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Selecting the limiter threshold for a loudspeaker power limiter.

Contents


Introduction.

This is a limiter threshold calculator I published around 1995.
After I took it off the web, I got several mails asking me to put it back up, so here it is.

The first section is how you calculate the limiter threshold.

Second section is a Java-script that will do the calculations for you.
This contain a list of amplifiers that are now out-of-date. The format is quite simple and to add new amplifiers, save the html file to disk and edit the embedded script.

The download section is a spread-sheet that will do the calculations. This allows you to save the calculations for several amplifier/speaker combinations.

Note that limiters are an aid in avoiding over-powering your speakers - they are not not a replacement for common sense.
The results you get from this calculations should be considered initial guess values.
There are a lot of parameters that are not included in the calculation ( limiter detector ( rms, peak or a combination ), detector time constants, etc. ).


Units.

All voltage and power levels used for these calculations are RMS values.

As I used to design broadcast equipment, I normally use the unit dBu:
1V=0dBV=2.2dBu.

Or:
0dBu=0.775V=-2.2dBV.

The exact value for 0 dBu is ( so you do not wonder when you find this in the spread-sheet ) :
0dBu=sqrt(0.6V).


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 and excursion 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.

Calculate the maximum allowable voltage across the loudspeaker:

VariableDescriptionExample
VSPMaximum long term voltage across loudspeaker in dBu.
PSPThe loudspeaker's long term RMS power handling in W.
This is usually half its AES pink noise power handling.
300 W
RSPThe loudspeaker's nominal impedance in Ω.8 Ω

VSP=20*log(sqrt(PSP*RSP/0.6)).
Example:
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:

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

Av=20*log(sqrt(PN*RN)/VI).
Example:
Av=20*log(sqrt(500W*8ohms)/1.5V).

Calculate required limiter threshold VTH:
VTH=VSP-Av.
Example:
VTH=36dBu-32.5dBu=3.5dBu.


Voltage for some speaker power ratings.

Power1 Ω2 Ω4 Ω8 Ω16 Ω
5 W9.2 dBu2.2 V12.2 dBu3.2 V15.2 dBu4.5 V18.2 dBu6.3 V21.2 dBu8.9 V
10 W12.2 dBu3.2 V15.2 dBu4.5 V18.2 dBu6.3 V21.2 dBu8.9 V24.3 dBu12.6 V
20 W15.2 dBu4.5 V18.2 dBu6.3 V21.2 dBu8.9 V24.3 dBu12.6 V27.3 dBu17.9 V
25 W16.2 dBu5.0 V19.2 dBu7.1 V22.2 dBu10.0 V25.2 dBu14.1 V28.2 dBu20.0 V
40 W18.2 dBu6.3 V21.2 dBu8.9 V24.3 dBu12.6 V27.3 dBu17.9 V30.3 dBu25.3 V
50 W19.2 dBu7.1 V22.2 dBu10.0 V25.2 dBu14.1 V28.2 dBu20.0 V31.2 dBu28.3 V
60 W20.0 dBu7.7 V23.0 dBu11.0 V26.0 dBu15.5 V29.0 dBu21.9 V32.0 dBu31.0 V
75 W21.0 dBu8.7 V24.0 dBu12.2 V27.0 dBu17.3 V30.0 dBu24.5 V33.0 dBu34.6 V
100 W22.2 dBu10.0 V25.2 dBu14.1 V28.2 dBu20.0 V31.2 dBu28.3 V34.3 dBu40.0 V
125 W23.2 dBu11.2 V26.2 dBu15.8 V29.2 dBu22.4 V32.2 dBu31.6 V35.2 dBu44.7 V
150 W24.0 dBu12.2 V27.0 dBu17.3 V30.0 dBu24.5 V33.0 dBu34.6 V36.0 dBu49.0 V
200 W25.2 dBu14.1 V28.2 dBu20.0 V31.2 dBu28.3 V34.3 dBu40.0 V37.3 dBu56.6 V
250 W26.2 dBu15.8 V29.2 dBu22.4 V32.2 dBu31.6 V35.2 dBu44.7 V38.2 dBu63.2 V
300 W27.0 dBu17.3 V30.0 dBu24.5 V33.0 dBu34.6 V36.0 dBu49.0 V39.0 dBu69.3 V
400 W28.2 dBu20.0 V31.2 dBu28.3 V34.3 dBu40.0 V37.3 dBu56.6 V40.3 dBu80.0 V
500 W29.2 dBu22.4 V32.2 dBu31.6 V35.2 dBu44.7 V38.2 dBu63.2 V41.2 dBu89.4 V
600 W30.0 dBu24.5 V33.0 dBu34.6 V36.0 dBu49.0 V39.0 dBu69.3 V42.0 dBu98.0 V
800 W31.2 dBu28.3 V34.3 dBu40.0 V37.3 dBu56.6 V40.3 dBu80.0 V43.3 dBu113.1 V
1000 W32.2 dBu31.6 V35.2 dBu44.7 V38.2 dBu63.2 V41.2 dBu89.4 V44.3 dBu126.5 V
1200 W33.0 dBu34.6 V36.0 dBu49.0 V39.0 dBu69.3 V42.0 dBu98.0 V45.1 dBu138.6 V
1500 W34.0 dBu38.7 V37.0 dBu54.8 V40.0 dBu77.5 V43.0 dBu109.5 V46.0 dBu154.9 V

