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Selecting the limiter threshold for a loudspeaker power limiter.
The two most common causes of loudspeaker damage are thermal overload and mechanical overload.
A power limiter can keep the power supplied to a loudspeaker within safe levels and thereby protect it from thermal overload. To protect a loudspeaker against mechanical overload, it must be driven only within the frequency range it is designed for.
To protect a loudspeaker from thermal overload, the limiter's threshold must be selected to match the loudspeaker's continuous power handling - normally half the power handling stated as continuous pink noise power handling - and the amplifier's voltage gain.

A JavaScript that calculates the limiter threshold is in Limiter threshold calculator for a loudspeaker power limiter.

Calculate the maximum allowable voltage across the loudspeaker:
VSPMaximum long term voltage across loudspeaker in dBu.
The unit dBu is a voltage unit with 0.775 V reference.
PSPThe loudspeaker's long term RMS power handling in Watts.
This is usually half its AES pink noise power handling.
300 W
RSPThe loudspeaker's nominal impedance in ohms.8 ohms

[VSP = 20 * log (SQRT(PSP*RSP/0.6))]
[VSP = 20 * log(SQRT(300W*8ohms/0.6)) = 36.0dBu]

Find the voltage gain of the amplifier used. Some manufacturers state this value in their data sheet.
Otherwise, the value can be calculated from the amplifier's nominal output power and its input sensitivity:

AvThe amplifier's voltage gain in dB.
PNThe amplifier's nominal output power in Watts.500 W
RNNominal load impedance for PN.8 ohms
VIAmplifier input sensitivity in volts.1.5 V

[Av = 20 * log(SQRT(PN*RN)/VI)]
[Av = 20 * log(SQRT(500W*8ohms)/1.5V)]
Calculate required limiter threshold VTH:
[VTH = VSP - Av]
[VTH = 36.0dBu - 32.5dBu = 3.5dBu]
This table shows the voltage for some typical speaker power ratings:
Power1 ohms2 ohms4 ohms8 ohms16 ohms
5 W9.2 dBu12.2 dBu15.2 dBu18.2 dBu21.2 dBu
10 W12.2 dBu15.2 dBu18.2 dBu21.2 dBu24.2 dBu
20 W15.2 dBu18.2 dBu21.2 dBu24.2 dBu27.3 dBu
25 W16.2 dBu19.2 dBu22.2 dBu25.2 dBu28.2 dBu
40 W18.2 dBu21.2 dBu24.2 dBu27.3 dBu30.3 dBu
50 W19.2 dBu22.2 dBu25.2 dBu28.2 dBu31.2 dBu
60 W20.0 dBu23.0 dBu26.0 dBu29.0 dBu32.0 dBu
75 W21.0 dBu24.0 dBu27.0 dBu30.0 dBu33.0 dBu
100 W22.2 dBu25.2 dBu28.2 dBu31.2 dBu34.3 dBu
125 W23.2 dBu26.2 dBu29.2 dBu32.2 dBu35.2 dBu
150 W24.0 dBu27.0 dBu30.0 dBu33.0 dBu36.0 dBu
200 W25.2 dBu28.2 dBu31.2 dBu34.3 dBu37.3 dBu
250 W26.2 dBu29.2 dBu32.2 dBu35.2 dBu38.3 dBu
300 W27.0 dBu30.0 dBu33.0 dBu36.0 dBu39.0 dBu
400 W28.2 dBu31.2 dBu34.3 dBu37.3 dBu40.3 dBu
500 W29.2 dBu32.2 dBu35.2 dBu38.2 dBu41.2 dBu
600 W30.0 dBu33.0 dBu36.0 dBu39.0 dBu42.0 dBu
800 W31.2 dBu34.3 dBu37.3 dBu40.3 dBu43.3 dBu
1000 W32.2 dBu35.2 dBu38.3 dBu41.2 dBu44.3 dBu
1200 W33.0 dBu36.0 dBu39.0 dBu42.0 dBu45.1 dBu
1500 W34.0 dBu37.0 dBu40.0 dBu43.0 dBu46.0 dBu

These pages lists the voltage gain for different power amplifiers: AB International, Amcron (Crown), Carver, Cerwin Vega, Dynacord, Harrison, HH Electronics, JBL, NAD.
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