Speaker Cabinet Design Considerations

[jsy]

Not really.
"sound absorbers" considerably reduce reflection from walls but not transmission through them.
It is a common mistake to think sound transmission will be effected a worthwhile amount (because of the name) but it isn't, you need mass (or blocking any tiny hole) to make a difference of consequence.

OK got it. So if you're building a cabinet, and trying to "deaden" the interior, really you're trying to make sure the walls are sufficiently stiff (materials/bracing) or transmission is blocked (dampening), not trying to reduce internal reflections? And absorbtive materials are introduced to cab interiors for tuning/volume purposes?

 

[Frank Dernie]

OK got it. So if you're building a cabinet, and trying to "deaden" the interior, really you're trying to make sure the walls are sufficiently stiff (materials/bracing) or transmission is blocked (dampening), not trying to reduce internal reflections? And absorbtive materials are introduced to cab interiors for tuning/volume purposes?

Stiffness isn't the issue, that just changes the resonant frequency of any panel resonance. Amplitude is reduced but frequency increased.
Damping reduces amplitude of panel vibration. Absorbing material in the box will reduce internal reflections so less likely for sound to come out of the reflex port or pass through the speaker cone.
It is worth reading the BBC papers and the KEF one on the LS50 that I posted up thread.

 

[SDC]

I found this when I was searching for the right amount of bracing for my speaker.
Not sure I understood correctly, but I used thicker MDF and reduced the number of bracing a bit after seeing this.

BTW, any measurements for those thin butyl+Alu CLD working in speakers? Heard they are no use for rigid speakers, saying 25T mdfs aren't sheet metal car doors...

 

[SDC]

https://www.diyaudio.com/forums/multi-way/347912-benefits-minimal-baffle.html

It is a common mistake to think sound transmission will be effected

Read the link. It is heart breaking to learn foam edges do nothing for wide baffles. I always thought, if the edges act as second sound source with delay we can simply absorb it and get better dispersion. And others aren't doing this for cosmetic reasons.

 

[jsy]

Thanks, guys, that's really helpful

I'll check out the papers and links and come back. Intuitively it seems strange to me that we can completely decouple things like resonance and damping, but hopefully some reading fills me in!

 

[Ron Texas]

Take one dead monkey and a sheet of mdf and build around the dead monkey.

 

[Biblob]

So what I've learned today is that you can use bracing, but it has to be much stiffer. So using aluminium or maybe HDF is better than using MDF.
Kef uses bracing at the center line of the speaker to support the woofer, so it can't move the baffle backwards. Also is the bracing connected in such a way that any movement is dampened, via a sort of CSD manner.

 

[Frank Dernie]

Intuitively it seems strange to me that we can completely decouple things like resonance and damping

You need to study this for a while then!
Resonances is complex in complex structures but often simple in mechanisms but damping is always the parameter that most strongly controls the amplitude at resonance.
If you didn't have an aptitude for physics as a child you may never understand it, it is often confusing even for engineers experienced and knowledgeable in other disciplines.
My first direct boss when I was doing noise and vibration research had had the job I had just taken before me, so I had no problem but his boss, the head of research, accepted what the maths predicted but always said it was counter intuitive and he couldn't visualise what was happening, and he was an experienced engineer with a pretty good reputation!

 

[jsy]

It is often confusing even for engineers experienced and knowledgeable in other disciplines.

This is me In fairness, my current reading is 0, so maybe I'll see it in a bit. If not, lump me with that confused boss!

 

[Juhazi]

I found this when I was searching for the right amount of bracing for my speaker.
Not sure I understood correctly, but I used thicker MDF and reduced the number of bracing a bit after seeing this.

BTW, any measurements for those thin butyl+Alu CLD working in speakers? Heard they are no use for rigid speakers, saying 25T mdfs aren't sheet metal car doors...

