Lanjutan…materi 3 Noise Measurement. 2 Dept. of Mech. Engineering University of Kentucky Equal Loudness Contours Sound level meters incorporate frequency.

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Presentation transcript:

Lanjutan…materi 3 Noise Measurement

2 Dept. of Mech. Engineering University of Kentucky Equal Loudness Contours Sound level meters incorporate frequency weighting to simulate this effect. (inverse of the ELC’s)

3 Dept. of Mech. Engineering University of Kentucky The A-weighted Sound Level The A-weighting is a filter that approximates the ear’s response at low levels. It filters and integrates the actual spectrum to yield a single value, e.g., 95 dB(A) overall sound level. A-weighting response 100 Hz 1000 Hz (Ear’s response at approximately 40 dB) 19 dB

4 Dept. of Mech. Engineering University of Kentucky Example of A-Weighting

5 Dept. of Mech. Engineering University of Kentucky The A-weighting Curve Approximation A-weighting response 100 Hz 1000 Hz 19 dB 0 dB An approximate function that can be used to calculate the response of the A-weighting curve is: This function is accurate to within 0.6 dB maximum error at any f.

6 Dept. of Mech. Engineering University of Kentucky Historical Notes Octave and one-third octave filters predate by many years the use of the FFT (introduced in the early 1970’s) to obtain the frequency spectrum of complex sounds. Many standards were written around octave and one-third octave filter frequency analysis and have yet to be fully updated. The A-weighting system is, strictly speaking, only valid at low sound levels (around 40 dB), but it is commonly used to rate sounds having levels over 100 dB. The A-weighting curve is often mistakenly used to rate noise for “annoyance” or other subjective (psychological) measures of sound quality. It is, in fact, only a measure of the physical response of the ear mechanism.

7 Dept. of Mech. Engineering University of Kentucky Working with Decibels Inasmuch as decibels are logarithmic quantities, we cannot manipulate them arithmetically. Instead, we must convert decibels to their “base” values, perform the arithmetic operation, and then convert back to decibels Addition of sound levels. Sound levels are added by adding their mean-square sound (i.e., rms 2 ) pressures:

8 Dept. of Mech. Engineering University of Kentucky Examples 1.Two sources each have a sound pressure level of 95 dB at a distance of 5 m. What is the combined level?

9 Dept. of Mech. Engineering University of Kentucky Examples (2) 2.A machine has a sound pressure level of 95 dB in the presence of background noise. A background noise measurement made with the machine off yields a level of 92 dB. What is the true level of the machine?

10 Dept. of Mech. Engineering University of Kentucky 3.A machine has a sound pressure level of 86 dB at a distance of 3 m. If 6 identical machines are arranged in a circle about the measurement point, what is the total level? Examples (3) For each machine: