BBC-style thin-walled cabinets vs. tight bass?

[fdieck]

Re: BBC-style thin-walled cabinets vs. tight bass?

I must confess I have built both sand and bitumen damped 3/4 and 1/5 ply enclosures and the amount of damping really influences the sound. I think it is a valid approach.

[Pete Basel]

Re: BBC-style thin-walled cabinets vs. tight bass?

From a David Smith interview:
"Cabinet construction frequently gets to the heart of audiophile beliefs and common misunderstandings. In audiophile circles, if a little wall thickness is good then a lot is always better. The physics are actually at odds with that. We need to lower the Q of cabinet resonances and higher mass or higher rigidity diminish the effect of any damping we apply. Damping is the key and we want a high ratio of damping material to wall mass or rigidity. The upshot is that thicker cabinet walls will always raise the Q of resonances and make their damping harder to achieve. Raising the resonance frequencies with more rigid walls will seldom get them above audibility, more likely they will just move into a range where they are more audible. This is at odds with many audiophiles understanding so it tends to lead to spirited arguments on the forums, but the physics is clear. "

Already discussed here:
http://techtalk.parts-express.com/sh...96#post1959396

http://techtalk.parts-express.com/sh...31#post1959331


I've read a lot from David Smith and I don't think I've found anything that I did not agree with.

I built half inch ply cabinets over 20 years ago simply to keep the weight down and used plenty of bracing which worked fine. I've thought for years that speaker boxes should be more like airplane wings with thin walls and more bracing mainly to keep the weight down.

I'm going with David Smith on this one but I doubt that the effect is even audible for well constructed cabinets of either type. My intuitive feeling is that for a given enclosure weight allowance one can do better with thinner walls with bracing/damping than thick walls.

If you are going to measure wall vibration you have to translate it into how much acoustic energy is radiated since this is what matters and I've not seen anyone do this.

I like what you're doing Ron_E and I've been using PE prefab cabinets lately but if I do some from scratch they'll be similar to yours. Did you consider one or two shelf braces in those builds, seems a bit simpler than what you're doing?

[fastbike1]

Re: BBC-style thin-walled cabinets vs. tight bass?

Well Jeff, I didn't make it up. http://www.harbeth.co.uk/usergroup/s...Why-so-special

That statement doesn't line up with Harwood's papers, nor does it line up with measurements of Alan Shaw's enclosures. See the current issue of Stereophile for an example. I will say this though, their crossover appears to be well implemented.

[Jeff B.]

Re: BBC-style thin-walled cabinets vs. tight bass?

Well Jeff, I didn't make it up. http://www.harbeth.co.uk/usergroup/s...Why-so-special

Oh, I believed you were quoting him accurately. I was just saying the measurements on the one tested in this month's issue of Stereophile showed some pretty high amplitude resonances in the lower midrange. From the review there seemed to be some degree of coloration associated with it too.

Now, I think hifi or audiophile monitors are a completely different animal than speakers used for bands and music amplification. I understand designing them for particular resonances.

[Pete Schumacher]

Re: BBC-style thin-walled cabinets vs. tight bass?

Panel resonances are only one of the sound transfer mechanisms. There's also sound transmitted directly through the material that has nothing to do with panel resonances. That's one reason you can hear conversations in the next apartment when there's inadequate sound deadening between the walls. Same thing holds true for speaker enclosures. Walls that don't move will still allow sound from inside the cabinet to make its way outside.

Constrained layer construction takes care of both methods of sound transmission.

[wolfgang]

Re: BBC-style thin-walled cabinets vs. tight bass?

To be clear, I am talking only about the midrange, i.e., 300hz to 3khz. In other words, the topic of building a dedicated midrange cabinet that is completely separate from the bass cabinet.

