Discussion on what materials to build speakers out of

[rick57]

Usually cotteon or wool-felt. Sometimes polyfluff.

. . I thought you might have meant *just air . .

(With say a ‘typical’ vented midwoofer) how do you decide what amount to use/ start experimenting with?

Thanks

 

[TNT]

The actual air moving inside the box actually has much, much less energy to drive a panel resonace than the mecanhcal mechanism that attahes the driver to the box... this is why push-push is so effective, The majority of that energy is actively cancelled. And airspace energy to drive a panel resonance declines very rapidly with frequency.

And further, a lot of the stuff used in practice to damp panels, stores energy.

dave

Yepp - it really works, on these I can go flat out but not feel any vibration - stainless steel is nice ;-)

http://www.diyaudio.com/forums/showthread.php?postid=989942#post989942

/

 

[Ed LaFontaine]

I appreciate what Ricky offers:

Decide what thickness cabinet you want. Let's say 30mm. That's 12mm of hardwood flat pressed to, wait for it, 18mm double density chipboard. One tip. Put a layer of veneer on one side of the chipboard. This will remove the "bendy spoon" element

I'd like to broaden several points touched upon in describing the use of "thick veneer".

If success is measured by the veneer remaining crack-free and with tight joints, that success will depend upon several things.

Limit the changes in humidity the case experiences. These will drive the woods expansion or contraction.
Limit the width of panel. Width will determine the total amount of movement that might be experienced.
Limit the thickness of veneer. Thicker veneer will behave more like wood. It will contain greater forces of expansion & contraction and could overcome the adhesive's ability to keep it from moving.
Grain orientation in the veneer. Quartered or rift-sawn wood moves less than flat sawn.

 

[MJL21193]

The actual air moving inside the box actually has much, much less energy to drive a panel resonace than the mecanhcal mechanism that attahes the driver to the box... this is why push-push is so effective, The majority of that energy is actively cancelled. And airspace energy to drive a panel resonance declines vwey rapidly with frequency.

And further, a lot of the stuff used in practice to damp panels, stores energy.

dave

Your push-push allignment, if wired in phase, cancels sound waves too, not just mechanical movement, so you really can use that as an example.
Energy from mechanical movement of the driver is SWALLOWED by the sound energy.
Yes, higher frequencies have less energy to drive panel resonance, but that's not all you deal with in a speaker.
Driving the resonance frequency up increases decay time. The higher the frequency, the longer it will resonate before decaying. Panel damping speeds up this process.

 

[MJL21193]

No. You haven't grooked what i'm trying to do.

If we assume that a panel will resonant no matter what we do (true), then what i am trying to do is move the resoances to a place where they will never be excited. If excited by an external source (ie a whack with a steel rod) it is not silent ... but that is not something that happens when you are listening to music.

Think of it as magic, sleight of hand.

Not hard to understand what you are trying to do Dave. What we have to assume is that there will be energy released into the box which has to be dealt with in an effective manner.
My repertoire of tools and methods to fix this problem does not include magic. I'm more of a hard science, show me how it works, type of guy.
I hear the term "energy storage" wrongly used to discribe decay time. I see "Q" used to add a little scientific sound to the arguement.
Energy, from whatever souce, has to be dissapated, it will not leave of it's own accord. Effective damping will do this.
Long and short of this is that you use damping material - wool felt, cotton. You made it seem as though no damping was used at all, just air. To fill an air space(even partly) with damping material, you are damping panel resonance.

 

[MJL21193]

John,
did you finish the two boxes, mdf/bb and measured them?

Cheers
Peter

Peter! See earlier in this thread pics of finished baltic birch box. Also as mention earlier, I need to get a sheet of 1/2 MDF to build that one. I thought I had some, but it was 5/8". I will be getting busier with work over the summer and will not have as much time to devote to the "hobby", but will be getting the other two boxes together shortly.
Also, for me, I have another pair to finish - threeway ripoff design of the 3A Integral, a shape I admire. Started these after Xmas, still don't have the woofers.
BTW, I will be measuring nearfield frequency response of each material, to determine the audible qualities of it's resonance. I have(as yet) no means to measure actual resonance.

 

[Svante]

Hej Svante!
Could it be Svante as in the Edge sim?

MVH
Peter

Javisst!

 

[Svante]

PS: when John Atkinsom measures panel rsonances with his little device... what kind of device is that and where would one get one (or get something to repurpose)?

It must be an accelerometer. It is basically a piezoelectric crystal and an attached mass. The piezo produces a charge that is proportional to the applied force, and the force is, through F=ma, proportional to acceleration. The device should be connected to a charge amplifier, which produces a voltage that is proportional to charge.

Net result: a voltage proportional to acceleration.

I don't know where to get one, but a google for "accelerometer" would probably help. It is somewhat tricky to make accelerometers sensitive in one direction only, so there is a reason they can be expensive.

 

[claudio]

I’m not suggesting you’re wrong, but has this been measured?

On another aspect, might the ratio of internal sound energy to panel resonance differ for sealed and vented? Or not because were so far above the vent's range

Hi Rick,
Dave is right in my opinion. Some measurements about it were done by Moriyasu on Axpress 02/2002.

A sealed enclosure has more damping material inside than a reflex one, therefore the air trasmitted vibrations are lower in amplitude.

 

[claudio]

It must be an accelerometer. It is basically a piezoelectric crystal and an attached mass. The piezo produces a charge that is proportional to the applied force, and the force is, through F=ma, proportional to acceleration. The device should be connected to a charge amplifier, which produces a voltage that is proportional to charge.

Net result: a voltage proportional to acceleration.

