Appendix A
Basic Concepts in Acoustics and Noise

This appendix is not intended to give a complete description of all of the quantities used in acoustics and noise control—information that is available in a wide variety of textbooks and handbooks (e.g., Rossing, 2007; Vér and Beranek, 2006; Crocker, 2007).

A few key concepts are described in this appendix:

  • immission and emission

  • quantities used in noise control

  • frequency weighting

  • levels and the decibel

IMMISSION VERSUS EMISSION

When most people mention noise levels, they are speaking of immission—the sound they hear. The sound may come from a specific source or from a number of sources at the same time. There is little distinction between the two. That is why the sound pressure level in decibels is used as the descriptor. It is, however, necessary to make a clear distinction between sound emitted by a source (i.e., noise emission) and the sound heard by an observer (i.e., noise immission). The former is relatively independent of the environment in which the noise source is located (outdoors, in a room, etc.). There are standard methods of determining the noise emission of stationary sources as well as of moving sources such as cars, trucks, and airplanes.

Noise immission may come from several sources and is always dependent on the environment in which the sources are located. The position of a source in a room, the size of the room, and the amount of sound absorption in the room all influence noise immission. Outdoors, immission levels can be influenced by the nature of the terrain, sound absorption by the ground, and wind and temperature gradients—among other effects.

Quantities Used in Noise Control

Sound pressure is the small variation above and below atmospheric pressure created by the passage of a sound wave; this is what most people think of as noise. Pressure sensed by a microphone on a sound-level meter is generally converted to a mean square pressure or pressure level by the measuring instrument. The level indicated by the sound-level meter fluctuates depending on the averaging time of the measuring system. More details are given in the section below.

In some cases, the sound pressure can be used as a metric for the noise emission of a source. The sound pressure may be converted into a metric that more closely relates to human response—such as the effective perceived noise level used to specify the noise emissions of airplanes. Or it may be the maximum sound pressure level during a vehicle pass-by under controlled measurement conditions. A more common descriptor of noise emission for stationary sources is the sound power level, a measure of the total sound energy emitted by a source. Sound intensity, the power per unit area, can be determined—usually by a measurement of pressure gradient—by instrumentation systems and is now used to determine noise emission by tire/road interaction. The method is called the onboard sound intensity method.

Frequency Weighting

The sound pressure as measured by a microphone varies in time and can also be described in terms of the frequency of the sound. The ear has different sensitivities to sounds of different frequencies, and a frequency weighting is often applied to the signal to make it more representative of the sound perceived by a listener. The most common weighting is A-weighting, which was originally derived in the 1930s by determining the loudness of sounds. The A-weighting curve is described in most textbooks and handbooks on



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Appendix A Basic Concepts in Acoustics and Noise Quantities used in Noise Control This appendix is not intended to give a complete de- scription of all of the quantities used in acoustics and noise Sound pressure is the small variation above and below at- control—information that is available in a wide variety of mospheric pressure created by the passage of a sound wave; textbooks and handbooks (e.g., Rossing, 2007; Vér and Be- this is what most people think of as noise. Pressure sensed by ranek, 2006; Crocker, 2007). a microphone on a sound-level meter is generally converted A few key concepts are described in this appendix: to a mean square pressure or pressure level by the measur- ing instrument. The level indicated by the sound-level meter • immission and emission fluctuates depending on the averaging time of the measuring • quantities used in noise control system. More details are given in the section below. • frequency weighting In some cases, the sound pressure can be used as a met- • levels and the decibel ric for the noise emission of a source. The sound pressure may be converted into a metric that more closely relates IMMISSION VERSuS EMISSION to human response—such as the effectie perceied noise leel used to specify the noise emissions of airplanes. Or it When most people mention noise levels, they are speaking may be the maximum sound pressure leel during a vehicle of immission—the sound they hear. The sound may come pass-by under controlled measurement conditions. A more from a specific source or from a number of sources at the common descriptor of noise emission for stationary sources same time. There is little distinction between the two. That is the sound power leel, a measure of the total sound energy is why the sound pressure level in decibels is used as the de- emitted by a source. Sound intensity, the power per unit area, scriptor. It is, however, necessary to make a clear distinction can be determined—usually by a measurement of pressure between sound emitted by a source (i.e., noise emission) and gradient—by instrumentation systems and is now used to de- the sound heard by an observer (i.e., noise immission). The termine noise emission by tire/road interaction. The method former is relatively independent of the environment in which is called the onboard sound intensity method. the noise source is located (outdoors, in a room, etc.). There are standard methods of determining the noise emission of Frequency Weighting stationary sources as well as of moving sources such as cars, trucks, and airplanes. The sound pressure as measured by a microphone varies Noise immission may come from several sources and is in time and can also be described in terms of the frequency always dependent on the environment in which the sources of the sound. The ear has different sensitivities to sounds are located. The position of a source in a room, the size of of different frequencies, and a frequency weighting is often the room, and the amount of sound absorption in the room applied to the signal to make it more representative of the all influence noise immission. Outdoors, immission levels can sound perceived by a listener. The most common weighting be influenced by the nature of the terrain, sound absorption is A-weighting, which was originally derived in the 1930s by the ground, and wind and temperature gradients—among by determining the loudness of sounds. The A-weighting other effects. curve is described in most textbooks and handbooks on 4

