Diffusors behind Bose 901's?

jdmccall

Super Member
I just put my 901's back into use after about a five month hiatus. Sound is glorious, but...I am having some issues with hot-spotting, or beaming of highs. What happens is this: Sitting in the sweet spot everything may be hunky-dory, but if I move my head left or right a couple of feet or lean forward a couple, it's like somebody just cranked the treble up 6-9 dB. This is only really a problem on some recordings that happen to have a lot of energy in a specific higher frequency band. Joan Baez' upper register and hotly recorded cymbals, for example. I've tried pulling the speakers out further from the front wall, which has helped, but the problem still persists. I've also moved the listening seat a few inches further from the speakers, but I can only go a few inches more. Likewise, I can only pull the speakers about three more inches from the front wall. They're set up on the long wall and I'm running out of real estate. So I'm wondering if diffusor panels behind the speakers would help. Thoughts?
 
Possibly...but there could be other acoustic artifacts in the room creating the boost in HF you're hearing. I've used diffusers behind dipole speakers and have been happy with the results they had on sound stage and image resolution, but never used them to deal with a problem such as what you're describing. I've seen many claim they've achieved some noticeably positive results using a large plastic plant behind a dipole speaker to help scatter the rear wave. Might be cheaper and/or easier to try that rather than buying diffusers.
 
Check speaker wiring. All the symptoms of speakers out of phase.

Cheers,

David
Interesting. I've heard bass drop out in out of phase speakers.
Could be the imbalance just makes the high frequencies seem louder?
I can see where the high frequencies could be boosted (or cut) with comb filtering, relative to the other frequencies, so maybe I see where you're going with this.
 
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Bass and imaging are just fine so I'm sure speaker phasing is correct. The problem seems to be with the frequencies, above which, the speaker's 4.5" drivers start to beam, which I believe works out to around 3kHz. Correct me if I'm wrong.

The plant idea sounds promising. I don't know why I didn't think of it! Right now I only have 15" between the back of the speaker cabinet and the front wall. 18" is about the most I can get, but I'll look for something that will fit and not look too stupid. Of course the back driver arrays are angled away from each other, so I may need two plants behind each speaker. Bose 901's can present some interesting challenges as far as set-up goes, that's for sure. But the payoff can make it all worthwhile, IMHO, of course! :)

Thanks for the ideas!
 
I never understood imaging with this speaker. You have one direct radiator and eight whose purpose is to bounce sound all around the room. This always seemed counter-productive to me. You wouldn't think high frequencies would be as prone to peaks and valleys as bass frequencies. Good luck in your search of the problem. Sounds like a big task.
 
I have found the bass to be the easy part of 901 set-up. Since they are usually placed away from room boundaries, room resonances tend to not be excited. But I've had this high frequency beaming problem in this room before. That's partly why I moved them into my family room for a while. That and just to hear how they sounded in there. But, I like the Klipsch RF-82's better in that room, so the 901's are back in the man-cave. They sound great in the man-cave but there is this persistant beaming issue. Getting the speakers as far from the front walls as possible is helping, and I really think tomlinmgt's decorative plant idea might finish taking care of the problem. I'm gonna try a couple and see how they work. I might add another couple later. Fun!
 
Suspending 901s from the ceiling as described in the owner's manual will cover a multitude of sins. Once I did that to mine, all the issues I had with them (well, most issues!) were solved. The reasons are obvious.

GeeDeeEmm
 
Suspending 901s from the ceiling as described in the owner's manual will cover a multitude of sins. Once I did that to mine, all the issues I had with them (well, most issues!) were solved. The reasons are obvious.

GeeDeeEmm
I believe you speak truth! I had'em hanging from the ceiling in the same room a few years back in a short-wall set-up. No beaming issues; probably just because my ears were out of the line of fire! I just decided this time I didn't want to mess with that. I have them on 22" stands which may actually make the beaming worse. I always thought the 18" off the floor Bose recommendation was for bass, but maybe not. Maybe it's more about keeping your ears out of the kill zone.
 
Exactly! Everything you wrote is the key to getting the best from the Bose. Being directly in front of the front-firing driver puts the listener right in the line of fire, with no time or room for the front waves to incorporate with the "direct reflecting" waves from the back. Practically everything the 901 does is intended to be off axis. Following your own keen observations will solve the problem.

If one does not have the exact mounting circumstances dictated by Bose, the best use for them is turning them around backward and using them like a conventional speaker. Don't laugh. The 802 is one of their most successful pro sound speakers, and that's exactly how it works, minus the lone ranger driver.

Good luck, and happy listening, my friend!

GeeDeeEmm
 
Every time I see a picture of backwards 901's, I cringe. They may sound great but that ain't how they're designed to work! Of course, my fully developed OCD may be in play here. :crazy: I like the 802's though, and was hot for about 5 minutes to get a pair to dj with...accuracy check...I have never actually DJ'd, but I have provided music for events. You could call me the silent DJ.

