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DEA3500: Ambient Environment: Basic Acoustics


Every element of building design and construction affects its acoustical characteristics.

Acoustic principles should influence the choice of finish materials in rooms, the location of these materials in a building, and the building design.


Almost every situation can be described in terms of a

Factors Influencing Acoustical Environments

  1. How is space to be used? e.g. office, hospital, theater, auditorium, library reading room etc.
  2. Type of noise sources? voice, music, impact sounds.
  3. Predictability of source? expected sounds less stressful than unexpected sound.
  4. Sound absorbing/reflecting qualities of interior surfaces.
  5. Reverberation time - echoes vs. "dead" space
  6. Source location/locations
  7. Receiver characteristics/expectations
  8. Signal/noise ratio.

Nature of Sound


Noise is usually defined as "unwanted sound".

Sound waves - are described by following parameters:

  1. frequency - number of complete cycles/sec. (Hz). Normal human ear can hear between 20-20KHz. 20 Hz = infrasound. >20KHz = ultrasound. 600 Hz - 4KHz = human speech, most sensitive 3KHz.

  2. amplitude - magnitude of pressure variation (pmax - pmin)

  3. phase - portion of the cycle through which the wave has progressed.

  4. wavelength - peak to peak distance.

l = c / f where l = wavelength, f = frequency, c = speed of sound (1130 ft/sec). Therefore at 100 Hz, l = 1130 / 100 = 11.3 ft.

Complex periodic waves comprise energy at:

Subjective Responses

decibel (dB) = 10 log10 I1 / I0. dB is a measure of sound power.

Original unit was the bel, but this was too large, dB = 1/10 intensity ratio in bels. Because we can't measure sound power directly, we have to measure variations in air pressure.

Sound power is directly proportional to the square of the sound pressure

That is:

Sound pressure - is what is measured by sound-level meters. Typically each meter has 3 differently weighted scales - A B C, each of which corresponds to a different

Frequency-response curve - these scales vary by differently attenuating sounds of certain frequencies. Additionally, some meters have a D scale, designed primarily to measure aircraft noise. The 'A' scale is that which most closely approximates the frequency-response scale of the human ear.

Octave Bands 1 octave = doubling of frequency.

Octaves start from 22 Hz i.e. approx. threshold of hearing and have internationally agreed octave bands.

1/3 octave-bands - for more detailed analysis of sound spectrum a 1/3 octave-band analyzer is used and the 1/3 octave-band center frequencies give a more detailed profile of sound. 


Combined Effects of Sound

Because the decibel scale is logarithmic rather than arithmetic, a large increase in sound power will be reflected in a change of only a few decibels. For example:

To estimate what the combined noise will be from combined two noise sources you can calculate the difference in noise level between any two noise sources. If the difference is 0 or 1 dB add 3dB to the louder of the 2 noise sources; if it's 2 or 3 add 2 dB to the louder; if it's 4-10 add 1 dB to the louder, and if it's 11 or more then there's no impact on the louder noise.

In other words, suppose you have 2 factories each producing 52dB noise, the difference (52-52) is 0, which means that the combined effect of these 2 noises is 52 + 3 = 55dB.

Suppose we now add a 3rd factory producing 52dB. Now the difference is 55-52 (the combined noise minus the new noise), which is 3 db, so now the new combined noise of 3 factories will be 55 + 2 = 57dB.

Now let's add a fourth factory producing 52dB. The noise difference is 57-52, which is 5 dB. Thus, the combined effect will be57 + 1 = 58dB.

Now let's add a 5th factory at 52 dB. The noise difference is 58-52=6dB, so the combined noise is 58+1=59dB, and so on.

In short, you can keep adding 52dB noises until you reach a combined noise of 63dB (i.e. 62 - 52 = 11, at which level there's no addition noise energy over and above the loudest noise).

This means that you can have 20 factories producing 52dB noise and the combined effect will be a noise level of 63dB, which is well below occupational noise hazard levels, but probably in an annoying range for a residential setting. (There are often widespread noise complaints when community noise levels are consistently over 60dB).

A 10 dB increase is 10 times as much acoustical power, but it only sounds two times as loud to the listener. 

Loudness level - Various units of measurement have been proposed to represent the loudness of a noise.

Loudness - subjective intensity of sound, independent of any meaning the sound might have (loudness depends on frequency).

Phons - The phon indicates the loudness of a sound. This is numerically the sound intensity equivalent to the decibel level of a tone of 1000 Hz which is judged equivalent in loudness. Therefore phons are represented as

Equal-loudness- contours e.g. a 50 Hz, 62 dB tone has a loudness level of 40 phons. (This means that 50 hz at 62 dB sounds as loud as 1000 Hz at 40 dB.) [NB-1KHz is the reference level.]

Sones - The phon tells us only about the subjective equality of various sounds, but it doesn't tell us anything about the relative subjective loudness - of different sounds i.e. a 40 phon sound isn't twice as loud as a 20 phon sound. Therefore a ratio scale of loudness, the sone scale, is used. One sone is defined as the loudness of a 1KHz tone of 40 dB (40 phons). A sound that is judged to be twice as loud as the reference sound has a loudness of 2 sones. A sound that is judged to be half as loud as the reference sound has a loudness of 0.5 sones and so on.

Noise Annoyance - Many factors affect this.

Noisiness - defined as the subjective impression of the unwantedness of a sound.

Noisiness = annoyance. However, two types of noisiness:

  1. Unwanted sound that carries information about the sound source that signifies unpleasantness e.g. baby crying.
  2. Unwanted sound that is annoying because of the physical sound, not its meaning e.g. aircraft noise. Higher frequency sounds of the same loudness are more annoying than low frequency sounds of the same loudness.

Noy - a subjective unit of noisiness. A sound of 2 noys is twice as noisy as a sound of 1 noy and half as noisy as a sound of 4 noys.

PLdB - see course textbook

PNdB - perceived noise level. An increase of 10 PNdB in sound is equal to doubling its noy value.

But just knowing the noisiness of a sound doesn't tell us how that may be affecting us detrimentally.

Equivalent sound level (Leq) - In real-life situations we may be exposed to noises of varying intensity over time and we need a measure of cumulative noise exposure.

Leq = 10 log10 1/n ( 10 Li/10), where Li = SPL of each 1 second interval over time , i = 1 second to n = nth second. (i.e.)- Leq is proportional to the sum of the energy of the 1-second SPL over a fixed, specified period of time e.g. 1 hour, 1 day, 1 year, etc.

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