Sensitivity | PerkAudio – Live Sound Consulting

Amplifier power vs Speaker Sensitivity

There are many aspect that would dictate the selection of an amplifier and speaker application. One of these aspect that I wish to address is the selection is the speaker sensitivity. The graph below shows the requirements in reference to the sensitivity of a speaker. In my case I already have my existing amplifiers and I wish to select my speakers to be used for my application. The first thing is to decide the dB level that you wish to provide for your application. The choices that I have provided in the graph below is 30, 50, 70 and 90 feet. These could represent:

• The 30 feet would address a small club
• The 50 feet would address a large club
• The 70 feet might address a small hall
• The 90 feet might address an outdoor event

Using the Crown calculation, the results provides the power requirement for each of conditions. For each distance the dB level requirement was selected to be 90 dBs. In my case, I have a 300 watt RMS amplifier that I used in my application and will use this value as a reference for the explanation that I am trying to provide. Since I have the amplifier, my speaker rating should be in the range of 150 watt RMS / 300 watt program power.

Now the question is, what will be my minimum speaker sensitivity for each of these requirements.
• For the 30 feet for a small club, I would select one that is greater than 88 dB.
• For the 50 feet for a small club, I would select one that is greater than 93 dB.
• For the 70 feet for a small club, I would select one that is greater than 96 dB.
• For the 90 feet for a small club, I would select one that is greater than 98 dB.

Note: For every 3 dB increase in sensitivity the power requirement is decreased by 50%.

Notice that the greater the speaker sensitivity the less power that is required. So it would be in your best interest to select one that is as large as possible to provide the additional headroom for your application.

This is only addressing the typical bass reflux speaker that manufactures have most of their focus on. In my case, my focus is concentrating on the loading of the speaker as well. Last year I build a couple of folded horn subwoofers and the sensitivity of these cabinet increased by 8 to 10 dBs.

I loaded these cabinets with a speaker that was rated at sensitivity 90 dBs and the result of cabinet loading increased this speaker application up to 98 to100 dBs. The end result decreased my power requirement 2 to 3 times. To explain further, this 90 dB speaker could provide a 50 foot application but would require 487 watts to do this. Now with the loading of these subwoofers with this same speaker, I can provide a 90 foot application and would only require 158 at 100 dB to 250 at 98 dB.

Lloyd Perkins – PerkAudio – Live Sound Consulting & Custom Speaker Design.

Speaker Sensitivity represents one of the most useful specifications published for any transducer.
Loudspeaker manufacturers follow different rules to obtaining this value with most expressing this value as the average output across the usable frequency when applying 1W/1M into a nominal impedance.

This represents the efficiency and volume that can be expected from a speaker when applying 1 watt into the nominal impedance and measuring the dBSPL at a distance of 1 meter with a reference voltage for this measurement is 2.83V into 8 ohms.

Why is this so important? When selecting a speaker or speaker cabinet, if I can increase the sensitivity by 3 dB, you will reduce my power requirement by half and this can be a considerable cost savings when purchasing a power amplifier.

This value can be used to determine the required amplifier wattage required to provide a certain given dB level at a given distance for an application.

This equations can be used to calculate the required wattage by first using:
dBW = Lreq – Lsens + 20 * Log (D2/Dref) + HR

Then using the dBW result to get the required wattage with:
Watts = 10 to the power of (dBW / 10)

Where:
Lreq = required SPL at listener
Lsens = loudspeaker sensitivity (1W/1M)
D2 = loudspeaker-to-listener distance
Dref = reference distance
HR = desired amplifier headroom
dBW = ratio of power referenced to 1 watt
W = power required

I do this in a spreadsheet for comparison calculations.

Doing this calculation yourself is not required, an app is provided the Crown site at: www.crownaudio.com/en/tools/calculators
and select the Amplifier Power Required.