The scenario for this hypothetical question is as follows:
You want to completely eliminate Driver's back wave. What would you do?
A.
Build a speaker enclosure out of a very very long pipe and ask your neighbors to kindly leave the window open.
B.
Build a very elaborate Nautilus construction. Lawrence Dickie's style
C.
Use an overgrown DIY Gun Suppressor.
D.
Install passive Radiators behind the The Active Driver.
E. Go Planar 😉
You want to completely eliminate Driver's back wave. What would you do?
A.
Build a speaker enclosure out of a very very long pipe and ask your neighbors to kindly leave the window open.
B.
Build a very elaborate Nautilus construction. Lawrence Dickie's style
C.
Use an overgrown DIY Gun Suppressor.
D.
Install passive Radiators behind the The Active Driver.
E. Go Planar 😉
F. Absorber ( my preference is "ultra denim" in US / 'absorbant Metiss' in France/EU)
Noam, you need a context for the kind of load the box will see ( BR, sealed,...) or answers won't hold true. Of course i'm talking sealed and mid/high intended freq range.
Shape of cavity could play a role too imho.
For BR a (thinner) sheet of same kind of material hanging in the middle of the cabinet is what Technics used in my SB-M2 ( BR loaded). It's effective. Simple but effective. 😉
Noam, you need a context for the kind of load the box will see ( BR, sealed,...) or answers won't hold true. Of course i'm talking sealed and mid/high intended freq range.
Shape of cavity could play a role too imho.
For BR a (thinner) sheet of same kind of material hanging in the middle of the cabinet is what Technics used in my SB-M2 ( BR loaded). It's effective. Simple but effective. 😉
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Thank you Krivium for pointing this material which is at least in France easily available and cheap.
Here is a link (found in quick search) to a video of an acoustic panel use : DIY panel with Metisse coton wool ... video in french but quite self explaining.
Do you know if this kind of solution is used for open back speakers in the idea to reduce the reflection effect in case of a too close wall? It is something I am thinking about for DML.
Christian
G - use front radiation to kill the back radiation, passive cardioid. Needs some damping and tuning the construct, and perhaps additional bass driver for the reduced lows
H - nothing, open baffle is quite logical solution for all kinds of speaker problems, but perhaps not suitable for all applications such as on-wall positioning
H - nothing, open baffle is quite logical solution for all kinds of speaker problems, but perhaps not suitable for all applications such as on-wall positioning
how far do you get just filling the rear chamber with absorption? If the chamber is deep enough, that should work.
B and C, I have used " ladders " and I have used layers of different types of foam. I've also used perforated mdf baffles. Eliminating the backwave ( as best as possible ) is a bit if an obsession of mine, on some very early speakers I made I isolated the driver vibration from the box ( possibly another obsession... ) and was shocked at how clear the effect of refections was. My latest speakers will have a Christmas tree diffuser behind a baffle.
Or conversely shallow enough, too shallow for a wave to form and bounce back at the highest frequency of the bas/mid driver, possibly an overlooked aspect of the BBC Ls35a.
Suppressing the reflected (HF) back waves of such things like a woofer is relatively easy, and has been done in vaious ways for decades.
Sealed, air-suspension designs, or ported - a generous yet thoughtfully added addition of acousticlly absorbent padding//stuffing.
I use polyester pillow filling and it works just fine.
Or a wool based sheet glued to the sides and rear.
Sealed, air-suspension designs, or ported - a generous yet thoughtfully added addition of acousticlly absorbent padding//stuffing.
I use polyester pillow filling and it works just fine.
Or a wool based sheet glued to the sides and rear.
I have a pair of Dahlquist speakers that work nicely, and use a ported rear chamber that is divided using a panel of open cell foam, creating something like an aperiodic TL.
Other speakers have benefited from being almost completely filled with an open cell foam, it depends on the response what a certain driver needs. Another speaker here only performed well when the originally supplied scant amounts of filler had been left in place.
Have been able to tame bass peaks by using loosely rolled and taped bubble wrap on many instances. Another one (that also works good inside car doors) is the closed cell acoustic foam meant to take care of frequencies centered around 5khz, placed behind the cone.
Other speakers have benefited from being almost completely filled with an open cell foam, it depends on the response what a certain driver needs. Another speaker here only performed well when the originally supplied scant amounts of filler had been left in place.
Have been able to tame bass peaks by using loosely rolled and taped bubble wrap on many instances. Another one (that also works good inside car doors) is the closed cell acoustic foam meant to take care of frequencies centered around 5khz, placed behind the cone.
Hi Christian,
I have almost zero experience with OB speakers so i can not tell ( heard planar a few times but in 'big rooms' and they were located far away from wall -esl57 and some Martin Logan, OB a few times too but it is not my thing).
Absorber material like this are often used for acoustic treatments but they are 'bandwidth limited': you can expect them to work on mid/high when put right against a wall ( eg: rockfon is ok from 1k and up) and if you want to increase efficiency in low end you have to create a gap/plenum between wall and absorber.
Iow it could work but don't expect miracles especially below 1khz if you cannot space them enough from walls.
If i were you i would look at diffusor instead: my take on OB/dipole is you rely on the backwave to give an 'effect' (note to readers: i'm not interested into a war about dipole rendering from users) so i would not try to lessen this effect, rather increase it and diffusor can help. That said their effectiveness is linked to the distance to the listener so the efficient bandwith they have ( lower freq at which they diffuse) have to be estimated beforehand.
The issue in small room is early reflections: if they are too short ( small room) they'll invariably 'color' the message. In the case of dipole the backwave is out of phase so maybe it is less objectionable but still you'll face comb filtering from it. I fear there is not many things to do about it and to me this kind of loudspeakers should be used in big rooms. But i might be wrong, my experience is limited with them...
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Yamaha later approach (ns5000) have to be mentioned too: they implement multi tuned helmotz resonators within the box.
A 45 degree panel within the cabinet behind the driver that reflects all the back wave down into a well damped fiber filled cabinet. Higher freq will go right through a driver but not through a wooden cabinet wall so if the back wave is guided away from the cone backside and absorbed in the rest of the cabinet it helps.
Perhaps a bit simplistic. At LF it will not be seen, at highest frequencies incidence angles are all over the place, so then to are reflections. Something more complex might be useful.
dave
dave
The highs frequencies come out of the tweeter which does not reflect behind the driver as it’s a monopole. Low frequencies are not the problem with reflected sound coming back through the cone, it’s the upper bass and mids that really are the problem. Most of the mid energy is reflected away from the come using the 45 degree inner reflector. Simple yet elegant in its result. It also breaks up standing waves in the cabinet at no extra charge!