Limiter threshold calculator for a loudspeaker power limiter.

This JavaScript will calculate the required setting for the limiter. All JavaScript code is embedded in this html file, so you can save it to disk and use it off-line.

Any use of data obtained from this program is on your own risk and responsibility.

The calculator is divided into the 3 sections Loudspeaker, Amplifier and Limiter.
The Loudspeaker section calculates the maximum allowed voltage across the loudspeaker based on its nominal impedance and power handling. If the maximum allowed voltage is known, just type it directly into the 'Max. voltage' field. The units Volt, dBV or dBu can be selected for this field.
The Amplifier section calculates the amplifier's voltage gain based on its input sensitivity and nominal output power. If the manufacturer states the amplifier's voltage gain, type this value directly into the 'Voltage gain' field. The units dB or * can be selected for this field.
The program has a database with data for some amplifiers. The data in the list is obtained from amplifier manuals, service-manuals, sales brochures and from measurements made on amplifiers. Some amplifiers will display the program's default for some parameters ( typically a 1 watt output power ), if I do not have the information.
The Limiter section calculates the required limiter threshold setting based on the maximum allowed voltage across the loudspeaker and the amplifier's voltage gain. The units Volt, dBV or dBu can be selected for this field. Typing a value into the 'Threshold' field will update the Loudspeaker 'Max. power' and 'Max. voltage' fields based on this value.


Loudspeaker.
Impedance Ohms
Max. power Watt
Max. voltage
Amplifier.
Model
Input sensitivity
Output power Watt
Nominal load Ohms
Voltage gain
Limiter.
Threshold

Downloads.

Spread-sheet with limiter calculations.


Copyright and disclaimer.

Copyright Notice.
This web-page, including but not limited to all text, drawings and photos, is the intellectual property of Poul Petersen, and is Copyright ©.
Reproduction or re-publication by any means whatsoever is strictly prohibited under International Copyright laws.
The author grants the reader the right to use this information for personal use only.
Any commercial use is prohibited without express written authorization from Poul Petersen.

The information is provided on an "as-is" basis and is believed to be correct, however any use of the information is your own responsibility.

This web-site may contain links to web-sites outside Poul Petersen domain ( www.poulpetersen.dk ).
Poul Petersen has no control over and assumes no responsibility for the content of any web-site outside Poul Petersen own domain.

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Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
E-mail: pp@poulpetersen.dk

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