How much power is required?
- I’m playing folk music in a coffee shop. How much amplifier power do I need?
- Our rock group will be playing in a 2000-seat concert hall. How many watts will we need?
- I just bought some PA speakers. I want to play them as loud as they can get without blowing them up. Which amplifier should I get?
Amplifier manufacturers are often are asked similar questions, and here I provide some answers. But please research this for yourself and prove me wrong.
If you want to play as loud as possible without destroying your speakers then this sight and take a WAG on the sizing of the amplifier. This is not the professional approach. This WAG method dos not take in consideration some of the parameters that makes a big difference.
The correct and professional approach to this is to use the power vs application approach. This approach is to define the required loudness at a given distance in SPLdB for the selected venue? The major contributing factor in the calculation is the sensitivity of the speaker cabinet.
The bigger the room, the louder the sound system and more power is required. Depending on the cabinet design, improvements on the raw speaker efficiency can be made. If you can increase the efficiency of the speaker cabinet by 3 dB’s you can reduce the power requirement by half.
Typical loudspeaker sensitivity is 95 dB SPL/W/m for small system, 100-105 dB for medium system and 110 dB for large system. It is recommended to provide for a signal peaks of 6 dB for rock music that is highly limited or compressed.
Power requirement for typical application:
- Rock music at a small outdoor festival
- Distance = 50 feet
- Desired SPL = 100 dB
- Speaker Sensitivity = 95 dB
- Headroom = 6 dB
- Results = 3075 Watts per speaker
- Alternative: Speaker Sensitivity = 100 dB
- Results = 1000 Watts
If one loudspeaker won’t handle the total power required, you need to divide the total power among multiple loudspeakers and multiple amplifier channels.
Power Calculator – A calculation that determines the amplifier power requirements for given application to achieve the desired SPL at a certain distance. This calculation accounts for the headroom requirements (in dBs) to compensate for peaks without clipping. This calculation requires additional parameters thta include the loudspeaker sensitivity, required listening distance, and the desired SPL level at that distance.
Explanation of some of the factors in the calculation include:
Sensitivity – The loudspeaker specification that is tested at 1m@1W of power. Typical sensitivity for a PA loudspeaker is 95 to 110 dB-SPL/watt/meter. Please note that you want to use the cabinet sensitivity specification for this calculation. The reason for this is because the cabinet design has a factor due to loading and can increase the efficiency of the speaker which will increase the sensitivity of the speaker. The example here would be a folded horn where the speaker sensitivity is 95 dB and the loading increased the efficiency by 9 dB. Now the cabinet sensitivity is now 104 dB meaning that every 3 dB increase will change my power requirement by half.
Example: The calculation results was 1000 watts for the raw speaker of 95 dB but the loading increased this to 104 dB for this subwoofer application.
- 1000 watt with an increase of 3 dB results a 500 watt requirement
- 500 watt with an increase of 3 dB results a 250 watt requirement
- 250 watt with an increase of 3 dB results a 125 watt requirement
Peak headroom – Because music has transient peaks that are 6 to 25 dB above the average level, the power amplifier needs to produce enough power to handle those peaks without distortion. In the calculation the minimum value is 6 dB but this requires incorporating a limiter in your application to eliminate the clipping of peaks above the 6 dB. It would be cheaper to use a limiter at a cost of $100 to $150 instead of compensating with additional power. An increase for every 3 dB above the 6 dB will double the power requirements an that would be very expensive. Note: The use of no limiter is not an option.
Listener distance – The listening distance from the loudspeaker to the farthest listener is required. If you don’t know this distance, you can make a rough estimate from the typical values below. Be sure to enter the distance in meters (m). Examples include:
- Small club or auditorium: 35 feet
- Medium club, auditorium or small outdoor festival: 50 feet
- 2000-seat concert hall: 125 feet
- Small stadium or arena: 250 feet
Desired SPL – The SPL level in dB (C-weighting, slow response) at the listening distance. A minimum of 10 dB above background noise level is required to achieve a good signal-to-noise ratio. Typical sound pressure levels (SPLs) for various types of music include Pop at 90-95 dB, Rock: at 95-110 dB and Heavy metal: 110 dB.
Room reverberation will increase the SPL typically by 6 dB. You can use this room gain as extra headroom.
In a Bi-Amp or Tri-Amp application, it is required to apply this calculation to each amplifier that will service unique set of frequencies.
When selecting an amplifier, round up your power requirement results or select the next size of amp above the calculated power. This will compensate for large venues or allow you to increase the volume due to an unexpected high background noise level.
The power distribution for the amplifiers is define by the frequency range because the lower the frequency the more power required to produce the sound. The typical frequencies breakdown and % power requirement is defined as follows:
|Frequency (Hz):||35 to 150||150 to 800||800 to 5k||5k to 18k Hz|
|% Total Power:||50%||25%||15%||10%|
Power Amplifier Recommendations:
If not using embedded DSP Controls, a Loudspeaker Management System can provide the
same functionality for existing non DSP amplifiers or purchased of non DSP amplifiers.