still resonating....after all those years!
Hi Panomaniac,
I agree with you.... not the good doctor!
The first time I heard an open baffle I knew that all sealed, ported, TL, etc had major cabinet resonance problems.
The really annoying thing was I had just spent two years and about 20 grand (the UK £ was really worth something back then!) developing a killer (OverKill !) box cabinet for my Encore speaker, which did win 2 best in show awards, Norway print Magazine "High Fidelity" and "6 Moons" on line magazine at its launch in the HiFi News show London 2004.
The bass cabinet was a 120Kg multi layered (33 layers of 22mm birch ply stacked and glued) affair with each and every layer individually CNC machined to give random asymmetry inside and a beautiful curved exterior. By any standard the 12 inch Beyma bass driver performed astonishingly well in this beast of a cabinet. The rep from Adam Hall, the UK beyma distributor, said he had never heard the driver sound as good.
But the real turning point came VISUALLY whilst watching the ripples on the Acrylic sides of a prototype open baffle speaker during a sine wave sweep to measure the room response.
Due to strong backlighting from floor to ceiling windows, bright sunlight and becasue the sides of the speaker were acrylic, all was revealed...! The acrylic was all very heavy 25mm (baffle, base and top) to 16mm (sides) but still it really rippled when the big 20Hz to 300Hz (that's the range the drivers in this cabinet were covering) pulses were sent through the drivers at 100dB SPL)
To eliminate air bourne vibrations I disconnected the cabinet from the amp blasted the sides with 100dB from the other cabinet and the side cheeks did not vibrate at all. This point of reference implies that more energy is mechanically transmitted to the baffle / cabinet than is acoustically output from the drivers.
This experiment motivated me to develop the VPL open baffle "Angel" speaker.
So whilst you can mute most box cabinet resonance if you use a lot of mass, bracing, damping and a vary the types of wood and materials , this costs a lot of time and money, results in a big heavy box...
Now the real box killer... despite our best efforts, the internal sound energy seeks the path of least resistance and bounces back out the thin paper cone!
Oh and if anyone actually believes that the inside of a box speaker cabinet is some how (due to lower impedance...?) a calm quiet environment, just drill a small whole in the back and pop in the tip of your....SPL meter!!!!!
If I was still selling speakers commercially they would not be box designs....!
All the best
Derek.
Hi Panomaniac,
I agree with you.... not the good doctor!
The first time I heard an open baffle I knew that all sealed, ported, TL, etc had major cabinet resonance problems.
The really annoying thing was I had just spent two years and about 20 grand (the UK £ was really worth something back then!) developing a killer (OverKill !) box cabinet for my Encore speaker, which did win 2 best in show awards, Norway print Magazine "High Fidelity" and "6 Moons" on line magazine at its launch in the HiFi News show London 2004.
The bass cabinet was a 120Kg multi layered (33 layers of 22mm birch ply stacked and glued) affair with each and every layer individually CNC machined to give random asymmetry inside and a beautiful curved exterior. By any standard the 12 inch Beyma bass driver performed astonishingly well in this beast of a cabinet. The rep from Adam Hall, the UK beyma distributor, said he had never heard the driver sound as good.
But the real turning point came VISUALLY whilst watching the ripples on the Acrylic sides of a prototype open baffle speaker during a sine wave sweep to measure the room response.
Due to strong backlighting from floor to ceiling windows, bright sunlight and becasue the sides of the speaker were acrylic, all was revealed...! The acrylic was all very heavy 25mm (baffle, base and top) to 16mm (sides) but still it really rippled when the big 20Hz to 300Hz (that's the range the drivers in this cabinet were covering) pulses were sent through the drivers at 100dB SPL)
To eliminate air bourne vibrations I disconnected the cabinet from the amp blasted the sides with 100dB from the other cabinet and the side cheeks did not vibrate at all. This point of reference implies that more energy is mechanically transmitted to the baffle / cabinet than is acoustically output from the drivers.
This experiment motivated me to develop the VPL open baffle "Angel" speaker.
So whilst you can mute most box cabinet resonance if you use a lot of mass, bracing, damping and a vary the types of wood and materials , this costs a lot of time and money, results in a big heavy box...
Now the real box killer... despite our best efforts, the internal sound energy seeks the path of least resistance and bounces back out the thin paper cone!