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46 TECHNOLOGY FOR A QUIETER AMERICA acoustics and noise control, and frequency weighting in A-frequency weighting and averaging the mean square pres- general—including weighting curves—is described in the sure over 24 hours with an increase in the amplification of the measuring system of 10 dB during the nighttime hours.1 This online encyclopedia Wikipedia (http://en.wikipedia.org/wiki/ Frequency_weighting). Octave and one-third octave fre- quantity is the day-night average sound level, Ldn (DNL). To quency bands (http://www.diracdelta.co.uk/science/source/ add further complication, it is common European practice to o/c/octae/source.html) are also used for a more complete use a 5-dB amplification in the measuring system during the description of the frequency spectrum of noise (see examples evening hours and a 10-dB gain during the nighttime hours. in this report). The result is the day-evening-night level, Lden. Another important quantity is sound exposure and the cor- responding sound exposure leel in decibels. This measure The Decibel is useful for assessing the noise produced by single events The decibel, unfortunately for public comprehension, such as an airplane flyover or vehicle pass-by. Here, the is used in a variety of ways in noise control and other quantity Q is the time integral of the squared pressure over branches of engineering. That it involves a logarithm makes the time interval of the event. The reference quantities are math-averse individuals uncomfortable. The decibel was 20 micropascals as the reference pressure and 1 second as originally used in the Bell telephone system to describe the the reference time. attenuation of a mile of “standard cable.” It is also commonly The decibel is also used in noise control for sound intensi- used to describe the gain of an amplifier and the power deliv- ty and sound power, which are common descriptors of noise ered to an electrical load. The online encyclopedia Wikipedia emission. For sound intensity level, the quantity is sound intensity and the reference quantity is 10–12 W/m2. For sound is a good source of information for a basic understanding of the concept (http://en.wikipedia.org/wiki/Decibel). power level, the quantity is sound power and the reference quantity is 10–12 W. In the information technology industry, The decibel is firmly entrenched in the language of noise, as in “how many decibels of noise is that?” Noise “ther- the sound power level is commonly expressed in bels, B (10 mometers” are frequently published showing the decibel dB = 1 B) to avoid confusion between sound pressure level level of noise for various sources. Examples are given in and sound power level. This has not been widely adopted, Chapter 1. These levels are almost always measures of noise however. For example, European requirements on outdoor immission. equipment are based on the sound power level in decibels. Fundamentally, the decibel is a unit of level and is defined as 10 log Q/Qref, where Q is a quantity related to energy and Sound Level Qref is a reference quantity. It is the fact that both Q and Qref can be different quantities (squared pressure, power, Throughout this report, the terms sound pressure leel, intensity, etc.) that makes general use of the decibel even sound intensity leel, and sound power leel are used to more confusing to the public. The mean square pressure is clarify which level is being discussed. The term sound leel the quantity most commonly used to describe noise, and its is sometimes used when sound pressure is implied—such corresponding reference quantity is (20 micropascals)2 or, in as in day-night average sound level. It is also used in con- terms of Newton per meter squared, (2 · 10–5 N/m2)2. Given nection with instruments—such as sound-level meter—and the range of mean square pressures commonly encountered when the quantity being discussed could be either pressure, when dealing with noise, the sound pressure level generally intensity, or power. ranges from about 0 to 140 dB. The corresponding pressures are only a tiny fraction of atmospheric pressure. Although the REFERENCES A-frequency weighting described above applies to the signal Crocker, M.J., ed. 2007. Handbook of Noise and Vibration Control. Hobo- and not to the unit (dB), the A-weighted sound pressure level ken, N.J.: John Wiley and Sons. is often expressed as dB(A) or dBA. Rossing, T., ed. 2007. Springer Handbook on Acoustics. New York: Springer Even with one definition of Q as the mean square pres- Science+Business Media LLC. sure, different averaging times lead to different decibel val- Vér, I.L., and L.L. Beranek, eds. 2006. Noise and Vibration Control Engi - ues—which causes further complication. For example, in the neering. New York: John Wiley and Sons. See, for example, Beranek, L.L., Chapter 1, Basic Acoustical Quantities: Levels and Decibels. evaluation of hazardous noise in the workplace, an 8-hour average is commonly used. For environmental noise out- doors, a day-night average sound level is computed by using 1Two different uses of the decibel in one sentence!