Cabinet phase

[tapehead ted]

Thanks everyone again so much for your time!

I can see plainly enough that any really low bleed from the back will wrap around and swamp the lows from the front, so I'm abandoning the idea of trying to use the sound from the back of the speaker like you would with an openback guitar cab, which isn't handling these low lows.

So I've been listening.

What I am hearing, seems to me, after a whole lot of time playing with mics and learning about phase conflicts-

There are places where it seems really phase healthy, and places where it sounds just like, in my experience, two sources with conflicting phase are summing at my ears.

Right in back it seems phase healthy. As I walk around the sides I hear the characteristic "wow" as two sources shift in phase relative to each other. Real slow and smeary, like "Whaooeoooww, Man..." Classic phase shifting, unmistakeable. I'm thinking this is the bleed through the crummy 7/16th plywood cab canceling with the sound from the front as it wraps around.

Again, there are no walls nearby, this is outdoors, so where else could the conflicting phase be coming from?

If I told you my cabinet was 1/8" plywood, but adequately braced, would you believe sound was leaking through the side? 1/4" plywood? why not 7/16" plywood?

I'm thinking a thicker, denser material will begin to drop the bleed down in decibels until it really ceases to matter. Maybe if I were performing these listening exercises with a well-braced 3/4" cab, I wouldn't be hearing this.

I'm hearing a different timbre that sounds, well, like plywood, that sounds distinctly different than the direct driver output or the port (which sound different too, and are noticeably different in phase relation to each other from different angles. This is clear enough in the higher frequencies, but it's harder to distinguish lower down, and I can't distinguish the two clearly towards the bottom of the midrange and below.

So right now I'm thinking some 1 1/4" hardwood, maple or walnut, would be a big improvement on this plywood, both harmonically and in decibels of bleed.

One of the main reasons I'm pursuing having a cab built to my specs is that all the commercially available cabs sound like plywood. This may be more obvious once you get the grill out of the way!

 

[walterw]

"sounds like plywood"?

what would you prefer for bass? (or PA for that matter)

solid wood (fragile, way more resonant, wildly expensive) or particle board (non-resonant but soft, heavy and won't survive getting wet)?

 

[Jim C]

Duke,
The ocean waves is a great visual analogy; thanks for that.

 

[billfitzmaurice]

So right now I'm thinking some 1 1/4" hardwood, maple or walnut, would be a big improvement on this plywood, both harmonically and in decibels of bleed.

There is absolutely no reason to use anything thicker than 1/2" plywood. All you'll get using 1 1/4" hardwood is a cab that you need a forklift to move. Bracing is the key. A properly braced cab looks like this inside:

If I told you my cabinet was 1/8" plywood, but adequately braced, would you believe sound was leaking through the side?

I have designed many cabs with parts made of 1/8" plywood, properly braced of course. They do not 'leak'.
Sound does not pass through solid materials per se. Sound will cause solid materials to vibrate, and that vibration is the source of sound heard on the opposite side of that solid material. If the material is made stiff enough so that it cannot vibrate, via a combination of mass and bracing, no sound will be heard on the opposite side. Doing so to low frequencies with large structures, like walls and floors, is very difficult. That's why bass is easily heard through them. Doing so with small structures, like speaker cabinets, is very easy. It's true that even the most well braced cab will vibrate, and that vibration will be manifested as sound outside the cab. But when the level of that sound is on the order of 30dB lower than that created by the cone, an easily achievable result, it's inaudible.

 

[tapehead ted]

Thanks again- I especially appreciate the illustration.

I know one function of the bracing is to push the resonant frequencies of the panels up out of the boomy range. I'm thinking this must be why I'm hearing the plywood sound so clearly in just that register- that must be the plywood resonating away.

The phase artifacts I'm hearing must be the cancellation between this plywood resonance and the driver output. It's not at all subtle. It's very certainly audible. I certainly can't just walk away and say it sounds fine, no problem. More to the point, I can't get interested in playing an instrument that sounds like this (that would include every other bass rig I've ever heard save one... in an unbraced pine cabinet with a single D-130... yeah I blew it up.) So I continue to burn the midnight lamp... hopefully not all alone.

The other reason I'm thinking hardwood is that softwoods tend to resonate more at the fundamental, and hardwoods at the harmonics. Also tonewoods sound good when they resonate- I certainly can't say that for plywood. I'd rather have more of a resonance that sounds good than less of one that sounds bad.

I wonder what happens to the phase of the sound as it passes through the wood... I'm guessing it's slowed down but barely.

Thanks again! I appreciate the education.

 

[rpsands]

Got your cabinet lined well enough on the inside?

 

[tapehead ted]

Got your cabinet lined well enough on the inside?

