Chapter 3 – Measurement of Sound
•
Stimulation of sensation—the presentation of sound to evoke a
subjective response to it
•
Function relating the output to the input allows characterization of
the performance of a particular system
•
Various parameters of sound – amplitude, frequency, and phase
•
Concepts underlying the pertinent procedural matters and the
practical units of measure of sound
•
Important to understand how the change in one parameter of the
sound stimulus can affect another, purely on a physical basis
1.
Amplitude Revisited and More
•
Amplitude—the difference between the maximum instantaneous
value and equilibrium
•
Most practical measure for quantifying sound—sound pressure
•
Peak sound pressure—the difference between the peak
condensation pressure and the ambient, or atmospheric, pressure
•
Peaktopeak amplitude—the difference between the maximum and
minimum values (peak condensation and rarefaction, for sound
pressure)
•
Effective or rootmeansquare (RMS)—a measure that better
reflects the overall power of the sound or vibration
•
Question: What measure of the AC voltage yields a value
equivalent to the DC voltage, such that the light bulb burns equally
brightly? Answer: the RMS voltage
•
In the case of a sinusoid, the magnitude at each instant during one
halfcycle mirrors the corresponding magnitude during the opposite
halfcycle—each is equal in value but of opposite sign
•
Sinusoidal sound (pure tones) are frequently used in hearing
science
•
Computation of the RMS magnitude of a sinusoid is simple
o
Task: determine a single value that best reflects the area under
the curves or waveforms of the two
•
Alternating pressure changes comprising the sound represent the
same overall power as that of a constantly applied pressure
•
A pure tone with a peaktopeak sound pressure of 3 N/m
2
has an
RMS sound pressure of approximately 1 N/m
2
—these relationships
only hold up for sinusoids
2.
Decibel Notation
A.
SOUND LEVELS
•
Decibel (dB)—the logarithm of the ratio of two quantities
•
Ratios of like quantities are dimensionless—the dimensions of the
quantities cancel one another in the process of calculating a ratio
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•
Measurement of acoustic intensity in decibels yilds the acoustic
intensity level (IL) of the sound
•
Measurement of sound pressure in decibels yields the sound
pressure level (SPL)
•
Most common sound measuring device—sound level meter—
indicates sound magnitude in dB SPL, not necessarily units of
sound pressure
•
The greatest sound pressure that can be tolerated is greater than
10,000,000 times that of the least detectable sound pressure
•
The smallest sound pressure detectable by the normal human
listener is about 0.00002 or 2 X 10
5
N/m
2
B.
THE LOGARITHM: FRIEND OR FOE?
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 Spring '11
 Durrant
 Sound pressure, iL, acoustic intensity

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