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8A - 8A ANALYSIS OF COMPLEX SOUNDS Amplitude loudness and...

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P108 Lab 8a: page 1 8A. ANALYSIS OF COMPLEX SOUNDS Amplitude, loudness, and decibels Last week we found that we could synthesize complex sounds with a particular frequency, f, by adding together sine waves from the harmonic series with a fundamental equal to the frequency ( f, 2f, 3f, 4f,…). We can reverse the process: a complex sound with particular frequency can be analyzed and quantified by the amplitude spectrum: the relative amplitudes of the harmonics. Our goal today is to understand how the pitch, loudness, and timbre of a sound are represented in a spectrum. However, rather than use an analog Band Filter, we will use the P RAAT program, which has powerful analysis software built in to calculate and graph the spectrum of a recorded sound. For the last two weeks we have used the amplitudes to represent the different waves. We can represent these in a table (the choice for the amplitude of the fundamental wave to be 2V is completely arbitrary): Table 1: The spectrum of a square wave Error Error Error Error 440 880 1320 1760 2200 2640 3080 3520 3960 1 2 3 4 5 6 7 8 9 2.000 0.000 0.667 0.000 0.400 0.000 0.286 0.000 0.222 1.000 0.000 0.333 0.000 0.200 0.000 0.143 0.000 0.111 1.0000 0.0000 0.1111 0.0000 0.0400 0.0000 0.0204 0.0000 0.0123 0.0 -9.5 -14.0 -16.9 -19.1 Harmonic 1 2 3 4 5 6 7 8 9 frequency (Hz) Harmonic number=fn/f1 Amplitude (Volt) Relative amplitude=An/A1 Relative Power=(An/A1)^2 Relative Power (dB) Another way to represent the relative amplitudes, which is used by P RAAT , is to graph the power for each harmonic. Since the sound power is proportional to the square of the amplitude for each harmonic, these numbers often get quite small for the higher harmonics. For this reason, the power is often expressed as a decibel. The power for each harmonic in decibels (dB) is: Relative Power (dB) = P n = 10 log 10 [ ( A n /A 1 ) 2 ] = 20 log 10 [ ( A n /A 1 ) ] Since the logarithm of zero is negative infinity, the Relative Power (dB) for the even harmonics is "Error".
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