Hi Mark,
Not sure I follow ? The timing reference is run through a separate speaker (I use a 3" full range 'mini' speaker from a 5.1 in a box type thing) Basically the 'timing' channel plays a chirp through a speaker so that the REW can lock onto it for timing before the sweep starts on the 'measurement' channel. (I send the left channel to my little full range speaker for timing, and the right channel goes to the speaker I want to measure)
You have to leave the mic and timing speaker in the same position whilst you do measurements of the loudspeaker under test. So on a 3 way tweet,mid, bass drivers will all be referenced to the timing loudspeaker to calculate differences in signal path.
I'm still very interested in the wavelet stuff for aligning phase between a mid and tweet, are you saying it's not worth me pursuing ?
Thanks,
Rob.
Oh Rob, I see.... kinda an audio distance finder. Cool.
I've read in big live-sound setups, that guys have used small piezo tweeters that can be stuck onto subs dust caps flown high in the air to determines distance like that. (until laser range finders came out)
To the extent the 3"s acoustic center matches whatever driver time-of-flight from its acoustic center you're measuring, it should work well it seems.
Problem is, i thinks it's a pretty coarse estimate for inter-driver phase/time alignment. Because distance to drivers acoustic centers vary so much by size/passband....it's damn hard to know/find.
If you knew the physical location of acoustic center of a driver (as mounted) and could put the 3" the same distance from mic, then it should work ok maybe.
I guess it all comes down to what we're trying to do....if the idea is to get a measurement of an entire speaker, and then do room tuning...i imagine acoustic ref can work fine.
I say imagine, i dunno, .....because I've only use measurements for aligning multi-ways...in which case acoustic ref timing simply doesn't work.
I've read in big live-sound setups, that guys have used small piezo tweeters that can be stuck onto subs dust caps flown high in the air to determines distance like that. (until laser range finders came out)
To the extent the 3"s acoustic center matches whatever driver time-of-flight from its acoustic center you're measuring, it should work well it seems.
Problem is, i thinks it's a pretty coarse estimate for inter-driver phase/time alignment. Because distance to drivers acoustic centers vary so much by size/passband....it's damn hard to know/find.
If you knew the physical location of acoustic center of a driver (as mounted) and could put the 3" the same distance from mic, then it should work ok maybe.
I guess it all comes down to what we're trying to do....if the idea is to get a measurement of an entire speaker, and then do room tuning...i imagine acoustic ref can work fine.
I say imagine, i dunno, .....because I've only use measurements for aligning multi-ways...in which case acoustic ref timing simply doesn't work.
All that matters is that it's the same between measurements, which it will be. After that, if your crossover style involves you separating some of the phase difference into delay for one of the ranges, then it's easy enough to do by sight on the measurement.
Hi Mark,
I don't know the inner workings of REW, but assume REW has a piece of audio it plays which is the timing chirp and the sweep, and the program knows the exact timing of the gap between the chirp and the sweep. When the USB mic picks up the timing chirp from the timing speaker it can compare the measured sweep on the test speaker and calculate any difference. Once that calculation is done, any other drivers measured can have their distances calculated as well. ie, if the timing speaker is 4 foot from mic and test tweeter is 2 foot from mic, REW can calculate that. Then if mid is 2.2 foot from mic REW calculates that, and we know there's a 0.2" offset. I used this method to set up the mids to tweets on my MEH btw.
Maybe I'm looking at this too simply ?
Rob.
Thinking about it I don't see why they could not make a USB mic that comes with a second channel to do a loopback. After all, most audio interfaces are USB with 2 channels. Just use a 2 channel interface , loopback the second channel and make it a closed box affair with a calibration file and driver. Plug and play.
