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Lanjutan … materi 3

Lanjutan … materi 3. Noise Measurement. Equal Loudness Contours. Sound level meters incorporate frequency weighting to simulate this effect. (inverse of the ELC’s). A-weighting response. 19 dB. (Ear’s response at approximately 40 dB). 1000 Hz. 100 Hz. The A-weighted Sound Level.

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Lanjutan … materi 3

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  1. Lanjutan…materi 3 Noise Measurement

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

  3. A-weighting response 19 dB (Ear’s response at approximately 40 dB) 1000 Hz 100 Hz 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. 3

  4. Example of A-Weighting 4

  5. 0 dB A-weighting response 19 dB 1000 Hz 100 Hz The A-weighting Curve Approximation 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. 5

  6. 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. 6

  7. 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., rms2) pressures: 7

  8. Examples • Two sources each have a sound pressure level of 95 dB at a distance of 5 m. What is the combined level? 8

  9. 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? 9

  10. Examples (3) 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? For each machine: 10

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