Designing an active 2-way speaker crossover

[morpheusX]

This might be useful for someone with calibrated USB mic: its a simple process to calibrate a mic using an already calibrated mic, in this case, using a Cross Spectrum Labs calibrated UMIK-1 to create the calibration file for the ECM8000.

 

[Rick Sykora]

So if i understood correctly, using the UMIK-1 with REW acoustic timing ref, will provide accurate measurement for crossover design in VituixCAD.

Is the above correct? There is so many contradictory information on this subject that its difficult to understand what the actual truth is ...

Is this mic custom calibrated?
Given the uncertainty regarding the USB mic (i have a cross spectrum calibrated UMIK-1), i'm considering getting an EMM-6, but they seem out of stock since 2019 on Cross Spectrum.

Yes and yes.

More specifically, the EMM-6 in individually calibrated (just as with UMIKs).

 

[morpheusX]

Yes and yes.

More specifically, the EMM-6 in individually calibrated (just as with UMIKs).

I'm sorry to insist, but does your EMM-6 use the "generic" calibration file provided by Dayton, or its calibrated by Cross Spectrum (or other similar company)?

 

[Rick Sykora]

I'm sorry to insist, but does your EMM-6 use the "generic" calibration file provided by Dayton, or its calibrated by Cross Spectrum (or other similar company)?

It has a Dayton-supplied serialized calibration file that (AFAIK) is unique to it.

I have nothing that indicates further calibration is required by a 3rd party company. If I were doing some professional work that required precision, might consider. When I see how much measurements change with small mic position movements, am more focused on making consistent relative measurements.

 

[McFly]

I keep getting waaack measurements when I go more than 150 degrees, it looks like the reflections are much higher than the speaker and REW is defaulting the IR window to wherever the measurement/IR is loudest, even though I have a loopback 2 channel setup with a 48V mic. What is the setting to get the IR start to where it should be? I see when you select no acoustic timing reference you are given options to choose where to set IR start but these options go away when loopback is selected.

 

[headshake]

I keep getting waaack measurements when I go more than 150 degrees, it looks like the reflections are much higher than the speaker and REW is defaulting the IR window to wherever the measurement/IR is loudest, even though I have a loopback 2 channel setup with a 48V mic. What is the setting to get the IR start to where it should be? I see when you select no acoustic timing reference you are given options to choose where to set IR start but these options go away when loopback is selected.

Did you try the "ir window" button at the top after you measured it?

 

[McFly]

yeah IR windows set 2ms left 4.5ms right, and all meas look normal until i get to 150 to 180 they all need the IR start point manually moved to capture the driver and not the louder reflections

 

[Perish]

USB mics do not have any means of providing a loopback connection, so can't be used with the normal timing reference mode used for measurement. The acoustic timing ref was added to address that, and works well. The timing reference is a 5 kHz - 20 kHz linear sweep detected by cross-correlation with the source signal to a precision well below the sample interval, typically within a few microseconds. There may be issues in highly reflective environments or if the mic is far off the axis of the speaker providing the timing reference as in both cases there might be reflections at high frequencies stronger than the direct sound. The off-axis problem can be addressed by using a separate speaker to provide the timing reference, assuming the mic and reference speaker stay in the same place while the speaker being measured is rotated on its axis.

A second issue that can affect USB mics is a clock rate difference to the signal source. The mic gets its clock from the USB interface and some are less accurate than others. REW has an option to detect and correct that through the use of a second timing reference signal at the end of the measurement sweep.

Sorry for bringing up a well discussed topic up again.
I just can’t wrap my head round the acoustic timing reference and how it works.
I get what it’s doing with the sweep but what’s happening in the background?

I can imagine REW says:
1. I have every value zeroed out. That’s my initial reference
2. Now I make the 5-20 kHz timing sweep.
3. the values measured with the timing sweep are my new zero values
4. the actual measurement sweep is created with the values displayed in REW

Is that basically how it works?

 

[JohnPM]

No. The location of the timing signal (or signals if clock rate correction is enabled) is determined by cross correlation of the signal data with the captured data.

 

[Perish]

No. The location of the timing signal (or signals if clock rate correction is enabled) is determined by cross correlation of the signal data with the captured data.

With this kind of correlation its possible to get a correct absolute phase measurement needed?

 

[Rick Sykora]

With this kind of correlation its possible to get a correct absolute phase measurement needed?

Afaik, the answer has not changed since his response in post #35. @JohnPM please correct if am incorrect.

 

[Jukka]

I've successfully used USB mics with acoustic timing reference, but I noticed that I get best results when I used a small speaker placed close to the microphone and not move them around between measurements (DUT can be rotated if doing off-axis). I previously used the other speaker of a stereo set, but it was placed too far away to give consistent results and timing was always off. I think you should mention this in your Getting Started with REW guide @JohnPM . MiniDSP has similar instructions on Driver time align, good read.

The last time I did this acoustic timing reference, the reference signal seemed to be frequency range limited by the start and end frequencies of the actual sweep. If memory serves, using a small speaker for reference didn't play it correctly for woofer measurements and would leave it hanging on "waiting for reference signal". I guess you have changed that since then, if it really is now in 5k-20k range.

 

[dualazmak]

As for reliable and reproducible time alignment measurements and tunings of SP drivers, my primitive approach/methods would be also of your reference and interest; I used a USB microphone for simple recording using independent second PC, and the recorded tone signals were analyzed by Adobe Audition 3.0.1.