To my understanding, rigidity is the last thing you want in such a design because, while decreasing the amplitude of vibrations, it also increases their frequency, thus increasing the resonant frequency of the overall structure. This has to do with the "springiness" of the material versus its "inertia", or mass. Mass actually decreases both amplitude and resonant frequency, hence bitumen or vinyl as sources of flexible mass with hardly any springiness to them. So the thin panel concept is not about exploiting cabinet resonances or having a less overall cabinet mass (although the BBC initially explored thin-wall cabinets for this very reason), but about having a less stiff structure in general with a resonance peak below the critical 300hz to 3k range.

So for midrange, mass and damping are both good; stiffness is bad. Anything rigid that makes an attractive exterior layer for a speaker box, whether Baltic birch or MDF, is going to bring a lot of stiffness along with its mass. Hence using a thinner wall, which still gives an attractive exterior, but tuning it downward in pitch by adding bitumen, which both mass loads and damps at the same time.

[wolfgang]

Re: BBC-style thin-walled cabinets vs. tight bass?

Walls that don't move will still allow sound from inside the cabinet to make its way outside.

I get what you're saying and agree, but just wanted to add that a wall that truly didn't move, i.e., an infinitely rigid wall, would not transfer sound as sound is simply mechanical energy that displaces the molecules within a particular medium. If there's no displacement, there's no transfer of energy. I only bring this up to point out why I don't agree with the concrete or marble wall enclosure, as it will end up reflecting acoustical energy back through the driver at some point, even if attenuated by internal damping (unless the damping layer is so thick that it absorbs 100% of the mechanical energy of the sound wave as it travels through once and then again after being reflected).

So an infinitely rigid wall would only serve to change the direction of the sound wave; it would not remove an energy from it at all. Since realistically our enclosures will never be the perfectly 100%-efficient mechanical-to-heat transducers that we would like them to be, some sound is always going to leave them. I think the concrete wall vs. thin wall debate is really about whether you want that sound leaving through the enclosure walls or through the driver. I'd rather have the former with walls that flex a little and help out by converting a little energy to heat themselves (as long as the whole system is well-designed of course) rather than the latter with walls whose only purpose is to change the direction of a wave.

[Pete Basel]

Re: BBC-style thin-walled cabinets vs. tight bass?

You need to define what you are trying to do better. You did start with the LS-3/5a as an
example and it does not have a limited range as you are now suggesting.
What are the dimensions of the enclosure that you're talking about?

In that same design where I used half inch plywood for the large bass cabinet, I used 1.5"
MDF for the very small mid enclosure because there would be no need for bracing and it
was so small that I could afford the weight.

[Pete Basel]

Re: BBC-style thin-walled cabinets vs. tight bass?

Panel resonances are only one of the sound transfer mechanisms. There's also sound transmitted directly through the material that has nothing to do with panel resonances. That's one reason you can hear conversations in the next apartment when there's inadequate sound deadening between the walls. Same thing holds true for speaker enclosures. Walls that don't move will still allow sound from inside the cabinet to make its way outside.

Constrained layer construction takes care of both methods of sound transmission.

ARe you sure that you worded this correctly?
"Walls that don't move will still allow sound from inside the cabinet to make its way outside."

The wall must move to move the air outside, so this makes no sense unless you are suggesting
that the wall is porous, which I do not believe.
Or are you suggesting re-radiation through the driver cone, or leaks?

If the wall does not move then what is the mechanism for moving air?

[craigk]

Re: BBC-style thin-walled cabinets vs. tight bass?

To be clear, I am talking only about the midrange, i.e., 300hz to 3khz. In other words, the topic of building a dedicated midrange cabinet that is completely separate from the bass cabinet.

To my understanding, rigidity is the last thing you want in such a design because, while decreasing the amplitude of vibrations, it also increases their frequency, thus increasing the resonant frequency of the overall structure. This has to do with the "springiness" of the material versus its "inertia", or mass. Mass actually decreases both amplitude and resonant frequency, hence bitumen or vinyl as sources of flexible mass with hardly any springiness to them. So the thin panel concept is not about exploiting cabinet resonances or having a less overall cabinet mass (although the BBC initially explored thin-wall cabinets for this very reason), but about having a less stiff structure in general with a resonance peak below the critical 300hz to 3k range.

