Basic Sciences and Acoustics
Sound as a physical quantity
Sound is a pressure wave; the decibel is a ratio, not an absolute unit. Sound pressure level uses 20 µPa as reference, so dB SPL = 20 log10(p/p₀). Intensity level uses 10 log10(I/I₀) with I₀ = 10⁻¹² W/m². A 6 dB increase in SPL doubles pressure; a 10 dB increase roughly doubles perceived loudness. Combining two equal, incoherent sources adds about 3 dB; two coherent sources in phase add 6 dB.
Frequency and the audiogram
Frequency (Hz) is the perceptual correlate of pitch; octaves double frequency. The audiogram plots dB HL, a reference-referenced scale normalized to average normal hearing thresholds (ANSI S3.6), not dB SPL. Know that 0 dB HL is not zero energy.
Key concepts to master
- Wavelength = speed of sound (≈343 m/s in air) ÷ frequency; low frequencies have long wavelengths and diffract around the head, high frequencies are shadowed by the pinna and head.
- Inverse square law: doubling distance from a point source drops level ~6 dB.
- Signal-to-noise ratio, masking, and the concept of the critical band.
- Resonance: tubes closed at one end (ear canal) resonate at odd quarter-wavelengths; a ~2.5 cm canal resonates near 3 kHz, which shapes the real-ear unaided response.
- Speech acoustics: vowels are low-frequency, high-intensity; consonants (fricatives) are high-frequency, low-intensity and carry much of intelligibility.
Traps
Questions confuse dB SPL with dB HL, or apply the 3 dB rule where the 6 dB pressure rule belongs. Watch for items asking about a doubling of distance vs doubling of sources. Another trap: describing dB as linear.
Rule of thumb
Two equal sources → +3 dB (incoherent). Double the pressure → +6 dB. Double the distance → −6 dB.