Acoustical engineers organize the audio spectrum into sound octave bands. The weighting switch on the Sound Level Meter (SLM-BTA, discontinued) or selection of the A- and C-weighted channels on the Go Direct® Sound Sensor (GDX-SND) is used to add or subtract different values to the octave band components.
The A-weighted sound level discriminates against low frequencies, in a manner similar to the response of the ear. In this setting, the meter primarily measures in the 500 to 10,000 Hz range. It is the weighting scale most commonly used for OSHA and DEQ regulatory measurements. The C-weighted sound level does not discriminate against low frequencies and measures uniformly over the frequency range of 30 to 10,000 Hz. This weighting scale is useful for monitoring sources such as engines, explosions, and machinery. The sound levels measured with these two weightings have units of “dBA” and “dBC”, respectively.
The table below shows how each weighting adjusts the sound level reading for each octave band.
Standard Octave Band Frequencies Ranges with Corresponding A and C Weighting Values
|
Center Frequency (Hz) |
Effective Band (Hz) |
A Weighting (dB) |
C Weighting (dB) |
|
31.5 |
22.1-44.2 |
-39.4 |
-3.0 |
|
63 |
44.2-88.4 |
-26.2 |
-0.8 |
|
125 |
88.4-177 |
-16.1 |
-0.2 |
|
250 |
177-354 |
-8.6 |
0 |
|
500 |
354-707 |
-3.2 |
0 |
|
1000 |
707-1,414 |
0 |
0 |
|
2000 |
1,414-2,828 |
1.2 |
-0.2 |
|
4000 |
2,828-5,657 |
1.0 |
-0.8 |
|
8000 |
5,657-11,314 |
-1.1 |
-3.0 |
In most classroom situations, you’ll want to measure sound levels A-weighted. To simplify its operation, the Sound Level Sensor (SLS-BTA) is A-weighted by default; there is no provision for changing its weighting. If you have the Go Direct® Sound Sensor (GDX-SND), you will primarily want to use the “Sound Level A-weighted” channel for measuring sound levels.