8. Psychoacoustics (Perception Layer) · Concept 13 of 18
Volley Theory
The idea that groups of nerves take turns firing so they can together keep up with fast sounds.
Three nerves fire in rotation; their summed volleys hit one spike per wave cycle, tracking a tone no single fibre could.
What it is
Volley Theory: hearing nerves take turns firing in rotation so the team tracks a sound wave faster than any single nerve could alone.
Key facts
Single auditory nerve fibre caps at ~500-1000 spikes/sec (a ~1 ms refractory rest limits it)
Phase-locking (firing at the same point each wave cycle) holds to ~1-2 kHz, fades by ~4-5 kHz
Volley Principle (Wever & Bray, 1937): fibres interleave volleys to encode pitch by timing up to ~4-5 kHz
Timing/volley coding handles LOW freqs; place coding (which spot on the cochlea moves) handles HIGH freqs
Human hearing range: 20 Hz to 20,000 Hz (20 kHz); upper limit drops with age
Period = 1 / frequency. 1 kHz tone = 1 ms per cycle; 100 Hz tone = 10 ms per cycle
Refractory period = the ~1 ms a nerve must rest before it can fire again
Cochlea = snail-shaped inner-ear organ; its basilar membrane sorts pitch by place; hair cells make the spikes
Speed of sound in air ~343 m/s at 20 degC (rises ~0.6 m/s per degree C)
+10 dB sounds ~twice as loud; +6 dB = double sound pressure; +3 dB = double power
How it works
A sound wave vibrates the basilar membrane in the cochlea.
Each hair cell triggers its nerve fibre to fire at the same point in the wave cycle (phase-locking).
One fibre can't fire every cycle for fast tones because it needs ~1 ms to recover.
So different fibres fire on different cycles, taking turns.
Stacked together, these interleaved volleys reconstruct the full fast rhythm.
The brain reads the combined spike timing as pitch up to ~4-5 kHz.
Real examples
A 2 kHz vocal tone: no single fibre fires all 2000 times/sec, but a team in rotation encodes its timing.
Hearing the pitch of a kick drum or bass guitar (50-200 Hz) relies on timing/volley coding, not just place.
Telephone speech (300 Hz-3.4 kHz) sits right in the volley-coded timing range.
Tuning a low E string (~82 Hz) by ear works because timing coding nails low-frequency pitch.
Above ~5 kHz (cymbal shimmer, hiss) timing fails and the ear leans on place coding instead.
How it helps in live sound
Trust your ears most for pitch/tuning below ~4-5 kHz, where timing coding is strongest.
Low-end (kick/bass) pitch is timing-coded, so phase and time-alignment of subs matters for clarity.
Use a 1 kHz reference tone (1 ms period) as a clean phase-lock test signal when ringing out a system.
Above 5 kHz, ears judge brightness over exact pitch, so EQ the air band by tone not note.
Comb filtering from delayed speakers smears the timing cues for low-mid pitch, so time-align your drivers.
Everyday analogy
Like a firing squad shooting in rotating volleys so the gunfire never stops, even though no single soldier can reload fast enough to keep it going alone.
Watch out
Myth: one nerve fires fast enough to track every wave. Truth: a single fibre caps near 500-1000 spikes/sec, so nerves must volley in rotation to encode anything above that.
Fun fact
Back in 1930 Wever and Bray wired a cat's auditory nerve to a telephone receiver, and the nerve signals replayed speech so clearly that people in the next room could understand the words. That result later helped inspire the volley theory.
Key takeaways
No single nerve fires fast enough for high tones; the team takes turns.
Phase-locked volleys encode pitch by timing up to ~4-5 kHz.
Timing coding rules low freqs; place coding rules high freqs.
Refractory period (~1 ms) is the bottleneck volleys work around.
Period = 1 / frequency: 1 kHz = 1 ms per cycle.
It explains why low-end tuning by ear is reliable.