post #37 of 38 2/25/10
[h=3]Issues with sensitivity specifications[/h]
This is an interesting thread. There seems to be a lot of misconceptions regarding headphone sensitivity, efficiency, impedance and the voltage and current needed to drive the headphone without clipping and distortion.
The confusion is compounded by the fact that manufactures are inconsistent in the manner that they communicate specifications. Sometimes headphone sensitivity is listed in dB (SPL) * when 1 milliwatt of power is applied to the headphones (this is sometimes written as dB/mW). Sometimes headphone sensitivity is listed in dB (SPL) when 1 Vrms of voltage is applied to the headphones (this is sometimes written as dB/V). For example, on their website, Sennheiser gives the following specifications for their HD 650:
Nominal impedance: 300 ohms
Sound pressure level (SPL): 103 dB (1 Vrms).
Sensitivities in dB/mW and dB/V are not always directly comparable to each other. To compare dB/V to dB/mW, we need to know the nominal impedance of the headphones. By my calculations, 103 dB/V (at 1 Vrms into 300 ohms) is equivalent to 97.8 dB/mW (at 1 mW).
And sometimes headphone sensitivity is listed in dB (SPL) WITH NO REFERENCE TO AMOUNT OF POWER OR VOLTAGE APPLIED. For example, on their website, Sennheiser gives the following specifications for their RS-180:
Impedance: 32 ohms
Sound pressure level (SPL): 106 dB
This type specification is not very useful. The specification 106 dB, by itself, says nothing about the sensitivity of the headphones. We are left to assume that they mean 106 dB (at 1 Vrms into 32 ohms). This assumption seems reasonable since Sennheiser list other headphones this way. Under that assumption, the RS 180 sensitivity is 91.1 dB/mW (at 1 mW).
* SPL is an acronym for Sound Pressure Level
==== Extra Credit =============
For the geeks out there, the equation to convert dB/V to dB/mW is
dB/mW = dB/V + 10*log(R*P/V^2)
where
dB/V is the sensitivity in dB (SPL) at 1 Vrms of voltage into impedance R, 103 dB/V into 300 ohms in this case
R is the nominal impedance, 300 ohms in this case
V is the reference voltage, 1 Vrms in this case
P is the reference power, 0.001 watt in this case
dB/mW is the sensitivity in dB (SPL) at 0.001 watt of power (that is 1 milliwatt)
log is the logarithm base 10
In the above example we have:
dB/mW = 103+10*log(300*0.001/1^2) = 98.7