1. Time-shifted multiple frequency tone burst sequence method
2. Simultaneous multiple frequency tone burst matching method
3. "Sine wave shape" precise matching (0.1 msec precision) method

Spoiler

On my project thread, you would please find;

- Precision measurement and adjustment of time alignment for speaker (SP) units: Part-1_ Precision pulse wave matching method: #493
- Precision measurement and adjustment of time alignment for speaker (SP) units: Part-2_ Energy peak matching method: #494
- Precision measurement and adjustment of time alignment for speaker (SP) units: Part-3_ Precision single sine wave matching method in 0.1 msec accuracy: #504, #507
- Measurement of transient characteristics of Yamaha 30 cm woofer JA-3058 in sealed cabinet and Yamaha active sub-woofer YST-SW1000: #495, #497, #503, #507
In case if you would be seriously interested in using the tone burst signal tracks I prepared and used in the above measurements and tunings, I will be happy to share them with you; please simply PM me if this would be the case.

- Perfect (0.1 msec precision) time alignment of all the SP drivers greatly contributes to amazing disappearance of SPs, tightness and cleanliness of the sound, and superior 3D sound stage: #520
- Not only the precision (0.1 msec level) time alignment over all the SP drivers but also SP facing directions and sound-deadening space behind the SPs plus behind our listening position would be critically important for effective (perfect?) disappearance of speakers: #687

 

[Perish]

Afaik, the answer has not changed since his response in post #35. @JohnPM please correct if am incorrect.

Well there’s not a single mention about phase

 

[Rick Sykora]

Well there’s not a single mention about phase

This thread was intended to be about how rather than why. A deeper discussion is going on here:

UMIK-1 or any USB MIC unsuitable for timing measurements?

I have both a USB and 2 channel mic for measurements. I go to the USB one for convenience most of the time. The issue with USB is you have no reference timing marker for measuring the "flight time" for the impulse, which will give you absolute phase information between different driver...

www.audiosciencereview.com

 

[dcibel]

An acoustic timing reference in REW can be "functional" for timing information for speaker measurements, but far from ideal. The problem is that the specific timing is highly dependent on the setup, so repeatability is lacking. Every time you set up your measurement rig, timing reference may be different from physical location of timing speaker, and any movement in mic location to measure a different speaker requires diligence to ensure the timing reference distance remains constant. Pack up your gear and set up another day, capturing the same timing reference may be a challenge. Perhaps some elaborate acoustic reference jig can alleviate this, but if you're going to that length why not just get a 2ch audio interface, normal XLR mic and avoid the pain.

The benefit that an electrical loopback reference has is simplicity of set-up, and high repeatability since it doesn't rely on any external acoustics. Just plug and play, set mic at same distance and results will always be the same. Setting up the measurement rig another day for additional measurements, I have simply repeated a measurement from the prior day and used the phase plot for accurate location of the mic at the same distance.

So conclusion of acoustic timing reference for loudspeaker design should be "functional but not recommended".

 

[gnarly]

An acoustic timing reference in REW can be "functional" for timing information for speaker measurements, but far from ideal. The problem is that the specific timing is highly dependent on the setup, so repeatability is lacking. Every time you set up your measurement rig, timing reference may be different from physical location of timing speaker, and any movement in mic location to measure a different speaker requires diligence to ensure the timing reference distance remains constant. Pack up your gear and set up another day, capturing the same timing reference may be a challenge. Perhaps some elaborate acoustic reference jig can alleviate this, but if you're going to that length why not just get a 2ch audio interface, normal XLR mic and avoid the pain.

The benefit that an electrical loopback reference has is simplicity of set-up, and high repeatability since it doesn't rely on any external acoustics. Just plug and play, set mic at same distance and results will always be the same. Setting up the measurement rig another day for additional measurements, I have simply repeated a measurement from the prior day and used the phase plot for accurate location of the mic at the same distance.

So conclusion of acoustic timing reference for loudspeaker design should be "functional but not recommended".

I don't get the distinction you are making about specific timing being highly dependent on setup.
Both acoustic timing reference and loopback, have the same time-of-flight dependency in their measurements. Both will be non-repeatable with any mic location movements.
So i think both are equally highly dependent on setup.

The problem with an acoustic timing reference is that it uses a sweep of only 5kHz up to establish where the impulse response is in time. (At least so with REW).
You can simulate the same problem with dual channel.....simply make the measurement sweep start at 5kHz....

I don't think we should ever try/expect to get the exact same measurement on subsequent sessions, if we don't indeed have a fixed locked-in-place setup for both mic and speaker/driver.....whether dual-channel or single w acoustic ref.

And sorry Rick, ....I won't belabor the mic topic in this thread any further...

 

[dcibel]

I don't get the distinction you are making about specific timing being highly dependent on setup.
Both acoustic timing reference and loopback, have the same time-of-flight dependency in their measurements. Both will be non-repeatable with any mic location movements.
So i think both are equally highly dependent on setup.

Electrical loopback reference will always be constant, it never moves. So time from t=0 to impulse measurement is always a function of physical distance of mic to DUT.

With an acoustic reference, the time from t=0 to the recorded impulse measurement is the difference in time of arrival of acoustic reference vs DUT. time reference only remains constant if acoustic reference distance to mic always remains the same. It makes for a less easily repeated measurement, and more finicky movement of mic location to retain accurate timing reference. I suppose you could build a jig with a tweeter that attaches to your mic boom or something, but the process is quite simplified with electrical loopback, all you need is a patch cable.

The problem with an acoustic timing reference is that it uses a sweep of only 5kHz up to establish where the impulse response is in time. (At least so with REW).
You can simulate the same problem with dual channel.....simply make the measurement sweep start at 5kHz....

Acoustic reference should be high frequency so the impulse peak is a sharp point as a timing reference. I'm not sure the problem you're trying to convey.

 

[gnarly]

Hi dcibel, I'm going to reply to this on the other UMIK-1 thread ....that one Rick linked above and we've both already posted in..ok?