Setting Speaker Levels & Distance in a Surround Sound System
Adjusting the delay between the front and rear speakers is important when calibrating your system. Setting the delay too long may result in an echoed, unnatural surround field, or cause imaging shifts in the soundfield. However, adjusting the delay too short may result in a flat 2-dimensional sound field between the front and rear speakers.
Consider the following:
The speed of sound in air is: 1100 ft/sec
1100 (ft/sec)*1(sec/10^3ms) = 1.1 ft
Every 1 ms of delay added corresponds to 1.1 ft of increased distance of your rear speakers relative to your listening position.
For Example: Setting the rear delay to 25ms would result equivalently placing the surround speakers 27.5 ft away from your listening position!
As a rule of thumb:
- If your surround speakers are located close to your listening position, adjust the rear delay short (5-15ms).
- If your surround speakers are located far away from your listening position (20-30ft), adjust the rear delay long (15-35ms).
Note : It is important to set the delay correctly so that proper time synchronization occurs.
Audio Level Adjustments
Level calibration between all of your speakers in a 5.1 system is one of the most important adjustable parameters. Many people tend to boost the rear speaker levels too high relative to the fronts and center channels. This sometimes tends to overemphasis the rear channels resulting in an unnatural surround field that is easily localized by the ear. Over emphasizing a particular channel sound level will diminish the balance in the system. Doing so undermines what the recording engineering intended the mix to sound like for the movie and/or music CD.
I recommend the following procedure for proper level calibration of your 5.1 Surround System:
- Select a common listening position.
Adjust the master volume of your Receiver/Preamp to a common listening level you are accustomed to (say 75dB).
- Initiate the Receiver/Preamp test tone.
- Allow the test tone to sweep each speaker in your system.
- Adjust the volume levels of each channel until they sound similar in volume. It is preferable to use an SPL meter (C-weighing, Slow Response) for greater accuracy.
This is the first step in calibrating the sound levels of your 5.1 system. Please remember that these levels will need to be tweaked depending on the source recording and/or surround scheme.
For example: Some recordings may boost the rear channels 1-3db higher than nominal due to poor mixing methods or deliberate wow factor. If the rear levels sound too loud when watching a movie or listening to a 5.1/6.1/7.1 audio mix, simply lower them until they sound balanced relative to the front and center channels.
I recommend acquiring a multi-channel set-up disc such as the one from Avia, Sound & Vision, or DTS. It will help you calibrate your listening levels of your speakers and subwoofer as it sweeps frequencies 20Hz to 20KHz for all channels.
If you don't trust your ears, you may wish to purchase a Radio Shack SPL meter. When operating the test tone of your Receiver/Preamp, calibrate the volume levels within 1 dB relative to each channel. Hold the unit so that the microphone is pointed at the ceiling and position the microphone as closely to ear level as possible (at the sweet spot listening position) when running this test.
Note: Make sure you set the scale to "C-Weighted" on the SPL meter as this closely matches a flat frequency response curve throughout the audible band. The early AVIA test discs used to recommend the "fast" setting on the SPL meter but we've found it easier to get more consistent results using the "slow' setting which allows easier readability.
Technical Advances Update: Most receivers/processors built since 2003-2004 enable you to merely configure the distance from each speaker to the listening position. The mathematical delay calculations are then performed by the receiver or pre-processor, thus saving you a lot of headache and time. be sure to calculate distances to a singular point, the sweet spot, even though you may have listening positions throughout the listening room.
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