Basics of Sound and Acoustic Treatment

Sound. A disturbance in an elastic medium resulting in an audible sensation.

Noise is by definition “unwanted sound”.


The basic unit of level in acoustics is the “decibel” (abbreviated dB). In acoustics, the term “level” is used to designated that the quantity is referred to some reference value, which is either stated or implied. The decibel (dB), as used in acoustics, is a unit expressing the ratio of two quantities that are proportional to power. The decibel level is equal to 10 times the common logarithm of the power ratio; or

In this equation P2 is the absolute value of the power under evaluation and P1 is an absolute value of a power reference quantity with the same units. If the power P1 is the accepted standard reference value, the decibels are standardized to that reference value. In acoustics, the decibel is used to quantify sound pressure levels that people hear, sound power levels radiated by sound sources, the sound transmission loss through a wall, and in other uses, such as simply “a noise reduction of 15 dB” (a reduction relative to the original sound level condition). Decibels are always related to logarithms to the base 10, so the notation 10 is usually omitted.

Decibel addition. In many cases cumulative effects of multiple acoustical sources have to be evaluated. In this case the individual sound levels should be summed. Decibel levels are added logarithmically and not algebraically. For example, 70 dB plus

70 dB does not equal 140 dB, but only 73 dB. A very simple, but usually adequate, schedule for obtaining the sum of two decibel values is:

hen several decibel values to be added equation 2 should be used.

Eq: 2

In the special case where decibel levels of equal magnitudes are to be added, the cumulative equation can be determined in equation 3.

Eq: 3

Decibel subtraction. In some case it is necessary to subtract decibel levels. For example if the cumulative level of several sources is known, what would the cumulative level be if one of the sources were reduced?  Decibel subtraction is given by equation 4.

Eq: 4

Decibel averaging. Strictly speaking decibels should be averaged logarithmatically not arithmetically.  Equation B-5 should be used for decibel averaging.

Eq: 5

Octave frequency bands. Typically, a piece of mechanical equipment, such as a diesel engine, a fan, or a cooling tower, generates and radiates some noise over the entire audible range of hearing. The amount and frequency distribution of the total noise is determined by measuring it with an octave band analyzer, which is a set of contiguous filters covering essentially the full frequency range of human hearing.

A-, B- & C-weighted sound levels. Sound level meters are usually equipped with “weighting circuits” that tend to represent the frequency characteristics of the average human ear for various sound intensities. The frequency characteristics of the A-, B-, and C-weighting networks are shown in figure. The relative frequency response of the average ear approximates the A curve when sound pressure levels of about 20 to 30 Db are heard. For such quiet sounds, the ear has fairly poor sensitivity in the low-frequency region, The B curve represents approximately the frequency response of hearing sensitivity for sounds having 60 to 70-dB sound pressure level, and the C curve shows the almost flat frequency response of the ear for loud sounds in the range of about 90 to 100 dB. Annoyance usually occurs when an unwanted noise intrudes into an otherwise generally quiet environment. At such times, the ear is listening with a sensitivity resembling the A curve. Sound levels taken on the A-, B-, and C-weighted networks have usually been designated by dB(A), dB(B), and dB(C), respectively. The parentheses are sometimes omitted, as in dBA.

Approximate Electrical Frequency Response of the A-, B-, and C-weighted Networks of Sound Level Meters.


The speed of sound in air is given by equation 15 where c is the speed of sound in air in ft./set, and tF is the temperature in degrees Fahrenheit. c = 49.03 x (460 + tF) 1/2                                   

Wavelength. The wavelength of sound in air is given by equation

Wavelength = c/f            

Noise Criteria In Buildings.  Room Criteria (RC) and Noise Criteria (NC) are two widely recognized criteria used in the evaluation of the suitability of intrusive mechanical equipment noise into indoor occupied spaces. The Speech Interference Level (SIL) is used to evaluate the adverse effects of noise on speech communication.

Noise Criterion (NC) Curves. Figure presents the NC curves. NC curves have been used to set or evaluate suitable indoor sound levels resulting from the operation of building mechanical equipment. These curves give sound pressure levels (SPLs) as a function of the octave frequency bands.                         

Room Criterion (RC) Curves. Figure  presents the Room Criterion (RC) curves. RC curves, like NC curves, are currently being used to set or evaluate indoor sound levels resulting from the operation of mechanical equipment

Speech interference levels. The speech interference level (SIL) of a noise is the arithmetic average of the SPLs of the noise in the 500‐, 1000‐, and 2000‐Hz octave bands.

Speech Interference Levels

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