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#13181 - 07/09/03 05:40 PM Re: Request for advice from a novice
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10634
It's roughly 1/10th of the 80hz crossover wavelength(for near maximum reinforcement)Masa; for lower frequencies the effect occurs at correspondingly greater distances.
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#13182 - 07/09/03 07:41 PM Re: Request for advice from a novice
pmbuko Offline
shareholder in the making

Registered: 04/02/03
Posts: 16437
Loc: Ben Lomond, California
This is interesting to me. Can you explain the logic behind it? In other words,

1. why 1/10th, and
2. why use the crossover frequency as a basis for that measurement?
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#13183 - 07/09/03 07:55 PM Re: Request for advice from a novice
rcvecc Offline
aficionado

Registered: 04/17/03
Posts: 608
Loc: east haven ct
hi mc1,for an oppinion on your tweeter height,i have always liked my tweeters at or as close to in height as possible,it seems to give a more seamless soundstage,higher tones are easier to localize so i like the tweeters to be about the same,try it ,it works for me..good luck....ron

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#13184 - 07/09/03 09:28 PM Re: Request for advice from a novice
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10634
Okay Peter, I'll give it a shot. I suppose that I learned this stuff as much as 20 years ago, a lot of it from articles in the late and lamented Audio magazine, which combined technical and non-technical approaches quite well. Although I also have enough technical training to be able to understand some of the stuff in technical journals, I prefer it when the math doesn't get much beyond 2+2.

This area involves two separate acoustical phenomena:1.standing waves(modes, resonances)which affect speakers placed close to room surfaces at frequencies in relation to the dimensions of the room in wavelengths; 2.boundary reinforcement(and cancellation)which affects all frequencies(not just the standing wave frequencies)where the speaker is "very close"(hence the 1/10th rule-of-thumb)to the room boundaries. As far as I know, the Audio material isn't available online. One thing I did find was this somewhat awkward translation from the Swedish of a discussion of the two phenomena as they relate to sub placement.

Some of the Audio articles, which my library has, and which I suppose your library has or can get photocopies, etc. of if you're interested are : Weinberg and Ferstler,"Bass Versus Space", Audio,July/August 1999;Allison,"The Best Place For Your Speakers? Your Computer Knows!", Audio, August 1994; Noussaine,"Birth Of The Boom" Audio, June 1998,and also "2 subs in a corner beats 5 in The Round", Audio,June 1996, and still further "The Lowdown On Getting The Most Out Of Your Bass Box", Stereo Review, January 1995.

Much of this is based on research done almost 50 years ago by Waterhouse("Output of a Sound Source in a Reverberation Chamber and other Reflecting Environments",Journal of the Acoustical Society of America, Vol.30,No.1,January 1958). So this isn't just my theory, but it appears valid to me and the bass sounds good.
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#13185 - 07/10/03 09:35 AM Re: Request for advice from a novice
MC1 Offline
frequent flier

Registered: 07/01/03
Posts: 10
Loc: San Diego, CA
I just wanted to thank everyone for their informative responses. I've just received my Axioms and I'm waiting on my Pioneer receiver. Also, I'm having some walls constructed to enclose a room for home theater use. I'm excited to set it all up!

Thanks Again to all

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#13186 - 07/10/03 02:04 PM Re: Request for advice from a novice
sushi Offline
connoisseur

Registered: 02/09/03
Posts: 1490
Loc: Dallas, TX
John,

Obviously I haven't yet read any of the offline articles you cited. But I do have major objections to some of core assertions in the Swedish article:

The article states: By choosing a corner placement you excite all existing resonances in the room and the frequency response will not become appreciable inferior compared to a completely free placement.

This will be the case ONLY IF you are lucky -- in that (1) you don't happen to have multiple different modes with resonance frequencies close to each other, which does occur commonly and will result in a big hump in that region; (2) the Q (quality factor) of every room mode is low enough so that they are all nicely tamed when overlapped with each other, without resulting in narrow humps/dips.

There is absolutely no guarantee that these conditions are met in YOUR room.

