Hi all, hope this is the right forum for finding help with a measurement I'm trying to make with a scope.
I'm sending a one-cycle pulse to a speaker, and trying to measure the time delay as picked up by a mic set up 1-2 mm from the dustcap.
The speaker is a little TC9, so there's maybe 1.5" at most from mic to voice coil.
I would expect the timing between the pulse and the signal from the mic, to be the very short distance mic to speaker....maybe 0.1ms or so.
But i get close to a full millisecond, which is over 12 inches of distance.
And simply can't figure out why.
Here's the scope trace. Horiz grid is 0.5ms per division.
First peak is the pulse signal, second is the mic's.
Further details of test setup, if that's where the devil is:
Signal from arb generator into Ch1 of mixer, which is sent to amplifier and speaker.
Signal from mic into Ch2 of mixer, summed with Ch1 , and sent to scope.
Pulse is a single 1kHz Gaussian burst.
The mixer doesn't appear to be introducing any delay between Channel's summation.
Thanks for any help...I'm really scratching my head why the delay is so long.
I'm sending a one-cycle pulse to a speaker, and trying to measure the time delay as picked up by a mic set up 1-2 mm from the dustcap.
The speaker is a little TC9, so there's maybe 1.5" at most from mic to voice coil.
I would expect the timing between the pulse and the signal from the mic, to be the very short distance mic to speaker....maybe 0.1ms or so.
But i get close to a full millisecond, which is over 12 inches of distance.
And simply can't figure out why.
Here's the scope trace. Horiz grid is 0.5ms per division.
First peak is the pulse signal, second is the mic's.
Further details of test setup, if that's where the devil is:
Signal from arb generator into Ch1 of mixer, which is sent to amplifier and speaker.
Signal from mic into Ch2 of mixer, summed with Ch1 , and sent to scope.
Pulse is a single 1kHz Gaussian burst.
The mixer doesn't appear to be introducing any delay between Channel's summation.
Thanks for any help...I'm really scratching my head why the delay is so long.
I'm not an acoustics guy so i'm shooting in the dark. You'll probably get better advice from other members.
Your mic is very close to the speaker. The 1ms gaussian pulse duration is longer than the expected delay.
Is it possible the second pulse is from a nearby reflection? Try a series of experiments with the mic progressively farther from the source and see if observed incremental acoustic delay make sense.
Does shape of mic waveform change with distance? With changes in Gaussian duration?
Experimentation may lend insight.
Your mic is very close to the speaker. The 1ms gaussian pulse duration is longer than the expected delay.
Is it possible the second pulse is from a nearby reflection? Try a series of experiments with the mic progressively farther from the source and see if observed incremental acoustic delay make sense.
Does shape of mic waveform change with distance? With changes in Gaussian duration?
Experimentation may lend insight.
Is there an expanded time base, or a delayed scan, function turned on in one channel?
Try swapping the probes and see if the display changes.
Can't be a reflection, the pulse is too clean, and a reflection would likely be inverted.
Try swapping the probes and see if the display changes.
Can't be a reflection, the pulse is too clean, and a reflection would likely be inverted.
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Thx for the reply.
I backed the mic up, and the delay increases incrementally with distance as expected.....above the starting unexplained 1ms.
So for example, with mic about 6.5" from cone, total delay is 1.5ms.
The pulse is clearly from the speaker, and keeps the gaussian form with distance.
Tried changing Gaussian width ( underlying frequency) and same patterns continue. Still ?????
Thx for the reply.
I'm just using a single scope channel.
It's displaying the stimulus pulse, summed with the microphone pickup, so i can see both peaks across time.
I agree, can't be a reflection.
Interestingly, for a sanity check, i just ran a regular dual channel FFT measurement with the mic at the cone,
and got a time-of-flight to mic equal to 0.15ms....which is what i expected to see between peaks on the scope.
I suspect the delay arises from the cascade of two transducers, but I'm too ignorant the physics to offer any cogent explanation.
I know the driving generator signal has to accelerate the mass of the speaker; the speaker has to couple to the air; the moving air must accelerated the mass of the microphone to generate its signal output. There are surely phase shifts and delays associated with these energy transfers, but I can't begin to justify them mathematically.
Sorry for this completely inadequate blather.
I know the driving generator signal has to accelerate the mass of the speaker; the speaker has to couple to the air; the moving air must accelerated the mass of the microphone to generate its signal output. There are surely phase shifts and delays associated with these energy transfers, but I can't begin to justify them mathematically.
Sorry for this completely inadequate blather.
A loudspeaker is a stiff low-pass followed by a differentiator.
Coil resistance through motor's flux against moving mass gives a falling response pretty much at the bottom of the main bandwidth. Around 140Hz in this case.
The coupling from mechanical to air is an integrator up to about 1.5kHz when the wave is finally cone-sized.
So a 1.14 milliSecond delay may be exactly right.
Coil resistance through motor's flux against moving mass gives a falling response pretty much at the bottom of the main bandwidth. Around 140Hz in this case.
The coupling from mechanical to air is an integrator up to about 1.5kHz when the wave is finally cone-sized.
