I have been experimenting with a small open baffle for midrange and treble duties. I tried putting several layers of absorbent (dusters, felt, whatever) at the back of the driver. What I found was that the measured response was flatter, and the sound to my ears was more focused and cleaner. Also the stereo imaging was better. Has anyone else tried this. Apologies if this is a repeat thread.
Sounds to me like you might have the speaker too close to the rear wall. I've played with spacing all over the place and that's the only way I could ever think of the rear wave being a negative.
Too me it's the opposite...I think it adds a lot to recreate a live sounding reproduction of recorded event. Realism.
Another possibility could be how live the room is. Maybe your needing some softer properties.
EV3
Too me it's the opposite...I think it adds a lot to recreate a live sounding reproduction of recorded event. Realism.
Another possibility could be how live the room is. Maybe your needing some softer properties.
EV3
This is one of those 'tweaks' that 'if you don't learn from history you're doomed to repeat it'. 😉
The earliest 'closed' box speakers were large open baffles that had been folded into horns with an open back box [filter chamber] ~filled with insulation to try and mimic the large baffle's tonal 'signature' except with more gain over a narrow BW.
Shortly thereafter, just the damped filter chamber was used to make compact systems, which in turn was pretty quickly developed into the bass reflex, allowing minimal damping and an extra half octave of ~ flat LF extension.
Through it all, conventional wisdom has dictated that OBs and even large IBs be damped in this manner for best overall performance, but statistically, none bother anymore, so thanks for the reminder!
GM
The earliest 'closed' box speakers were large open baffles that had been folded into horns with an open back box [filter chamber] ~filled with insulation to try and mimic the large baffle's tonal 'signature' except with more gain over a narrow BW.
Shortly thereafter, just the damped filter chamber was used to make compact systems, which in turn was pretty quickly developed into the bass reflex, allowing minimal damping and an extra half octave of ~ flat LF extension.
Through it all, conventional wisdom has dictated that OBs and even large IBs be damped in this manner for best overall performance, but statistically, none bother anymore, so thanks for the reminder!
GM
Done right, which means a lot of tweaking to get the speaker and the room voiced together some attenuation combined with speaker and listener placement can really make OB, Dipole, Bipole, systems vividly real. This can include both some absorbing directly behind the driver and various absorption and reflection treatment on the room walls. It can certainly help if you have a few select recordings where you know the live sound signature. Get it right and you will have that special satisfaction of seeing your friends jaw drop open when they hear what's possible. And NO you don't have to spend a fortune to get there.
That "gray stuff with rubber" I suppose is the same thing that I've found inside
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I have been down this path too and have found that an open box design with some absorbing material behind the driver is the optimal to my ears. Without absorbing material it doesn't sound right and with the box closed it also doesn't sound right (although it was originally designed as a closed box).
I have been down this path too and have found that an open box design with some absorbing material behind the driver is the optimal to my ears. Without absorbing material it doesn't sound right and with the box closed it also doesn't sound right (although it was originally designed as a closed box).
Bigun, have just waded through your Audio Nirvana Super 15 speaker design thread. Quite a journey!.
I remember trying a small open box with a midrange driver, and some stuffing in the box, and a felt back panel. Don't know why I did not pursue the design; it sounded surprisingly good. I lack your staying power.
I remember trying a small open box with a midrange driver, and some stuffing in the box, and a felt back panel. Don't know why I did not pursue the design; it sounded surprisingly good. I lack your staying power.
The basic concept had a pretty nice write up back in ------ wait for it ------Gramophone Magazine March 1935!
The Hartley-Turner Boffle
The Hartley-Turner boffle sets out to absorb as much as possible of the sound produced from the back of the speaker and at the same time to prevent the wood of the box from being set into vibration.
It succeeds in both objects. At regular intervals across the inside of the boffie sheets of sound-absorbing material are stretched. In the centre of each sheet a hole is cut, the holes decreasing in size from the back of the speaker to the back of the box, the air column thus formed being approximately exponential. There is thus the equivalent of an exponential air column behind the horn (which itself is useful in imposing an acoustic load on the cone), and in addition a series of air cavities between the absorbent sheets to act as a sound absorbing wave-filter.
So successful is the arrangement that with the speaker working at large volume only very faint sounds, and those of very low pitch, can be heard coming from the back of the speaker even when the ear is placed within a few inches of the back of the boffle ; whilst to the touch the boffle itself is as dead as the proverbial mutton. All within an 18-inch cube, mind you.
We understand that other sizes of boifie are available. One measures 18 in. by 18 in. by 12 in. and costs £2. Another measures 21-inch cube and costs £3 15s.
There can be no doubt that this boffle is a substantial contribution to loudspeaker design and to the cause of undefiled reproduction generally.
The Hartley-Turner Boffle
The Hartley-Turner boffle sets out to absorb as much as possible of the sound produced from the back of the speaker and at the same time to prevent the wood of the box from being set into vibration.
