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Old 21st April 2011, 09:55 AM   #1
webby is offline webby  United Kingdom
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Join Date: Jul 2004
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Thumbs up Help to Tune / measure open baffle H frames

Hi All,
I would like some help with my DIY open baffle speakers.
I have just started using HOLMimpulse to measure these speakers.
They are Eminence Beta 15 in a 40cm sq H fame powered by a Dayton plate amp with adjustable gain 40Hz - 180Hz crossover.
Above is an Audio Nirvana super 8 full range, with no crossover, driven by low powered valve amps.
I really don't know how to tune the speakers, or what to do with this data?
I can adjust the output of both amps individually, and the plate amp has a 40Hz - 180Hz filter.
Other options;
I can change the baffle of the audio nirvana super 8's, Enable them. add passive components. but i came to this design from a large rectangular baffle (see pic), and this small baffle improves the imaging / sound stage. But may be adversely effecting the frequency response?
Any suggestions please?
Thanks
Ian
Attached Images
File Type: jpg p1050526sm.jpg (329.4 KB, 508 views)
File Type: jpg dsc_0256.resized.jpg (212.1 KB, 492 views)
File Type: jpg DSC_0653.resized.JPG (142.4 KB, 482 views)
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Old 21st April 2011, 10:02 AM   #2
webby is offline webby  United Kingdom
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Default measurements

Here are the HOLMimpulse measurements
The room dimensions are;
Room LxWxH 410x340x190 cm approx
Both drivers measured at 10cm, .5m, 1m distance.
But i wasn't sure how to gate the beta15, so i used 200Hz & 20Hz.
Attached Images
File Type: png an 8 10cm 1m.png (86.1 KB, 471 views)
File Type: png beta15 10cm 1m 200Hz gating.png (79.1 KB, 447 views)
File Type: png beta15 10cm 1m 20Hz gating.png (82.2 KB, 73 views)
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Old 21st April 2011, 10:06 AM   #3
webby is offline webby  United Kingdom
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Now the combined drivers;
So now I would like some help.
I really don't know how to tune the speakers, or what to do with this data?
I can adjust the output of both amps individually, and the plate amp has a 40Hz - 180Hz filter.
Other options;
I can change the baffle of the audio nirvana super 8's, Enable them. add passive components. but i came to this design from a large rectangular baffle, and this small baffle improves the imaging / sound stage. But may be adversely effecting the frequency response?
Any suggestions please?
Thanks
Ian
Attached Images
File Type: png an8+beta15+ob1@1m 200Hz gated.png (80.8 KB, 87 views)
File Type: png ob1 10cm 1m 200Hz gating.png (86.3 KB, 67 views)
File Type: png beta15 10cm 1m 20Hz gating.png (82.2 KB, 50 views)
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Old 21st April 2011, 01:05 PM   #4
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Join Date: Jan 2007
Hi Ian,

1. Are these measurements on-axis? At ear height? If you measure 15-ish degrees off-axis, it should help flatten the high frequencies. Definitely measure at ear height.

2. What distance to the back wall? OB needs a good distance and the photo implies not much distance.

3. Are you primarily trying to get more bass?

4. Do you have any EQ options (e.g., are you using a computer for a source, do the plate amps have any bass boost or parametric?)

5. When you make a tweak, if you do a before/after, the graph will most likely show the results. So do an extreme tweak (e.g. on axis vs. 30 degrees off-axis) and then you'll definitely see.
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Old 21st April 2011, 03:55 PM   #5
webby is offline webby  United Kingdom
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Hi rjbond3rd,
Thanks for the reply.
1. They were measured on axis, with the mic at phase plug height. which is about ear height.
2. The first photo is the old baffle, and they are pushed up against the wall when not in use, hence the photo.
I use them with 3ft gap from the back wall, and when I measured one I had it in the middle of the room, cushions to the sides where possible and some absorbing panels behind.
3. I thought I could try and better match the drivers together, as well as improve the baffle if need be. Same with adding components etc. I am just not sure where to start other than adjusting crossover on the plate amp, and amp volume etc.
4. My plate amps only have gain, phase & 40-180Hz adjustments. no EQ available. I used CD, vinyl, and radio tuner.
5. I will try this out
Do the current measurements tell you anything about the settings?
Many thanks
Ian
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Old 21st April 2011, 11:27 PM   #6
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Ian,

Nice job on the speakers!

"I really don't know how to tune the speakers, or what to do with this data"

What is it about the sound that makes you think that they need to be 'tuned'? There must be something. Do they not sound right in some way? If we knew what that was, perhaps we could be more helpful.

Just off hand, I would suspect that the small circular baffle (middle pic) is too small to allow a crossover at or below 180 Hz. The AN Super 8's have a very strong motor, so response is already down a few db by 180 Hz. The baffle in the right pic looks better in this respect.

