Ambient Environment: Basic Acoustics

Aedryan Methyus

Hey Kids,
Here is some good information about sound that I borrowed from Cornell University Ergonomics Web. I thought this would make a good reference for those of us who are trying to further our understanding of how sound works, in relationship to electronic voice phenomenon. I think it's very important that we all try and learn as much as we can about how sound works if we wish to further the study of spirit voices. If anyone forms some solid ideas about how to incorporate any of these principals into EVP monitoring and classification please post your thoughts here.

DEA350: Ambient Environment: Basic Acoustics

ACOUSTICS
speed of sound v. = square root of (pressure / density of air)
frequency x wavelength = speed (constant for sound)
high frequency - short wavelength
low frequency - long wavelength
person is 44 dB at quietest (respiration and heartbeat)
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.

ACOUSTIC SITUATIONS
Almost every situation can be described in terms of a

Source - of sound
Path- transmission medium for sound
Receiver- recipient of sound Acoustical problems can be tackled at any or all of these 3 points.
Factors Influencing Acoustical Environments
How is space to be used? e.g. office, hospital, theater, auditorium, library reading room etc.
Type of noise sources? voice, music, impact sounds.
Predictability of source? expected sounds less stressful than unexpected sound.
Sound absorbing/reflecting qualities of interior surfaces.
Reverberation time - echoes vs. "dead" space
Source location/locations
Receiver characteristics/expectations
Signal/noise ratio.
Nature of Sound
Sound - pressure changes in an "elastic" medium which are capable of being detected by the ear. Pressure fluctuations set up as "traveling" wave.
Sound waves - vibrating source emits sound waves in front and behind source. As pressure wave expands so it becomes virtually spherical and this spherical expansion causes a rapid decrease in sound intensity as distance from source increases. Note: with a sound wave the air between source and listener does not move but molecules vibrate from side to side. For sounds of average loudness this deviation is equal to 1 millionth normal atmospheric pressure (15 lbs sq.in.).
Directionality. Many sources emit sounds of greater intensity in one direction e.g. human voice. Polar diagram of human voice shows that for high frequencies the voice is less than one half as loud from behind than in front.

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NOISE
Noise is usually defined as "unwanted sound".

Sound waves - are described by following parameters:

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.

amplitude - magnitude of pressure variation (pmax - pmin)

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

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:

fundamental frequency - periodic common denominator of frequencies

harmonics - integral multiples of fundamental e.g. waveform of three frequencies, 1000, 1100, and 1200 Hz, is periodic and has fundamental at 100 Hz (and therefore will hear periodic increased loudness).

Subjective Responses
Frequency - we seldom hear pure tones, such as those produced by a tuning fork, but we tend to hear a mixture of frequencies.
Timbre - (tonal quality) - purity of the frequency
Pitch - periodic complex sounds evoke a sensation similar to the response to a pure tone of a given frequency. Pitch is highness / lowness of sound. High frequency - high pitch, low frequency - low pitch.
Amplitude - ear can respond to an enormous range of sound intensities. Ratio of power of loudest : weakest sound is approximately 1013 : 1. To encompass this vast scale, a logarithmic scale expressing the ratio of two intensities is used (reference intensity = I0 [10 -12 wm -2] where w = approx. threshold of hearing for 1KHz pure tone). Other intensity = I1.
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 Level (SPC) (dB) = 10 log P21 / P2 0
P20 = standard reference sound pressure squared that represents 0 decibels (= 20 µ Nm2 [microNewtons per m2] or .0002 microbars or 20 µ Pa (micropascals))
p21 = measured sound pressure level.
This equation simplifies to: SPL (dB) = 20 log p1 / p0, which deals directly with sound pressure rather than sound pressure squared.
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:

one TV set operates at 65 dB
two TV sets, at 65 dB each = 68 dB
ten TV sets, at 65 dB each = 75 dB
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.

40 phons = 1 sone
50 phons = 2 sones
60 phons = 4 sones
30 phons = 0.5 sones
20 phons = 0.25 sones

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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:

Unwanted sound that carries information about the sound source that signifies unpleasantness e.g. baby crying.
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.

Yours In Strange Happenings,

Aedryan
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