Lowmid frequency bump - Home Theater Forum and Systems - HomeTheaterShack.com

Old 04-19-10, 07:32 AM Thread Starter
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xmor

Join Date: Dec 2007
Location: GREECE
Posts: 21
Lowmid frequency bump

Recently I was making some measurements for a friend of mine using REW. He's having a pair of Tannoy RHR LE playing in a quite large space (7 X 6 X 2.7m approx.). Both speakers are firing the long distance of the room and they are at least 1.2m away from the fron and side walls. The room is acoustically treated with 3 RPG's Modex Plate Bass Absorbers, RPG's Abffusors and RPG's BAD Panel. The reverberation time is quite flat (0,45s average in all frequencies). Could someone explain me the huge bump that appears in the low mid frequencies? Is it a result of wall-speaker loading? Could this phenomenon be connected with the bump in the impulse response from 6-11 ms? What about the "huge" roll-off at approx. 60Hz? These speakers are supposed to go down to 30Hz. I've tried to move the speakers further away from the front wall without any big success... Results were quite similar... Any kind of comment would be greatly appreciated.

Harry
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Last edited by xmor; 04-19-10 at 09:25 AM.
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Old 04-19-10, 09:06 AM
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Bill

Join Date: Sep 2009
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Re: Lowmid frequency bump

Welcome to the forum, Harry. I'm still trying to relate what I see to my impulse graphs to my frequency response graphs, and I am finding it challenging. So take my comments as just guesses.

Playing with the REW Impulse graph, I see that your lump from 6-10 msec corresponds to distances of 2.0-3.4m. 2.0m is a full wave length at 172 Hz. 3.4m is a full wave length at 101Hz, 2 wavelengths at 202Hz. So it is certainly plausible that this broad lump in the impulse is related the higher levels you see in the frequency response between 100Hz and 240Hz. In terms of the room dimensions you've given, these might be related to the 1.2m distance to the front wall, or to the distance to the ceiling.

I've only tried looking at individual peaks in the curves, not at a broad tendency such as you show. You might find the Energy-Time graph gives cleaner peaks to analyze than does the impulse curve directly. You could then associate the delay time to the peaks to distances. (REW has a feature for this in the Impulse and Energy-Time graphs where you can start with the cursor at zero, hold down Shift and the right mouse button, drag it to the target pulse, and it shows you exact times and distance in m and ft. See bruce's picture here.) Given the times/distances, I've tried associating these with peaks and dips in the frequency response curve. Sometimes the dips are sharper, easier to pinpoint in frequency, and easier to analyze and confirm that you are seeing a reflection at a particular distance. (Obviously an odd half wavelength multiple for a dip.)

You may have already figured this out already. Your 1.8msec peaks in the impulse may represent the delay between the tweeter drivers and the woofer drivers in the speakers.

Bill

Last edited by laser188139; 04-19-10 at 10:05 AM.
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Old 04-19-10, 02:56 PM Thread Starter
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xmor

Join Date: Dec 2007
Location: GREECE
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Re: Lowmid frequency bump

Thanks for your writing Bill. I guess you've been through some measurements realizing that it's a quite "voodoo" thing...Of course I checked the ETC curve but still I cannot justify what I really see on the graphs. Sure, maybe I'm not an expert but believe me theory is not always working so great in some cases. I guess the only way is "trial and error"... Unfortunately I'm always asking for explantations....

Thanks,

harry
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Old 04-19-10, 03:30 PM
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Jean

Join Date: Aug 2009
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Re: Lowmid frequency bump

Have you tried to put them closer to the front wall, even against it? My Onken / Iwata works better close to the back wall.
Jean
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Old 04-20-10, 05:06 AM Thread Starter
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xmor

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Re: Lowmid frequency bump

No, Jean I haven't. I still have work to do on positioning. My main problem is to first realize why I get this acoustic picture... Isn't it a bit awkward? I believe that by getting the speaker closer to the front wall I may boost even more these low mid frequencies. On the other hand I will raise the low ones which seem to be truly problematic...

Harry
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Old 04-20-10, 09:30 AM
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Greg

Join Date: Nov 2007
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Re: Lowmid frequency bump

Are you feeding the AVR one channel from REW or two?

-Greg

Don't worry... nothing new here, I've already made that mistake. Trust me.
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Old 04-20-10, 09:40 AM Thread Starter
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xmor

Join Date: Dec 2007
Location: GREECE
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Re: Lowmid frequency bump

I'm feeding these monster amps separately. Red = Left Channel, Blue = Right Channel.

Harry
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Old 04-21-10, 04:23 AM
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Jean

Join Date: Aug 2009
Location: france
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Re: Lowmid frequency bump

Quote:
xmor wrote: View Post
No, Jean I haven't. I still have work to do on positioning. My main problem is to first realize why I get this acoustic picture... Isn't it a bit awkward? I believe that by getting the speaker closer to the front wall I may boost even more these low mid frequencies. On the other hand I will raise the low ones which seem to be truly problematic...

Harry
In 1994, i made some measurements (with a radio shack meter and a cd of frequencies) to try see the difference i was hearing. It's not as accurate as the REW measurements, i should do it again now that i use REW, but anyway i join the "graph". You can't avoid peaks and valleys but they are more even when the speaker is close to the wall, if the speaker moves away there is a big hole and a big peak, i prefer the 1st case. I verified it a few weeks back at a friend's home; he had new speakers (TAD) and the bass was bad with a big hump, i told him to move them back and the hump disappeared. It's only empirical and i don't want to make a theory about it, it goes against what many people say, but to throw in a valid recommendation i remember Jean Hiraga saying in an article many years back that for big speakers with horns a good position was against the wall. G A Briggs was saying that the best position was in the corners, it's better to excite all room modes and then ajust the bass level than excite one or two modes only - and TACT says the same when you bi-amp with the RCS.
Best regards,
Jean
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Old 04-21-10, 08:23 AM
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Greg

Join Date: Nov 2007
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Re: Lowmid frequency bump

Exciting all modes may be preferable if you have no way to tame them, and you can find a set of positions where they're all excited equally.

But much better still is to excite none of them, or tame the ones you do excite, with a combination of positioning, treatments, EQ, etc.

-Greg

Don't worry... nothing new here, I've already made that mistake. Trust me.
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Old 04-22-10, 12:24 PM
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Jean

Join Date: Aug 2009
Location: france
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Re: Lowmid frequency bump

Quote:
glaufman wrote: View Post
Exciting all modes may be preferable if you have no way to tame them, and you can find a set of positions where they're all excited equally.

But much better still is to excite none of them, or tame the ones you do excite, with a combination of positioning, treatments, EQ, etc.
I agree with you, i was just answering to the original poster and his measurements.

He said: "The room is acoustically treated with 3 RPG's Modex Plate Bass Absorbers, RPG's Abffusors and RPG's BAD Panel. The reverberation time is quite flat (0,45s average in all frequencies)" and also that the speakers are "1.2m away from the front and side walls".

The room seems good, so moving the speakers seemed a good option at this moment.
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