[jules]

No information from either of you but merely the hint that if you could spare the time, my goodness, could you rip into the facts!

There has been much said here about the evils of cabinet resonance. "Resonace is colour"!!!

Well, what is the blindingly obvious message of the speaker-in-the sand experiment? You immerse a speaker in sand, you dramatically reduce the output from the port. So, the reason you had sound coming out of the port before was that you have [controlled] forced resonance. So, controlled cabinet resonance works in concert with the air column to enhance the bass production of the box. This is the essence of bass reflex design!

I have to admit that my understanding of this subject has increased the more I've thought about it so now, I take back the suggestion I made early in the thread about strategic use of a lead weight on a wall. The speaker-in-the-sand clearly demonstrates the error of overdamping and there's no reason why this reasoning should not extend to the reality that an overly rigid cabinet can have a reduced ability to make use of bass enhancement. In fact I understand there was a period when double walled cabinets with a sand shell were popular but they were based on a misunderstanding of bass resonance principles and were abandoned.

So, I'm about to go and enjoy my weekend but I believe there were a number of basic errors in this thread that shouldn't be allowed to become part of audio lore.

I have not got around to challenging various views on damping via cabinet stuffing but I see the same basic lack of understanding in that area as has been evident in the issue of speaker panel movement [from the micro to the macro level]

have a good weekend

Jules

[Daryl]

An infinitely rigid surface (forget about where it would come from) acts as an acoustic mirror to sound.

Sound simply is deflected upon striking a solid surface like light striking a mirror.

Now if sound were to strike a surface that is not infinitely rigid but say the exact opposite infinitely compliant the situation is somewhat different.

Now the sound is still reflected like a mirror but the difference is that the polarity of the reflection is reversed.

This phenomenon is taken advantage of in Labirynth and Transmission line cabinets.

If the surface is in between these unachievable extremes it's acoustic impedance will be resistive and/or reactive (all practical surfaces will be resistive AND reactive) then the reflected energy will be reduced in amplitude and phase shifted depending upon the specific acoustic impedance of the surface.

If the impedance of the surface being struck by sound is resistive and it's acoustic impedance identical to that of air the sound wave will be absorbed by the surface and no reflection will occur.

Keeping the acoustic impedance of the surface resistive but increasing it's acoustic impedance (rigidity) somewhat above that of air will cause a partial reflection to occur, the rest of the energy being absorbed.

As the acoustic impedance is increased further above that of air the amount of reflected energy will increase.

Complete reflection of all energy occurs when the acoustic impedance (rigidity) reaches infinity.

When the acoustic impedance of the surface goes below that of air the same is true but the reflection is in the opposite polarity.

Partial reflection occurs but in the opposite polarity with the amount of energy reflected increasing as acoustic impedance approaches zero.

A surfaces acoustic impedance also will be reactive.

The inertia of mass is reactive like an inductor and a spring (compliance) is reactive like a capacitor.

Both of these mechanical properties will impede the movement of a surface but neither absorbs any energy.

If the acoustic impedance of a surface were completely reactive it would not absorb any energy and all energy would be reflected completely.

If a surface had a completely reactive acoustic impedance and it's magnitude was the same as that of air all energy striking it would be reflected completely but the phase would be shifted 90 degrees (half way between normal and inverted polarity) either leading or lagging depending upon whether the surfaces reactance was inertia or compliance.

Keeping the surfaces impedance completely reactive but uncreasing it's magnitude above that of air maintains the complete reflection of energy but the phase shifts toward normal polarity.

When the impedance reaches infinity the phase of the reflected energy is completely restored to normal polarity.

The same is true when the acoustic impedance falls below that of air for a surface with a completely reactive acoustic impedance except the phase shifts toward inverted.

When the impedance reaches zero the phase of the reflected energy is completely in the opposite polarity.

A practical surface will have an acoustic impedance that is both resistive and reactive.

Some of the energy will be absorbed and some reflected.

Daryl

[Scotty]

