Discussion on what materials to build speakers out of

[Ricky]

Let's no add another layer of complexity, and assume (reasonably) that sound does not travel along the grain structure.

AGREED!!!! That's what veneer's there for! (LOL)

 

[MJL21193]

It is the rubber backing that acts as a panel damper. Commercial carpet fibres will have little air space damping and will have little affect on standing waves. And in a subwoofer of that size, it is unlikely that any box dimensions are large enuff to get excited by the subs bandwidth... what XO frequency are you using?

dave

Correct Dave, the carpet in this application is only for panel damping purposes, not for standing waves. It's my belief (and quite a few others) that standing waves (due to the long wavelength) are not an issue in a sub woofer.
Crossover is integrated into HT receiver and I have it at 80Hz.

 

[MJL21193]

It is the rubber backing that acts as a panel damper. Commercial carpet fibres will have little air space damping and will have little affect on standing waves. And in a subwoofer of that size, it is unlikely that any box dimensions are large enuff to get excited by the subs bandwidth... what XO frequency are you using?

dave

All fibres have some effect. I use other methods of dealing with standing waves also, the usual suspects: non-paralell walls, stuffing. I am not, however a big follower of the golden ratio, prefering to deal with the standing waves with the methods above.

 

[planet10]

I had a discussion with Lawrie Fincham of the original KEF and there are also issues that hardwood is in fact porous at certain frequencies.

As is MDF... it is quite a site to see a piece of MDF laid down on a vaccuum hold-down CNC and then see the pieces being routed placed on top of it and still very firmly held in place.

dave

 

[planet10]

It's my belief (and quite a few others) that standing waves (due to the long wavelength) are not an issue in a sub woofer.

It is not... the dimensions of a subwoofer tend to be too small to support a standing wave at the frequencies it opperates in... this isn't a belief, it is physics.

For a box to support a standing wave at 80 Hz it would need to have a dimension of 85". At 160 Hz this would be 42.5" and we do see sunbs with dimensions of that order... the woofer signal would be down 18 dB with a 3rd order XO at 80 Hz thou.

dave

 

[johninCR]

Let's no add another layer of complexity, and assume (reasonably) that sound does not travel along the grain structure.

I don't think I can agree with that at all. I'm getting significant sound through the edges of these little dipole baffles, and it has to be coming from the driver output at the cutout. Plus I've hammered quite a few wooden poles into the ground without the sound of the strike changing all that much. This goes for pilings too. I'm sure the ground typically presents pretty good damping, yet I have no doubt that sound travels right through the wooden poles to the other end. I've also noted reduced vibrations across the grain from the cutout compared to the output along the edges along the grain of the wood.

I've thought about trying a constrained layer to damp these solid wood baffles, but I think it will be a waste of time for the most part.

 

[MJL21193]

It is not... the dimensions of a subwoofer tend to be too small to support a standing wave at the frequencies it opperates in... this isn't a belief, it is physics.

I was distracted over at the fan subwoofer thread. Dave, you might want to check out some of THEIR ideas. They really got it going on.
Physics? Really? Here in the DIYAudio forum physics is a pliable concept, not a branch of science.
Ok, let's see, what else can we argue about?

 

[MJL21193]

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.

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.

John: You say there is sound coming from the baffle, it must be vibrating, you need to damp it. I don't think this is a specific problem to solid wood - MDF and plywwod would behave the same way.Also, you say sound is traveling through it, like a pipe, from the driver cutout to the edge. This is also possible (and likely, sound travels through all materials). Once again, damping should correct this .
Your plan to decouple the driver frm the bafle wll solve mechanical transfer (if it truely is decoupled), but will not solve the sound wave transfer, unless the material you use to isolate is a good damping material, which will act to partially damp the baffle.
Make sense? If all else fails, cover with orange shag carpet.

 

[MJL21193]

I've thought about trying a constrained layer to damp these solid wood baffles, but I think it will be a waste of time for the most part.

Constrained layer is a method of panel damping. How could it be a waste of time?

BTW, when I said it doesn't travel along the grain I meant that it travels relatively evenly throughout the wood.

 

[MJL21193]

Plus I've hammered quite a few wooden poles into the ground without the sound of the strike changing all that much... I'm sure the ground typically presents pretty good damping, yet I have no doubt that sound travels right through the wooden poles to the other end.

No doubt the sound of your hammer on the end of the pole went though to the other end, but did it echo? The earth is damping the sound.
Here's a thought: Take that same pole and suspend it horizontily by strings in mid-air. Now strike the end with that same hammer. My guess is that it will continue to emit sound for a short while afterwards, ringing much like a bell would. To arrest this, grab it with your hand. It stops, you've damped the vibration.
Striking it on the side will give a different sound, you are producing a different sound wave. It will still vibrate for the same period.

 

[johninCR]

MJL,

You're caught up in the resonant ringing. Hold it in your hand while striking it and try to tell me sound isn't transmitted to the other end despite your hand damping it. Do the same with an MDF stick, or even a plywood stick, and neither will transmit the sound very well compared to the solid wood. Do it with a cross grained cut of wood too while you're at it.