It's stuffed with that white fiberglas substitute. I've read that real fiberglas is far preferable, but I hate that stuff... I also read that long haired wool is the best of all, and as it happens I have some right handy... washed, so it's not all greasy...

I'll try replacing the stuffing.

 

[billfitzmaurice]

I know one function of the bracing is to push the resonant frequencies of the panels up out of the boomy range.

Myth. The purpose of bracing is to stop the panels from vibrating. In the process of so doing the panel resonant frequency goes higher. It's a side effect, you can't do the one without having the other. If a high panel resonance mattered you could make cabs out of 1/32" sheet metal.

 

[walterw]

If a high panel resonance mattered you could make cabs out of 1/32" sheet metal.

you lost me here;

wouldn't this thin sheet-metal cab have a hideously low resonance, flapping and generally misbehaving right in the middle of the bass frequencies? (how thick is the sheet metal used to make "thunder sheets" for foley effects?)

 

[will33]

you lost me here;

wouldn't this thin sheet-metal cab have a hideously low resonance, flapping and generally misbehaving right in the middle of the bass frequencies? (how thick is the sheet metal used to make "thunder sheets" for foley effects?)

Think of blowing across a tightly streched reed, or blade of grass. Low frequencies ain't going to get it moving....higher ones will.

 

[walterw]

Think of blowing across a tightly streched reed, or blade of grass. Low frequencies ain't going to get it moving....higher ones will.

in this example, think of a thin cymbal vs. a thick one; the thinner cymbal definitely has a lower pitch.

the material is thinner, but not "tightly stretched".

 

[will33]

That's a good point. I'm not the guru here, just playing along, although, I've always heard heavier cymbals as having a fuller (deeper) tone with a higher "hit" or attack. Thin ones don't have the fullness or body to them....seems to be all their middle sound....althoigh, there's a real possibility it had more to do with the drummer playing them.

I was with my now current drummer, several years ago, when he was shopping cymbals. The guy showed me stuff and explained cymbal sound to me I would've never otherwise understood. I can play next to this guys kit all night and it sounds like music and tone at any volume. Some folks stuff just sounds like noise...it's harsh and not musical.

This is a more "meat-n-potatos" guy on drums, like I am on bass. He can be Bonham when it's called for, yet never has any of those little stick dents in his heads no matter how loud he plays them.

 

[billfitzmaurice]

you lost me here;

wouldn't this thin sheet-metal cab have a hideously low resonance,

Rap the outside of your car door. The sound made is not sourced in the bass. Now think about how car doors are made to work better as speaker enclosures: they're lined with dense damping materials like Dynamat. Dynamat doesn't raise the resonant frequency of door panels, it lowers it, reducing their propensity to vibrate.

how thick is the sheet metal used to make "thunder sheets" for foley effects?

1/32 to 1/16 inch. And they don't create low frequencies, they provide the midrange and high frequency content of the simulated thunder. The low frequencies come from a tone generator, the two sources are mixed for the final result.

 

[1958Bassman]

Thanks everyone again so much for your time!

I can see plainly enough that any really low bleed from the back will wrap around and swamp the lows from the front, so I'm abandoning the idea of trying to use the sound from the back of the speaker like you would with an openback guitar cab, which isn't handling these low lows.

So I've been listening.

What I am hearing, seems to me, after a whole lot of time playing with mics and learning about phase conflicts-

There are places where it seems really phase healthy, and places where it sounds just like, in my experience, two sources with conflicting phase are summing at my ears.

Right in back it seems phase healthy. As I walk around the sides I hear the characteristic "wow" as two sources shift in phase relative to each other. Real slow and smeary, like "Whaooeoooww, Man..." Classic phase shifting, unmistakeable. I'm thinking this is the bleed through the crummy 7/16th plywood cab canceling with the sound from the front as it wraps around.

Again, there are no walls nearby, this is outdoors, so where else could the conflicting phase be coming from?

If I told you my cabinet was 1/8" plywood, but adequately braced, would you believe sound was leaking through the side? 1/4" plywood? why not 7/16" plywood?

I'm thinking a thicker, denser material will begin to drop the bleed down in decibels until it really ceases to matter. Maybe if I were performing these listening exercises with a well-braced 3/4" cab, I wouldn't be hearing this.

I'm hearing a different timbre that sounds, well, like plywood, that sounds distinctly different than the direct driver output or the port (which sound different too, and are noticeably different in phase relation to each other from different angles. This is clear enough in the higher frequencies, but it's harder to distinguish lower down, and I can't distinguish the two clearly towards the bottom of the midrange and below.

So right now I'm thinking some 1 1/4" hardwood, maple or walnut, would be a big improvement on this plywood, both harmonically and in decibels of bleed.

One of the main reasons I'm pursuing having a cab built to my specs is that all the commercially available cabs sound like plywood. This may be more obvious once you get the grill out of the way!

