Larson Davis 831C-FFT, Справочное руководство

Страница: 252 / 254

SoundAdvisor Model 831C C-17
Threshold Sound Level (Lt)
The A-weighted sound level below which the sound produces little or no Noise Dose accumula-
tion and may be disregarded. It is used for hearing damage risk assessment.
Standard: ANSI S1.25
Time Weighted Average Sound Level (TWA, LTWA(TC))
It is the level of a constant sound over the Criterion Duration, that would expose a person to the
same Noise Dose as the actual (unsteady) sound over the same period. If the Exchange Rate is 3
dB then the TWA is equal to the Leq.
Where TC=T
2
-T
1
and K is the Exchange Rate Factor. It is used for hearing damage risk assessment.
Standard: ANSI S12.19
Time Weighting
The response speed of the detector in a sound level meter. There are several speeds used.
Slow
: The time constant is 1 second (1000 ms). This is the slowest and is commonly used in envi-
ronmental noise measurements.
Fast
: The time constant is 1/8 second (125 ms). This is a less commonly used weighting but will
detect 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 rea-
son for the double constant is to allow the very short signal to be captured and displayed.
Tone Seek Delta
This FFT and Tonality setting—associated with the 1996-2 Tonality standard—is used to
distinguish noise pauses from masking noise in the measurement. The default value is 1
dB. Refer to ISO 1996-2 C.4.2 to determine which value to use for your measurement.
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
L
TWA TC
 
Klog
10
1
T
---
10
L
p
t
   
K

t d
T
1
T
2

 
 
 
 
=

c
f
-- =