Fundamentals of Acoustics and Noise Essay example

4656 Words May 5th, 2016 19 Pages
Fundamentals of Acoustics and Noise

Unit 4
Frequency analysis,
Frequency bands,
Decibel scales,
Descriptors for time varying noise levels Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-1

Contents
Frequency Analysis of Sound Pressure Signals

Constant Proportion Bandwidth Frequency Bands

Constant Bandwidth Frequency Bands

Decibel Scales
Descriptors for Time Varying Noise Levels

Equivalent Continuous Sound Level

Sound Exposure Level

Percentile Exceeded Sound Level

4-2

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

Frequency Analysis of Sound Pressure Signals
A microphone is constructed to produce a voltage
Figure 4.1

Time domain representation of a pure tone.

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-3

Figure 4.2

Frequency domain representation of a pure tone.

When the sound is a simple pure tone, its magnitude may be represented by its amplitude, which in this case is also the same as its peak value. The average value of the sound pressure over any period of more than a few cycles will be zero because the positive half-cycles will cancel the negative pressure half-cycles. For more complex waveforms, such as harmonic, transient or random noise, the expression of magnitude is not as simple, but the time-averaged value is still zero.
A commonly used expression of magnitude is the RMS value of the sound pressure. This gives a non-zero average, corresponding to the square root of the mean (average) of the square of the pressure. Figure 4.3 Processing stages of a complex sound pressure signal to determine its RMS value.

Consider now a complex sound pressure obtained by the superposition of two harmonically related pure tones of frequencies mf and nf. p(t ) = Pm cos 2πmft + Pn cos 2πnft

(4.2)

The frequency domain representation of this acoustic pressure is shown in Figure 4.4.
It is evident that the strengths of the individual components of frequencies mf and nf, in terms of their RMS values are pmrms = Pm / 2 and

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