This is an important pic. Notice how amplitude of resonances gets reduced

 

[jsy]

KEF Reference white paper cites Olsen with more info (linked, attachment too large)
http://www.kef.com/uploads/files/THE_REFERENCE/REF_White_Paper_preview_path_200514.pdf

 

[Biblob]

I have been modelling some in Sketchup. Inspirated by wat Kef did in the LS50 and what Gedlee suggests. The circles would be Sorbothane pads. The 0531525-30-10 to be exact. They also have an online vibration calculator, and based on that it would this design would help reduce <300 hz vibration a lot.
Maybe all for nothing, but hey, it's fun. Heck, I might make it anyways.
Feel free to share thoughts.

 

[jsy]

The circles would be Sorbothane pads. The 0531525-30-10 to be exact.

Looks pretty cool! What's the sorbothane pad parallel to the baffle for? Does it contact the baffle?

The KEF design seems to jam damping between the braces and the panels, but your contacts look glued flush. Any reason not to damp at these joints?

 

[HammerSandwich]

Stiffness isn't the issue, that just changes the resonant frequency of any panel resonance. Amplitude is reduced but frequency increased.

Frank will know this, but I'd like to clarify something for other readers.

Reduced panel amplitude does not mean that the box's SPL is lower. At a fixed SPL, a speaker driver will have 1/4 the excursion at twice the frequency (one octave higher). IOW, a stiff box that moves less - but at a higher frequency - can be just as loud. And, thanks to hearing sensitivity, potentially more audible.

The situation's complicated, of course. Bracing will divide a panel into smaller areas and so forth. Just remember that "less panel motion" does not imply "less audible cabinet."

 

[jsy]

Frank will know this, but I'd like to clarify something for other readers.

Reduced panel amplitude does not mean that the box's SPL is lower. At a fixed SPL, a speaker driver will have 1/4 the excursion at twice the frequency (one octave higher). IOW, a stiff box that moves less - but at a higher frequency - can be just as loud. And, thanks to hearing sensitivity, potentially more audible.

The situation's complicated, of course. Bracing will divide a panel into smaller areas and so forth. Just remember that "less panel motion" does not imply "less audible cabinet."

I think I understand. You may be trading amplitude for a higher resonant frequency and end up with the same loudness. I presume it should be theoretically possible to continue until your frequency is ultrasonic though?

 

[Frank Dernie]

I presume it should be theoretically possible to continue until your frequency is ultrasonic though?

Not with a box. As I have written before nothing is rigid over the whole range of audible frequencies.
There are tweeter domes whose first mode is above 20kHz, some cartridge cantilever/stylus mass are but nothing else in audio I am aware of.
More rigid can be worse in a lot of situations but it isn't simple and a proper analysis needs doing to know where nodes and anti-nodes are to see whether "damage" is likely to be done in the way of audible colouration.

 

[jsy]

Not with a box. As I have written before nothing is rigid over the whole range of audible frequencies.

Yes sir, I read what you wrote before. I've read the KEF attachments, the BBC papers, and on your advice revisited my high school and college physics in an effort to keep up. My understanding is about the same as it was before, in that:

- Mechanical stiffness, by definition, describes an object's ability to resist physical deformation in response to force. Stiffness is the inverse of compliance
- Mechanical damping, by definition, describes the loss of energy in a moving system, such as created by a shock absorber

If I have these definitions incorrect, that would certainly explain why I don't understand what you're saying. I was attempting to describe that, under theoretical "ideal" conditions:
- an infinitely stiff material cannot resonate, by definition, because it cannot deform. Spring mechanics break down in the absence of compliance
- a system with infinite damping cannot resonate, because motion is brought to an instantaneous halt

I recognize that neither of these ideal conditions is achievable under practical circumstances. Having said this, I don't agree with your assertion that a box cannot be theoretically rigid enough to prevent resonance over the whole range of audible frequencies, unless you're imposing additional constraints in your definition of a box (e.g. panels of xyz width/length/thickness, made of abc materials). Naively, I could construct a box consisting of one cubic mile of steel, hollow out a 2 inch square, affix an absurdly thick baffle to the hollow, and vent it. I would expect my mile-high steel enclosure to exhibit a Helmholtz resonance, but no cabinet resonance in the audible frequencies, since my system has sufficiently high stiffness and damping as not to exhibit meaningful spring behavior.

More rigid can be worse in a lot of situations but it isn't simple and a proper analysis needs doing to know where nodes and anti-nodes are to see whether "damage" is likely to be done in the way of audible colouration.