I don't know the inner workings of REW, but assume REW has a piece of audio it plays which is the timing chirp and the sweep, and the program knows the exact timing of the gap between the chirp and the sweep. When the USB mic picks up the timing chirp from the timing speaker it can compare the measured sweep on the test speaker and calculate any difference. Once that calculation is done, any other drivers measured can have their distances calculated as well. ie, if the timing speaker is 4 foot from mic and test tweeter is 2 foot from mic, REW can calculate that. Then if mid is 2.2 foot from mic REW calculates that, and we know there's a 0.2" offset. I used this method to set up the mids to tweets on my MEH btw.
Maybe I'm looking at this too simply ?
Rob.
Thinking about it I don't see why they could not make a USB mic that comes with a second channel to do a loopback. After all, most audio interfaces are USB with 2 channels. Just use a 2 channel interface , loopback the second channel and make it a closed box affair with a calibration file and driver. Plug and play.
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If i understand you, i don't think it works well.
I think it doesn't work well when you have an acoustic time reference determining overall speaker response, from which you try to glean the various phase response of the individual driver sections.
Problem is, that full spectrum mag and phase traces are bound to the timing of the VHF. And that the measurement in its entirety has to make sense out of a presumably continuous flow from that timing.
These problems can be seen when driver traces are taken individually . They quite most often don't lie on top of each other in time....but they give that appearance in an overall measurement.
Separate driver section measurements and phase traces overlays, is the most time honored way of alignment I know of.
Which is something acoustic ref simply can't do, because it can't handle drivers timing accurately, that have less than VHF response.
Purchase:
1) Calibrated xlr microphone - Dayton EMM-6 is a good bargain in the US, not sure about UK although it is available at Soundimports. A search of diyaudio will find plenty of mic recommedations.
2) Preamp - Behringer UMC202, Steinberg UR22mkii, Motu M4, etc. The Behringer is on sale at a nice price at Sweetwater but probably irrelevant for the UK.
3) Some mic and other cables (and maybe adapters) for the loopback wiring
Free:
REW is nice to get started. Pretty easy to use and easy to get to graphs.
ARTA is nice but a little more complicated in my opinion. Free version does not allow saving files or something. More important to me was that the paid version allows the automated measurement and recording of off-axis measurements for "spinorama" so I paid for the license, around $90. Start with REW to learn and move on from there as needed.
Optional:
Mic stand - convenient but you can always make something DIY.
Turntable - if you move on to the spinorama off-axis measurements a heavy-duty TV swivel stand can work great.
A little. It doesn't know how far the reference speaker is, hence the need to listen and after that it still doesn't know the distance.. but it doesn't need to as long as it's the same between measurements.
@mark100 While I'm not trying to convince anyone to buy a USB mic, when done the way REW has created there's no reason it shouldn't work. All it needs is one distance reference. It's not necessary to know exactly why it needs to chirp a whole range of frequencies to do this but it's only taking away a single reference distance, not a whole reference phase plot.
Hi Rob, i think the entire issue is REW's acoustic ref sweep starts at 5Hz...
How you gonna use it for drivers that don't go that high?
Good question. I think because the measurement program is using an internal USB loopback already.
The thing with acoustic ref imo, and imo only, is that to get something that is really user friendly and shows very consistent times, it HAS to be keyed off VHF.
Anybody who spends time with dual-channel measurements learns how difficult it is to get consistent measurement timing , as freq decreases.
(That's what wavelets are about imo, a way to get more precise low frequency timing measurements. A microphone in a fixed spot ( either Umik or XLR ) and a scope are best wavelet tools i think)
There is a function in REW to calculate and remove the delay of the initial timing chirp, though I have never needed it for my projects. Like you say, as long as we don't move the mic or timing reference speaker it is all relative.
Since the distance of the reference speaker is arbitrary it won't be very useful to subtract that distance specifically, nor will it hurt.
To do this right, you need to remove/add the same amount of delay to each of the measurements. This way they are correct no matter how much you add/subtract. Obviously you want to find an amount that makes it easy to see what you're doing with phase when crossing. You want to find an amount that doesn't cut the start off the measurement with the earliest arrival. Apart from that there is no need to be exact so just do it using best judgement.
