Horses for courses - Optimal boxes for different passbands

[rick57]

I’ve spent a while reading on this, there is more than one theory, but does this sound right?

If you are going to the trouble of making separate boxes (or more than one system):

Midrange - for a natural articulate midrange ~
For mid boxes or satellites (eg 120 - 2500+ Hz), the BBC approach, as in eg speakers by Spendor Rogers etc, should give the most natural midrange for eg vocals.
In this approach, I understand that in approx order of importance:
- Low mass ie thin walls, to minimise energy storage hence less delayed energy release. walls only 9-12 mm (3/8 – ½ inch?)
- Plywood’s composite nature, to spread resonances
- Heavily damped with eg bituminous pads to dampen panel resonances. (They also lower Hz, which must be balanced overall)
- Rigid. Rigidity could come from closer spaced, stiffer (hardwood/ MDF and or deeper eg 75 mm * 25 mm) bracing.

If someone can chime in on two questions on the mids:

- Ply options - a local void free wood is (dearer but) 38% *less dense than the Baltic ply ~
I would think that other things being equal, if low mass is better, for a speaker aiming for really good sound, and not too large (ie low material cost), that the less dense low mass option would be better – is that right?

- as bracing pushes the resonances up, this is only a benefit if the higher resonances can more easily be tamed by a stuffing eg fiberglass, above . . Hz?


Subs eg < 80 Hz. I’m less sure here, but think they benefit from:
- high mass, ie here use the *heaviest available void free ply.
- if a larger box, close enough bracing to push the resonances above the passband.

A full range box . .
. . by trying to do more, is harder to optimize. . choose between optimizing the bass or optimizing the midrange.

Also
- being a composite, chipboard is better than MDF, but not as good as void free ply.

- extra thickness ie mass only benefits the bass (I think)??

Bracing . .
should be spaced so that unbraced panel distances correspond to a resonant Hz either outside the passband, or in a range where it can be effectively absorbed by a lining eg fiberglass.
. . Any more is overkill

Please let me know where the above is wrong!

Thanks

Rick

[owdi]

I'm going dipole for my midrange. With the right size baffle I can apply zero baffle step compensation, and get an 87db/watt efficient speaker.

I'm also keeping the crossover extremly simple. 2nd order passive for the lowpass, and 2nd order line level passive for the highpass.

Dan

[rick57]

Dan, was that in the right thread?

Anyone agree or disagree that midrange boxes are better with the “opposite” construction I described, to subs?

Thanks

[roddyama]

Hi Rick,

Your suggestions will move the resonances to different frequencies and this will help,... somewhat. However, ultimately you would only get partial results from just moving the panels resonance.

Every panel will resonate at some frequency. There's no way around that fact. Getting back to basics, there are only 3 ways to deal with a resonance; move it to a different frequency as you suggest by changing the values of the mass or spring or both, removing the excitation which you suggest by adding damping in the box, or applying damping.

For speakers boxes, moving the resonance is only a partial solution as the panel will still resonate at the new frequency. So unless you move the resonance completely out of the audible range, it will still be an issue to one extent or another.

It's impossible to remove all the excitaions because the excitations are coming from the speaker itself through the inside of the box, transmitted through the box panels, and through the air from the other speakers.

The most effective method is to damp the panels and the excitation source where possible (like your damping inside the box). This is where the material from which the panel is made can make a huge difference. Materials with high internal damping like carbon fiber can work very well in this regard. I've seen suggestion on the forum here to use various plastics. This too could work very well as many plastics have high internal dampings. I know there are some that don't like it but MDF has relatively high internal damping and is easy to get, relatively cheap to use, and easy to make into boxes.

One approach the takes advantage of each of the 3 methods to a greater or lessor extent is to use different shapes like curved panels. The analysis of a curved panel would be much more complex but the resonance would be moved upward, the panel would receive the excitation at different angles/times so the effects of the excitation would be less, and I'm not certain but I believe the damping would be effectively increased due to the complexity of the resonance modes in the curved panel.

[AMV8]

Hi

When I built my last speaker system I had a box for the mid and treble and a separate bass unit.

I found that the critical box was the mid box. Here as you state in post 1 you need neutrality. I found this best achieved by using a concrete/corian box for the mid/treble unit. I shaped mine as a foward sloping pyramid. I left the interior walls rough and did not use any filler material.

For the base unit I used a rigid box 30mm walls with bracing made of hdf. I have used condrete in the past for a base unit but I did not find that the extra rigidity helped much. ( And the lighter weight of wood/hdf certainly does help when it comes to moving them around!!!)

Don

[Hezz]

Rick,

Even though the designs that copied the BBC monitors had good sound for thier day they are bettered by other more modern techniques. Looking for the best approach you must look at the really expensive speakers. Basically you have to use exotic materials or exotic construction techniques or heavy overbuild using MDF and Plywood.

