How to use it
Put on headphones and follow the short set-up: a left/right check and a comfortable volume. Loudspeakers do not work: each ear would hear both channels and the differences between the ears would be lost.
In Explore, press Play and move the two sliders while short bursts repeat. The drawing of a head from above marks where such a sound is typically heard: a rough guide only, because people differ and over headphones the sound usually stays inside the head. Measure finds the smallest difference you can detect.
Controls
- Time difference (ITD): which ear gets the sound first, up to 800 microseconds (µs) either way, in 5 µs steps.
- Level difference (ILD): which ear gets it louder, up to 20 dB. Half is added to one ear and half taken from the other.
- Sound: low tone (500 Hz), high tone (4 kHz), noise burst or click train.
- Presets: ready-made examples, including a trading preset (time points left, level points right).
- Keyboard: Space plays or stops; Esc stops all sound. In a run the Left and Right arrow keys answer, Space repeats the pair and Esc pauses.
- Volume: a slider with a fixed ceiling (−6 dBFS).
How it works
A sound from the right reaches the right ear first and is louder there. The interaural time difference (ITD) is tiny: for an adult head the largest natural value, for a source directly to one side, is about 0.6 to 0.7 milliseconds. The interaural level difference (ILD) arises because the head casts an acoustic shadow.
For angles the page uses Woodworth’s formula for a rigid sphere of radius a = 8.75 cm, a distant source at angle θ from straight ahead and the speed of sound c = 343 m/s.
This gives 656 µs at 90° and about 9 µs per degree near the front.
Which cue the brain uses depends on frequency: the duplex theory, published by Lord Rayleigh in 1907. Low tones have waves much longer than the head (69 cm at 500 Hz), so they bend around it and are almost equally loud at both ears; but nerve cells fire in step with the wave, and its timing at the two ears can be compared. Above roughly 1.5 kHz this fails: the cycles follow each other too fast for the comparison to work, and a whole cycle (667 µs at 1.5 kHz) fits inside the largest natural ITD, so the phase is ambiguous. High tones are short waves (8.6 cm at 4 kHz) that the head blocks, producing level differences that can reach about 20 dB.
Noise and clicks can be moved by a time difference again: they contain low frequencies, and their onsets and envelope carry timing even at high pitch. Over headphones a level difference also works for low tones, although a real head hardly shadows them.
Every burst is computed sample by sample, so the delay is exact to a fraction of a microsecond. For tones only the wave is shifted, while the fade-in and fade-out happen at the same instant in both ears. For noise and clicks the whole burst is delayed.
Measure mode uses a two-down, one-up staircase. Each trial plays a centred sound, then a shifted one, and you say which way it moved. Two correct answers in a row make the shift smaller, one mistake makes it larger, and the run ends after 10 reversals or 50 trials, or earlier if the largest shift is missed four times. This rule homes in on the shift you get right 70.7 % of the time; the result is the geometric mean of the last six reversals. Practised listeners reach roughly 10 to 20 µs and about 1 dB under good conditions (approximate laboratory values); a first run is usually several times larger.
Things to try
- Low tone, time difference only: find the smallest shift that still moves the sound.
- Keep that time difference and switch to the high tone: the sound returns to the middle. Then try noise.
- High tone at exactly 250 µs: one full cycle at 4 kHz, so both ears receive identical signals again.
- Trading preset: move the level slider until the sound is centred. That is how many decibels cancel 300 µs for you.
- Measure your ITD threshold with the low tone and with noise.
Frequently asked questions
Why does the sound stay inside my head?
Headphones bypass the outer ear, the head and the room, which normally filter and echo every sound, and the sound does not change when you turn your head. Without these signs of an outside source, the brain places the sound between the ears.
Can it move a sound up, down or behind me?
No. ITD and ILD only tell left from right: all directions on a cone around the line through the ears give nearly the same pair of values, the cone of confusion. Up, down, front and back are told apart mainly by the folds of the outer ear, which filter high frequencies differently for each direction. The page does not simulate that.
What angle is my threshold?
About 10 µs corresponds to roughly 1°, about the smallest change of direction listeners can detect straight ahead.
Is this a hearing test?
No, it is not a medical test. The result depends on your headphones, practice and attention, and says nothing about the health of your hearing. A sudden drop in hearing is a reason to see a doctor the same day.