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Why two speakers can arrive out of step
Sound travels at a finite speed, about 343 metres per second in air at room temperature. That sounds fast, and it is, but it is not instant. If one speaker is 2 metres from you and another is 5 metres away, the farther speaker’s sound takes about 9 milliseconds longer to reach you. Nine milliseconds is enough to smear a drum hit, thin out a bass note, or make a voice sound hollow, because the two copies of the same sound arrive slightly out of step and partly cancel each other.
Live sound engineers fix this with delay. They delay the nearer speaker by exactly the travel-time difference, so both sounds arrive together. The same fix applies at home: in a home cinema with a rear speaker far behind you, in a garden with speakers at different distances, or on a stage with front fills near the audience and main speakers further back. This calculator works out that delay in milliseconds and in audio samples, so you can enter it into a delay unit, a digital mixer, or a speaker processor.
How to use the calculator
Measure both distances to the same listening spot, in metres. It does not matter which spot you pick, as long as it is the place that matters most, usually the main seat or the centre of the audience. Enter the nearer distance, the farther distance, the air temperature, and your sample rate. Temperature matters because sound travels faster in warm air: about 331 metres per second at freezing point, rising by roughly 0.6 metres per second for each degree Celsius.
The result tells you how much to delay the nearer speaker. Delaying the nearer one may feel backwards at first, but it is correct: the nearer sound would otherwise arrive early, so you hold it back until the farther sound catches up. If both distances are equal, the delay is zero, and no correction is needed. The sample count is for digital systems that set delay in samples rather than milliseconds: multiply the delay in seconds by the sample rate, which the tool does for you.
The formula, in plain words
Delay equals the distance gap divided by the speed of sound. The distance gap is simply the farther distance minus the nearer distance. At 20 degrees Celsius, sound travels about 343 metres per second, so a gap of 3.43 metres gives a delay of 10 milliseconds. A gap of 0.34 metres gives about 1 millisecond. That rule of thumb is handy on site: roughly 1 millisecond of delay per 34 centimetres of distance gap, or about 1 millisecond per foot.
Sample rate turns milliseconds into the numbers a digital delay wants. At 48,000 samples per second, 10 milliseconds is 480 samples. At 44,100 samples per second, the same delay is 441 samples. If your processor only accepts whole samples, round to the nearest sample; an error of one sample is about 0.02 milliseconds, far too small to hear. If your processor accepts milliseconds with one decimal place, that precision is plenty too.
A worked example
Suppose your main speakers are 5.43 metres from your seat and a near fill speaker is 2 metres away. The gap is 3.43 metres. At 20 degrees, the speed of sound is about 343.4 metres per second, so the delay is 3.43 divided by 343.4, which is 0.01 seconds, or 10 milliseconds. At 48 kHz, that is 480 samples. Enter 10 milliseconds of delay on the near fill, and both speakers now reach your seat together.
Now move outdoors on a warm day, 30 degrees. Sound travels faster, about 349.5 metres per second, so the same 3.43 metre gap needs only about 9.8 milliseconds. The difference is small, about 0.2 milliseconds, and most listeners would never notice it. Temperature matters more over long distances: for a speaker 34 metres away, the same 10 degree change shifts the delay by about 3 milliseconds, which is worth correcting in a large venue.
Limits to keep in mind
This calculator aligns arrival time at one listening spot. It cannot align every seat at once, because distances differ across a room or an audience. In a venue, engineers choose a compromise point, often the centre, and accept small errors elsewhere. At home, align for the main seat and let the other seats fall where they may. Also, delay only fixes timing. It does not fix volume differences, room reflections, or a speaker pointed the wrong way.
Check the result by ear. Play a short, sharp sound, such as a click or a snare hit, through both speakers, and listen from the chosen spot. With the right delay, the hit sounds single and solid. With the wrong delay, it sounds doubled or thin. If your system has no delay control, moving the nearer speaker back by the same distance gap achieves the same timing, though it is rarely as practical. Either way, timing alignment is one of the cheapest sound improvements there is, because it costs no new equipment, only a measurement and a number.
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