Acoustic Characteristics
Since room acoustics have been mentioned repeatedly,
here is a brief introduction to some basic factors involved
in acoustics.
Sound Waves – Sound waves consist of pressure variations
traveling through the air. When the sound wave travels, it
compresses air molecules together at one point. This is
called the high pressure zone or positive component(+).
After the compression, an expansion of molecules occurs.
This is the low pressure zone or negative component(-).
This process continues along the path of the sound wave
until its energy becomes too weak to hear. If you could view
the sound wave of a pure tone traveling through air, it
would appear as a smooth, regular variation of pressure
that could be drawn as a sine wave. The diagram shows
the relationship of the air molecules and a sine wave.
Frequency, Wavelength, and the Speed of Sound –
The frequency of a sound wave indicates the rate of
pressure variations or cycles. One cycle is a change from
high pressure
to low pressure
and back to high
pressure. The
number of cycles
per second is
called Hertz,
abbreviated “Hz.”
So, a 1,000Hz
tone has 1,000
cycles per second.
The wavelength of a sound is the physical distance from the
start of one cycle to the start of the next cycle. Wavelength
is related to frequency by the speed of sound. The speed of
sound in air is 1130 feet per second or 344 meters/second.
The speed of sound is constant no matter what the
frequency. You can determine the wavelength of a sound
wave of any frequency if you understand these relationships:
Loudness –
The fluctuation
of air pressure
created by sound
is a change above
and below normal
atmospheric
pressure. This is
what the human
ear responds to.
The varying amount
of pressure of the
air molecules
compressing and
expanding is related
to the apparent
loudness at the
human ear. The greater the pressure change, the louder
the sound. Under ideal conditions the human ear can
sense a pressure change as small as .0002 microbar. One
microbar is equal to one millionth of atmospheric pressure.
The threshold of pain is about 200 microbar. Obviously,
the human ear responds to a wide range of amplitude of
sound. This amplitude range is more commonly referred
to in decibels. Sound Pressure Level (dB SPL), relative to
.0002 microbar (0dB SPL). 0 dB SPL is the threshold of
hearing and 120 dB SPL is the threshold of pain. 1 dB is
about the smallest change in SPL that can be heard.
A 3 dB change is generally noticeable, while a 6 dB
change is very noticeable. A 10 dB SPL increase is
perceived to be twice as loud!
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RECORDING
Microphone Techniques
for
Ambient sounds
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rarefaction rarefactioncompression compression
wave motion
Approximate wavelengths of common
frequencies:
100 Hz: about 10 feet
1000 Hz: about 1 foot
10,000 Hz: about 1 inch
The Wave Equation: c = f • l
speed of sound = frequency • wavelength
or
wavelength =
speed of sound
frequency
for a 500Hz sound wave:
wavelength =
1,130 feet per second
500Hz
wavelength = 2.26 feet
Wave amplitude
peak-to-peak
peak
rms
one cycle or one period