For instance - your output at 100mm spacing is only ~0.8dB higher that at 200mm.....
Yet you moved the posterior driver of the clamshell 20% further away from your mike.....
(500mm measuring distance to the front of the anterior driver)
could this not be accounted for by simply the increased distance from the rear driver?
Yet you moved the posterior driver of the clamshell 20% further away from your mike.....
(500mm measuring distance to the front of the anterior driver)
could this not be accounted for by simply the increased distance from the rear driver?
As distance between the drivers increases you will get to a point where there is rarefication in front of driver A when it is traveling rearward. The air coming off the back of the driver will go straight around to the front, significantly reducing D. When the driver spacing is small the air from the back of driver A will head around to the back of driver B, a low pressure point. To me it appears this begins to stop happening when the spacing of the drivers is a bit less than their diameter.
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I predict the toilet paper will rustle, not be sucked in and out. Eagerly awaiting your video!
Can I post videos here - does anyone know?
If not I could upload to you-tube and post a link?
Nice!
Looks like it will be DIY wind tunnel for me tonight
Any other suggestions - gratefully received!
I think it should be "Efharistó" !
BM, you could also compress the video file, and upload it as a zip.
vous êtes les bienvenus (“parakalo”)
Interesting topic.
I had built (*) two small sized woofer enclosures (internal dim. In mm: Width 470, Height 420, Depth 155) intended as multimode woofer jig for 12inch woofers.
One woofer fixed on front panel, one woofer fixed on rear panel.
That rear panel could be removed (turning into one woofer open back enclosure) or replaced with a normal panel (turning into one woofer sealed enclosure).
When with two woofers, both could be driven, in series, in parallel or individually, in phase, out of phase. When one woofer only driven, the electric terminals of the other could be left open, shorted or loaded with a resistance.
All these can be tested on this rim to rim compound woofer at discussion (with small excursion woofers, no spacer is required).
I have withdrawn those boxes from use. I had better results with a single woofer (*) on an open baffle
IMHO for woofer and focusing on in-room bass, mic measurements give comparative results with acoustic feel only when mic is placed >>1m from the drivers, especially so when there is more than on woofer at play.
(*) as helper woofer for a FR on OB
George
Interesting topic.
I had built (*) two small sized woofer enclosures (internal dim. In mm: Width 470, Height 420, Depth 155) intended as multimode woofer jig for 12inch woofers.
One woofer fixed on front panel, one woofer fixed on rear panel.
That rear panel could be removed (turning into one woofer open back enclosure) or replaced with a normal panel (turning into one woofer sealed enclosure).
When with two woofers, both could be driven, in series, in parallel or individually, in phase, out of phase. When one woofer only driven, the electric terminals of the other could be left open, shorted or loaded with a resistance.
All these can be tested on this rim to rim compound woofer at discussion (with small excursion woofers, no spacer is required).
I have withdrawn those boxes from use. I had better results with a single woofer (*) on an open baffle
IMHO for woofer and focusing on in-room bass, mic measurements give comparative results with acoustic feel only when mic is placed >>1m from the drivers, especially so when there is more than on woofer at play.
(*) as helper woofer for a FR on OB
George
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Funny you should mention it!
I have only recently found them as it were - I have a pretty mad eclectic taste in music - from EDM, to Classical, to prog rock, all sorts.
I saw a tribut to them last year (I think it was their 50th anniversary of their first album or something) and thought - 'wow this is different!!'
Since then I have been working my way through their albums - and of course - if it came out on vinyl - then I want to hear the original!!!
Agreed - loving them! Had Three Friends on last night - they are such incredible musicians!
I have only recently found them as it were - I have a pretty mad eclectic taste in music - from EDM, to Classical, to prog rock, all sorts.
I saw a tribut to them last year (I think it was their 50th anniversary of their first album or something) and thought - 'wow this is different!!'
Since then I have been working my way through their albums - and of course - if it came out on vinyl - then I want to hear the original!!!
Agreed - loving them! Had Three Friends on last night - they are such incredible musicians!
Yeah - I think this is not real - further measurements after this confirmed 5-6dB fairly well, I actually wonder if this is to do with the dipole behaviour of this configuration ('super dipole' dispersion) and room modes - not something we have yet dared to try to measure! Or because the acoustic centre was altered by removing one driver......possible none of the above.
Really if we are trying to get to the bottom of this - it will require two identical amps, two identical drivers, and an outside measurement rig, to do proper polars outside at a decent distance over 1M - a fair bit of work....
Maybe over my summer holiday!
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Only because all amps are not made equal, how are we certain power outputs change proportionally at 2ohms, 4ohms, 6ohms, 8ohms, etc.
i.e. does your amp deliver 50 watts at 8 ohms, and 100 watts at 4 ohm,
or 50 watts at 8 ohms and 75 watts at 4 ohms?
For instance the new NAD C388 (first one I googled!) has these specs:
IHF Dynamic Power:
8 Ohm 250W
4 Ohm 350W
2 Ohm 400W
So I guess - just to remove any possible confounding factors (especially as we are dealing with a dB here, and a dB there!)
But you are probably right - just me being overly cautious.
i.e. does your amp deliver 50 watts at 8 ohms, and 100 watts at 4 ohm,
or 50 watts at 8 ohms and 75 watts at 4 ohms?
For instance the new NAD C388 (first one I googled!) has these specs:
IHF Dynamic Power:
8 Ohm 250W
4 Ohm 350W
2 Ohm 400W
So I guess - just to remove any possible confounding factors (especially as we are dealing with a dB here, and a dB there!)
But you are probably right - just me being overly cautious.
William - a couple of comments.
First, thanks for the measurements. The more data we get on different arrangements and with different sizes of drivers the better we will be able to understand what is going on. I hope to make my own set of measurements with a pair of 15" drivers in the next day or so.
I think we have to be careful at this point to draw conclusions. The dip could be related to the chassis diamater, or it could be that the distance means that, at that frequency, the sources are no longer "acoustically close", which was an underlying assumption in SL's model. Measurements over the same range of separation but with e.g. larger drivers might be able to differentiate between these two possibilities.
Attributing the effect to an increased rear-to-rear pathlength would seem to go against SL's model. I have pasted the page below. The separation distance between the driver centers (he uses d2 for that) does not contribute to the magnitude, H(f). Rather it is only d1, which is the effective front to back pathlength for a single driver. It seems counter-intuitive, but that is the result he presents.
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First, thanks for the measurements. The more data we get on different arrangements and with different sizes of drivers the better we will be able to understand what is going on. I hope to make my own set of measurements with a pair of 15" drivers in the next day or so.
[image]
I think we have to be careful at this point to draw conclusions. The dip could be related to the chassis diamater, or it could be that the distance means that, at that frequency, the sources are no longer "acoustically close", which was an underlying assumption in SL's model. Measurements over the same range of separation but with e.g. larger drivers might be able to differentiate between these two possibilities.
Attributing the effect to an increased rear-to-rear pathlength would seem to go against SL's model. I have pasted the page below. The separation distance between the driver centers (he uses d2 for that) does not contribute to the magnitude, H(f). Rather it is only d1, which is the effective front to back pathlength for a single driver. It seems counter-intuitive, but that is the result he presents.