Oh and if anyone actually believes that the inside of a box speaker cabinet is some how (due to lower impedance...?) a calm quiet environment, just drill a small whole in the back and pop in the tip of your....SPL meter!!!!!
If I was still selling speakers commercially they would not be box designs....!
All the best
Derek.
Re: Cabinet colouration
Thanks for the link. I've been poking around looking for subjectively low-coloration broadband absorbers for some years now. I like Gary Pimm's discovery of Bonded Logic Ultratouch Cotton, which sounds very good and is also quite reasonably priced, but the Angel Hair looks interesting as well.
There were a series of experiments I did back at Audionics in the late Seventies. The measurement equipment was crude (Altec 1/3 octave real-time analyzer with 1/2" microphone), but the results were revealing. I measured with the microphone inside the cabinet, close to the cone (1 cm), at 1 meter, and 2 meters.
Inside the cabinet it was a horror of standing-wave resonances. Lining the walls with foam helped a little, but not as much as I expected. Filling helped a little more. A vented cabinet, contrary to expectation, was actually a bit worse than a sealed cabinet, since the vent modes interacted with the cabinet modes in a complex way.
It looked somewhat better on the outside measuring the nearfield of the cone, but the same patterns were evident. At 1 meter, the cabinet resonances were much harder to see, and at 2 meters, more so. But they were still audible once you knew what they sounded like. So how to measure them?
That's where the BBC chopped-sinewave method came in handy. Although the cabinet resonances were not visible on the 1/3 octave RTA, and barely visible on swept sinewave, they were evident when the chopped-sinewave cut off and appeared as stored energy in the tail. Today, this is the familiar MLS CSD display.
Overkill Audio said:
Thanks for the link. I've been poking around looking for subjectively low-coloration broadband absorbers for some years now. I like Gary Pimm's discovery of Bonded Logic Ultratouch Cotton, which sounds very good and is also quite reasonably priced, but the Angel Hair looks interesting as well.
There were a series of experiments I did back at Audionics in the late Seventies. The measurement equipment was crude (Altec 1/3 octave real-time analyzer with 1/2" microphone), but the results were revealing. I measured with the microphone inside the cabinet, close to the cone (1 cm), at 1 meter, and 2 meters.
Inside the cabinet it was a horror of standing-wave resonances. Lining the walls with foam helped a little, but not as much as I expected. Filling helped a little more. A vented cabinet, contrary to expectation, was actually a bit worse than a sealed cabinet, since the vent modes interacted with the cabinet modes in a complex way.
It looked somewhat better on the outside measuring the nearfield of the cone, but the same patterns were evident. At 1 meter, the cabinet resonances were much harder to see, and at 2 meters, more so. But they were still audible once you knew what they sounded like. So how to measure them?
That's where the BBC chopped-sinewave method came in handy. Although the cabinet resonances were not visible on the 1/3 octave RTA, and barely visible on swept sinewave, they were evident when the chopped-sinewave cut off and appeared as stored energy in the tail. Today, this is the familiar MLS CSD display.
Regarding cone transparency, what matters for the sake of discussion isn't transparency at 10 kHz, but in the 200 to 800 Hz region, where cabinet modes dominate. If paper is such a good absorber, why aren't speaker cabinets made of cardboard, or recording studios use paper walls, like Japanese shoji?
Re: still resonating....after all those years!
Hey OA.
Man, doesn't it ever! But the results can be very, very good. Almost no box talk at all. Hard to sell giant, heavy cabinets like that, tho...
Fist time I built good open baffle I was amazed too. Sounded so much like the giant, heavy boxes.
Uh, 'cause it's "transperant?"
I did once build horns off 100% cotton paper. Very stiff. Sounded great, very little coloration. Don't know if that was the paper's fault.
BTW, I went over to Gary P's last night to hear the new system. Very impressive!
Hey OA.
Overkill Audio said:
Man, doesn't it ever! But the results can be very, very good. Almost no box talk at all. Hard to sell giant, heavy cabinets like that, tho...
Fist time I built good open baffle I was amazed too. Sounded so much like the giant, heavy boxes.
Lynn Olson said:
Uh, 'cause it's "transperant?"