Don't worry, it took me a while to catch on to the problem, and I have the physical items in hand. I have some beautiful solid wood stuff too, and the only way I think I can salvage them to my satisfaction is with a mounting ring of much more internally damped material like MDF that is decoupled from the baffle.

 

[MJL21193]

MJL,

You're caught up in the resonant ringing. Hold it in your hand while striking it and try to tell me sound isn't transmitted to the other end despite your hand damping it. Do the same with an MDF stick, or even a plywood stick, and neither will transmit the sound very well compared to the solid wood. Do it with a cross grained cut of wood too while you're at it.

How's the weather down there? I just came back from that region of the planet.
I AM caught up on resonant ringing - it does not belong in a speakers performance.
On the pole again:Of course the sound will travel to the other end. Damping doesn't stop sound waves - damping shortens the decay time of an oscillation. Sound or mechanical means produce the oscillation. The hammer strikes the pole, sound is produced, a shockwave of kinetic energy starts to travel through the pole, the pole starts to vibrate, you hand damps the vibrations by absorbing them, thereby shortening the decay time.
As Dave says, that's physics.

 

[tinitus]

Funny .... the other day I played with a woofer, no box nor baffle, just lying on the floor, and it made a HUGE bass ... but lifted up from the floor, bass completely gone

 

[MJL21193]

MJL,

You're caught up in the resonant ringing. Hold it in your hand while striking it and try to tell me sound isn't transmitted to the other end despite your hand damping it. Do the same with an MDF stick, or even a plywood stick, and neither will transmit the sound very well compared to the solid wood. Do it with a cross grained cut of wood too while you're at it.

If you were to construct a same sized pole from MDF, it would not sound much different. The resonant frequency of MDF is different from solid wood, but not by that much. Density is the key here. Use a pole made from steel for the same test, a higher and longer lasting "ring" will be heard. Higher resonant frequency due to higher density. Also internal damping from variations is the density will make a difference, but only in decay time.

 

[johninCR]

I'm glad we're in agreement then. I was just trying to make a point regarding a problem with using solid wood that I've never seen discussed, and the effect isn't subtle.

The weather is typical of dry season, perfect. I'm up in the mountains so it's ideal. Right now everyone is praying for rain. The national power company didn't expand it's output sufficiently and demand equalled output during dry season (while the hydro plants are at their lowest output each year). Equal at least until a big transformer blew at the principal dam, sending a shudder through the grid that fried some turbines at a couple of geothermal plants. Repairs can't be anytime soon, so we need rain to boost the hydro output. Until then we'll have to put up with blackouts alternating through the different areas giving everyone half day outtages 2-3 times per week. I have a generator, so it's more of an inconvenience than anything.

A drought would devastate this country in just one season, so if it comes I may have to head up to Dave's neck of the woods.

 

[Pit Hinder]

John in CR,
as to your OBs doing a merry song and dance...yes, they are prone to. In pre-electronic days, when thunder was needed during a theater performance they just shook a large piece of sheet metal (called a Donnerblech in German - calling an old lady of a certain attitude an "altes Donnerblech" can result in bodily harm).

Decoupling the driver by using a felt ring and having the mounting screws only finger tight, is a good idea here - and if you can find screws made of plastic instead of metal, even better.

Pit

P.S. Or, as they are the nut-and-bolt type, should I have written bolts? Tech English is a bit confusing for one who rather speaks hunnish...

 

[johninCR]

Pit,

That's the obvious stuff that I've already done, so I'm not getting sound transmission to the baffle from the driver frame. What I've found is that with the little full rangers I'm using that sound is entering these solid wood baffles through the inside edge of the driver cutout because the spl is so high within an inch or so of the driver cone.

I'll try lining the cutout with some modelling clay or duct seal, but I don't think I have room for sufficient material. The next step will be a larger ring using a material much better damped than the solid wood. Then decouple that in its attachment to the baffle. Then the solid wood will only be exposed to output travelling parallel to the baffle.

There's a slim chance that the vibrations and sound in the baffle are the product of edge diffraction, but I don't think so because the baffle edges are sanded to a point in hopes that the front and out of phase rear edge diffraction net to zero.

 

[454Casull]

If you were to construct a same sized pole from MDF, it would not sound much different. The resonant frequency of MDF is different from solid wood, but not by that much. Density is the key here. Use a pole made from steel for the same test, a higher and longer lasting "ring" will be heard. Higher resonant frequency due to higher density. Also internal damping from variations is the density will make a difference, but only in decay time.

I'm pretty sure the modulus of elasticity also is a factor in determining resonant frequency.

 

[rick57]

dave

Whoa this thread has run

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

Makes sense, though:

> I avoid both BR & sealed boxes in favour of aperiodic or TL boxes. They are low pressure.

Wouldn’t BR be close to TL, and aperiodic be a little lower than sealed?

wool felt, cotton "effective into the low frequencies"

Now even more intrigued. This is more effective than the bitumen pads used by the BBC?

I can see the benefit of low mass: I’ll try to hunt a copy of audioXpress February 2002, thanks Claudio.

Cheers

 

[MJL21193]

I'm pretty sure the modulus of elasticity also is a factor in determining resonant frequency.

It's a factor in how much abuse from John's hammer that pole can take.