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Speed of Sound Time Delay Table Hire Delays Reference Library Home

Speed of Sound - Introduction

You may be surprised to learn just how slowly sound travels in air. Whilst light is almost instantaneous at 299337 km (186000 miles) per second, sound crawls along at a mere 342 metres (1120 feet) per second.
It is the slow speed of sound which causes audible echos to bounce back from a flat surface eg a wall or mountain. The closer you are to the wall the shorter the echo. As you move further away the echo time lengthens. If you check echo lengths with the chart below, remember to double the length shown because the sound has to travel there and back.

The repeating sound you often hear from multiple speaker public address systems give an excellent illustration of time delay and the problems it creates for the sound designer or engineer. It's a common occurrence in railway stations because there are usually speakers placed in a straight line down the length of a platform. After the primary sound from the nearest speaker reaches your ears, the repeats you hear are the sounds arriving from speakers further away. Hearing multiple sound sources can result in the message becoming indistinct. For reasons of public safety, Performance Indicators are now used called RASTI which prescribe the desired intelligibility of announcements.

The standard method for improving system synchronisation and intelligibility is with the use of signal time delay units. Nowadays, these devices use a full bandwidth digital processor which "holds up" the signal  to the secondary speakers for a fraction of a second until it is in sync with the main source. Mono and stereo units are common but are often overlooked because they can be a bit fiddly to set up (in which case the chart below will be a big help). Some of the more clever time alignment units can be set up by punching in the distance between the delay speakers and source and then translating it to milliseconds. Many manufacturers now produce programmable DSP engines or software based systems which can be programmed to process multi-channel audio with compression, eq, filtering,  priority routing and other useful facilities.

Time Delay Table for loudspeakers

Metres delay in milliseconds   Feet delay in milliseconds
5 14.6   20 17.9
10 29.2   30 26.8
15 43.8   40 35.7
20 58.5   50 44.6
25 73.1   60 53.6
30 87.7   70 62.5
35 102.3   80 71.4
40 117.0   90 80.4
45 131.6   100 89.3
50 146.2   110 98.2
55 160.8   120 107.1
60 175.4   130 116.1
65 190.1   140 125.0
70 204.7   150 133.9
75 219.3   160 142.9
80 233.9   170 151.8
85 248.5   180 160.7
90 263.2   190 169.6
95 277.8   200 178.6

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