB to account for greater nighttime sensitivity to noise.

The State of California developed a 24-hour metric, the Community Noise Equivalent Level (CNEL), to document the ambient noise levels during a typical day. Rather than dividing the day into two periods like the Ldn metric, the CNEL metric includes a third period to address the time when people are likely to be engaged in outdoor activities around the home. Between the period of 7 p.m. and 10 p.m., the measured noise levels are increased by +5 dB to reflect additional annoyance noise causes during this time. In most cases, the difference between Ldn and CNEL is slight, and the two measures are fairly interchangeable.

State and local noise limits for intrusion across a property line have been established for most developed areas in the United States. Guidelines have also been published to help evaluate the appropriate use of specific land areas based on the ambient noise environment. In the absence of local noise limits, the land use table in Figure 1 can be a good resource for evaluating impact to adjacent properties.

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