Acoustics, Psychoacoustics, and Speech Science
This domain tests physical acoustics, psychoacoustic laws, and the acoustic analysis of speech. Questions require calculations, interpretation of graphs described in text, and links between acoustic cues and perception.
Core rules
- Sound pressure level: dB SPL = 20 log10(P/P0), where P0 = 20 micropascals. Intensity level: dB IL = 10 log10(I/I0), where I0 = 10^-12 W/m2.
- Doubling pressure adds about 6 dB; doubling intensity adds about 3 dB. A tenfold pressure increase adds 20 dB.
- Frequency f = 1/T; wavelength = c/f. A quarter-wave tube resonates when its length equals one quarter wavelength.
- Fourier analysis decomposes complex sounds into sinusoids. Harmonics are integer multiples of f0; formants are vocal-tract resonances and are not the same as harmonics.
- Masking, critical bands, equal-loudness contours, temporal integration, and just-noticeable differences explain how listeners detect signals in noise.
Worked example
A tone has pressure 200 micropascals. Since 200/20 = 10, dB SPL = 20 log10(10) = 20 dB SPL. If a second identical, uncorrelated source is added, intensity doubles and the level rises by about 3 dB to 23 dB SPL. If the two sources were coherent and added in phase, pressure would double and the level would rise by 6 dB. Reasoning: dB is logarithmic and depends on whether pressure or power is added.
Traps
Do not add dB values arithmetically. Do not treat 0 dB SPL as no sound; it is a reference. Do not confuse dB HL with dB SPL. Do not call formants harmonics. Do not apply the inverse-square law to a tube without considering boundary conditions.