Binaural Hearing

A sound from the left hits the near ear sooner and louder than the far ear; your brain compares the two (timing for lows, level for highs) and points to the source.

What it is

Using both ears together so your brain can pinpoint where a sound is coming from in space.

Key facts

• Speed of sound in air = 343 m/s at 20 degrees C (round to ~340 m/s).
• Ears are ~21 cm apart, so max time gap between ears (the ITD = Interaural Time Difference) is about 0.6 to 0.7 ms (640 microseconds).
• ITD = Interaural Time Difference: sound hits the near ear sooner; brain uses this BELOW ~1.5 kHz to locate.
• ILD = Interaural Level Difference: head shadows the far ear, making it quieter; brain uses this ABOVE ~1.5 kHz, gap can reach 15 to 20 dB.
• Duplex Theory (Lord Rayleigh, 1907): low frequencies located by ITD (timing), high frequencies by ILD (loudness).
• Minimum Audible Angle: humans resolve direction to about 1 to 2 degrees dead ahead (best case).
• Below ~80 Hz hearing is basically non-directional - bass is mono to your brain (why one sub works).
• Haas / Precedence Effect: the FIRST arrival sets perceived direction; a copy arriving 1 to 35 ms later is heard as the same sound, not an echo - past ~40 ms you hear a distinct echo.
• +6 dB = double the sound PRESSURE; +10 dB = roughly twice as LOUD; -3 dB = half the power (half-power point).
• HRTF = Head-Related Transfer Function: outer-ear (pinna) filtering tells you up/down and front/back, not just left/right.

How it works

1. A sound off to one side reaches the near ear first (timing cue = ITD).
2. The same sound is slightly louder at the near ear; the head shadows the far ear (level cue = ILD).
3. Your brain compares the two ear signals - timing for low notes, loudness for high notes.
4. Pinna (outer ear) shape filters the sound to add up/down and front/back info (HRTF).
5. Brain fuses both ears into ONE located source pointing at the origin.
6. In a mix, panning feeds each ear differently to fake this same left-right placement.

Real examples

• Closing one eye does nothing to hearing, but plugging one ear destroys your ability to tell left from right - that's binaural at work.
• A snare panned hard left in headphones lands almost entirely in your left ear because each ear gets a different feed.
• You can find a ringing phone in another room by turning your head until both ears hear it equally (centred).
• At a festival you can pick out which side a heckler is yelling from even with a wall of stage noise.
• Stereo recording works only because playback gives each ear its own channel, recreating the two-mic effect.

How it helps in live sound

• Keep ALL subs mono and ideally clustered/centred - bass below ~80 Hz carries no direction, so stereo subs just waste headroom and cause comb filtering.
• Use the Haas trick for delay/fill speakers: add 5 to 20 ms delay so the audience images the sound at the MAIN PA, not the nearby fill speaker overhead.
• Set delay-line distance: time = distance / 343 m/s (e.g. 20 m gap needs ~58 ms of delay to align with the mains).
• Don't over-pan key elements (lead vocal, kick, snare, bass) - keep them centred so the whole room hears them; the crowd is spread wide, not in a stereo sweet spot.
• Pan width is for headphone recordings - in a large PA think 'mono with flavour', hard pans vanish for anyone off-centre.
• A 6 dB level bump between two speakers, or a 1 ms timing offset, is enough to visibly shift where the audience hears the image.