The article states: An interesting side effect of placing the subwoofer in a corner is that the distortion is lowered significantly. The cause for this is that you get a reinforcement of fundamental tones from all the reflections in phase from the adjacent surfaces, while harmonics (which is distortion when it comes to a steeply low pass filtered subwoofer) are lacking the corner support, and become weaker in amplitude compared to the wanted fundamentals.

This completely self-contradicts the statement made earlier in the same article. You choose the corner placement with less than 1/8 - 1/10 wavelength distances from the three walls so that ALL frequencies in which the sub operates (e.g., 20-80Hz) get a boundary reinforcement. So, when the sub is playing, say, a ~20Hz fundamental, its 2nd, 3rd and 4th harmonics will all be similarly enhanced. In fact, if this isn't the case, you are in trouble because it means that you don't have a flat frequency response within the operating range.

The harmonic and other distortions generated by the sub at above 80Hz is certainly a different story. But again, as the article itself says, the higher harmonics may be cancelled or enhanced, depending on specific frequencies they happen to be at. I would in fact even speculate that this modulation may well be one of the biggest culprits which make a corner-placed subwoofer sometimes very boomy (like my own case).

In the end, the article says after all the lengthy discussions: "...then place the subwoofer, use a lot of experimenting." I wish that the AUDIO articles you cited are more coherent and convincing than this one at the very least.


[added in edit]

Oh, at least I now understand the "1/10 wavelength" recommendation. Thanks! Incidentally, I play flute, which is in fact the only orchestral woodwind instrument that acoustically behaves as a double open-ended, non-conical air column. Lo and behold, the mouth piece of a flute (where the "air reed" generates sound) is located at approximately 1/10 average air-column length from the top (left) end of the instrument.




Edited by sushi (07/10/03 02:28 PM)

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#13187 - 07/10/03 04:13 PM Re: Request for advice from a novice
JohnK Offline
shareholder in the making

Registered: 05/11/02
Posts: 10634
Masa, I do believe that your flute has established yet another proof!! I would strongly urge you to prepare a paper for submission to the AES. The regional convention was in Tokyo this year, which would have been ideal,but it concluded yesterday, so don't rush. The relative proportions of your flute(incidentally, in an earlier life, clarinet here), presumably arrived at empirically, certainly are powerful data. To clinch the argument, I'd suggest that you add a temporary extension to the mouthpiece end and see if they don't start yelling that they can't hear you, play louder!

On the coincidence of room modes, if the room dimensions vary significantly, the resonances should be spread sufficiently so that the overall response is acceptable(at least the testing described in Audio and elsewhere appears to indicate this).

On the lower distortion point, I haven't really seen this discussed much as a significant advantage. I vaguely recall that it was pointed out in something I read that the lower overall volume required to be delivered by the sub amp and driver with corner reinforcement could be viewed as a minor plus.
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#13188 - 07/10/03 05:06 PM Re: Request for advice from a novice
pmbuko Offline
shareholder in the making

Registered: 04/02/03
Posts: 16437
Loc: Ben Lomond, California
On that note, I think when I get around to building my own house, I'll design the home theater to be a massive cylinder lying on its side. Then I'll place a 10' subwoofer driver 1/10th of the way out from the front wall. Not that will be some good bass! Let's just hope it doesn't cause me to spontaneously void my bowels...
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#13189 - 07/10/03 05:38 PM Re: Request for advice from a novice
JasonG Offline
local

Registered: 04/29/02
Posts: 216
Loc: central Ohio
You could put a toilet in the sweet spot.

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#13190 - 07/10/03 06:13 PM Re: Request for advice from a novice
sushi Offline
connoisseur

Registered: 02/09/03
Posts: 1490
Loc: Dallas, TX
Talking further about musical instruments and "one-tenth wavelength" rule:

- The hammers in the grand piano are placed to hit the strings at about 1/7 from one end.
- A violinist or cellist bows the string at around 1/10-1/7 from the bridge (in fact, the louder he wants it the closer to the bridge he bows).
- Organs have the reeds placed at about 1/8 from the bottom of each pipe.

[incidentally, in all cases the string/air-column lengths represent a half wavelengths]

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