OB speakers and room acoustics

jimbones

Member
2004-11-08 5:18 pm
ny
Wasn't sure where to post this so if this needs to be moved i apologize in advance:

My speakers are a hybrid OB (sealed bass) OB mid tweeter. My room is rectangular 12x17ft I made the room reflective (or live) from the plane of the baffle back. In the seating area i made it a combination of diffuse and absorbtive. Does this sound correct? I think making the area behind the speaker absorbtive goes against the idea of an OB speaker. Suggestions appreciated.
 

jimbones

Member
2004-11-08 5:18 pm
ny
OK not sure if I was clear. From the baffle forward (into the listening area) It is a mix of absorbtive and diffuse. Details i did not provide are absorbtive panels at first reflection point (using mirror method) and diffuse on cieling and floor has carpet. Wall behind listener could be considered diffuse (book case :)

The reason I was not thinking absorbing sound behind speakers is because it would deaden the rear wave which is what we are after, no?
 

jimbones

Member
2004-11-08 5:18 pm
ny
distances

I also agree, more or less. Diffusive behind a dipole if you've got lots of space back there (and I mean a lot, not like a foot and a half), or if you must have them close to wall behind, absorptive.

I should post a room layout;)
The speakers are at least 3 ft from front wall and about just under 2 ft from side walls. I have GIK wall diffusors I can use. I also make 1 1/2 inch thich absorbtive panels.
 

StigErik

Member
2008-02-21 12:30 pm
So Stig, if you are going to kill off all the rear radiation, what is advantage of an OB compared to a sealed woofer? Directivity in the lower octaves?

Directivity is one thing. An other is that the dipole is a velocity source instead of a pressure source; properly placed it will excite the room modes less. And lastly, you avoid the speaker box (cavity) with all it's inherent problems.

The price you pay for this is very low efficiency, that's maybe why dipole bass is not common in commercial speakers I guess.
 
So Stig, if you are going to kill off all the rear radiation, what is advantage of an OB compared to a sealed woofer? Directivity in the lower octaves?

If the rear wave of an OB is considered detrimental then why use OBs in the first place?

Better LF directivity. There is no other way to get LF directivity with only one speaker. If this is important or not is the issue.
 
An other is that the dipole is a velocity source instead of a pressure source; properly placed it will excite the room modes less.

A velocity source, yes, but less room mode excitation than a monopole, that is a common misconception, but it is incorrect. Put either at the their particular "best" locations and the room mode excitation will be about the same. This "best" location is different for the two types however.
 

adason

Member
Paid Member
2004-11-10 8:31 pm
Maryland
The former is achieveable with other methods too. As far as I know the latter has never been shown to be a significant factor.

markus76, box colorations were always a factor for me
when I heard good open baffle speakers (audio artistry beethoven), I heard no box, since then I never went back, no matter how well that box is made, there is still some resonant character to the sound for me, call it boxy sound if you will, I always hear it
 
The former is achieveable with other methods too. As far as I know the latter has never been shown to be a significant factor.

Quite correct on your second point and I have done these tests. box coloration is a fallacy unless the box is junk.

And dipoles are not, in general, constant directivity across a wide bandwidth unless the drivers are very small. Otherwise the directivity will narrow with frequency just as it does in a close box. You do get some bandwidth of constant directivity - at a fairly low DI - but it is not "very wide". These are the old "Dipoles are better" fallacies.

Dipoles have a small directivity advantage at LFs but a BIG efficiency disadvantage at these same frequencies.
 
markus76, box colorations were always a factor for me
when I heard good open baffle speakers (audio artistry beethoven), I heard no box, since then I never went back, no matter how well that box is made, there is still some resonant character to the sound for me, call it boxy sound if you will, I always hear it

I think the difference in the indirect sound is causing such a sensation not the box itself. A blind listening test would shed some light on the matter.