You're hearing phase shift and modulation because you're moving your position, relative to the speaker. That's how a Leslie speaker works. If you were to put the speaker on a turntable and rotate it, you would hear the same thing. This has less to do with the cabinet "leaking" than the varying phase. If you want to demonstrate this on a very small scale, make sure you're in a quiet room- cover your mouth with your hand and move it away from your face and toward your face as you exhale through your open mouth. As you move your hand, listen for the phasing sound. Move your hand at different speeds, too.

A well-braced cabinet won't have a "sound" of its own. A cabinet that was made to hit a certain price point will. If the cabinet is built to "cost is no obstacle" specs, it should be able to produce the sound of whatever signal is fed to it, with whatever coloration the speaker imparts. If you put your hand on a cabinet and feel the panels vibrating strongly with even low SPL output, it's not properly braced for these frequencies.

 

[SurferJoe46]

Would a down-firing speaker help eliminate all this conjecture?

If 360º purely equal projection is needed, I don't see how a driver placed vertically can suffice.

 

[billfitzmaurice]

Would a down-firing speaker help eliminate all this conjecture?

If 360º purely equal projection is needed, I don't see how a driver placed vertically can suffice.

Down firing filters out the mids and highs. True 360 degree broadband dispersion requires something like this:
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[DukeLeJeune]

Now think about how car doors are made to work better as speaker enclosures: they're lined with dense damping materials like Dynamat. Dynamat doesn't raise the resonant frequency of door panels, it lowers it, reducing their propensity to vibrate.

Dynamat does a nice job on the backside of plastic horns, too. I think that the stiff metal outer layer combined with the lossy, sticky layer results in a constrained-layer damping effect. Anyway it reduces a plasticky finger-flick "panggggg" to a dull "thonk" on the horns I use.

 

[tapehead ted]

I'm a huge fan of Leslies, and have actually put a lot of time into speaker animation, with some really rewarding and inspirational results. In the guitar frequency range though, a piece of cake compared to real bass.

And I've spent hours entertaining myself with all kinds of animated sounds- spinning gongs and the like... I enjoy those sounds.

Not so much this. I'm convinced it's mostly bracing problems. This cab was definitely built to a price point. The bracing is 1/2 plywood too, and from this source I'm thinking it's just not doing the job.

(link didn't work- try www.audioholics.com and look for "a detailed look at proper cabinet bracing".

The side with the vertical duct in it (pointing 90 degrees away) sounds the best by far. I've been wondering about this, and I think it's likely that because there are two panels an inch apart with bracing on both sides of the inner one, it's just far better braced than the other sides and the top is definitely not well braced at all. And the top definitely sounds worst...

I wonder if the fact that the sound was shifting into phase in the duct, and hence on the other side of the outside panel, has anything to do with it. So to the extent it's still resonating it's doing so in phase with the driver output. Perhaps plywood has it's charms when it's resonating in phase!

I'm not feeling much vibration. Still it seems to be enough to color the sound yucky.

Thanks everyone for turning this into a very interesting thread.

FWIW I know Berlioz didn't have a tone generator! I wonder if that's scored for some big soft mallets on the bass drum too.

You're hearing phase shift and modulation because you're moving your position, relative to the speaker. That's how a Leslie speaker works. If you were to put the speaker on a turntable and rotate it, you would hear the same thing. This has less to do with the cabinet "leaking" than the varying phase. If you want to demonstrate this on a very small scale, make sure you're in a quiet room- cover your mouth with your hand and move it away from your face and toward your face as you exhale through your open mouth. As you move your hand, listen for the phasing sound. Move your hand at different speeds, too.

A well-braced cabinet won't have a "sound" of its own. A cabinet that was made to hit a certain price point will. If the cabinet is built to "cost is no obstacle" specs, it should be able to produce the sound of whatever signal is fed to it, with whatever coloration the speaker imparts. If you put your hand on a cabinet and feel the panels vibrating strongly with even low SPL output, it's not properly braced for these frequencies.

 

[walterw]

like duke and bill have pointed out, i don't know how you could begin to hear the side panels of the cab when the speaker was right there filling the air with real sound.

i imagine if you built some kind of iso box, you'd maybe hear what the side panels themselves sounded like.

a lot of this (plywood "sound" vs. solid wood, bass "traveling through" the cab walls, etc.) strikes me as conclusions looking for observations.

 

[billfitzmaurice]

i don't know how you could begin to hear the side panels of the cab when the speaker was right there filling the air with real sound.

You can't. That's why really sophisticated testing uses dial indicators to measure panel flex, and extrapolate from that if panel stiffness is adequate. But as far as that's concerned the old fashioned 'knuckle' test is still sufficient.