Surround Sound System Settings

Correct home theater speaker placement and properly balanced sound levels between the different channels in multi-channel audio, while crucial, are not enough to sustain a unified soundfield in a surround sound setup. A correctly set surround sound system requires that sound from all the different speakers reach the listener at exactly the same instant.  

This explains why all fronts, left, right and center channel speakers, should be set at exactly the same distance from the listening position. At the same time, in a typical home theater setup, the surround speakers are closer to the viewer than the fronts.

To compensate for this distance discrepancy, surround sound playback systems apply a split-second delay to the surround sound channels.

Setting the time delay in a surround sound system would effectively adjust the soundfield between the front and rear channels to ensure that simultaneous sounds from each speaker arrive at the listener's ear at about the same time.

However, in Dolby Pro-Logic surround sound systems, there is a second reason for applying delay to the surround channels.

In Pro-Logic mode, sound tends to leak between the front left and right channels and the rear surround channels as a result of the way the original multi-channel sound from the different sources is encoded over the two (stereo) right and left channels.

These leakages tend to disturb the proper perception of the sound field as a result of what is referred to as the Haas effect, also known as the law of the first wave front.

This effect will occur when the arrival difference between the two sources is within 10 to 40 milliseconds.

The Haas effect is basically the subjective perception by a listener that whenever identical sound arrives from two sources positioned at different distances, sound is perceived to be coming only from the closer source. Since surround speakers are closer to the listener, any sound leaking onto the surrounds but that should be coming from the fronts, would appear as coming from the surround speakers. This effect would distort the proper placement of the different sounds within the 'enveloping' soundfield in a Dolby Pro-Logic surround sound system.

In these circumstances, applying the correct amount of delay to a Pro-Logic encoded surround sound system would help ensure that even if you are physically sitting closer to the surround speakers, sounds that are supposed to be coming from the front - but that are also leaking onto the surround - will still be perceived as coming from the front speakers.

However, this necessitates the application of a slightly longer delay setting in a Pro-Logic surround sound system - typically in the region of 15 to 20 milliseconds - than that applied in Dolby Digital systems. This extended delay helps to improve the inherent lower channel separation found on these surround sound systems by reducing the audibility of sound leakages from the front to the surround.

This is not the case with Dolby Digital surround sound systems, where each channel is discrete, i.e. separate and therefore is not encoded over a second sound channel.

Adjusting the Time Delay Settings

Thanks to advancement in technology, time delay adjustments in the latest generation of home theater receivers have become much easier than ever. Some of the latest AV receivers incorporate an auto setup and calibration routine that uses a microphone for the multi-channel surround sound system to pick-up the different sounds and set the correct levels.

This does not mean that with these new systems, there is no need for further fine tuning. A home theater calibration disc and an inexpensive sound pressure level meter such as the Galaxy Audio CM130 SPL Meter, would still be required if you want to get the very best out of your system. At the same time, one has to admit that these built-in auto setup routines are more than adequate for a quick startup.

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More information on the use of calibration discs can be found at our Home Theater Set-Up Discs section.

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But even in AV receivers without an auto setup, setting delay times in a surround sound system has become easier than before. Up to a few years ago, the user had to calculate and then choose the right amount of delay in milliseconds, based on the positioning of the speakers with respect to the listener.

A good home theater amplifier or AV receiver without an auto set-up would simply ask you to key in the respective distances of each speaker from your listening position, and then the system would make the appropriate choice for you based on your response. No mathematics - just key in the distances and you are done!

Time Delay Calculations

Unfortunately, not all surround sound systems provide such a friendly user interface as detailed above. With some less intelligent systems, the user will have to key directly the delay settings in milliseconds.

To set the time delay between the fronts and the surrounds correctly, you need to determine the 'difference' in the distances between a front speaker and a surround speaker when this distance is measured with respect to your main viewing position.

To translate this in terms of delay, we make use of a simple physics equation, namely:

The speed of sound in air is approximately given by the following equation:

V_{sound in air} = [331.4 + 0.6 T_c]m/sec

where T_c is the air temperature in Celsius.

At a typical ambient temperature of 22 deg. Celsius (72 deg. Fahrenheit), the speed of sound in air is 345m/s or 1132ft/sec.

This means that if say the difference in the distances between the surround and the main front speakers measured with respect to your main viewing position is 15ft, the required delay is ≈ 13msec., or approximately 1 msec per foot.

Remember however that in the case of 'matrixed' surround sound formats such as Dolby Surround Pro Logic, the minimum delay should be set to at least 15msec and even longer to allow for the Haas Effect referred to earlier on in this discussion.