For the purpose of discussion, let’s assume that we have a 3-way loudspeaker system (loudspeakers with high, mid and low sub-sections), with the following AES power ratings and sensitivity ratings:

Table 1: Loudspeaker sensitivity and power ratings

The high and mid sub-sections of a single loudspeaker can handle the minimum SPL requirements of 139dB at 1 meter. However, the low frequency sub-section will require two loudspeakers.

And then, by doubling the number of loudspeakers, we will obtain a +6dB gain, which results in a low frequency peak SPL of 143dB. We can now go directly to our amplifier selection. In order to calculate the amount of power required, we need to use the following equation:

(4a) PWR(dB) = SPL Criteria peak - sensitivity + SPLdist-loss

(4b) PWR (watts) = 10 PWR(dB)/10

The peak SPL criteria was established earlier at 111dB SPL (96dB nominal + 6dB peaks + 10dB headroom). The loss due to distance is 28dB. By plugging these numbers into equations 4a and 4b (above), we obtain the following results:

Table 2: Amplifier power requirements

* One loudspeaker will be required to provide an SPL criteria peak of 105dB SPL since two loudspeakers will give us our required SPL criteria peak of 111dB SPL.

** This is peak power, not AES. The AES power handling would -6dB lower than this (divide by 4).

Console and processing output levels

Now that the amplifier size has been determined, the next thing to look at is processing level inputs and outputs. Most sound consoles can comfortably handle an output level between +18dBu and +24dBu.

This, in turn, will feed the processing equipment. Analog processors can usually handle +18dBu input and output signals. This is the first place in line where attenuation or a pad may be required. If you are using a console that can output +24dBu, you will want 6dB of attenuation at the input of the audio processor. This can usually be achieved by the input attenuators on the signal processor.

The outputs of the signal processors require a bit more discussion. Many DSP devices have either output switch settings or output jumper settings that can select between 0, +6dB, or +12dB, so the obvious questions are "Why are there different options?" and "When do you use them?". To answer this, we must first continue our discussion about amplifiers.

Amplifier Input Levels

Many manufacturers have input selection settings than can choose between 0.775V, 1.4V, X20 (or 26dB), or X40 (or 32dB). For the purpose of discussion, Table 3 (below) indicates the input level that 200-, 400-, and 800-watt amplifiers will accept before the amp clips.

The 0.775V and the 1.4V input level settings indicate that all amplifiers will clip at the same input level. For the X20 (26dB) or the X40 (32dB) selection settings, the size of the amplifier and the load on the amplifier will determine the level at which the amp will clip. It is very important to be able to understand the clip levels and gains of the amplifiers in both dB and in voltage.

For 0.775V or 1.4V input sensitivity

(5a) Gain (volts) = sqrt [Max power rating * load (ohms)] / input sensitivity
(5b) (dB) = 20 log[Gain (voltage)]
(5c) Clip Level (volts) = input sensitivity (0.775V or 1.4V)
(5d) Clip Level (dB) = 20 log [clip level (volts)]

For X20 (26dB) or X40 (32dB) gain

(6a) Clip level (volts) = sqrt [Max power rating * load (ohms)] / gain (20 or 40)
(6b) Clip level (dB) = 20 log[Clip level (volts) / 0.775V]
(6c) Gain (volts) = gain (20 or 40)
(6d) Gain (dB) = 20 log[gain(volts)]