dipoles in small rooms

[noodle_snacks]

what are dipoles like in smaller rooms placed relatively close to the walls (within 600mm), could acoustic treatment help?

[sdclc126]

If you haven't already, see Linkwitz's site about room placement for dipoles - I think you need a minimum of 2 ft from side walls and more like 4 feet from the rear - so 600mm might be too tight.

You need a minimum delay for the rear wave to get the dipole effect, otherwise the front and rear waves tend to cancel eachother. I doubt any kind of sound treatments for the room would help; indeed if you're talking about absorbing reflections that would really negate dipole altogether, as they need rear reflection to work properly.

Linkwitz's Pluto is designed for smaller rooms - not dipole but omni has similar characteristics - so perhaps consider something like that.

[noodle_snacks]

From: http://www.linkwitzlab.com/rooms.htm refering to dipoles

"The only difference is in the frequencies for which addition and subtraction occur. The best remedy is to move the speaker away from the wall, or to make the wall as sound absorptive or diffusive as possible. (FAQ31)"

In my room side wall distance is not a problem either side, but rear wall distance is. Do you think i'd be better off with a monopole design (ala pluto) or trying a dipole with diffusion or adsorbtion on the rear wall (i can have as much sound treatment on the wall as i like)

You need a minimum delay for the rear wave to get the dipole effect, otherwise the front and rear waves tend to cancel eachother. I doubt any kind of sound treatments for the room would help; indeed if you're talking about absorbing reflections that would really negate dipole altogether, as they need rear reflection to work properly.

I am not quite sure i understand this, i thought the radiation of a dipole was primarily related to angle and reflections from the rear wall combining with the direct sound to cause cancelations and peaks etc or something to that effect. Would someone be able to clarify my understanding?

[Tenson]

You need a bit of space around the baffle because you want the waves to go round the side of it and cannel each other out so you get less side wall reflections. For this you don't need the back wave to reflect from the back wall though, you just need to give it room to breath.

If you place the speaker 2 ft from any wall that should be enough.

You can then absorb/diffuse as much as possible on the back wall. This will reduce the comb filtering, but at the same time one of the nice things about di-pole - its spaciousness. This is caused by the back wave arriving slightly later than the front wave and giving your brain spatial cues.

So yes it can work, but it won't sound a airy as in a larger room with a purely diffusive wall behind them.

What I am going to do in my room is place diffusers along the back wall and then on top of that place a layer 2" rockwool. This will absorb a lot of the sound (especially with the diffuser making a gap behind the rockwool) and then the sound that is not absorbed will be diffused. The absorption will also broaden the Q of the diffuser which means it will be slightly less effective, but work over a larger range. Thats why I would not put more than one 2" layer of rockwool in front, and I mean in front, don’t shove it into the gaps in the diffuser. I’d suggest a QRD over a 3D diffuser like the Skyline.

[sdclc126]

"I thought the radiation of a dipole was primarily related to angle and reflections from the rear wall combining with the direct sound to cause cancelations and peaks etc or something to that effect. Would someone be able to clarify my understanding?"

You don't want the front and rear waves to cancel eachother - you want them to complement eachother. You actually are in effect creating an echo with dipole - the rear wave arrives at your ears at just enough of a delay from the front wave to create a 3-dimensional spaciousness to the music, more accurately replicating the original source, i.e. band, orchestra etc.

You need a minimum amount of space from the rear wall to create this effect. Perhaps two feet will work, but more is probably better, and will lessen the need for wall treatments.

Hey bottom line if you want to try it go for it - you might end up having to do a lot of experiemntation with diffusion/absorption, and it may be a lot of trial and error if you don't have any measuring equipment or software, but you just may end up being happy with the end result. It just might be a lot simpler doing something like the Pluto though.

[Tenson]

Originally posted by sdclc126
more accurately replicating the original source, i.e. band, orchestra etc.

You need a minimum amount of space from the rear wall to create this effect.

More accurate? Hardly true, but arguably sounds nicer!

If you want your mind to be able to tell the difference between the front wave and the back wave you need to have a delay between them of about 20ms. That would equate to them being a distance of just over 3m from the back wall.

Diffusion that works to a low frequency (say 500Hz) on the back wall will help tremendously. You could likely get away with 0.5m - 1m then as long as your room is no smaller than about 12ft x 12ft and doesn't have a low ceiling (not a good ratio I know). Some broadband absorption in the 4 corners of the room would not go amiss either in helping to control the frequencies below the operational range of the diffusers.

[sdclc126]

Tenson, with all due respect you just sound like you're trying to sell your products to this guy.

I'm speaking from what I've read on Linkwitz's site, that's all.

I'm not an expert but I am quoting one.

[Tenson]

I don’t actually see where we have disagreed particularly? We both say you need a reasonable distance behind them and I agree with Linkwitz that if space is tight, you need to absorb and/or diffuse as much as possible at the back wall. I'm not trying to sell anything or even undermine what you said; I'm just picking up on some things that I thought could be clarified.

The simple fact is, ones mind needs to have a certain amount of time between different sounds to tell them apart. That time is around 20ms (a commonly accepted figure). That means they need to be 3.4m from the wall at the back. You can probably get away with a bit less and like this no treatments would be needed.

If you then add diffusion the path the sound takes will not be a direct reflection from the wall anymore, it will go all around the room in many many different paths taking far longer to reach you. Thus they can be placed closer to the wall. As the sound is not reflecting straight back at the speaker you also won't get such surveer comb filtering compared with if no diffusion was in place.

The comment about di-poles being more accurate is simply not true because any information about the room the band played in will be on the recording (if intended) so adding your on rooms sonic signature is hardly more accurate. If you want to claim that in close mic'd recordings there is no venue on the recording then fine, but it is still not more 'accurate' as the fact is, they didn't play in a live room.

[Rudolf]

noodle_snacks,

to complicate things even further:
You have to to differentiate whether you are talking about a dipole or an open baffle. Both need not be the same!

For a dipole OB to work you want some distance to the back wall to allow for a real figure 8 radiation characteristic. If you put the open baffle too close to the wall, it will loose that figure 8, but still lack boxiness.

Another point of view that doesn´t relate to dipoles only, is the effect of front wall reflections: If the distance between OB and wall is less than 1 ft, the "echo" of the front wall reflection will integrate with the original sound as one singular acoustic event to the ear.
If the distance is 3 ft and more the "echo" will be noticed as a separate event, independent from the original sound, and interpreted as room related reflection.

Distances between 1 and 3 ft should be avoided since they will kind of "smear" the original sound with its echo.

So if you lean a very narrow but high OB against the wall, you don´t have a dipole radiation, but still a boxless sound. You do need some assistance in the bass due to the narrow baffle, but you will be free from the strong resonance peak a wide baffle will excite.

This picture may give you a hint, what I´m talking about:
http://img508.imageshack.us/my.php?i...41000229pk.jpg

[Tenson]

Those look really nice, especially in that room Very minimalist. What are the drivers?

I can't help feeling people are underestimating how much distance is needed behind a open baffle for the mind to truly differentiate between the front and reflected rear wave (if it is a direct reflection). I'd say 3ft is an absolute minimum if no diffusion is used. I'm not saying it won't still sound good, just that it will still be 'smeared' in comparison to say 6ft distance with no treatments.