Spartan 821/721 Manual Glossary C-19
Slow: The time constant is 1 second (1000 ms). This is the slowest speed. It is commonly
used in environmental noise measurements.
Fast: The time constant is 1/8 second (125 ms). This time weighting speed detects
changes in sound level more rapidly.
Impulse: The time constant is 35ms for the rise and 1.5 seconds (1500 ms) for the decay.
The reason for the double constant is to allow a very short signal to be captured and
displayed.
Vibration
The oscillatory movement of a mechanical system (generally taken to be solid). It is used
as a broad descriptor of oscillations.
Wavelength (l)
The distance between peaks of a propagating wave with a well defined frequency. It is
related to the frequency through the following equation
where c is the sound speed and f is the frequency in Hz. It has the dimensions of length.
Wavenumber (k)
A number that is related to the wavelength of sound and is used to compare the size of
objects relative to the wavelength or the time delay in sound propagation. It is related to
wavelength through the following equation
where l is the wavelength, c is the sound speed, f is the frequency in Hz, and w is the
radian frequency. It has the dimensions of inverse length.
Windscreen
Air (wind) blowing across the microphone generates pressure fluctuation and vibration
on the microphone diaphragm. Thus, your noise exposure reading would include wind
noise, an undesirable component. To achieve the best result with the 721 or 821, we
recommended using the provided WS001 windscreen.
c
f
--
=
k
2
------
2f
c
---------
c
----
= = =
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