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Properly Setting Sound System Gain Structure

Analyzing each device in relation to the others -- and in relation to the entire signal path...

By Chuck McGregor May 21, 2019

Image courtesy of Alexander Stein

In the example, a different pad would be needed for each output (assuming the amplifiers have equal input sensitivities). The top of the window of the device feeding the crossover is still matched to the top of the crossover’s window.

In the actual system, the amplifier input levels are adjusted to acoustically balance the system similarly to a multiple branch system. Use the frequency band that you want to turn up the most – typically the subwoofer (but of course you won’t turn it up – right?) as the reference output. Balance the other bands to it by turning DOWN their amplifier input level controls.

Once you have the system balanced to your acoustical liking, you may find that amplifier input level controls, in particular for horn amplifies, may be set too low for them to reach full output – even with a single frequency sine wave in their pass band. You can increase all the amplifier input level controls by the same amount to get some or all of this unusable capability back for limited frequency range signals.

Keep in mind, however, that this will have two consequences: It will raise the acoustic noise floor of the system and the capability for full-range signals will remain the same. However, some amplifiers will clip before the signal processing in the system.

This is another situation where you must accept a compromise or change amplifier sizes to get a better match in gain and capability between the different frequency bands.

Other Band-Limited Devices

There is a more general case, similar to the crossover scenario. If you have full-range signals at the input of a device that limits the frequency response—such as with high or low pass filters—there will be an energy loss from its input to output.

Calculate this loss using the same procedure outlined in the previous section on electronic crossovers. The significant energy of full range music signals effectively spans about 9 octaves (approximately 30 Hz to 15 kHz).

Example: An under balcony system band limited from 150 Hz to 5 kHz.

1) Multiply the lowest frequency limit of the device by 2 until you get to the highest frequency limit for the device. The number of times you multiplied = the number of octaves. Round off the results to the nearest whole octave [= 5].

2) Divide the number of octaves by the 9 full-range octaves [= 0.56].

3) Push the LOG key for this result [= -0.3].

4) Multiply this result by 10 to find the approximate loss [= -3 dB].

Now you must draw a horizontal line on the output side of the device’s window. The line is drawn at a distance below the top of the window equal to the loss in dB as found in #4 above. This line is used to match up the device’s window to the top of the window of the following device.

System Limiting

The purpose of a system limiter in a properly gain structured system is to prevent any signals from exceeding the system’s maximum level. As such, it is used as an “emergency” device meaning it is intended to provide a hard, never-to-exceed maximum output level.

Limiter/compressors with soft-knee thresholds are not as ideal for protection. You really want something that doesn’t do anything up to a certain point then stops any further increase cold in its tracks. Because you need some margin between the device’s dynamic range (30 dB above the noise floor to the clipping level). The result is 42 dB.

Measurements of the maximum dynamic ranges for acoustic instruments and voice yield maximum figures in excess of 40 dB.

The maximum input and its limiting threshold are a bit tricky to implement properly without compromising the system’s dynamic range.

Just as with any other device you must introduce the limiter using its input, output, noise floor specifications, and gain setting just as with any other device in the system. Because of the way they work, the threshold setting is used as the maximum input.

For proper functioning the threshold should be set to at least 3 dB lower than the maximum output signal from the device preceding it. The limiter’s output gain should be used to adjust its maximum output at threshold to about 2 dB below the input level of the device it feeds. This allows a little “margin for error” in the protection.

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