15mm vs 20mm Plywood for OB

[kaybee]

Observation
15mm vs 20mm Thickness Plywood
Sound Comparison for Open Baffle
Using Fostex 103e

Both boards were cut to same size, about
80 cm x 90 cm.

Conclusion first :
20mm Plywood is not too good a match
for this particular Fostex 103e fullrange spkr.
15mm Plywood sounds MUCH Cleaner.

By Cleaner, I mean to say that one can Effortlessly
Hear MICRO DETAILS on the thinner 15mm
Plywood, which are simply LOST on the thicker
20mm plywood. The 3 D Magic is Almost Gone
on the thicker plywood.

I would Guess that the heavier, and denser thickness
Means that certain Absorptions take place, and
Important MICRO DETAILS are gone, thereby
Greatly Killing the Reason of going OB in first place.

Micro Details means Micro Vibrations, carrying
Most meaningful Micro INFORMATION on
DETAILS recorded, be it low amplitude info,
Or Phase info, etc etc,

All this Important Golden Information is simply
Killed, or Damped by the thicker , heavier board.

Yes, ladies and gents.
It is becoming apparent that the Art of Loudspeaker
Design is very much the same as Art of Guitar or
Cello or Piano design�c. THE WOOD MATTERS
In many More ways than we care to think.

I would further like to Playfully Challenge Those
That are Wood Musical Instrument makers to try to
Make a very Accurate and very Musical Loudspeaker.
I bet they can.

In this observation, the Deadening effect of the 20mm plywood
Is not as bad as MDF, but it is going in that Same direction.

I am going back to 15mm Plywood for the moment.
Now it would be fun to test different Height vs Width
For this thickness For OB.

Oh. One more thing. I wish I had a sensitive accelerometer
Or some such device to see on my oscilloscope, what my ears
Are telling me.

When one Gently TAPS almost Anywhere on the 15mm
Plywood, one can hear Good Consistent Strong Tap sounds
Anywhere One taps, even if one ' s ear is at one extreme end
of the OB Board. Trying this same TAP test on 20 mm
tells one Two things immediately :

The Sound is totally different, by being Very Much Muffled.
Next, as you TAP progressively away from your ear
Position and Tap Onwards, the energy reaching your ears
Really dimishes fast for every few inches moved.

On 15mm Plywood, no matter where you Tap, the energy
Hardly dimishes�c

This means, that VIBRATION is needed to make OB work.
At least to some extent of achieving good Balance.

Put your Ear an inch away from the OB.
Put your left hand ON any part of OB, perhaps
The top edge.
Start tapping with Right hand knuckle.

As your Right hand moves away Tapping here and there,
Note that your LEFT hand tells you there is much
Vibration on the 15mm plywood no matter where you tap.

However, for the 20mm, as your Right hand moves away,
even by few inches at a time,
The vibration reaching your Left hand , really dimishes
Quickly.

I further guess that the resonant frequency or frequencies
Of the 15mm are one matter, where as the 20 mm is
Quite another matter. I am not a Pro Musician , but
The difference is like going down 1 or more octaves
On the 20 mm and
Very Much Muffled Damping going on also.

All this , is also about DAMPING, be it
Just Right, or OVER or UNDER. Call it
Ringing, or vibration or resonant points, or
Whatever, but the principle is similar to building
A good sounding guitar VS. a very poor sounding one.
The WOOD MATTERS a LOT.

Pls note the obvious for
Basic Plywood OB and 103e system such as this,
Pls make sure the bottom of OB IS TOUCHING THE FLOOR.
Full Contact must be made. This mechanical coupling is a must.

OB seems to be about FREEDOM and Simplest Design Possible,
Using Most Logical Best materials matched to the speaker at hand.

Be good

k.

[Cal Weldon]

Interesting results. I like someone that uses their ear to test. We seem to get caught up in the graphs all too often.

Tell us, was the driver front mounted, flush mounted and did you round off the inside of the hole?

[Geoff H]

I tend to agree with Cal on that. Perhaps I'm too stubborn to do all the calcs. But at the end of the day, it is our aural senses we wish to please, not the masses.

Comparing baffles with musical instuments. The sound board on a string instument is supposed to radiate sound. The strings alone don't radiate much. A baffle is supposed to be dead accustically speaking, or have I got it wrong with OBs.

If the drivers are front mounted, the thicker baffle will have a greater shielding effect. Try mounting the drivers on the rear, to see if the same applies. That's quicker than rebating or shamforing.

On the other hand, if weight or expense are an issue, a larger, thinner baffle may have an edge.

