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[George Reiswig]

Follow-up question on measuring the VS21 “vented and horn-loaded” subwoofer:

I cannot effectively get an impulse estimate of distance to subwoofers, and there since the driver itself is not loaded in the front, it’s hard to tell what delay I should tell the software to compensate for distance between the microphone and speaker when measuring it. Is there a reasonable point on the cabinet that I should be measuring to, then applying the relevant temperature/elevation calculations for calculating a delay to apply? E.g. to the middle of the cabinet where the driver sits, or to the front lip of the horn, or…?

[Mac Kerr]

Follow-up question on measuring the VS21 “vented and horn-loaded” subwoofer:

I cannot effectively get an impulse estimate of distance to subwoofers, and there since the driver itself is not loaded in the front, it’s hard to tell what delay I should tell the software to compensate for distance between the microphone and speaker when measuring it. Is there a reasonable point on the cabinet that I should be measuring to, then applying the relevant temperature/elevation calculations for calculating a delay to apply? E.g. to the middle of the cabinet where the driver sits, or to the front lip of the horn, or…?

You can't really get an implulse response of a subwoofer in its passband. A 100Hz wave is about 10' long, where on that is the peak? You can get an impulse outside the passband by arbitrarily using a higher frequency, but that really won't tell you the reality of below 100Hz. It is better, in SMARRT or some other FFT analyzer to get the response of the top speakers and then look at the phase response of each speaker and adjust the timing so the phase curves match up through the acoustic crossover range.

I also can't see any reason you couldn't use a PZM for this ind of set up.

Mac

[George Reiswig]

You can't really get an implulse response of a subwoofer in its passband. A 100Hz wave is about 10' long, where on that is the peak? You can get an impulse outside the passband by arbitrarily using a higher frequency, but that really won't tell you the reality of below 100Hz. It is better, in SMARRT or some other FFT analyzer to get the response of the top speakers and then look at the phase response of each speaker and adjust the timing so the phase curves match up through the acoustic crossover range.

I also can't see any reason you couldn't use a PZM for this ind of set up.

Mac

That’s what I am trying to do, which is why I’m asking the question about where to measure to for this type of subwoofer. If I use Merlijn van Veen‘s method of relative and absolute measurement, in his illustration he places the grills of the main and sub equidistant. Which makes sense for a sub that is built with a front-facing cone. But this doesn’t have that, and I’m trying to do this as accurately as possible.

[John Schalk]

That’s what I am trying to do, which is why I’m asking the question about where to measure to for this type of subwoofer. If I use Merlijn van Veen‘s method of relative and absolute measurement, in his illustration he places the grills of the main and sub equidistant. Which makes sense for a sub that is built with a front-facing cone. But this doesn’t have that, and I’m trying to do this as accurately as possible.

For Merlin's method, you should still line up the grills of your speakers.  The goal of his method is to phase align the main and sub speaker when they are next to each other so that, when you are in the field and the speakers are spaced well apart, you can use a simple distance measurement to determine the additional delay that is required.

Assume you are out at a gig.  You stand in the place where you want to align your main(s) with your sub.  You aim a laser measure at the left main speaker and note the distance.  Next, you aim the laser at the Bassboss sub, what part of the sub are you measuring to?  The grill, of course.  Does that make sense?

[Frank Koenig]

That’s what I am trying to do, which is why I’m asking the question about where to measure to for this type of subwoofer. If I use Merlijn van Veen‘s method of relative and absolute measurement, in his illustration he places the grills of the main and sub equidistant. Which makes sense for a sub that is built with a front-facing cone. But this doesn’t have that, and I’m trying to do this as accurately as possible.

This is just a reference delay. It really doesn't matter what point relative to the speaker enclosure you use so long as you use the same point later when calculating the delay. The face of the grille is an easy-to-remember choice.

On impulse responses: You certainly can obtain an accurate impulse response of any speaker, including a subwoofer, but its main practical use for us is archiving measurements. It's a sequence of real, as opposed to complex, numbers and is a complete representation of a single input and output of the system modeled as a linear-system. All other linear-system representations can be derived from it.

The frequency-domain representation (log-magnitude and phase for us audio folks, usually) is far more useful for time aligning speaker boxes with each other as Mac and others often remind us.

You can convert between the time- and frequency-domain representations using the Fourier Transform although there a few computational pitfalls to be aware of.

--Frank

[George Reiswig]

For Merlin's method, you should still line up the grills of your speakers.  The goal of his method is to phase align the main and sub speaker when they are next to each other so that, when you are in the field and the speakers are spaced well apart, you can use a simple distance measurement to determine the additional delay that is required.