To do this right, you need to remove/add the same amount of delay to each of the measurements. This way they are correct no matter how much you add/subtract. Obviously you want to find an amount that makes it easy to see what you're doing with phase when crossing. You want to find an amount that doesn't cut the start off the measurement with the earliest arrival. Apart from that there is no need to be exact so just do it using best judgement.
The timing chirp can be played through a full range speaker that is not the speaker being tested. I think REW calculates an impulse response from it that becomes the start of the measurement. (time = 0) then when the actual speaker being measured plays its sweep REW can calculate its time against the timing speaker.
Hi Allen, yep, i don't know why it uses the particular 5k-20k range it does either.
I do get the idea of it being from 5kHz all the way up though, because the more VHF info there is in the impulse response, the more precise/repeatable the timing.
So that's back to my simple speculation about it being easy and repeatable.
Why the acoustic timing ref doesn't/can't go lower...or be be bandwidth limited...no clue.
I'm pretty sure once you apply the offset it goes through all the subsequent measurements in a session. The distance measurements work in negatives aswell so there is no chance of the measurement being cut off too early.
Makes sense.
Are you saying that if one of the later ones is earlier, it will adjust the previous ones as well?
I do all my adjustments manually so I don't have any doubt.
Hi Mark,
If you take the first measurement and calculate the offset, then all further measurements are referenced to it.
If my timing reference speaker was 2 foot away from the mic, and I moved my bass driver 6 foot away REW would show a minus 4 foot distance.
I've got a feeling you think the timing chirp is being compared to the actual measurement sweep? It isn't. The timing chirp is an acoustic version of a loopback. It plays an impulse through one channel for the measurement to use as a timing reference and then the actual measurement sweep plays a few seconds later on the right channel through the test speaker. This just allows REW to remove any latency type issues as the chirp and sweep are played in one stereo stream.
If you take the first measurement and calculate the offset, then all further measurements are referenced to it.
If my timing reference speaker was 2 foot away from the mic, and I moved my bass driver 6 foot away REW would show a minus 4 foot distance.
I've got a feeling you think the timing chirp is being compared to the actual measurement sweep? It isn't. The timing chirp is an acoustic version of a loopback. It plays an impulse through one channel for the measurement to use as a timing reference and then the actual measurement sweep plays a few seconds later on the right channel through the test speaker. This just allows REW to remove any latency type issues as the chirp and sweep are played in one stereo stream.
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It is bandwidth limited to 5k up. All it needs to do is provide a reliable impulse chirp.
Rob.
Hi Rob, I ran a few quick tests using acoustic ref on a full range speaker to try to figure out how more exactly how it works , but didn't really get anywhere.
I was wondering if the >5kHz 700ms bleep establishes the timing for the subsequent full sweep measurement on it's own, or does timing come from comparison to the same frequency range in the full sweep. (hope that made sense.)
I think it must be establishing timing on its own against internal timing, because I told the signal generator to stop at 3KHz so there would be nothing to compare to, and I still got reasonable looking traces for the sweep that stopped at 3KHz.
Anyway, so do you keep the small 3" as part of your test-setup, along with the Umik?
If you were to measure a sub alone, where do you put the 3" relative to the sub?
Hi Mark,
The way I understand it (and I am not an expert on REW by any means) is that the USB interface has trouble with latency / timing issues so that the incoming signal from the microphone can have varying arrival times when compared to the outputted test signal. To work around this REW can send a timing chirp out on the same audio stream as the test signal, this way no matter if there is a varying delay between output and input, REW can work out the timing from the input signal alone, using the timing chirp as a reference for the actual sweep. In my mind it gives REW a time=0 to work from. AFAIK REW does not make any comparisons between the timing chirp and the measured sweep. The timing signal is there to give REW a definite 'pulse' to work from.
Imagine you have a stereo audio track playing. There is an impulse click on the left channel and then 5 seconds later a 5Hz to 20kHz sweep plays on the right.