[preiter]

With an open baffle speaker, it's possible to decouple the driver from the baffle. Look how Linkwitz recommends that drivers be mounted to the baffles on his speakers (pressure mounted from the back).

I know some have also experimented with suspending the driver by it's frame/magnet, and attaching the baffle by various acoustically isolated means.

If the drivers are decoupled from the baffle, the baffle materials shouldn't make much difference, you can even try cardboard or styrofoam as baffles.

[johninCR]

I agree with Cal. We really need to know how the driver was mounted and the shape of the driver cutout before any conclusions can be made. Without flush mounting AND a very open rounded backside of the driver cutout, the 103 will sound poor on a 20mm thick board, because the frame openings will be very much blocked by the driver cutout. I've experienced this problem with another shallow basket Fostex, the 108Esigma. On OB it sounded very "closed in" until I sufficiently opened up and smoothed the backside of the driver cutout.

I don't think plywood is good for making any musical instruments. I've experienced enough baffle vibration in test baffles to know that I don't want to attempt to build any type of "singing baffles".

[marec]

I've experienced enough baffle vibration in test baffles to know that I don't want to attempt to build any type of "singing baffles".

Hear Hear

[Geoff H]

"I don't think plywood is good for making any musical instruments"

What is the sound board of a grand piano made from?

Many guitars have a 3 ply top. Some, a single ply top. Even solid electric guitars are laminated. Here we want that panel to vibrate.

The baffle vibrates out of phase with the cone, if it is rigid to the frame.
That is loss of efficiency and attack. Both no no's in my book.

Not sure about cardboard providing isolation at low frequencies, which is what the baffle has to do in order to reinforce the bass.

[kaybee]

Thank you all for reading .

Pls note a most important fact.

i am trying to Compare the ( sound ) Difference
of the Two thicknesses of Plywood here.

The two fostex 103e units were mounted
in the SAME identical WAY. so.......

( in fact, towards the end.. i moved around
the same driver unit from one ob to the other,
so i can be convinced that the not so subtle
differences i was hearing, was not due to the two
driver differences or stereo tube amp L vs R
difference )

experimentally speaking, this means the only
difference that i am HEARING , should be
the OB plywood Thickness difference effect.

( yes, well, the 15mm and 20mm are different
by 5 mm, so of course, there is the question
of the composition of what that 5 mm is ..too...)

for your kind info,

both were merely Screwed in from FRONT, plainly and simply,
with the four supplied screws.
the Round HOLE is a clean diameter of 97mm.
nothing special done.

anything material on our planet,
has at least one resonance point.

FFT ananlysis, musical NOTES analysis, etc...all exist,
but unfortunately i dont have them at my disposal,
to thoroughly check the full story of a plank of wood.

perhaps this plank is a C sharp, and this is a G ?

again, pls be most aware of the
micro vibration Detail robbing properties of many
materials too.

again, the idea is
how to Balance all these factors and choices
to realize a final working unit to get
Realistic Musical Synergy and Events...

in other words, to get my foot tapping as if
only the music and i exist .

i believe
if i can learn enough from these OB
experiments, i can move onto more complex
designs and cabinets in future.

be good.

k.

[johninCR]

Kaybee,

I appreciate what you are trying to do, however, I'm confident that the significant difference in sound that you are hearing is related more to how much the 2 thicknesses restrict the rear wave as it leaves the driver, than microvibrations in the baffle itself. The thicker baffle blocks the rear wave from free expansion to such an extent that it is affecting the operation of the driver itself, in addition to the impact on the rear wave, because it loads cone different on the back side. The thin baffle is closer to free air operation of the driver.

Significantly open the backside of those driver cutouts if you want to make a better comparison of the different baffle thicknesses.

[chrisb]

Originally posted by johninCR
Kaybee,

I appreciate what you are trying to do, however, I'm confident that the significant difference in sound that you are hearing is related more to how much the 2 thicknesses restrict the rear wave as it leaves the driver, than microvibrations in the baffle itself. The thicker baffle blocks the rear wave from free expansion to such an extent that it is affecting the operation of the driver itself, in addition to the impact on the rear wave, because it loads cone different on the back side. The thin baffle is closer to free air operation of the driver.

Significantly open the backside of those driver cutouts if you want to make a better comparison of the different baffle thicknesses.

John, you're absolutely right about that; what would you say about rear mounting the driver and machining a chamfer or radius on the front. On 20mm thick panel you'd end up with a short "waveguide" of sorts, and while it could be opined that this would not be a perfectly balanced load to the driver cone, I'm sure it would "sound" far more open than the highly compressed conditions that you describe above.

This is more or less what resulted with one of our variations on supra-baffles for the Ron Clarke A126 horns.