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Author Topic: Wedges & Transfer Function Measurement: Meas. Mic Position?  (Read 5433 times)

Shane Ervin

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Wedges & Transfer Function Measurement: Meas. Mic Position?
« on: October 30, 2012, 11:36:48 am »

This thread applies to the roll-your-own crossover / DSP / LMS situation for bi-amp wedge owners.  In my case, I picked up some used 15" units from a nearby regional provider.

I'm looking for comments on measurement mic placement, and am interested in comparing the relative merits of placing the measurement mic:
  • where the performer's head would be, as contrasted with
  • alternate arrangements whereby the meas. mic is placed on the floor / ground.

Topics which I'm hoping to see in this discussion of wedges as the D.U.T. (Device Under Test), are covered in Mark Frink's article:
  • Half-Space coupling of the wedge with the floor.  (160 Hz area)
  • Baffle loading & directionality of LO driver (300 Hz area)
  • Use of wedges in pairs (1000 Hz area)
Also, it'd be great to see comments touching on:
  • Quasi-Anechoic technique of time-windowing, as it applies to a D.U.T. so close to a nearby reflecting surface, that it's touching it (namely, the floor), and
  • the obvious "weirdness" of elevating a floor wedge to an "above floor" position - done merely to achieve a measurement setup in which the meas. mic is placed on the floor / ground, and/or the D.U.T. is far enough away from a nearby surface such that quasi-anechoic time-windowing makes clearer sense.
So far, I've only conducted T.F. measurements by placing the wedge in the "as-used" position on the floor, and the meas. mic was at performer's head height, aiming down at the grill.

Propagation delay found, based on the HF driver (muting the LO), and used for T.F. measurements, I ran another impulse measurement on the LO to obtain an initial delay value to employ in the MiniDrive LO band.

I proceeded to apply gains, and some filtering, in both bands, to achieve a magnitude response picture suitable for identifying the acoustic crossover point.

Moving to the goal of achieving phase alignment through the crossover region, I captured a new T.F. trace of the (now) EQ'd HF driver, and proceeded to dial-in / fine-tune the LO delay by lining up the phase traces at f=acoustic x-over.

A 0.417 ms delay applied to the LO did the trick.
  • Electronic X-Over: f = 1.14 kHz (L-R 24)
  • Acoustic X-Over: f = 1.4 kHz

With both bands un-muted, it was very satisfying to sanity-check matters by reversing the polarity on one band, and observe a deep valley in the magnitude response at the crossover point.

P.S.  Kudos to Rational Acoustics; I'm enjoying the demo version of Smaart v7.4.  (... 3 cheers for Object-Oriented Programming!)
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Timo Beckman

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #1 on: October 31, 2012, 05:06:22 am »

You might want to try aligning @ the x-over via all-pass filters .
I would use as little delay as possible on a stage monitor or no delay at all if my processor has all-pass 1st and 2nd order filters .
But a 0,4 something delay would not be a problem so if it works fine......
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Shane Ervin

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Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #2 on: October 31, 2012, 09:03:21 am »

Hi Timo,

That delay was just time-aligning the woofer to the horn, since the acoustic centres are not lined up physically in these boxes.  I'm with you, of course, in avoiding the use of overall delay in floor wedges.  Indeed, since the horn is not delayed electronically, I wouldn't say that delay is applied, per se, to the wedge.

Back to the thread topic, what are your thoughts on the orientation of the D.U.T. and placement of meas. mic?  Are there are compelling reasons to go to the extra trouble of elevating the speaker, say, onto a pedestal and conducting T.F. measurements with the mic on the ground?

In the context of comparing measurement methods (and setups) with the use of an anechoic chamber, my interest is in an exploration of the merits of including the half-space loading effects in the T.F. measurement, since, after all, that's how the wedge is used in practice.  This, in preference - perhaps - to full anechoic testing.  Building on this idea, could it be that the optimum test method for floor wedge T.F. measurements is to use an anechoic chamber equipped with a conventional (i.e., not mesh) floor?
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Timo Beckman

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #3 on: October 31, 2012, 11:26:51 am »

If there are all-pass filters in the processor you do not need to use delay to align the low driver to the mid highs . Via all-pass 1st or 2nd order just delay the frequencies that need delaying and leave the rest untouched .
For mic placement i would go for the fast and easy by just putting the mic roughly where the listener would be or @ the middle point of both drivers really close and then verify at the listeners position if the alignment holds .
Before doing the alignment of the drivers may it be delay or allpass i would take out some of the peaks and maybe some heavy dips (if any are there) on the drivers via eq .
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Art Welter

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #4 on: October 31, 2012, 12:40:59 pm »


Back to the thread topic, what are your thoughts on the orientation of the D.U.T. and placement of meas. mic?  Are there are compelling reasons to go to the extra trouble of elevating the speaker, say, onto a pedestal and conducting T.F. measurements with the mic on the ground?