So for midrange, mass and damping are both good; stiffness is bad. Anything rigid that makes an attractive exterior layer for a speaker box, whether Baltic birch or MDF, is going to bring a lot of stiffness along with its mass. Hence using a thinner wall, which still gives an attractive exterior, but tuning it downward in pitch by adding bitumen, which both mass loads and damps at the same time.

rigidity is the quality of resisting change in form. stiffness is the ratio of a steady force acting on a deformable elastic medium to the resulting displacement. there is not a good or bad, there is just using them properly in a box design.

[Jeff B.]

Re: BBC-style thin-walled cabinets vs. tight bass?

To be clear, I am talking only about the midrange, i.e., 300hz to 3khz. In other words, the topic of building a dedicated midrange cabinet that is completely separate from the bass cabinet.

So, are you distinguishing "midrange" from vocal range? Because your band is pretty high for the vocal range, as there is a lot of vocal energy below 300 Hz and very little of it reaches 3 khz, which is solidly in the treble region.

I have Continuums made of solid maple, thin wall bamboo, and Corian. The Corian enclosure is lined with 1.5" acoustic foam on all side and then filled with poly fill. It measures the cleanest in terms of resonances and has the lowest measured stored energy. It also sounds the best to my ear with the highest level of resolution of detail in the midrange and the cleanest upper bass. I find no downside to using this material for this monitor, neither in my measurements or my listening.

[Pete Schumacher]

Re: BBC-style thin-walled cabinets vs. tight bass?

ARe you sure that you worded this correctly?
"Walls that don't move will still allow sound from inside the cabinet to make its way outside."

The wall must move to move the air outside, so this makes no sense unless you are suggesting
that the wall is porous, which I do not believe.
Or are you suggesting re-radiation through the driver cone, or leaks?

If the wall does not move then what is the mechanism for moving air?

I'm talking about panel resonances. While the molecules must move to transfer the sound, that doesn't mean the panel is undulating the way a panel resonance does.

Constrained layers will attenuate the transmitted sound better than a wall of a single material. That's why when someone builds a "sound proof" room, they will use hollow walls filled with another medium that dissipates the acoustic energy by changing the density of the medium multiple times between the inside of the enclosure and the outside.

[djg]

Re: BBC-style thin-walled cabinets vs. tight bass?

And also make a wall thicker than the 2x4 studs so that each stud only is attached to one side of the drywall, not both, alternate studs to alternate sides.

[craigk]

Re: BBC-style thin-walled cabinets vs. tight bass?

Panel resonances are only one of the sound transfer mechanisms. There's also sound transmitted directly through the material that has nothing to do with panel resonances. That's one reason you can hear conversations in the next apartment when there's inadequate sound deadening between the walls. Same thing holds true for speaker enclosures. Walls that don't move will still allow sound from inside the cabinet to make its way outside.

Constrained layer construction takes care of both methods of sound transmission.

Pete, Pete. you are absolutely correct.

[andy19191]

Re: BBC-style thin-walled cabinets vs. tight bass?

The BBC approach was a pragmatic solution to what was required to push cabinet vibration just below the threshold of audibility for a modest cost and weight. It made use of the fact that cabinet resonances are less easy to perceive at lower frequencies and so placed the largest first few modes relatively low in frequency. It is not an approach intended to produce a "high performance" cabinet but one that is just good enough to do the job.

To make a significantly more inert cabinet than a "BBC cabinet" following a different approach is not particularly difficult but is very likely to involve more cost and more weight. Simply doubling the thickness of the walls is usually not a good idea because it shifts the first few resonant modes higher in frequency but usually without significantly changing the level and so they become easier to perceive. To be equally loud at higher frequencies the wall displacement needs to be smaller (e.g. equally loud tweeter vs woofer) so the increased stiffness reducing the wall displacement is not necessarily going to help.