I did once build horns off 100% cotton paper. Very stiff. Sounded great, very little coloration. Don't know if that was the paper's fault.
BTW, I went over to Gary P's last night to hear the new system. Very impressive!
Re: still resonating....after all those years!
I agree with almost everything you say, except this portion. This would only be valid if you were using a method to perfectly couple all of the the airborne energy into the surface. There are other complexities as well. It's well known that the energy will largely travel around the obstacle rather than couple to it, there's an impedance mismatch.
BTW: The Bonded Logic website doesn't list all locations. My local Ganahl lumber carries it, but is not listed. I'm going to check it out tomorrow and probably buy a bag. I'll also be experimenting with bamboo batting. My hypothesis is that there would be less consistency with the bamboo fiber than with synthetics, and maybe act a little differently from cotton.
Overkill Audio said:
I agree with almost everything you say, except this portion. This would only be valid if you were using a method to perfectly couple all of the the airborne energy into the surface. There are other complexities as well. It's well known that the energy will largely travel around the obstacle rather than couple to it, there's an impedance mismatch.
BTW: The Bonded Logic website doesn't list all locations. My local Ganahl lumber carries it, but is not listed. I'm going to check it out tomorrow and probably buy a bag. I'll also be experimenting with bamboo batting. My hypothesis is that there would be less consistency with the bamboo fiber than with synthetics, and maybe act a little differently from cotton.
Good vibrations...
Hi Badman,
Good spot, I didn't think of that!
I suppose I could just increase the SPL of the "blasting cabinet" to compensate for lost sound energy.
What about bringing the "blasting" cabinet very close (50mm ish)
to the un-powered cabinet side?
Ah, then I couldn't see very much...!
I probably wont repeat the experiment but if anyone else is bored and wants to try a controlled experiment to put gather some accurate data it would be interesting.
Lynn : Also thanks for the ultratouch Cotton link.
Yes the 200Hz to 800Hz band is so easily damaged by slow decay resonance.
Fear not I have an instant solution to reduce this problem by 50% or more... Switch to an open baffle mounted Manger for all frequencies above 400 Hz!!!
Joking aside its a great option, especially if you use a shallow oval wave guide.
I have a test pair of WG that I got CNC milled out of two blocks of MDF and they bring the Manger up to nearly Edit :94dB Edit: sensitivity and add even more "easy dynamics" with effortless detail and a reach out and touch type of vocal realism.
I am going to start experimenting to see how deep I can make my wave guides before any sonic nasties start to become audible.
Second project how to double or triple the BL of the Manger.
The special twin voice coils are untouchable so I think it will all have to come from an increase in the magnetic force.
Any suggestions anyone?
Happy Easter to one and all!
Derek.
Hi Badman,
Good spot, I didn't think of that!
I suppose I could just increase the SPL of the "blasting cabinet" to compensate for lost sound energy.
What about bringing the "blasting" cabinet very close (50mm ish)
to the un-powered cabinet side?
Ah, then I couldn't see very much...!
I probably wont repeat the experiment but if anyone else is bored and wants to try a controlled experiment to put gather some accurate data it would be interesting.
Lynn : Also thanks for the ultratouch Cotton link.
Yes the 200Hz to 800Hz band is so easily damaged by slow decay resonance.
Fear not I have an instant solution to reduce this problem by 50% or more... Switch to an open baffle mounted Manger for all frequencies above 400 Hz!!!
Joking aside its a great option, especially if you use a shallow oval wave guide.
I have a test pair of WG that I got CNC milled out of two blocks of MDF and they bring the Manger up to nearly Edit :94dB Edit: sensitivity and add even more "easy dynamics" with effortless detail and a reach out and touch type of vocal realism.
I am going to start experimenting to see how deep I can make my wave guides before any sonic nasties start to become audible.
Second project how to double or triple the BL of the Manger.
The special twin voice coils are untouchable so I think it will all have to come from an increase in the magnetic force.
Any suggestions anyone?
Happy Easter to one and all!
Derek.
Lynn,
For me the transparency issue is not the source of contention. Rather is is the idea the open backed enclosures are significantly better than sealed boxes. As I said in a previous post, the resonances in an open backed system are still very sever. Check this out. It the front and rear SPL from my NaO U-frame woofer system before damping.
