dm
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Posts: 10 
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Post by dm on Jun 22, 2016 14:40:45 GMT
I assume - correct me if I'm wrong - that to ideally handle frequencies down to say, 27 Hz (A0), the depth of a CR should be at least 6.25 meters to handle half the wave length. If John (Brandt) or Ethan feel such a length is not necessary (my acoustic training is from the 1980's and I'm aware there's much new thinking since then), then obviously, I'd be most interested to hear about more up-to-date parameters.
But assuming for the moment that 6+ meters is ideal: If one does have the possibility of constructing a CR that deep because the back wall of a 5-meter-deep room happens to have a yard behind it, does the ENTIRE back wall have to come out for bass not to reflect back, or can a 'bay window' kind of extension do the trick? If so, what % of the back wall needs to come out so that you're not left with problems? And how would you treat the remaining 'corners' - with free-standing angled acoustic panels cut to size?
Thanks.
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Post by Ethan Winer on Jun 22, 2016 16:46:01 GMT
It's a myth that a room must be large enough to "contain" a long wave, or long enough for bass waves to "develop" to be heard. If that was true, closed-back headphones would have no output below 1 KHz.  But large rooms have other benefits, such as better and closer mode spacing. A back wall made of thin glass will pass the lowest frequencies to the outside, but then you won't have a place to put the bass traps you need for higher bass frequencies. And thin glass lets heat and cold, and sound, come through. So that's one argument against having a window in the rear of the room. Maybe John Brandt has a better suggestion. You can get sunlight and let the lowest bass escape from a window in the center of the wall, but make the perimeter drywall to hold bass traps in all four of those corners. --Ethan |
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dm
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Post by dm on Jun 23, 2016 16:10:20 GMT
Ah, I didn't mean have an actual window (with glass) at the back - I meant would having a partial aperture, say, taking up 60% of the footprint of the back wall, give all the benefits of removing 100% of the back wall? No window at all - just a bay-window SHAPE.
Let's say my back wall is 5.3 meters from the front of the CR. And let's say I have the option of breaking down that back wall completely and rebuilding it another meter further on to give a length of 6.3 meters. But if instead, I punch through through a large part of the 5.3 meter wall and build a bay extending 1 meter, I now have part of the back wall (where I punched through) 6.3 meters deep, and the non-punched-through perimeter remaining at 5.3 meters deep.
How would this affect bass in the room? Would the low-end frequencies down to 30 hz be sufficiently handled by the bay, or would they reflect back off the perimeter around the bay? I assume that if the bay took up 90% of the footprint of the wall, the effect of the perimeter bounce back would be negligible - whereas if the bay took up just 30% of the footprint, the perimeter around the bay would be so large that the bay wouldn't achieve that much.
But is that the case - and at what % footprint would the bay be very effective? And how to treat the perimeter so as to minimize the bounce back? |
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Post by Ethan Winer on Jun 24, 2016 18:20:36 GMT
A "stepped" rear wall helps some because it splits the reflection times between the two room depths. So instead of one big null at 80 Hz you have slightly smaller nulls at 80 Hz and 60 Hz, for example. But this doesn't reduce the strength of the reflections or avoid the need for bass traps. If this were my room I'd probably just make it longer overall, and add thick bass traps on the entire rear wall.
--Ethan
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But large rooms have other benefits, such as better and closer mode spacing.