In the context of comparing measurement methods (and setups) with the use of an anechoic chamber, my interest is in an exploration of the merits of including the half-space loading effects in the T.F. measurement, since, after all, that's how the wedge is used in practice.  This, in preference - perhaps - to full anechoic testing.  Building on this idea, could it be that the optimum test method for floor wedge T.F. measurements is to use an anechoic chamber equipped with a conventional (i.e., not mesh) floor?
The response of a wedge changes radically (more than +/- 3 dB) from a placement on the floor compared to in free space, unless you also use the wedges on poles, an anechoic response of a wedge is not of much use other than above the frequency of pattern control for the HF horn.
Even on a pole, the floor bounce lacking in an anechoic environment will be a real part of the sound heard, though it will change with distance.

A useful test position would be the wedge on the ground, outdoors, mic  approximating normal ear placement, wedge at least 20 feet from any walls or large objects.
If your wedges need shimming (Angular Discrepancy Adjustment Modules, AKA 2"x4", audio logs, etc.) to get to a proper listening angle (like 12AMs need) they should be tested both flat and with the shim, as the baffle to floor distance affects response considerably.

This will give you a "large hall" approximation, as boundaries in a large hall are far enough away so won't affect the monitors all that much, while on small stages you are often EQing as much for the room as the wedge.

As well as room problems, the performer's face, mouth,  eyeglasses, and hat all have huge effects on the eq needed to make a wedge work properly, a flat mid-band response is only a starting point, but is a helpful reference.

Art Welter
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Shane Ervin

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #5 on: October 31, 2012, 02:59:30 pm »

This is an interesting side discussion, and since you've twice raised the notion of using all-pass filters as an alternative to the application of delay in driver time-alignment, I thought I'd pick up on it.

If there are all-pass filters in the processor you do not need to use delay to align the low driver to the mid highs. Via all-pass 1st or 2nd order, just delay the frequencies that need delaying and leave the rest untouched.

Rather than it being a case in which "you do not need to use delay" as a means to align drivers, is not delay considered to be the method preferred over a phase shift introduced via an all-pass filter?  That is, at least as the first remedy one would apply prior to contemplating a supplemental phase shift filter to treat the phase relationship at frequencies on either side of the acoustic crossover?

This (older) article provides a compelling case that delay is the superior method to correct for physical misalignment of the drivers' acoustic centres.

And here's an old thread featuring some of PSW LABsters from years gone by, debating the same topic.

I'll add two more arguments in favour of delay (in preference over a phase shift filter) as a means to achieve driver alignment:
  • Not all DSP crossovers provide all-pass filters, whereas most, if not all, will offer a band o/p delay parameter.  Case in point: BSS MiniDrive.
  • In DSP crossovers where both techniques are available, it takes fewer DSP resources to implement a short time delay, than it does to implement a filter. (i.e., Z-1, and no MAC's).

(minor edit)
« Last Edit: October 31, 2012, 03:14:06 pm by Shane Ervin »
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Charlie Hughes

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #6 on: October 31, 2012, 05:21:38 pm »

For the application at hand, aligning the signal arrival of the woofer with a horn that is physically offset behind the woofer, delay is the proper tool to use.  Given enough all pass filters it is possible to get an equivalent delay within the pass band of the woofer.  However, IMHO, this is the long way to get to the solution.  Using delay for this application has absolutely no adverse side effects.

The signal from the horn is arriving 0.417 ms after the signal from the woofer.  Delay the woofer by this amount so that their arrivals are synchronized.  Only after doing this will it be possible to correctly determine the LP and HP filters which will provide complimentary magnitude & phase response for the LF & HF devices through the crossover region to yield the desired system response.