![url]](/community/proxy.php?image=http%3A%2F%2F%5Burl%5Dhttp%3A%2F%2Fwww.musicanddesign.com%2Fimages%2FNaO_II_U_frame1.gif%5B%2Furl%5D&hash=31c132907f2b29d96511ebbec6c99958)
There is a 15 dB leak at the first resonance , a 9dB peak at the second, and so on. These aren't of too much concern to me as they are above the band pass of the woofer system. But damping is required to achieve the correct phase response for quasi-cardioid behavior at low frequency as indicated from this figure after adding damping, the LP crossover filter and the required equalization for a 1st order gradient woofer. Now this u-frame has a length to width ratio of 2:1 and the height = width. If you want to reduce the Q and amplitude of the open backed enclosure resonances you must make the cross sectional dimensions larger compared to the length. Ultimately this tends to an OB.
For me the transparency issue is not the source of contention. Rather is is the idea the open backed enclosures are significantly better than sealed boxes. As I said in a previous post, the resonances in an open backed system are still very sever. Check this out. It the front and rear SPL from my NaO U-frame woofer system before damping.
There is a 15 dB leak at the first resonance , a 9dB peak at the second, and so on. These aren't of too much concern to me as they are above the band pass of the woofer system. But damping is required to achieve the correct phase response for quasi-cardioid behavior at low frequency as indicated from this figure after adding damping, the LP crossover filter and the required equalization for a 1st order gradient woofer. Now this u-frame has a length to width ratio of 2:1 and the height = width. If you want to reduce the Q and amplitude of the open backed enclosure resonances you must make the cross sectional dimensions larger compared to the length. Ultimately this tends to an OB.
Here's the correct image.
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Rear wave colouration.
Hi John K,
Taking a step back from the detailed debate on open baffle Vs box cabinet for a moment I have a feeling, (no data...) that the following two factors are of great significance, especially in the DIY arena.
(1) It is easier ( lower cost way to achieve top class result) to construct an OB U frame or derivative which transmits less of its energy into the baffle & sides than constructing a mass (ive) based box cabinet.
By the way I agree with your estimate of 500 to a 1,000 times Mms for cabinet mass if building a traditional box cabinet.
My Encore cabinet was 120Kg Vs the 100g Mms of the modified Beyma 12LX60 I used in it. I could have probably got away with 80 Kg to 90 Kg but Overkill by name Overkill by nature!
(2) It is also easier (but still far from easy!) to deal with the (slightly?) lower acoustic rear wave energy as it spreads out and away from the rear of the driver. The natural roll off with distance law will be working for us rather than us fighting an intense "convert or reflect"
sonic war (today is real metaphor day ...!) that takes place in the closed / ported box a few inches from the driver magnet.
Placing various room treatment, on wall, and in corners behind the speakers, a range of on cabinet damping materials and a range of absorbers / converters placed immediately behind the drivers, will give us several passes (longer time period) at converting and dispersing the rear wave energy as it is naturally loosing intensity.
This seems to me to be less challenging than the closed box battle where all the nasty bass and lower midrange energy that we cant convert in the first pass (short time period & high intensity of this internal energy) will bounce out through our thin paper driver.
As I said at the start of this post, I have no data on this theory, what do you think?
All the best
Derek.
Hi John K,
Taking a step back from the detailed debate on open baffle Vs box cabinet for a moment I have a feeling, (no data...) that the following two factors are of great significance, especially in the DIY arena.
(1) It is easier ( lower cost way to achieve top class result) to construct an OB U frame or derivative which transmits less of its energy into the baffle & sides than constructing a mass (ive) based box cabinet.
By the way I agree with your estimate of 500 to a 1,000 times Mms for cabinet mass if building a traditional box cabinet.
My Encore cabinet was 120Kg Vs the 100g Mms of the modified Beyma 12LX60 I used in it. I could have probably got away with 80 Kg to 90 Kg but Overkill by name Overkill by nature!
(2) It is also easier (but still far from easy!) to deal with the (slightly?) lower acoustic rear wave energy as it spreads out and away from the rear of the driver. The natural roll off with distance law will be working for us rather than us fighting an intense "convert or reflect"
sonic war (today is real metaphor day ...!) that takes place in the closed / ported box a few inches from the driver magnet.