It succeeds in both objects. At regular intervals across the inside of the boffie sheets of sound-absorbing material are stretched. In the centre of each sheet a hole is cut, the holes decreasing in size from the back of the speaker to the back of the box, the air column thus formed being approximately exponential. There is thus the equivalent of an exponential air column behind the horn (which itself is useful in imposing an acoustic load on the cone), and in addition a series of air cavities between the absorbent sheets to act as a sound absorbing wave-filter.
So successful is the arrangement that with the speaker working at large volume only very faint sounds, and those of very low pitch, can be heard coming from the back of the speaker even when the ear is placed within a few inches of the back of the boffle ; whilst to the touch the boffle itself is as dead as the proverbial mutton. All within an 18-inch cube, mind you.
We understand that other sizes of boifie are available. One measures 18 in. by 18 in. by 12 in. and costs £2. Another measures 21-inch cube and costs £3 15s.
There can be no doubt that this boffle is a substantial contribution to loudspeaker design and to the cause of undefiled reproduction generally.
While the original BOFFLE was designed to simulate an infinite baffle, my own experience is getting the rear wave attenuated by 15dB or so does the trick and keeps some of that rear reflection ambience like a concert hall. Of course your room your set up might require the proverbial tweaking. And aren't we all at DIY addicted to the tweaks! 🙂
My Quad ESL has a rear attenuation: a half inch pad of loose horse hair (i think it is that).
This sounds much better than some american panels that are curved even so much that at the back at about two feet there is a focal node of high frequencies ging rise to very unwanted reflection.
So now I'm slightly confused. In another thread I thought that one of the positive motivations for open baffle was the dipole characteristic - reducing reflections off the floor and ceiling etc.
Presumably if you absorb some of the rear radiation you lose that, and the speaker more resembles a conventional box with a wide baffle..? So is the motivation really to have all the directionality characteristics of infinite baffle without any of the disadvantages?
Presumably if you absorb some of the rear radiation you lose that, and the speaker more resembles a conventional box with a wide baffle..? So is the motivation really to have all the directionality characteristics of infinite baffle without any of the disadvantages?
^Most open baffles have a very narrow band of true dipole 8 radiation (usually below 500Hz depending on baffle narrowest diameter) - so by attenuating rear radiation you don't loose much!
Those who have measuring gear can most easily see these changes in EDT and decay waterfall/csd graphics. So, actually we are altering delayed reflections' energy/spl.
Those who have measuring gear can most easily see these changes in EDT and decay waterfall/csd graphics. So, actually we are altering delayed reflections' energy/spl.
Most people build "dipoles" with much to large baffles. This leads to strong reflections to the sides (30-60° preferably). It would be logical to build narrower baffles to avoid that and get a "true" dipole radiation pattern. The Linkwitz LX 521 and Kreskovkys Nao Note II follow that road and are applauded for their very good rendition.
Why do people instead try to loose that figure 8 pattern by attenuating the rear radiation?
I would suspect it's usually because of insufficient control of front wall reflection. True dipoles usually end up sounding too bright. And, realistically, because it is somewhere between impractical and impossible to get a true dipole pattern above 2-3 kHz with cone drivers anyway.
But that requires putting extra components in your signal to deal with baffle step. I prefer not doing that myself as I think it colors the real signal. Of course I've seen the crossover for Linkwitz's design, It looked like the mother board out of my Marantz surround sound receiver.
I think they attenuate it for the same reason I tried to, so it would give the clarity of an infinite baffle. But as they soon find out, it also cuts the amount of sound you get from your speakers in half.
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If I remember correctly both Linkwitz and Kreskovsky have stated for their latest dipoles (LX521 and Note II RS) that using the rear tweeter (or not) is more a decision of personal taste than an acoustical necessity. We should be aware that both loudspeakers have finally overcome the "tweeter bloom" problem of previous dome tweeter solutions by using their dome tweeters only above the blooming region.
I have done my share of polar measurements with cone, dome and planar drivers from 7" to 1" diameter. In wide baffles, narrow ones and no ones at all. I have seen how the rearward response of cone drivers rolls off in a quite predictable way. And I have seen how (too) large baffles inevitably increase the radiation to 30°-60° off axis to a much larger extent (with respect to the energy distributed into the room) than any attenuation of the rearward radiation alone can compensate.
Compromising the dipole 8 pattern by attenuating the radiation from the rear side of a driver is clearly the wrong way to get rid of bright treble in open baffles. Cutting off excessive baffle material is the way dipole physics demand imho.
Rudolf
What is really "coloring the signal"?
We would consider an amplifier "just good enough" if it has less than 0,1 % THD. We would consider a driver good enough if it delivers less than 1% THD. Are you aware that a bad polar pattern in combination with the room influence is easily responsible for 10-100% of THD?
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