As there was a lot of discussion about microphone distortion here recently and I'm actually working on a pressure chamber which can produce >160dBSpl ... I spent an afternoon to do some microphone measurements.
Setup
A pressure chamber is a device with a speaker membrane with a very small and 100% sealed front chamber. This pushes the resonance frequency of the 8" speaker to about 350Hz in my case - the volume is that small. But you can produce huge pressure changes now!
Measurements as always done with proper gear (APx515, Class1 Calibrator, Benchmark AHB2 Amp etc).
Measurement frequency is 250Hz (to be sure we are in 100% pressure chamber behaviour and it's the recommended calibration frequency by B&K and GRAS), stepped level sweep, 31 steps.
Check of the chamber - how much THD produces the speaker.
Such a pressure chamber produces stupendous SPL at very little membrane movement - so there is very little THD from the speaker. To check my limits I did the first 2 measurements with a GRAS 1/4" 40BD capsule with 2 different "preamps" (actually just impedance converters, they are just called preamps but there is no amplification).
That's the result.
There is a LOT of noise and some strange stuff between 115 and 135dBSpl going on with these 1/4" capsules - legit THD measurements are only starting from 140dBSpl and up. So we produce 150dBSpl with <1%THD and 160dBSpl <3%THD. We don't know if that's the speaker or the refmic! (I have an idea ...)
But we are sure about staying under 1% <150dBSpl and 3% <160dBSpl
So how are our microphones performing.
Here you see the linearity graph. 160dBSpl is easy for our 1/4" capsule. Not so for the other microphones ...
M50 gets to 145dBSpl - spec is 140dBSpl
M215 around 140dBSpl depending on capsule sensitivity -> preamp distortion. Spec is 135dBSpl.
I didn't bother to look after the Behringer Mic spec as it's just here as deterrent example - but >130dBSpl is actually not bad!
But what about THD?!
Here it is - the truth about reference microphone THD:
As expected ... don't use cheap measurement microphones for delicate measurements. They can't do them. The Behringer is out.
Interesting is that the Earthworks has higher THD as the M215! Noise area is up to about 105dBSpl (there is a lot of noise in the pressure chamber cause of it's sensitivity!!!) but then we get valid measurements. At the specified 140dBSpl max it has already 2% THD - that's A LOT! Studio microphones are normally specified with 0,5% THD, somtimes with 1% when the manufacturer wants to push it a little.
M215 - 135dBSpl is where the preamp starts distorting - that's not a limit of the capsule! Actually - it is when you take the 0,5% THD rule ...
BUT - one of the M215 amplifiers produces more noise as it should. Noise should be less as the M50 (dark green) but number 1 has more noise (light green). Will have to investigate, maybe just dirt in the high impedance area (these mics don't get pampered).
You can calculate your THD from this graph for lower SPL. -20dB -> 1/10th of THD. Even the Behringer follows that rule, it's how a condenser capsule SHOULD behave in theory. And they do!
So what about higher order harmonics?
tbc.
Setup
A pressure chamber is a device with a speaker membrane with a very small and 100% sealed front chamber. This pushes the resonance frequency of the 8" speaker to about 350Hz in my case - the volume is that small. But you can produce huge pressure changes now!
Measurements as always done with proper gear (APx515, Class1 Calibrator, Benchmark AHB2 Amp etc).
Measurement frequency is 250Hz (to be sure we are in 100% pressure chamber behaviour and it's the recommended calibration frequency by B&K and GRAS), stepped level sweep, 31 steps.
Check of the chamber - how much THD produces the speaker.
Such a pressure chamber produces stupendous SPL at very little membrane movement - so there is very little THD from the speaker. To check my limits I did the first 2 measurements with a GRAS 1/4" 40BD capsule with 2 different "preamps" (actually just impedance converters, they are just called preamps but there is no amplification).
That's the result.
There is a LOT of noise and some strange stuff between 115 and 135dBSpl going on with these 1/4" capsules - legit THD measurements are only starting from 140dBSpl and up. So we produce 150dBSpl with <1%THD and 160dBSpl <3%THD. We don't know if that's the speaker or the refmic! (I have an idea ...)
But we are sure about staying under 1% <150dBSpl and 3% <160dBSpl
So how are our microphones performing.
Here you see the linearity graph. 160dBSpl is easy for our 1/4" capsule. Not so for the other microphones ...
M50 gets to 145dBSpl - spec is 140dBSpl
M215 around 140dBSpl depending on capsule sensitivity -> preamp distortion. Spec is 135dBSpl.
I didn't bother to look after the Behringer Mic spec as it's just here as deterrent example - but >130dBSpl is actually not bad!
But what about THD?!
Here it is - the truth about reference microphone THD:
As expected ... don't use cheap measurement microphones for delicate measurements. They can't do them. The Behringer is out.
Interesting is that the Earthworks has higher THD as the M215! Noise area is up to about 105dBSpl (there is a lot of noise in the pressure chamber cause of it's sensitivity!!!) but then we get valid measurements. At the specified 140dBSpl max it has already 2% THD - that's A LOT! Studio microphones are normally specified with 0,5% THD, somtimes with 1% when the manufacturer wants to push it a little.
M215 - 135dBSpl is where the preamp starts distorting - that's not a limit of the capsule! Actually - it is when you take the 0,5% THD rule ...
BUT - one of the M215 amplifiers produces more noise as it should. Noise should be less as the M50 (dark green) but number 1 has more noise (light green). Will have to investigate, maybe just dirt in the high impedance area (these mics don't get pampered).
You can calculate your THD from this graph for lower SPL. -20dB -> 1/10th of THD. Even the Behringer follows that rule, it's how a condenser capsule SHOULD behave in theory. And they do!
So what about higher order harmonics?
tbc.