Back to my beamy 901's...I just got done re-doing my room with them and putting a new NAD amp in the cabinet to power them with, so I'm not too keen on the thought of re-doing the re-do. I probably should have strung'em up to begin with and I would not have this issue. I'd be sharing pics! But I'm lazy and then there's that OCD thing. I resist temptations to do anything requiring putting holes in walls and ceilings because in six months I want to change something again and I'm left with the holes to deal with. But in this case I may have to make an exception. In the meantime, I wonder where I might pick up some cheap plastic plants? :)
 
I resist temptations to do anything requiring putting holes in walls and ceilings because in six months I want to change something again and I'm left with the holes to deal with. But in this case I may have to make an exception. :)

Ha! Well, you know good and well that the nagging OCD will eventually force you to do this the right way, JD. Remember, holes can be plugged. But, then, you'd know in your heart that despite the looks, there is a hole there. What to do . . . what to do . . . . :dunno:

Try this: change the phase on the front speakers. Just reverse the terminals. If it doesn't help, change it back.

Or, how 'bout this: since the problem is coming out the front of the 901s, diffusers on the rear are not going to help. So, go the route Bose used on some of its other speakers and place a diffuser on the front speaker. Seen those car vents that rotate and have adjustable vents? Adapt one of those to the front speaker. (Won't be seen.) Then experiment.

(You know what? I shouldn't have told you this. Somebody is now going to develop an adjustable diffuser for 901s, patent it, and sell reams of them. Doesn't even matter if it works. Audio guys are willing to buy anything simply on the promise that revolutions in audio are at hand. Ask Monster cable!)

I'm quite sure that there is a way to solve this problem. Just requires a little imagination and some tinkering.

GeeDeeEmm
 
I started doubting my advice about changing the phasing of the front driver based on my 901 IIs, as changing the phase would throw off the cabinet tuning, but then remembered that somewhere along the line the front driver picked up its own separately-tuned chamber. Changing the front driver phasing wouldn't affect the other drivers at all. It may be worth the try.

GeeDeeEmm

Bose 901 mk1-3.jpg
 
Me no wants to tinker with individual driver phasing. I've had my current 901's seven years and I've never even pulled the back grilles off, if that gives you any idea about how likely I am to tear into the innards of a perfectly functioning speaker! I'm sure the problem is from the reflected highs from the inside four drivers of each speaker. What to do about it is the question. I may try the plant idea and I may still hang'em from the ceiling...eventually. I think I'll scrounge around the nooks and crannies to see if I can turn up something to experiment with diffusion of that back wave before I actually spend money on a bunch of fake plants, though.
 
The 901 is not something to practice "sweet spot" listening with. Dr. Bose designed it specifically to kill off the sweet spot. The fact that you have found a sweet spot and good imaging is purely an anachronism. They were never intended to do either of these things. Since the design uses the walls to distribute most of the sound, you will get an enormous amount of "comb filtering" effects which sound exactly like the problem you are experiencing.

Putting diffusers on the walls might help to subdue any direct sound, but you have to remember what the 901 was for, and that was for Dr. Bose to settle a bet with himself about the viability of diffuse sound, especially to give a more predictable sound in a room with bad acoustics. Unfortunately, his idea didn't quite work the way he intended, but I suppose was an improvement.

FWIW, great speakers of fairly conventional layout can beam too, when placed in an acoustically unfriendly room. I had this problem, and since I have hung acoustic traps in the room, the "beaming" has reduced considerably.

Here is a sample. Note that although I move the camera (and microphone), the response of the room changes very little, except for when I move the camera under the desk and in front of the Preamp, and the tweeter's line-of-sight is broken.



You could do the same, but since the 901 relies on reflection to do its thing, hanging traps would be like shooting yourself in the foot.

You may be looking at getting a different pair of speakers, and maybe a little room treatment, because I don't think you are going to get what you seek with 901's.
 
No replacement for my 901's is being sought! I love'em, man! :biggrin: I listened to Rosanne Cash's first two albums on them today and the sound was glorious. There was a little hot-spotting that was noticable if I leaned way to the left or right from my perch in the sweet zone (not spot!:)), but it wasn't bad. The worst offender I've heard so far was an old Joan Baez LP. Mercy, if I did the left/right lean with that rekkid, I was in danger of getting my ears singed. :eek: So it seems to be "progam dependant", as they say.

I have to disagree with you about the 901, as far as "sweet spot" listening goes. It is still 2-channel stereo, after all. There is ideally, always gonna be a sweet spot. It's just a question of how big it can be and maintain some semblance of life-like imaging. I don't know for sure but I suspect old Amar was seeking to eliminate the head-in-a-vice type sweet spot, and instead have a much larger sweet "zone" (see above:D).

Really just out of curiosity, I placed a 2x2 absorbant panel on the wall behind the inside rear drivers of both speakers. Whoa, that really dulled things down, just like I expected. Lesson learned: The wall immediately behind the speakers (901's, that is) has an immense effect on the overall sound. I think some diffusion in that location, at least above 3 kHz, is what I need to try. Most diffusion devices on the market cost way more than I want to spend, so it looks like DIY, or go the the fake plant route. Nice fake plants aren't too cheap either, for that matter. :(
 
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