In order to illustrate how delay controls work, let’s look at two examples. The first case: You install a television lengthways in a rectangular room. Your main and rear speakers thus find themselves against walls that are facing each other. Your lounge is smack in the centre of the room. Second case: same configuration, except that your lounge is against the back wall of the room. In analysing these situations, it appears that in a film, when a plane passes, for example, from front to back, the back speakers relay the front ones. In the first case, You are further from your Surround speakers, and their sound will take more time to arrive at your ears than in the second where the Surround speakers are much closer to you. This time difference is very minimal, measured in milliseconds, but it has its importance. If your amplifier is adjusted on the same "Surround delay" setting, you will have more the impression of a front to rear passage in the first case than in the second. Effectively, what gives the "flying-over" impression is the fact that the sound is firstly in the front speakers, then the back ones. If the sound arrives virtually at the same time at the back as in the front, it will give more the impression of a "stationary" flight than the front-to-back fly-over desired. Conclusion:
In the case that you’re very close to your speakers, it is necessary to artificially elongate the delay between the front and rear speakers. Inversely, the more equidistant you are from the front and rear speakers, the more you will have to shorten this delay, or else you’ll run the risk of having residual sound of the plane from the rear speakers, even though the scene is over. NB: The higher the quality of the amplifier, the more it will be likely to have precise delay controls, which can be configured down to the last millisecond. Generally, this configuration is effectuated 5 milliseconds by 5 milliseconds on a scale of 0 to 30. Particular cases:
Certain Dolby Surround amplifiers, in particular those possessing a Dolby Digital decoder, allow for the adjustment of "Centre Delay", which is the delay between the central and main speakers, in the case where the central speaker is further away than the main ones. For the same reasons of speed of sound reaching the listener, taking account of their distance from each speaker, it is thus possible to artificially elongate the delay with which the central speaker will react compared to the main ones. By the same token, it is also possible to shorten the delay in the case where the central speaker is too distant. Certain other amplifiers go as far as offering this possibility for the bass box. In theory, there is a mathematical rule allowing one to calculate, in comparison with the speed of sound and the distance between each speaker, the ideal delay in milliseconds to apply to speakers to obtain the most coherent results. The best method is to test it for yourself. Hint :
We owe it to the manufacturers, who simplify the task for us by giving the possibility to tell the amplifier what the distance is in metres (or in feet in some cases), between each speaker and the listener. The amplifier then takes care of calculating all the details itself. "