Assume you are out at a gig.  You stand in the place where you want to align your main(s) with your sub.  You aim a laser measure at the left main speaker and note the distance.  Next, you aim the laser at the Bassboss sub, what part of the sub are you measuring to?  The grill, of course.  Does that make sense?

That does make sense, I believe. IOW, the important factor here is which surface you're taking your distance measurement from. Not so much "where does the sound start at this speaker" measurement. And I'm guessing that at the wavelengths involved in the crossover region, a few inches off for the subwoofer won't make too much of a difference? If my understanding is right, you'd have to be almost 2' off at 100Hz in order to be out of phase by 60 degrees?

On impulse responses: You certainly can obtain an accurate impulse response of any speaker, including a subwoofer, but its main practical use for us is archiving measurements. It's a sequence of real, as opposed to complex, numbers and is a complete representation of a single input and output of the system modeled as a linear-system. All other linear-system representations can be derived from it.(SNIP)
--Frank

Sorry, Frank...I think we're talking about two different things. I'm not talking about shooting an IR of the speaker. I'm talking about "Impulse" in the sense that Open Sound Meter is using it. As I understand it, it's basically the way the software estimates how far away the speaker is by timing a point in the reference signal to when that same point registers at the measurement mic. It's THAT which I can't get the software to register when it comes to a subwoofer, for the reasons that Mac Kerr said above, I believe. So I believe what Merlijn van Veen‘s method is doing is basically putting both speakers in approximately the same equidistant plane from the mic (at least as far as a laser measure would regard it), then using the full-range speaker to measure that impulse (timing) and applying it to both the full-range speaker and the sub.

[Frank Koenig]

Sorry, Frank...I think we're talking about two different things. I'm not talking about shooting an IR of the speaker. I'm talking about "Impulse" in the sense that Open Sound Meter is using it.

Ah, very good. I'm not familiar with Open Sound Meter but faced with the problem of determining an absolute delay in a narrow band of frequencies I would use a tone-burst test-signal (which could be a physical test signal or a "virtual" test signal in a numerical model of the system based on the impulse response) with a carrier frequency at the frequency of interest and an envelope such as a raised-cosine (Von Hann) window. Then compute the envelope of the response of the system to the tone-burst by taking the magnitude of the analytic signal* corresponding to the real response. Then compute the time difference between the peaks of the input and output envelopes. (The width of the window controls the time-frequency uncertainty tradeoff.)

I wonder if this is what Open Sound Meter does? The above approach for determining delays gives generally robust results even for "messy" real systems. After a few years of fooling with this stuff I've come to think of tone bursts as often the best conceptual test signals and representations for many audio problems. Tone bursts are a good model for the waveforms of percussive musical instruments, for what it's worth.

The group delay of a system at a particular frequency can be thought of as the delay of the envelope of a tone burst with a carrier at that frequency, just as above. The group delay is minus the slope of the phase and that is what we attempt to match, along with the phase, when aligning speaker pass-bands. So it all fits together.

*An analytic signal is one in which the amplitude of its Fourier transform is zero for all negative frequencies. The imaginary part of the analytic signal in the above example can be derived from the real system response by means of a Hilbert transform. You can think of the above method of getting an envelope as creating a complex signal whose imaginary part is a frequency-independent 90 degree phase shifted version of (or in quadrature to) the real part, which is the original signal. Then take the magnitude of that complex signal. Probably more than anyone wanted to know about penguins...

--Frank

[Art Welter]

And I'm guessing that at the wavelengths involved in the crossover region, a few inches off for the subwoofer won't make too much of a difference? If my understanding is right, you'd have to be almost 2' off at 100Hz in order to be out of phase by 60 degrees?

George,

Yes, a few inches won't make any audible difference in phase response at 100Hz.
At 72 degrees F, sound travels 1130' (feet) per second, 1.13' per millisecond, 0.885 ms per foot.
1130/100=11.3' for one wave length, 360 degrees, 10ms
360/60 =6
11.3/6=1.88'
You could correct a 60 degree phase misalignment in the crossover region by moving the "late" speaker forward physically by 1.88' or applying a delay of 2.44ms to the speaker that arrives "too soon".

At 100Hz, 5ms is equal to 180 degrees of phase change,  a polarity reverse for alignment can avoid adding excess delay.

Art

[Frank Koenig]

At 100Hz, 0.5ms is equal to 180 degrees of phase change,  a polarity reverse for alignment can avoid adding excess delay.

Decimal point, Art 

[George Reiswig]

Decimal point, Art 

Yess! That sure made me go “hmmmm!”

[ProSoundWeb Community]