speaker measurement at lower frequencies

almikel

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Sep 18, 2010
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I posted a larger version of this in the DEQX thread as there were specific DEQX questions, but I've cut out the general questions and re-posted them here - apologies for any double up but I think the DEQX forum has many cobwebs:

Normally I do my measurements outdoors with the speaker elevated 2m off the ground facing up, with the mic 1m above.
That setup provides the most distance between the direct sound and the first reflection.

My planned mid woofers, for use between 50Hz and 300Hz will be very heavy, so lifting them high off the ground to perform measurements is really not an option (safely), 1m off the ground would be OK, but ground plane easier again.
The planned tapped horn will operate between 25 and 50Hz, and also be heavy and unwieldy, so again either 1 m off the ground or ground plane measurements will be done.

Q1) Given the ground bounce will be the first reflection, and very soon after the direct sound, is there benefit from measuring outdoors?
Is having the ground bounce the only reflection better than lots of reflections soon after the ground bounce for meaningful results?

Q2) is close micing recommended? Since I'm not measuring a multi way speaker there's no requirement to have the Mic 1-2 m away and could have the mic 20mm off the driver for the mid bass and right at the horn mouth for the tapped horn?

Key goal is to measure Frequency Response, Phase, Group Delay, distortion

Cheers
Mike
 

amirm

Banned
Apr 2, 2010
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I have not done this type of work but one method used is to put the mic on the ground to eliminate the floor reflection:

 

2ears2hear

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Apr 16, 2012
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Q1) Given the ground bounce will be the first reflection, and very soon after the direct sound, is there benefit from measuring outdoors?
Is having the ground bounce the only reflection better than lots of reflections soon after the ground bounce for meaningful results?

Q2) is close micing recommended? Since I'm not measuring a multi way speaker there's no requirement to have the Mic 1-2 m away and could have the mic 20mm off the driver for the mid bass and right at the horn mouth for the tapped horn?

Key goal is to measure Frequency Response, Phase, Group Delay, distortion


Hi Mike
Inside the room you will always have the room resonances, even if you measure very close to the cone. You will always have a floor bounce too, that even changes woofer loading. With the (pressure zone) mic on the floor you need to consider vertical room resonances too.
If your measurement software allows time windowing, you may not need to go outside.

Why not dig a hole and have the cabinet/woofer flush-mounted with the ground (firing to the sky)? You can place the mic over ist and there is no Allison dip (1/4 wavelength distance cancellation). Most of the measurements are free from side effects then.
Cheers
Hans
 

DonH50

Member Sponsor & WBF Technical Expert
Jun 22, 2010
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Amir, I'm no expert, but putting the mic on the floor places it at a boundary, not usually the most accurate place to measure...

Disclaimer: I have not made detailed measurements for years.

I have had mixed success with close micing. As has been said, the room all too often comes into play, so I have always gotten the most accurate results in an anechoic chamber (our local college has one and it is available cheaply) or outdoors (assuming you can find a place with low enough background noise). For single aperture system it sounds like a reasonable idea to me, but outdoors as you've been doing is what I would do if you can't get to an anechoic chamber.

The fewer the reflections, and the later they arrive, the better. For impulse tests you can gate out the reflection(s) but measuring steady-state distortion and such is always a challenge. I think most measurement software includes a time gating function (mine does, RPlusD).

One thing I have done in the past is to lay the speaker outdoors on its back (usually on a small riser to get it off the ground and provide room for the cables). Then, suspend the mic above it to take measurements. Essentially the same as on an infinite wall, but at least any reflections are equal and since the driver is pointing up they are a long time coming (easy to ignore).

FWIWFM - Don
 
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almikel

New Member
Sep 18, 2010
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Hi Mike
Why not dig a hole and have the cabinet/woofer flush-mounted with the ground (firing to the sky)? You can place the mic over ist and there is no Allison dip (1/4 wavelength distance cancellation). Most of the measurements are free from side effects then.
Cheers
Hans

I've considered a hole in the ground - but a hole to fit the tapped horn would make the neighbours think something different!
Also thought about a box trailer full of mulch with the speaker buried flush.

Thanks guys - it seems outdoor measurements are still the go.

Mike
 

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
Sep 6, 2010
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www.genesisloudspeakers.com
Mike,
Accurately measuring FR and distortion at low frequencies is almost impossible without an excellent anechoic chamber. This is because you need to set a time window that will reject the first reflection, and still be long enough to provide the resolution you need at the frequency you are measuring at.

A 10ms window, for example, only provides enough resolution for 1kHz and above. However, a 10ms time window will reject first reflections from about 1.7m away.

To measure 100Hz, you need a time window of at least 100ms - and the first reflection point needs to be 17m away.

As a general rule, the time window is inversely proportional to the frequency you want to measure.

With outdoor measurements, you need a pretty tall crane to hoist the loudspeaker up above the ground.
 

Nyal Mellor

Industry Expert
Jul 14, 2010
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SF Bay Area, CA, USA
The way the DEQX is designed you ideally need to keep the mic a constant distance from the loudspeaker whilst doing the speaker correction.

Below a certain frequency what you hear in room is dominated by the effect of room modes and speaker boundary interference anyway. The DEQX's linear phase frequency / group delay correction is most useful in the region where the speaker dominates what you hear.

Generally it is only possible to measure and correct down to 150HZ or so even if you take the speakers outside. To get a good measurement down to 150Hz you need ~15ms between the direct sound and any reflections. That means elevating the speakers quite high off the ground.

Gary I am not sure where your figures come from for the relationship between time window and frequency. They seem excessively long, though it does depend what type of IR window truncation you are using. Wavelength of 150Hz is 7.5ft (takes 8.25ms for a complete wave cycle) so not sure why you would need a 100ms window.

What most people try and do with the DEQX is run the speaker correction down too low and what ends up happening is that they correct for measurement artifacts and anomalies, not the true speaker response.
 

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