I don't know where to get one, but a google for "accelerometer" would probably help. It is somewhat tricky to make accelerometers sensitive in one direction only, so there is a reason they can be expensive.

Cheap uncalibrated accelerometer, the same brand and model sold by Audiomatica: MSI ACH-01-03/10, available at Mouser P#824-ACH-01-03/10

 

[planet10]

On another aspect, might the ratio of internal sound energy to panel resonance differ for sealed and vented?

Here i think we need to change from sealed & vented, to high pressure & low pressure boxes. I avoid both BR & sealed boxes in favour of aperiodic or TL boxes. They are low pressure.

dave

 

[planet10]

Long and short of this is that you use damping material - wool felt, cotton. You made it seem as though no damping was used at all, just air. To fill an air space(even partly) with damping material, you are damping panel resonance.

That is airspace damping, not panel damping... i was quite clear about that... none of those are effective into the low frequencies and do little to damp a panel (you only get an affect of less excitation of panel resonances if the panel resonances are high enuff to be above the effective minimum of the damping materials frequency range) ... when we are talking about panel damping we are talking things like the bituminum pads used by the BBC, tar-like substances applied to the inside of the panels, etc.

dave

 

[planet10]

Driving the resonance frequency up increases decay time. The higher the frequency, the longer it will resonate before decaying. Panel damping speeds up this process.

Why is that?

dave

 

[planet10]

Your push-push allignment, if wired in phase, cancels sound waves too, not just mechanical movement, so you really can use that as an example.
Energy from mechanical movement of the driver is SWALLOWED by the sound energy.

Any internal cancellation is minor in the scheme of things and only happens at very specific frequencies related to the distance the drivers are apart -- one hopes that these are high enuff that the airspace damping is dealing with them. At low frequencies the air only sees one compund driver.

Have you ever seen a subwoofer walk -- you know that isn't caused by the air moving around.

dave

 

[planet10]

My repertoire of tools and methods to fix this problem does not include magic.

And neither does mine... it is just as magic as the magicians card trick... no voodoo going on at all... only looking at the problem from a different perspective.

There is much less panel resonance because the panel resonances are not excited.

dave

 

[planet10]

Not hard to understand what you are trying to do Dave.

But from all you comments it is clear you still haven't grasped the concept.

dave

 

[planet10]

Cheap uncalibrated accelerometer, the same brand and model sold by Audiomatica: MSI ACH-01-03/10, available at Mouser P#824-ACH-01-03/10

Thanx... i'll look that up.

dave

 

[johninCR]

I use real wood because I build mostly OB's, so other than the mechanical operation of the drivers there aren't a lot of forces on the panels, at least with flat baffles, and leaks from a split wouldn't be the end of the world. Recently I've been using magnet mounts and decoupling the driver frame from the baffle, yet I still have a problem with vibration and sound coming from these solid wood baffles.

What I'm finding is that a lot of sonic energy is getting into the wood at the driver cutout. It doesn't seem to have anything to do with the resonant frequency of the panel, just how well the wood itself transmits sound. SPL's are extremely high right at the cutout because it is so close to the driver, so sound enters there and spreads throughout the baffle. It's not something I have the tools to measure, but I can easily feel the vibrations and hear a lot of sound coming through the wood with the screwdriver handle to the ear trick.

I've been chasing this baffle vibration thing for a while, and have gone to extremes to ensure they aren't the result of direct transmission from the driver frame. Plywood prototypes end up sounding much better than identical dimensioned solid wood, because of the internal damping of the ply. I thought I was going crazy at first, when the prototypes sounded much better.

My plan to counter this problem is to use an MDF ring right at the driver, then decouple that from the real wood baffle. It also give me the added benefit of being able to easily swap out different drivers in a finished baffle, even with flush mounting.

 

[MJL21193]

Here i think we need to change from sealed & vented, to high pressure & low pressure boxes.

I could have sworn you said mechanical vibration from the drivers operation was the biggest sourse of energy in the box. If so, low pressure, high pressure - doesn't matter.

That is airspace damping, not panel damping... i was quite clear about that... none of those are effective into the low frequencies and do little to damp a panel (you only get an affect of less excitation of panel resonances if the panel resonances are high enuff to be above the effective minimum of the damping materials frequency range) ... when we are talking about panel damping we are talking things like the bituminum pads used by the BBC, tar-like substances applied to the inside of the panels, etc.

dave

Damping is damping in any form. I provided a pretty accurate definition of damping earlier, didn't you read it?. Panel damping happens when a damping material contacts the panel (tuning fork I mentioned earlier). Or are you using "slight of hand" to make it float in the airspace without touching the panels?

But from all you comments it is clear you still haven't grasped the concept.

dave

I have a good grasp of the concept. Anyone who reads (not skims over) what I have written here will see that.

 

[MJL21193]

Any internal cancellation is minor in the scheme of things and only happens at very specific frequencies related to the distance the drivers are apart -- one hopes that these are high enuff that the airspace damping is dealing with them. At low frequencies the air only sees one compund driver.

Have you ever seen a subwoofer walk -- you know that isn't caused by the air moving around.

Two identical sound waves, of the same magnitude and phase approaching each other will cancel each other out, even at low frequencies. Are you disputing this?
My favourite panel damping is and always has been carpet, not asphalt impregnated roofing felt. It's effective for breakup of standing waves and also is a very effective panel damping solution It seems you have associated the use of damping with inferiior box construction. It is possible to build boxes with undersized structure and damp the resulting vibration problem, but I am not advocating that. I say build the box well, keeping in mind the amount of energy that needs to be disappated, then use damping to deal with it. Damping is not only effective, it crucial for vibration controll. Without it the boxes will ring like bells, especially if you drive resonance upwards.