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-   -   Is enclousure strengthening needed? (http://www.diyaudio.com/forums/multi-way/218981-enclousure-strengthening-needed.html)

weiyan 3rd September 2012 01:05 AM

Is enclousure strengthening needed?
 
As I browsed many "diy" speaker set vendor, most of them advertised their enclosures are strengthened with a square frame.

This is a typical example:

5" 2-way unit:
H(300mm) X W(180mm) X D(269MM)

Material: 18mm MDF.

For 6.5" 2-way unit:
H420mm*W230mm*D296mm


The rest are the same with 5" enclosure.

One I talked to a person of Silver Flute, he said its a market gimmick. Given the material is strength enough(I guess he refers to 18mm MDF), no strengthening needed.

Thanks for any suggestion.

Bare 3rd September 2012 01:10 AM

Obviously the "Silver Flute" person knows everything.
Send him ALL your $$$.

twest820 3rd September 2012 02:08 AM

Depends on the speaker design. Conventional box speakers tend to have trouble with the Fourier series resonances supported by rectangular enclosures, requiring bracing to manage mechanical resonances and damping for acoustic ones. The most complex alternatives support something approximating Legendre polynomials---look at the B&W Nautilus---but Bessel type solutions like the Linkwitz Pluto are pretty easy to build.

Another mechanically simple alternative is dipoles as panel resonance is not generally too hard to control. Or just go with a nude dipole.

weiyan 3rd September 2012 03:04 AM

Quote:

Originally Posted by twest820 (Post 3149820)
Depends on the speaker design. Conventional box speakers tend to have trouble with the Fourier series resonances supported by rectangular enclosures, requiring bracing to manage mechanical resonances and damping for acoustic ones. The most complex alternatives support something approximating Legendre polynomials---look at the B&W Nautilus---but Bessel type solutions like the Linkwitz Pluto are pretty easy to build.

Another mechanically simple alternative is dipoles as panel resonance is not generally too hard to control. Or just go with a nude dipole.

Dipole may consider in future project.

Do you think a square frame inside the box can damp the resonance?

nigelwright7557 3rd September 2012 03:16 AM

For 12inch or bigger speakers I always use 18mm plywood and 2inch square bracing.
I have a 2 by 12 inch 450WRMS speaker cabinet and a 650WRMS 18 inch eminence bass speaker cabinet.
They need to be strong though because they sometimes get taken out on the road to do discos or guitar work.
I guess the bracing needs to stop cabinet vibrations mostly and after that not fall apart if moved.

twest820 3rd September 2012 04:29 AM

Quote:

Originally Posted by weiyan (Post 3149854)
Do you think a square frame inside the box can damp the resonance?

Depends on the frame and box you have in mind; as posed this question is too vague to answer meaningfully. If you're not familiar with Fourier basis functions your best option for predicting what'll happen is probably FEM simulation. If you can share 3D renderings or plan drawings of the design that's likely sufficient for informed speculation but, ultimately, that's no substitute for taking the time to develop your own design and analysis skills.

Very broadly speaking, what I was saying in the previous post is it's often easier to change the geometry to avoid problems than it is to solve them head on. If you're asking about typical rectilinear bracing in typical rectilinear enclosures usually it's more about shifting resonances out of band, though how feasible that is depends on the enclosure geometry, crossover frequencies and slopes, and material and bracing choices.

Baseballbat 3rd September 2012 07:02 AM

Hi,

Quote:

Originally Posted by weiyan (Post 3149854)
Do you think a square frame inside the box can damp the resonance?

no, it can not. It will shift the panel resonance to higher frequencies, and will probably even increase the Q, so the resonance may become more harmful.

If you're lucky the resonances will be shifted out of band, but that is very difficult, and nearly impossible with wooden enclosures. "Out of band" means above 20 kHz so you won't here them any more. It does not mean "above operating range of the woofer", because the midrange and even the tweeter may excite the panel resonances. I tested this last friday with an accelerometer, and this was very surprising to me (2 6.5" very light midranges were exciting the panel resonance as strong as 2 10" heavy woofers, and even the tweeter did excite them).

Read also the posts of Speaker Dave on this topic, he put me in the right direction recently.

Bracing is not completely useless, though. If you build a big floorstand, it will most probably collapse (either instantly or after some time) without bracing. Do not use thicker material instead, which is even worse than bracing.

In such a case, I would use thin wood (9 to 12 mm plywood), careful bracing so that the enclosure does not collapse, and apply a big amount of damping material on each inner surface, at least same thickness as the wood. It may be useful to strenghten the edges of the enclosure, especially with the 9 mm plywood. Decouple all drivers, at least with rubber grommets.

Baseballbat

weiyan 3rd September 2012 03:11 PM

Usually diy box maker us 25mm mdf face and 18mm mdf for the rest. There are hard wood box. Are these good material for speaker?

twest820 3rd September 2012 05:05 PM

Quote:

Originally Posted by Baseballbat (Post 3150000)
I tested this last friday with an accelerometer, and this was very surprising to me (2 6.5" very light midranges were exciting the panel resonance as strong as 2 10" heavy woofers, and even the tweeter did excite them).

Any chance you could post the data somewhere? Good accelerometer measurements are rather hard to find, particularly on three ways.

planet10 3rd September 2012 07:12 PM

Quote:

Originally Posted by Baseballbat (Post 3150000)
no, it can not. It will shift the panel resonance to higher frequencies, and will probably even increase the Q, so the resonance may become more harmful.

The opposite is more typically true. Raising the frequency of a (potential) panel resonance higher will (usually) make it less likely to get excited (as will increasing its Q).

dave


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