Effect from front baffle material?

[Oscar]

A friend of mine (no really!) is thinking about putting a plate of aluminum on the front baffle of his speakers. Might the aluminum material affect the sound? He claims it doesn't, I believe it does.

Any opinions?

[SY]

Well, if it rattles, it will certainly affect the sound. If it's bolted on good and tight, it will stiffen the baffle a bit which may or may not improve the sound, depending on how stiff the baffle was to begin with.

[roddyama]

The problem with aluminum and (most metals) is they have little or no intenal damping so they will tend to resonate (usually in the audible range). So, eventhough he may be adding stiffness, he will also need to effectively damp the aluminum plate so it doesn't resonate.

[Timn8ter]

So if a material like aluminum on the front baffle does not adversely effect the sound what is the benefit of using a soft material as I've seen done?

[Oscar]

And what about reflections? Might the sound waves that are reflected off the front baffle behave differently whether it's wood or aluminum? Or as in my speakers' case, little plastic pyramids...

[SY]

If the aluminum is bolted down or adhered so it doesn't rattle, the internal damping is irrelevant.

The issue with baffle materials isn't softness, it's acoustic absorbance versus reflectance. That's why felt and open-cell elastomeric foams are so popular; they won't necessarily improve sound, but they will certainly change it.

The little pyramids are mostly for show. They may have some effect in the 10-20K octave, but that's of relatively little importance- and the tweeter will be pretty beamy at that point so that the baffle won't have much effect anyway.

[roddyama]

Originally posted by Timn8ter
So if a material like aluminum on the front baffle does not adversely effect the sound what is the benefit of using a soft material as I've seen done?

It may very well have an adverse effect on the sound, or it may not, depending on how it is implemented. A stand alone aluminum plate will ring (resonate), you know what that sounds like. When installed as a speaker baffle, the ringing will need to be damped to an inaudible level. This is not as easy as it may appear on the surface.

Originally posted by Oscar
And what about reflections? Might the sound waves that are reflected off the front baffle behave differently whether it's wood or aluminum? Or as in my speakers' case, little plastic pyramids...

The aluminum will reflect the sound just as wood or plastic. It presents a smooth hard surface for the wave to form. Sometimes this is desired, sometimes not. Your speakers have some sound damping material attached to the baffle. This is typically to "control" unwanted reflected sound at certain frequencies. The designer must have found that the reflected sound was a problem so he tried to eliminate it, or the marketing dept. came in and said "these little pyramids on the front would look cool".

[SY]

Rodd, the wording of the original question implied (to me, anyway) that the aluminum wasn't a "stand-alone" but was being attached to an existing baffle. But I may have read it wrong, in which case you're 100% right. Unless the aluminum is honeycomb...

[Oscar]

Originally posted by roddyama
Your speakers have some sound damping material attached to the baffle.
...
or the marketing dept. came in and said "these little pyramids on the front would look cool".

Yeah, I guess it's mostly for show. Still, the B&W Matrix 3 are a fairly serious set of speakers.

Originally posted by SY
Rodd, the wording of the original question implied (to me, anyway) that the aluminum wasn't a "stand-alone" but was being attached to an existing baffle.

Correct. It would be a normal MDF box and then a flat aluminum plate would be bolted onto it, only for looks. So, if he managed to attach it very tightly so it will resonate in inaudible frequencies, there wouldn't be a problem?

Next question then, how to attach? Using lots of bolts maybe could work? But that might cause diffraction since you then have stuff sticking out, right? Glue? (Lots of it.)

[SY]

Screws can be rebated or countersunk. I'd also use some sort of gasket between the Al and the wood, preferably an adhesive. If you have access to 3M 468 sheet adhesive, it could serve very nicely. If not, a very thin sheet of elastomer will prove to be beneficial.