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