As to the measurement setup, I'd do exactly as a Art suggested.  Place the wedge in a position just as it will be used, but outdoors as far away from any boundaries or objects as possible.  Place the mic close to ear height and make some measurements.  If multiple mics are available, place them in different positions, but at the same height.  This should yield some info about how the response of the floor wedge changes at off-axis positions.

Hope this helps.
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Shane Ervin

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Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #7 on: November 01, 2012, 10:42:33 am »

Hi Art, Charlie, and Timo,

Excellent discussion going here; thanks for your posts!

As I mentioned in the kickoff post, the interesting aspect of a floor wedge as a D.U.T. is the consideration given to placement of the meas. mic placed elsewhere than on the ground - a bit unsettling at first.  Then you start to think of the underlying rationale (no pun intended) for the test activity, and you can reasonably justify the departure from standard practice.

It would be a good side discussion to cover saving, in a DSP preset, notch filters for use with certain specific vocal mics, or rooms, or performers.  I did that at a gig in late Sept. and left the EQ's at home, knowing I had adequate GBF with a tried and tested custom preset.

Our side discussion on all-pass phase shift filters, as compared with straightforward application of delay is also welcome.  I still own a BSS FDS-360 with flush-mounted phase-adjustment pots.  Remember those old forum posts on the "poor man's technique" for driver alignment?  Using a sine tone at the crossover frequency and polarity-reversing cable, you'd adjust for the deepest null - out front - then remove the pol-rev cable.  IIRC, BSS described the technique in their product manual, or other literature.

Fun story to relate:  One day, years ago at the sound company warehouse, when phase-aligning for a sidefill's 18" to the 10" mid at 250 Hz, I did an ad-hoc demo of beam-steering!  We could make loose objects rattle on their shelves in different locations around the warehouse - just by action on the phase-adjustment pot.  Ah, the good old days of swept sine testing!

So time rolls on and thanks to advances in technology, and we now have access - at low cost - to so much more in the realm of test gear, and techniques.  It's been 21 years since I attended the Vancouver chapter AES meeting where Audio Precision demo'd their (then new) "Porta-One", conducting a quasi-anechoic T.F. measurement on a small speaker.  Then came MLSSA from DRA Labs, and others with TDS techniques.

And in all that time, I'd never stopped to consider floor wedge monitors as a special case for consideration w.r.t. test setup.  We'll have to add the internet in that list of measurement-related resources we now enjoy.
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Frank Koenig

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #8 on: November 09, 2012, 01:14:00 pm »

Interesting discussion. I, too, have had this question as to the most relevant way to measure floor monitors.

The measure-it-how-you-use-it approach has logical appeal even though it runs counter to the widely held belief that the frequency response of the first sound to arrive is of the greatest perceptual importance. But in this case the reflection off the floor is so early that it may well be perceptually more part of the the speaker than part of the room.

In my limited experiments, I ended up basing my settings on a pseudo-anechoic measurement (speaker in the air, windowed impulse response), figuring if the uncompensated hole in the response caused by the floor reflection was objectionable, it could be EQ'd in use.

So far musicians have liked the sound, and I haven't had any feedback problems, but then, I haven't been in any very demanding situations either.

I'm very interested to learn from you folks who've done this more than I have.

--Frank
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Mac Kerr

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Re: Wedges & Transfer Function Measurement: Meas. Mic Position?
« Reply #9 on: November 09, 2012, 01:41:27 pm »

The measure-it-how-you-use-it approach has logical appeal even though it runs counter to the widely held belief that the frequency response of the first sound to arrive is of the greatest perceptual importance. But in this case the reflection off the floor is so early that it may well be perceptually more part of the the speaker than part of the room.

I am not aware of this widely held belief. I am aware of a widely held belief that says the apparent source will be the first arrival assuming certain other conditions, like near equal level and a limited time differential.

Quote
In my limited experiments, I ended up basing my settings on a pseudo-anechoic measurement (speaker in the air, windowed impulse response), figuring if the uncompensated hole in the response caused by the floor reflection was objectionable, it could be EQ'd in use.

How do you eq the hole caused by reflection back in? All you can do is be aware of it or change the setup so it doesn't happen. You cannot correct a reflection caused suckout with eq.

Mac

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