Interpreting frequency response graphs with respect to actual sound quality is difficult, even with experience. It requires some off axis measurements (in addition to on axis), and careful listening. IMO, first, identify the flaws through careful listening, then try to relate those to the measurements. Naturally, you will also listen for flaws that were measured, but not initially heard.

Regards,

Bob
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Old 21st April 2011, 11:44 PM   #7
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I'm not sure which graph correlates with which measurement.

But it does look to me like you have a steadily rising response. You want to shoot for something flatter. I would definitely try toe-ing them in to bring down the treble peaks.

It also looks like the bass is steadily falling. Do you feel the treble is too hot, and the bass a bit weak? It could still sound musically decent of course.
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Old 22nd April 2011, 12:19 AM   #8
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Quote:
Originally Posted by webby View Post
Hi rjbond3rd,
Thanks for the reply.
1. They were measured on axis, with the mic at phase plug height. which is about ear height.
2. The first photo is the old baffle, and they are pushed up against the wall when not in use, hence the photo.
I use them with 3ft gap from the back wall, and when I measured one I had it in the middle of the room, cushions to the sides where possible and some absorbing panels behind.
3. I thought I could try and better match the drivers together, as well as improve the baffle if need be. Same with adding components etc. I am just not sure where to start other than adjusting crossover on the plate amp, and amp volume etc.
4. My plate amps only have gain, phase & 40-180Hz adjustments. no EQ available. I used CD, vinyl, and radio tuner.
5. I will try this out
Do the current measurements tell you anything about the settings?
Many thanks
Ian
Perhaps you can describe what you are hearing then we can try and correlate it to the FR plot, but as already mentioned, there seems to be a rising HF response, which could be adjusted by increasing the LF output from the plate amp and/or lowering the HF output for starters.

Have you modeled the speaker with software like MJK's Quarter-Wave? Playing around with the H-frame, baffle size and the crossover parameters could provide some clues for your tuning (a bit backwards since you already build the speakers...) but hey it is never too late.

Jaz
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Old 23rd April 2011, 11:10 AM   #9
T101 is offline T101  Bulgaria
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Very nice approach to the full range baffle. When I stated modeling mine OB's with the Edge software, I found out it was unaware of the existence of floor or other boundary underneath, so the only way to achieve the same response as predicted is to have a suspended OB that has all it's edges open.

Here is what I get with similar baffle in the Edge.

Hm, quite nice, a 5 db notch at 800 hz and - ~1 db overall will lead to 180 hz@ - 3 db - quite nice for crossover there...

A nice dedicated BSC with a large value capacitor (should be in the 15-20uf range) could quite do it...

And a H frame is a H frame... the smaller size means less air to load the membrane, but means greater coupling of the membrane to the available air. And you can always tweak with wadding to the rear opening of the H-frame. - For example 1/2 inch of loose wool wading attached to acoustically transparent cloth stretched over the rear opening.
The cloth over the opening beneath the OB serves for reducing part of the acoustic short circuit as it is.
The resulting acoustic blanket at the rear of the H-frame might increase system resonance a little, but will reduce the short circuit with a little too - let's hope with little more... It could lead to better transients and impulse response too.

See if you can obtain a frequency response graph of the full range driver, so we could consider it when designing the BSC which will actually serve more kinda as a notch filter and just a little as a BSC (a db or two). Best wish is a hole in the response at 800 hz and a peak at 1800 hz - both are quite possible.

Best regards!
Attached Images
File Type: jpg DIYnirvanacircbaffle.jpg (114.3 KB, 88 views)

Last edited by T101; 23rd April 2011 at 11:16 AM.
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Old 25th April 2011, 05:00 PM   #10
webby is offline webby  United Kingdom
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Wow!
Thanks for all the info coming my way, I am new to all this measuring stuff, but I'll try and explain a bit further.
After the large rectangular baffle, I read about the MJK H frame and copied that for the Beta 15. At the same time, it was suggested to try a smaller baffle for the AN8. This was the result. The bass improved with the H frame, and the imaging / sound stage improved with the smaller full range baffle. I am guessing the mid range suffered, as it does sound full in the lower bass ( upping the plate amp output) and high end. Male voices sound a little thin at the low end.
naively, I thought it must just be the settings on the plate amp. Not taking into account the smaller baffle might affect this so much.
The first graph in post #2 is the AN8, on axis at 10cm, .5m & 1m.
I have just measured the combined drivers at 1m an axis, 15 deg & 30 deg, see below;
And i will go and measure just the AN8 at 15 deg & 30 deg.
Let me know what you think.
Many Thanks
Ian
Attached Images
File Type: png 1m on axis.png (78.5 KB, 72 views)
File Type: png 1m 15 deg.png (78.4 KB, 40 views)
File Type: png 1m 30 deg.png (78.7 KB, 29 views)
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