I agree. It strikes me as sensible that damping is preferred to stiffness in a box the size of a usual loudspeaker, and that increasing stiffness without considering your overall effects is a bad idea. I don't think pursuing both stiffness and damping in an enclosure is a bad idea - if that's not what you were implying, perhaps I've misunderstood.

Also, earlier, I had said "Intuitively it seems strange to me that we can completely decouple things like resonance and damping", and you had replied that I should hit the books. Having done this, I still don't understand how we could decouple resonance and damping, since damping seems to describe the ability to attenuate resonance over time.

 

[Biblob]

Looks pretty cool! What's the sorbothane pad parallel to the baffle for? Does it contact the baffle?

The KEF design seems to jam damping between the braces and the panels, but your contacts look glued flush. Any reason not to damp at these joints?

The parallel pad would be to dampen the driver movement.

I was quick to realise that my sketch is bad design. It's because of what I read on the diyaudio.com forum on CLD. Gedlee posted a patent on CLD that is worthwile to read as well. I will update the design on these principles and post them here.

 

[jsy]

The parallel pad would be to dampen the driver movement.

I was quick to realise that my sketch is bad design. It's because of what I read on the diyaudio.com forum on CLD. Gedlee posted a patent on CLD that is worthwile to read as well. I will update the design on these principles and post them here.

I was just reading that thread! The one with folks taking random shots at the poor Canadian guy just trying to make a sandwich, right?

I was wondering how his idea would work with hard glue at the panel edge joints, but didn't want to ask there, because I still don't understand the formal boundaries for what is or isn't "CLD". Nice to find another reader in this thread!

 

[Frank Dernie]

Yes sir, I read what you wrote before. I've read the KEF attachments, the BBC papers, and on your advice revisited my high school and college physics in an effort to keep up. My understanding is about the same as it was before, in that:

- Mechanical stiffness, by definition, describes an object's ability to resist physical deformation in response to force. Stiffness is the inverse of compliance
- Mechanical damping, by definition, describes the loss of energy in a moving system, such as created by a shock absorber

If I have these definitions incorrect, that would certainly explain why I don't understand what you're saying. I was attempting to describe that, under theoretical "ideal" conditions:
- an infinitely stiff material cannot resonate, by definition, because it cannot deform. Spring mechanics break down in the absence of compliance
- a system with infinite damping cannot resonate, because motion is brought to an instantaneous halt

I recognize that neither of these ideal conditions is achievable under practical circumstances. Having said this, I don't agree with your assertion that a box cannot be theoretically rigid enough to prevent resonance over the whole range of audible frequencies, unless you're imposing additional constraints in your definition of a box (e.g. panels of xyz width/length/thickness, made of abc materials). Naively, I could construct a box consisting of one cubic mile of steel, hollow out a 2 inch square, affix an absurdly thick baffle to the hollow, and vent it. I would expect my mile-high steel enclosure to exhibit a Helmholtz resonance, but no cabinet resonance in the audible frequencies, since my system has sufficiently high stiffness and damping as not to exhibit meaningful spring behavior.



I agree. It strikes me as sensible that damping is preferred to stiffness in a box the size of a usual loudspeaker, and that increasing stiffness without considering your overall effects is a bad idea. I don't think pursuing both stiffness and damping in an enclosure is a bad idea - if that's not what you were implying, perhaps I've misunderstood.

Also, earlier, I had said "Intuitively it seems strange to me that we can completely decouple things like resonance and damping", and you had replied that I should hit the books. Having done this, I still don't understand how we could decouple resonance and damping, since damping seems to describe the ability to attenuate resonance over time.

Well I am an engineer so I am referring to useful structures, ie a box with a big enough volume to correctly load the bass unit and panels stiff enough for their first mode of resonance to exceed 20kHz.
Not sure your mile high box of steel would comply but I certainly can't be arsed to do the sum.
I perhaps didn't understand your comment about decoupling resonance and damping.
Resonance occurs whether there is damping or not (and most metals have so little internal damping its effect is minuscule) Damping reduces amplitude at resonance (and increases it away from resonance.