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0nL7_0n9 11th December 2005 02:00 AM

Miscelleanous questions
 
Hi! I would like to know if metal (aluminium for example) is not recommended for building a sealed box? I didn't see any box in metal so i wonder why? Do you think a F200A could sound good into a ~100 L sealed box (with foam)? Thanks for any answer.

ron.eddy 11th December 2005 02:23 AM

Enclosures made of metal would not be capable of damping the backwaves off of the loudspeaker. If these back waves are not absorbed or dampened by the enclosure, they will "escape" from the box and work to cancel the sound from the front of the driver. This is much like playing a loudspeaker in no box at all, where you can notice it doesn't have nearly the same volume or low frequency response of the enclosed version of the same speaker.

Building boxes with MDF or particle board, etc... are much better since they are much better at damping sound.

http://www.silcom.com/~aludwig/Louds...a_sound_sponge

I know nothing of the F200A.

- Ron

0nL7_0n9 11th December 2005 11:35 AM

The thing I didn't tell is that I was thinking about replacing a 20 mm MDF by an equivalent mass in steal or aluminium. Is this really cannot separate the backwave from the frontware? For damping the backwave I wanna use a waving-foam (see http://img-europe.electrocomponents....R408822-01.jpg). For the F200A I talking about the Fostex F200A (see http://www.fostexinternational.com/d.../pdf/f200a.pdf).
What do you think about this?

ron.eddy 11th December 2005 05:32 PM

It is possible with the foam added in there that the back wave will be damped out, but what you could do if you want the metal look is to affix thin metal sheets to the current box. I did some searching and could not find anywhere which specifically said anything about using metal boxes.

It looks like, from my searches, that any metal used near sound is usually "sound damping metal" which is two sheets of metal with damping material in between. Nowhere can I find anyone using solid metal.

However, I also looked into various sound absorption coefficients from wood, metal, etc... It seems as though most woods for speaker building have a coefficient of around 0.5 for most audible frequencies. Metal, from all the charts I have seen is actually around the same as far as sound damping, it would seem.

BUT, while wood and metal have similar sound absorption rates, they still differ in that wood will mostly only pass longitudinal vibrations (pressure waves), where as metal will pass both longitudinal vibrations (pressure waves) and transverse vibrations (shear waves). What I think this means is that metal may work, but it would need to be braced VERY heavily so as the panels themselves do not vibrate, however I can't be certain. Please somebody correct me on this as its been awhile since I had accoustics class.

I do know the damping of a material is frequency dependent and it looks like the F200A has a flat frequency response through much of the 20-20kHz range, but you should only really need to worry about the lower frequencies getting through (which incidentally are most likely to vibrate the panels themselves).

Hopefully someone else who may have more hands on experience with this will reply to this thread, as I am going strictly on numbers and theory...

Regards,
Ron Eddy

ron.eddy 11th December 2005 05:43 PM

Oh, and I completely ignored the fact that metal reflects a lot of sound as well, as can be seen by using the acoustic impedance calculator:

Acoustic Impedance Calculator

With the addition of that damping foam you would eliminate much of the reflected waves, but since metal reflect much more sound than wood, it still won't be as effective as it would be in a wood enclosure.

- Ron Eddy

maxro 11th December 2005 11:26 PM

Quote:

Originally posted by ron.eddy
Oh, and I completely ignored the fact that metal reflects a lot of sound as well, as can be seen by using the acoustic impedance calculator:

Acoustic Impedance Calculator

With the addition of that damping foam you would eliminate much of the reflected waves, but since metal reflect much more sound than wood, it still won't be as effective as it would be in a wood enclosure.

- Ron Eddy

Awesome choices of materials in that applet. I'm thinking a lucite and beef speaker cabinet would look wikid.

max

ron.eddy 12th December 2005 01:54 AM

Quote:

Originally posted by maxro


Awesome choices of materials in that applet. I'm thinking a lucite and beef speaker cabinet would look wikid.

max


Haha, yea there aren't very good predefined choices, but you can enter in velocity and density yourself too. Beef?? Why? Wierd.

- Ron

Nanook 12th December 2005 03:59 AM

metal enclosures...
 
2 Attachment(s)
Celestion created an aluminum speaker with Aerolam, a membrane/honeycomb/ membrane. It was the SL700

Stereophile's SL700 review

enjoy the read.


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