Play the left channel 'click' through a full range speaker placed 1m to the left of a measurement mic. Play the sweep through the speaker under test 1m in front of the mic.
REW will use the left channel click as its time =0 marker, and then can calculate the difference between the measured sweep from the test loudspeaker, and what it knows is the actual time distance between the chirp and the sweep on the signal it sent out.
I don't keep a fullrange speaker for test purposes, just grab whatever I have to hand. (indoors I use a little full range, measuring in my garden I'd grab a bookshelf from the shed system 😀)
Measuring a sub alone I'd just sit the full range on top of the sub as the wavelengths involved would not even notice it. However if I was measuring just a sub on its own, with no other speakers I would only be interested in response, and not be worried about keeping a timing reference for alignment with other speakers.
Regards,
Rob.
The way I understand it (and I am not an expert on REW by any means) is that the USB interface has trouble with latency / timing issues so that the incoming signal from the microphone can have varying arrival times when compared to the outputted test signal. To work around this REW can send a timing chirp out on the same audio stream as the test signal, this way no matter if there is a varying delay between output and input, REW can work out the timing from the input signal alone, using the timing chirp as a reference for the actual sweep. In my mind it gives REW a time=0 to work from. AFAIK REW does not make any comparisons between the timing chirp and the measured sweep. The timing signal is there to give REW a definite 'pulse' to work from.
Imagine you have a stereo audio track playing. There is an impulse click on the left channel and then 5 seconds later a 5Hz to 20kHz sweep plays on the right.
Play the left channel 'click' through a full range speaker placed 1m to the left of a measurement mic. Play the sweep through the speaker under test 1m in front of the mic.
REW will use the left channel click as its time =0 marker, and then can calculate the difference between the measured sweep from the test loudspeaker, and what it knows is the actual time distance between the chirp and the sweep on the signal it sent out.
I don't keep a fullrange speaker for test purposes, just grab whatever I have to hand. (indoors I use a little full range, measuring in my garden I'd grab a bookshelf from the shed system 😀)
Measuring a sub alone I'd just sit the full range on top of the sub as the wavelengths involved would not even notice it. However if I was measuring just a sub on its own, with no other speakers I would only be interested in response, and not be worried about keeping a timing reference for alignment with other speakers.
Regards,
Rob.
Yes. (..I got mine from Thomann US.)
https://www.thomannmusic.com/behringer_u_phoria_umc204hd.htm
..and the "Premium +" version of this mic. (unless you are very interested in distortion testing)":
https://www.cross-spectrum.com/measurement/calibrated_dayton.html
(..the "Premium" version for the mic.s sensitivity measurement; see the "Pricing" tab/portion of that page.)
For semi-dual channel testing you'll also need a small (length) 1/4" mono to 1/4" mono patch cable (top of the 2nd page "Right Out to Right Input"):
https://kimmosaunisto.net/Software/VituixCAD/VituixCAD_Measurement_ARTA.pdf
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Thx Rob, I wasn't fully aware of the possible USB syncing issues. That all makes sense.
It sounds like you've found a process using a full-range proxy speaker to set a fixed timing distance, that gives you good results.
I can see how that can work well, as in the end I think what we need are measurements of each driver section from one fixed reference point.
Good you posted your method for folks to see, again.
One thing i really do like about Umiks is their calibrated SPL. It's annoying to have to have to calibrate an XLR mic with an external calibrator whenever I switch measurement programs or sound card without recallable preamps etc. ( for when measured SPL matters)
Apart from thinking loopback timing is more easily used, and more accurate for multi-way sections on their own,...a big part of the reason I like XLR is extended lengths of the mic cables. I run REW or Smaart from my office with 50ft XLR lines running out into the listening room.
And outdoors, finding/making shade to see laptop screen is a problem for me, without a long mic line.
Also like loopback because the soundcard can double as an easy interface interface for making electrical line-level measurements...I study xovers, and filter alot that way. I quess a computer sound card could do that too...but a USB soundcards I/O hookups make it easier than a full set of mini-jack adapter cables.