Having read most of this thread I have noticed that confusion exists about the definition of damping of a speaker enclosures walls for the purpose of suppressing internal resonances at frequencies other than that which the enclosure is tuned for and suppressing enclosure flexure and unwanted radiation by the walls of the enclosure at system resonance and above.
  What has not been mentioned in this string is the term QL which in Theile-Small theory is the the term related to box loss. The example that this term directly applies to in this thread is that of the box buried in sand. The boundry between
the wall of the box and the air outside the box is a discontinuity which prevents acoustic energy that is inside the box and energy that is in the walls from easily
leaving the enclosure. Even when the the walls are vibrating little of the energy that is in the air of the enclosure is actually exiting the box through this path. Most of the energy is exiting the port and contributing to bass reinforcement at the desired frequency. When the box is buried in the sand a better impedance match between the acoustical energy in the enclosure and the enviroment outside exists which provides a path of lower resistance for the acoustical energy to leave the enclosure without contributing as much energy to the port
radiation. The box now looses more energy through it's walls than perhaps was allowed for with the original QL term value. The condition of burying the loudspeaker in sand could be allowed for as part of the design which would result in adequate port output for bass reinforcement.
The above speaker in the sand analogy is obviously flawed. The speaker should have been buried in concrete that was allowed to set. The walls of the enclosure
would not be able to move and a substantial discontinuity and impedance mismatch would exist between the air in the box and the surrounding concrete.
Theoretically the port output should acually increase as a result of the enclosures greater rigidity and lower QL figure.
  Ideally speaking the enclosure should loose no energy through absortion into it's structure or through transmission through it's walls at or below the frequency which the box is tuned to. An ideal enclosure would also not radiate acoustical energy through it's walls or from it's vent at frequencies above the vents design frequency. The design objective is to minimally absorb or dissipate
energy in the enclosure at or below the operational frequency of the vent while
maximally attenuating radiation from the walls of the enclosure and the port  above the ports tuning frequency. An enclosure could be said to be overdamped as soon as a drop off
of port output was noticed upon the addition just a little bit more dampening material. The port output should be measured with a swept tone with no dampening material in the enclosure. This is the best case scenario for maximum
bass reinforcement from the port. The ideal dampening material would not absorb bass frequencies at or below the ports design frequency and would be
highly absorptive at higher frequencies. Increasing the enclosures rigidity through bracing of the walls and breaking up the size of panels that can resonate into smaller sizes of unequal area is advantageous. Smaller areas of the enclosures walls can vibrate and the frequency that they can vibrate at will be higher and
thus less likely to be excited by the frequency that the enclosure is tuned to.   
Wall flexure at or below the tuning frequency that the enclosure was designed
for is not a desirable condition as acoustical  energy is being dissipated in the form of mostly incoherent radiation from the walls instead of reinforcing the bass output from the port in a phase coherent manner. Significant acoustical radiation from walls of the enclosure above the tuning frequency of the system
can lead to the perception of coloration and a loss of detail in the lower midrange and above.
  When someone tells you that they have taken wall flexure into account and incorporated it into their design they are saying that dampening material on the walls of the enclosure is cheaper to do and weighs less than a speaker design
with proper bracing and rigidity.
  A word about rigidity and bouncing. A sound wave can be considered analogous to a SuperBall. A SuperBall will bounce higher if dropped onto a rigid concrete floor than if it is dropped onto an elastic surface like a trampoline.
Try it and observe the results. Elastic deformation of a surface always results in a loss of energy and less reflected energy. Elastic deformation of a material is completely unrelated to and is not required in order for a ball to bounce or a sound wave or light wave to reflect off of a surface it impinges upon.
The effect of cabinet construction on the efficiency of a vented design and the size of the cabinet for a given design is not insignificant and is accounted for in Theile-Small theory by the QL term.
Nice dissertation on mechanical impedance and reflection Daryl.
Scotty

[JohninCR]

Daryl & Scotty,

Thank you for the detailed explanations.

[jules]

Scotty,

I'm a bit busy at the moment so I'll only reply to a part of what you've said.

quote Scotty:

"Elastic deformation of a material is completely unrelated to and is not required in order for a ball to bounce or a sound wave or light wave to reflect off of a surface it impinges upon."

This is much the same assertion that Daryl made but consider this ... If the super ball hit your hand what would happen? Would you feel the force or would it, as you are suggesting, be an internal thing? In order for the ball to bounce, there is an exchange of energy. Sorry, basic physics.

jules

[Daryl]

Hi Jules,

There is always an exchange of energy, however.

If the surface being struck by the ball or sound were infinitely rigid no energy exchange would occur between the ball or acoustic wave and the surface.

Where the ball is concerned the exchange of energy in the bounce is from the kinetic energy of the balls inertia to the static energy stored in the ball when it strikes the surface and it's shape is deformed.

When the ball comes to a complete stop the entire inertial kinetic energy has been converted to static energy stored in the deformed ball.

The ball regaining it's original shape gives up all of it's static energy converting it again to kinetic energy as it propels itself in the opposite direction to the original velocity.

The energy exchange was between the deflection of the rubber and the inertia of the balls mass only.

An acoustic wave is a constant energy exchange static, kinetic, static, kinetic (pressure, velocity, pressure, velocity) repeating every wave length.

When an infinitly rigid surface is struck the pressure at the surface builds up to twice the pressure of the wave in free air.

The pressure must equalize and since it is up against a fixed surface it equalizes in the opposite direction.

The energy exchange is in the medium only.

Using the mirror analogy the fixed surface being an acoustic mirror creates the existence of a phantom driver on the other side of the surface the same distance away from the surface as the actual driver.

At the fixed surface both the actuall and the phantom driver are equidistant and the amplitude of sound is doubled due to the contribution of the phantom driver.

Daryl

[jules]

An illustrated point about "infinite rigity" ...

What we are talking about here is relative rigidity. Now I know that articles on speaker boxes refer to a "rigid" box but ...

