Acoustics and Hearing
Acoustics and Hearing moves from one wave to a room full of them, and then to the ear-brain system that interprets them. The exam expects you to calculate reverberation, explain why rooms colour sound, and name the psychoacoustic effects that guide microphone and monitor placement.
Room acoustics
- Absorption coefficient (0 to 1) describes how much incident energy a surface absorbs; the noise reduction coefficient averages the mid-band values.
- Reverberation time RT60 is the time for sound to decay 60 dB after the source stops. Sabine form for metric units: RT60 = 0.161 V / A, where V is room volume in m³ and A is total absorption in sabins (m²).
- Direct sound falls 6 dB per doubling of distance; the reverberant field is roughly constant, so a critical distance exists where both are equal.
- Standing waves (room modes) occur when a dimension equals a half-wavelength; they cause boomy bass and uneven response.
- Flutter echo bounces between parallel hard surfaces; comb filtering is the notched response produced when a delayed reflection sums with the direct signal.
Hearing and psychoacoustics
- Equal-loudness contours show sensitivity peaking around 3 to 4 kHz and falling at low frequencies.
- Masking: a loud sound hides a quieter one nearby in frequency or time.
- Precedence (Haas) effect: a reflection arriving within about 30 ms is fused with the direct sound and heard as loudness or space, not as a separate echo.
- Localisation uses interaural time and intensity differences; binaural cues create stereo imaging.
Worked example. A 120 m³ studio has 20 sabins of total absorption. RT60 = 0.161 × 120 / 20 = 0.966 s, about 0.97 s. Doubling absorption to 40 sabins halves it to 0.48 s, which suits speech.
Traps
- Using the wrong Sabine constant (0.161 for metres, 0.049 for feet).
- Treating an echo and a precedence-effect reflection as the same thing.
- Saying "more absorption always improves the room" without considering dead, lifeless results.