Environmental Acoustics

Determining if what one hears is “sound” or “noise” can be a difficult task. What is music to one person is a loud racket to another – just ask any parents with teenage children! It also depends on the time of day that the sound is heard. Sounds heard during the day can often be ignored, but the same sound at night can be extremely annoying (such as your neighbor’s barking dog, or a low flying jet).

During the planning process for a project, noise is just one of the many components reviewed. Will the project increase the community noise levels? Will there be a noise impact on the neighbors during the construction process? Environmental Impact Reports (EIRs) must include an assessment of the future noise impacts.

The following provides a summary of the various terms used when discussing environmental noise. Most likely you have heard the term decibel, but perhaps LEQ, dBA, or LDN are less familiar.

To understand the subjective response to sound, we have to understand three basic aspects of sound: level, frequency content, and time varying characteristics.

The first term, level, is the first one that many people think of when describing a sound. It is a measure of intensity or loudness, generally expressed as decibels (dB).

Another way people speak of a sound is “high and whining”, or “low and rumbly”. This refers to the pitch, or frequency. Frequency is expressed in Hertz (Hz).

Because the human ear perceives extremely low and high frequency sounds as less loud than mid frequency sounds, weighting scales have been developed. Environmental measurements are commonly made using the “A-weighting” scale. This scale represents the subjective “noisiness” of a sound and more closely corresponds to the sensitivity of human hearing. Sound levels measured with an A-weighted scale are referenced as “dBA”.

Although the A-weighted noise level may adequately indicate the level of environmental noise at any instant in time, community noise levels vary continuously. This leads us to the third characteristic, time varying characteristics, which look at how sound levels change over time and how sensitive humans are to those changes. Most environmental noise includes a combination of noises from distant sources (traffic, wind in the trees, waves crashing on the beach, etc.) These create a relatively steady “background noise” in which no one source is identifiable. The amount of noise above the “background noise” will vary over time, therefore it is helpful to know the A-weighted noise level that was statistically exceeded during 10%, 50%, and 90% of a stated time period. These levels are referenced as L10, L50, and L90. A single number descriptor called the LEQ is also widely used. The LEQ is the equivalent Aweighted noise level during a stated period of time.

Adding another element into this mix is the fact that noises tolerable at 3 pm are unbearable at 11 pm. Light traffic on a nearby street may only be 70 dB, but at night, the exterior background noises are so much lower that the same amount of traffic becomes noticeable. To account for this human sensitivity to nighttime noise levels, a descriptor, LDN (day/night average sound level), was developed. The LDN divides the 24-hour day into the daytime hours (7 am – 10 pm) and nighttime hours (10 pm – 7 am).

The nighttime noise levels are “penalized” or weighted 10 dB higher than the daytime noise levels (thus a sound that would be weighted at 40 dB during the day is given a rating of 50 dB at night). Another 24- hour average sound level, the Community Noise Equivalent Level (CNEL) is also used and includes both an evening (7 pm – 10 pm, 5 dB Penalty) and nighttime weighting (10 dB Penalty).

We hope this helped clarify some of the issues surrounding environmental acoustics.