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.
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 ...
Yes and yes.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).
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)?
Did you try the "ir window" button at the top after you measured it?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.
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.
With this kind of correlation its possible to get a correct absolute phase measurement needed?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.
Well there’s not a single mention about phaseAfaik, 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
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".
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.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.
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.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....