Placing various room treatment, on wall, and in corners behind the speakers, a range of on cabinet damping materials and a range of absorbers / converters placed immediately behind the drivers, will give us several passes (longer time period) at converting and dispersing the rear wave energy as it is naturally loosing intensity.
This seems to me to be less challenging than the closed box battle where all the nasty bass and lower midrange energy that we cant convert in the first pass (short time period & high intensity of this internal energy) will bounce out through our thin paper driver.
As I said at the start of this post, I have no data on this theory, what do you think?
All the best
Derek.
In general
I would like to know how a 100cy bass wave coming off of the back of the drivers cone would not reflect off of the inside of the cabinet and then pass thru the paper thin cone and go out into the room? Also I wonder what Dave Wilson of Wilson Audio would think of cabinet radiation not being a major issue in speaker design? I can feel the vibration of my cabinet while it is making music, if there is vibration of an object it is moving air, moving air makes sound. It all seems so simple, sound goes thru low mass barriers, vibrating objects produce sound.
I would like to know how a 100cy bass wave coming off of the back of the drivers cone would not reflect off of the inside of the cabinet and then pass thru the paper thin cone and go out into the room? Also I wonder what Dave Wilson of Wilson Audio would think of cabinet radiation not being a major issue in speaker design? I can feel the vibration of my cabinet while it is making music, if there is vibration of an object it is moving air, moving air makes sound. It all seems so simple, sound goes thru low mass barriers, vibrating objects produce sound.
100 Hz wave reflection
Hi Rick,
Well I'm with you on this one and to be honest I think most people will be as well.
With great effort it is possible to reduce the problem in box cabinets but as for a total cure.....
At 100Hz and lower the wave lengths are to long to fully develop inside the box (well most boxes!), as Lynn points out a few posts ago real trouble begins in the low mids and up to 800Hz give or take a wee bit.
But even at 100Hz there is a lot of colouration in 99% of box speakers. i.e. Listen to low piano, cello, bowed double, bass, organ etc from any sealed box ( or worse a ported or ABR) sub, and then compare that to a good 15 or 18 inch bass driver on an open baffle U frame or VPL Uframe.
The difference is astonishing and so much more life like when you loose the box.
As per my last post I think its easier to deal with all this in an open baffle set up instead of a box.
Cheers
Derek.
Hi Rick,
Well I'm with you on this one and to be honest I think most people will be as well.
With great effort it is possible to reduce the problem in box cabinets but as for a total cure.....
At 100Hz and lower the wave lengths are to long to fully develop inside the box (well most boxes!), as Lynn points out a few posts ago real trouble begins in the low mids and up to 800Hz give or take a wee bit.
But even at 100Hz there is a lot of colouration in 99% of box speakers. i.e. Listen to low piano, cello, bowed double, bass, organ etc from any sealed box ( or worse a ported or ABR) sub, and then compare that to a good 15 or 18 inch bass driver on an open baffle U frame or VPL Uframe.
The difference is astonishing and so much more life like when you loose the box.
As per my last post I think its easier to deal with all this in an open baffle set up instead of a box.
Cheers
Derek.
I though I woud post these without comment. They are for a ScanSpeak 8554 mounted in sealed box, U-frame (box open at rear) and open baffle. All are near field measurement (1/4" off dust cap) of the on axis response. The sealed box abd U-frame are undamped. U-frame is internallt 7 x 7 x 8 deep. The sealed box is the U with a back panel.
Sealed box:
U-frame:
Open Baffle:
Sealed box:
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U-frame:
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Open Baffle:
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gedlee said:
A cabinet vibrates and it moves air and it creates a sound, how is this not important? We are talking about sound reproducers not musical instruments. It is a problem to reproduce sound from an unwanted source; only the cones should produce sound not the enclosure.
Of course some people think that all caps sound the same and are not important even though Walt Jung measured the differenced over 25yrs ago. Some people think that the 7th harmonic is not important.
I hope that the only thing you are saying that is not important is the LEVEL of the sound coming from the enclosure. That the level is so small that it is un-important at that small level compared to the main cone output.
I feel that enclosure vibration should be minimized, as it is audible. Why are so many speaker companies trying to reduce vibration if it is not important? Vibration moves air, right? Moving air creates sound right? Why is this not important?