Using a condenser microphone and the FFT HP3561A analyzer, a 200 Hz square wave signal was fed into an ESL (Martin Logan Sequel II electrostatic loudspeaker):
This is the electrostatic speaker:
FT signal of the 200 Hz square wave of the function generator:
FT signal of the condenser measuring microphone:
This is the electrostatic speaker:
FT signal of the 200 Hz square wave of the function generator:
FT signal of the condenser measuring microphone:
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Please post a clear photo of the actual square wave at 1kHz.
The woofer crosses over at 250Hz.
My question was:
Could I use an ESL to generate an acoustic square wave signal to study the response of different microphones?
To check quantitatively the quality of this acoustic square wave I have used this FFT analyzer.
As you can easily see, this acoustic square is of no good quality.
Could I use an ESL to generate an acoustic square wave signal to study the response of different microphones?
To check quantitatively the quality of this acoustic square wave I have used this FFT analyzer.
As you can easily see, this acoustic square is of no good quality.
The ECM8000 isn't flat at the top end (typical), which could cause some overshoot. If you can EQ the mic it would be interesting to see what you get.
Unlikely, a speaker with a usefully uniform frequency response to do this probably does not exist.
The same procedure for a 1 kHz square wave:
FT signal of the 1 kHz square wave of the function generator:
FT signal of the condenser measuring microphone:
FT signal of the 1 kHz square wave of the function generator:
FT signal of the condenser measuring microphone:
As with motional feedback for subs, THIS is where the rubber meets the road and is the basic way to see sound output. Great experiment. I've been trying unsuccessfully to see square waves for decades. A friend with older Quads claims he can do it but never has sent me the pix.
BTW, here's the rule of thumb. For square waves on an oscilloscope to look square, the signal has to be OK in amplitude and phase from one-tenth to 10X the basic freq. So it really is a very meaningful test esp when applied to electronic gear.
But for speakers too? Too bad "you can't get there from here" because the waves that hit the mic (at least at higher frequencies) are all over the map in phase after leaving the diaphragm. Yes, while the mic image then looks funny, luckily your ear doesn't care. So my amateur impression is there's no way to get an oscilloscope picture that meaningfully looks like a square wave except by accident of mic placement.
The FFT, on the other hand and within the limits of the gear as others have commented, does reflect the goodness of the speaker. But that's not too much different than a garden-variety frequency plot.
B.
BTW, here's the rule of thumb. For square waves on an oscilloscope to look square, the signal has to be OK in amplitude and phase from one-tenth to 10X the basic freq. So it really is a very meaningful test esp when applied to electronic gear.
But for speakers too? Too bad "you can't get there from here" because the waves that hit the mic (at least at higher frequencies) are all over the map in phase after leaving the diaphragm. Yes, while the mic image then looks funny, luckily your ear doesn't care. So my amateur impression is there's no way to get an oscilloscope picture that meaningfully looks like a square wave except by accident of mic placement.
The FFT, on the other hand and within the limits of the gear as others have commented, does reflect the goodness of the speaker. But that's not too much different than a garden-variety frequency plot.
B.
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Apparently it's perfectly possible to get a square wave from a speaker: PM90 gig reports
Even in different mic positions.
I suspect, however, that you do need a wide open space to do that sort of measurement - reflections will mess things up.
I recently got a set of Powersoft T-series amps which have lots of FIR processing. I'll give this a try and report back.
Chris
Can't find the square wave pix. Can you please post or link.
I suppose you could diddle with a DSP in order to make good looking square waves at one spot in space and that would demonstrate something. Let's see.
B.
It's linear-phase processing, where you can change the frequency and phase response completely separately.
I didn't see any pictures either, but I believe him when he says it happened. Some digging around on Peter Morris's speaker designs might turn something up.
Good controlled-directivity speakers will sound the same everywhere, so I see no reason why you couldn't have a square wave in lots of mic positions.
Chris
This is Digital Filter / Digital Signal Processing domain.
Fan, please try to show us a single turn of the square
wave curve form at two or three different frequencies.
Single wavelength contains all the information, to show
a long trace is useless. It seems your speaker impulse
response is not too bad.
Microphone evaluation will be done in a different way though.
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The Martin Logan ESL with a 200 Hz square wave:
Signal of the square wave into the amplifier input at the top, 20 mV/cm, 40 mVpp.
At the center of the screen the microphone signal 5 mV/cm, 27 mVpp, is shown.
At the bottom of the screen the current signal 1 A/cm, 5 App is shown, (output current of the amplifier into the input of the Martin Logan ESL).
FT of the current signal 5 app, center frequency 1 kHz:
FT of the current signal 5 app, center frequency 5 kHz:
I am looking forward to your comments.
Signal of the square wave into the amplifier input at the top, 20 mV/cm, 40 mVpp.
At the center of the screen the microphone signal 5 mV/cm, 27 mVpp, is shown.
At the bottom of the screen the current signal 1 A/cm, 5 App is shown, (output current of the amplifier into the input of the Martin Logan ESL).
FT of the current signal 5 app, center frequency 1 kHz:
FT of the current signal 5 app, center frequency 5 kHz:
I am looking forward to your comments.
If you can show a shot of a single period it will be more instructive.
It has all the information (for this frequency) but higher visual resolution.
It has all the information (for this frequency) but higher visual resolution.
It goes me some consolation to see my inability to capture square waves is shared. Perhaps all the frequencies are there and in proportion and maybe it sounds like it should but the mic output sure doesn't look square.
B.
B.
This is a square wave sweep of a DIY synergy horn with a passive crossover. Essentially the same result happened at off-axis points as well, though the mic had to be kept relatively close to the horn to overcome room effects getting into it too much.
YouTube
Same speaker with a slightly revised crossover -- YouTube
Of course this can be done with most any single-point/coaxial type speaker measured on axis if a FIR filter is used to fix up its phase response.
YouTube
YouTube
Same speaker with a slightly revised crossover -- YouTube
Of course this can be done with most any single-point/coaxial type speaker measured on axis if a FIR filter is used to fix up its phase response.
YouTube
The Martin Logan ESL with 200 Hz sine, the same experimental setup as the 200 Hz square wave:
Signal of the sine wave into the amplifier input at the top, 20 mV/cm, 40 mVpp
Center: microphone signal 5 mV/cm, 17 mVpp
At the bottom: current signal 200 mA/cm, 900 mApp (output current of the amplifier into the input of the Martin Logan ESL).
As you can see, in contrast to the square wave, there is full agreement of the waveforms of all three signals (amplifier input, amplifier output or loudspeaker input and microphone signal)
FT of the current signal 900 mApp, center frequency 1 kHz:
Spectrum-pure sine of 200 Hz of the current signal 900 mApp.
Signal of the sine wave into the amplifier input at the top, 20 mV/cm, 40 mVpp
Center: microphone signal 5 mV/cm, 17 mVpp
At the bottom: current signal 200 mA/cm, 900 mApp (output current of the amplifier into the input of the Martin Logan ESL).
As you can see, in contrast to the square wave, there is full agreement of the waveforms of all three signals (amplifier input, amplifier output or loudspeaker input and microphone signal)
FT of the current signal 900 mApp, center frequency 1 kHz:
Spectrum-pure sine of 200 Hz of the current signal 900 mApp.
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