As far as I can say it works better for the mid/tweeter cabinet to be high mass/high internal dampening and rigid. By doing this you can move the resonances high into the overtones range and out of the fundementals where they do the most damage. This is also easy to accomplish with thick MDF and dampening and bracing. ALso, with acoustic foam and dampening pads attached to the walls some of the resonances can be attenuated by turning the air motion into physical motion in the foam which can attenuate the resonance by turning it into heat. Also the dampening can attenuate panel born resonances.

Also, cabinet designs like aperiodic ones work fantastic for the midrange driver because it lowers the internal air pressures in the cabinet which helps to keep internal air pressures from exciting the cabinet. For fantastic midrange sound avoid the tradition of putting the driver in a small sealed housing unless it's large enough for the driver to be critically damped or near critically damped.

For the bass speaker rigidity and strength is more important than mass. So I would use plywood in the bass cabinet with lots of bracing. The better quality the plywood the better since the better grades are more rigid. But I have found that doubled up plywood in the bass cabinet still sounds better than single sheets due to being more rigid. Now if you can move the panel resonances in the bass module high enough that the driver SPL is way down at those frequncies then there is a whole lot less problem and the sound quality improves a lot. And if the bass cabinet is decoupled from the mid/tweet cabinet it also helps to keep the bass resonances out of the mid/tweet cabinet.

[Hezz]

Another technique that I have considered for the mid/tweet cabinet is to make the basic cabinet from a mix of epoxy resin and lead shot. Poured and cured in a cheap foam mold. And then skin both the inside and out with fiberglass or carbon fiber and epoxy resin. Thicker walls the better for the inside of the wall. The outer skin adds a lot of rigidity and can be made as thick as is practicle.

The molded part can also be made of epoxy and sand or crushed and washed rocks. THe more dense the better.

[rick57]

Hi Rodd

Thanks for your comments and suggestions

“there are only 3 ways to deal with a resonance; move it to a different frequency as you suggest by changing the values of the mass or spring or both, removing the excitation which you suggest by adding damping in the box, or applying damping.”

Could you explain the difference between “adding damping in the box, or applying damping”

Your comment on internal damping got me googling, and I found a formula:

http://www.snippets.org/pipermail/di...er/003539.html

but to be useful (the catch) ~ you need to know the MOE (modulus of elasticity) the Poisson's ratio of your candidates (I’m now waiting on them on two shortlisted timbers).
If you do, looks like the formula in the link can be used to *estimate with some science appropriate timber thicknesses, the effects of bracing on resonances . .


Does anyone know the MOEs or PRs of the normal contenders chipboard/ masonite/ marine ply etc?? PRs for several materials including concrete . . but no woods are here
http://en.wikipedia.org/wiki/Poisson's_ratio

According to Dickason, MDF has ‘relatively poor’ damping, anyone got any numbers or view on the contenders; I think MOE is damping?

Cheers

[rick57]

Hi Don

Thanks for your comments and suggestions

I’ve thought of concrete, I might well try it in a future project, by cutting up concrete stormwater pipes. there’d probably be a lot of material wastage (they are longer than you need) but you get non-parallel “sides & top & bottom”.

Using timber at the front & back would leave only one dimension of parallel walls. Martin King has said on this forum that if the drivers are centred on the front baffle, front to back modes will *all be excited – so
- place off centre,
- maybe put a ‘wedge’ on the inside of the back wall to make the front-to-back non-parallel, and
- most stuffing on the front or back walls . .?

Cheers

[rick57]

Hi Hezz

Thanks for your comments and suggestions too . .

“For the bass speaker rigidity and strength is more important than mass.”

I want to distinguish between bass and mid boxes - thank you!

“if the bass cabinet is decoupled from the mid/tweet cabinet it helps keep the bass resonances out of the mid/tweet cabinet”

Definitely

Aperiodic boxes to lowers the internal air pressures sounds a good idea - you compared with/ without?

The BBC monitor designers could have gone “high mass”, maybe not with MDF, but with other dense/ thick materials.
Two of the (many) very good commercial designs are very low mass - from the early 80s the low mass Celestion SL-600 in construction terms is probably still ahead of many current speakers (made it into Stereophile's list of "Best top 25 speakers of past 40 years"); and nowadays the super dear Wilson Benesch with carbon fibre cabinets.


Better than either low or high mass may be constrained layer damping construction. It works with MDF http://www.silcom.com/~aludwig/Louds...struction.html

Anyone know if CLD works better with ply than MDF??


“I’d use plywood in the bass cabinet with lots of bracing”

Linkwitz in the 50 litre Thor uses Baltic birch ply, a small pinhole (I guess that’s aperiodic), but just bracing along the joins . .


Cheers