So a 1.14 milliSecond delay may be exactly right.
Thx guys!
BSST, yes, surely beyond my physics understanding too.
PRR, why do you think i get a time of flight of 0.15ms, that seems like the correct mic to speaker distance, when using either REW or Smaart or ARTA etc?
Seems like these programs would show the larger 1ms delay as per scope, if there is an electromechanical delay in play?
Also, any frequency I pulse with, I've tried from 100Hz to 5kHz, gives the same scope delay of about 1ms with mic on dust cap.
BSST, yes, surely beyond my physics understanding too.
PRR, why do you think i get a time of flight of 0.15ms, that seems like the correct mic to speaker distance, when using either REW or Smaart or ARTA etc?
Seems like these programs would show the larger 1ms delay as per scope, if there is an electromechanical delay in play?
Also, any frequency I pulse with, I've tried from 100Hz to 5kHz, gives the same scope delay of about 1ms with mic on dust cap.
Yep, tried that. Same thing with 4fe35.
Same good looking pulse.
Have tried different pulses, more dirac like....timing stays the same .....???
Same good looking pulse.
Have tried different pulses, more dirac like....timing stays the same .....???
Then it may be the inherent time delay in the driver, which would be related to the bandwidth.
For an ideal low pass system, Td ~ 1/bandwidth.
For an ideal low pass system, Td ~ 1/bandwidth.
Time of flight..... if using REW for instance ...it's delay time in REW relative to loopback.
Assuming loopback is taken from a point in the signal chain where the isn't any further processing or DAC, latency, etc.
I believe i found the timing discrepancy.
Seems it is in the digital mixer. The mic input must add some undocumented delay.
I swapped it out for an analog mixer, and timing between the electrical pulse, and the subsequent mic capture, is right about 0.15ms as expected.
Thx again to all who relied to help.
Assuming loopback is taken from a point in the signal chain where the isn't any further processing or DAC, latency, etc.
I believe i found the timing discrepancy.
Seems it is in the digital mixer. The mic input must add some undocumented delay.
I swapped it out for an analog mixer, and timing between the electrical pulse, and the subsequent mic capture, is right about 0.15ms as expected.
Thx again to all who relied to help.
What kind of mic? Show the whole signal chain - if there's any digital section in the
signal path that's the source of delay, modern ADCs and DACs have large latencies.
signal path that's the source of delay, modern ADCs and DACs have large latencies.
Ah, didn't notice the "digital mixer" there - yes that's the latency. However if all the inputs to it are set the same there ought to be the same latency on each signal - so the mic input must use a different option.
Add an analog mic preamp and put both signals in at line-level?
Modern audio ADCs and DACs are sigma-delta which involves considerable delay due to the decimation and interpolation digital filters involved, typically dozens of samples delay in each direction. If you want minimal latency in digital audio you need to avoid sigma-delta ADCs and DACs
Add an analog mic preamp and put both signals in at line-level?
Modern audio ADCs and DACs are sigma-delta which involves considerable delay due to the decimation and interpolation digital filters involved, typically dozens of samples delay in each direction. If you want minimal latency in digital audio you need to avoid sigma-delta ADCs and DACs
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Thx!
Digital mixer was a X-32, which has 0.8ms local latency.
But like you say, it's not an issue when both inputs have same latency on each channel.
I'm still trying to figure out where the extra mic latency came from, because a line level signal input in lieu of the mic input , shows outputs all in perfect time sync.
Yes, adding an analog mic preamp was the idea i came to, too.
But the only one I had was in an analog mixer, so it was easier to just use the analog mixer for the whole experiment.
Now I will try using the analog mic preamp with the digital mixer at line level, like you suggest, to get a handle on how the digital mixer works.
Btw, the point of all this is to get a better grasp of how drivers in a synergy interact with each other.
I'm applying a pulse to mid-low drivers mounted on the synergy horn walls, and using the compression driver as a mic to capture the energy that goes back into the throat of the horn.
I was getting way too long a time using the digital mixer....twas why i went to trouble shooting on a little open baffle TC9, where i knew what timing to expect. Hope that made sense...
Digital mixer was a X-32, which has 0.8ms local latency.
But like you say, it's not an issue when both inputs have same latency on each channel.
I'm still trying to figure out where the extra mic latency came from, because a line level signal input in lieu of the mic input , shows outputs all in perfect time sync.
Yes, adding an analog mic preamp was the idea i came to, too.
But the only one I had was in an analog mixer, so it was easier to just use the analog mixer for the whole experiment.
Now I will try using the analog mic preamp with the digital mixer at line level, like you suggest, to get a handle on how the digital mixer works.
Btw, the point of all this is to get a better grasp of how drivers in a synergy interact with each other.
I'm applying a pulse to mid-low drivers mounted on the synergy horn walls, and using the compression driver as a mic to capture the energy that goes back into the throat of the horn.
I was getting way too long a time using the digital mixer....twas why i went to trouble shooting on a little open baffle TC9, where i knew what timing to expect. Hope that made sense...