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 of 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 equation 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)
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 myself for comparison calculations.
My reference is to provide 90 dB listening level at 90 feet with the minimum 3 dB headroom
This defines my minimum speaker sensitivity requirement is 97 dB.
This allows me to use a 315 watts amplifier power to provide my minimum requirement
Some of the relationship that will dictate my requirement include:
- An increase in 3dB of my speaker sensitivity will reduce my power requirements down to 158 watts.
- An increase of 3 dB headroom will increase my power requirement to 630 watts
- A decrease in distance by 30% will decrease my power requirement to 157 watts
- An Increase in listening level by 3 dB will increase my power requirement to 630 watts
For another reference, I will choose difference distances as relationship for power requirements related to speaker sensitivity. My constants for this reference shall be the listening level of 90 dB and 3 dB headroom. I have a 300 watt RMS amplifier that I use for this application.
The following provides minimum speaker or cabinet dB requirement of for 90 dB listening level for a given distance while used the above 300 W amplifier.
- At 30 feet my minimum sensitivity is 87.6 dB
- At 50 feet my minimum sensitivity is 92 dB
- At 70 feet my minimum sensitivity is 95 dB
- At 90 feet my minimum sensitivity is 97.2 dB
These distance value that I listed above could represent the following:
- 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
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 was increased by 8 to 10 dB.
I loaded these cabinets with a speaker that was rated at 90 dBs sensitivity 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 watts at 100 dB to 250 watts at 98 dB.