If a mouse dances on a 4X2 supported at both ends on bricks a few metres apart ... it's going to reckon that's rigid. An elephant would disagree.

Let's imagine an infinitely rigid piece of string. You could lay it between two bricks a mile apart, put a mack truck on it and you'd get no deflection.

A rigid object would have no frequency repsonse and no ability to deflect. Now, if you are right Daryl, this makes it a 100% reflector BUT if an exchange of energy is required the infinitely rigid objst just can't do it in my book. For sound to travel from a speaker and out through a port, it has to bounce of the walls of the cabinet. If it can't bounce, it doesn't come out.

Daryl, as I was typing your reply came in ... it contradicts one of your assertions earlier in this thread which I'm not going to chase up right now as I want to get away from here, that there was NOT an exchange of energy.

jules

ps I think you might have done some reading since last time

[Daryl]

I think I probably meant that no exchange of energy was necessary between the medium and the surface.

But I'm not shure to exactly which statement you are reffering.

When I read my posts over I constantly notice not only mispellings and missing/transposed words but that I could have said it more clearly another way.

I'm shure everyone else feels the same.

One neat thing about this board is that you can go back and correct your posts.

Daryl

[jules]

what are you implying here Daryl ...

quote:

"One neat thing about this board is that you can go back and correct your posts."

jules

[Daryl]

No implications Jules.

But I like to fix spelling and such.

No I wouldn't do anything dishonest or covert if that's your angle.

Daryl

[Scotty]

Jules, If you are convinced that in order for a sound wave to reflect back from a surface the surface must first be elastic there is nothing I can do about it.
 I would suggest further conversations with Prof. Wolfe concerning loudspeakers  with particular emphasis on the application of Theile-Small theory to vented loudspeaker design. He should be able to clear up any misconceptions about why
loudspeakers function as they do or be able to suggest printed reference works
which deal in depth with this subject which is beyond the scope of an online discussion. For those interested in doing further reading on the subject of loudspeaker design here is a link to a list of useful resources on the subject.
http://tinyurl.com/rc372
Scotty

[jules]

Scotty,

there is no such thing as a substance that is not elastic.

I also suggest you read the contribution from Prof. Wolfe more closely.

jules

[Daryl]

Hi Jules,

Scotty,

there is no such thing as a substance that is not elastic.

I also suggest you read the contribution from Prof. Wolfe more closely.

jules

We discussed this already this thread.

Everyone understands nothing is perfectly rigid and no one would suggest that.

By using the perfectly rigid example to illistrate the fundamentals we are merely giving you credit for understanding that obvously nothing could be and also understanding that while none of us may be as intelligent as yourself all of us understand that as well.

The point is simply that as the enclosure becomes more rigid it's helholtz resonance becomes more efficient and closer to theoretical.

Perfect rigidity though unachievable is simply the most extreme case of cabinet rigidity.

Constantly reverting back to the idea that nothing can be perfectly rigid could give someone the idea that you were trying to pigeon hole others in this thread and we all hold you in too high esteem to allow that.

On the other hand if the cabinet were very flimsy the mechanical Q of the helmholtz system would be reduced considerably.

None of us would build a cabinet so poorly.

Daryl

[Scotty]

jules, if you are using the term elastic in the context of a physics discusion some clarification is called for. The soundwaves impacting the interior of the enclosure are subject to the conditions governing inelastic collisions. An inelastic collision is one in which part of the kinetic energy is changed to some other form of energy in the collision. In this case heat or the physical vibration of the cabinet walls.
The inelastic nature of the collisions in the box are what causes us to have to build the box larger than the theoretical ideal. The term QL is used in Theile-Small
equations to account for box losses from this phenomenon. The more rigid we can make the box the more closely we will approach the ideal behavior of an elastic
collision between sound waves and the walls of the box. For more information
about inelastic collisions see these links
http://hyperphysics.phy-astr.gsu.edu/HBASE/elacol.html
http://hyperphysics.phy-astr.gsu.edu/HBASE/thermo/inteng.html#c2
I hope this clears up some of the confusion in this thread.
Scotty

[jules]

Daryl,

I find it ironic that you have the nerve to get sarcastic about esteem ....

quote Daryl:

"we all hold you in too high esteem to allow that."

Earlier in this thread you were absolutely clear in saying that there was no transfer of energy between a sound wave and the cabinet material. It is obvious that you went away and did a little reading up on the topic because you then had the gall to come back and give me a mini lecture on the exact physics you had bagged me for earlier.

If you'd had anything like courage, you would have given me a simple apology at that point.

Your followers dilligence doesn't seem to have extended to the height of noticing that you have done a complete about face so and I don't share your concern for having a cloud of adoring but uncritical fans.

Scotty, Consult a guru ... I suggest you pm Daryl since the case you're putting is pretty much the one he put before he had a re-think.

To all of you 

jules

[DSK]

Hey guys, regardless of who is right (if anyone) and who is wrong, there has been a lot of thought and mental gymnastics in this thread. By being challenged we think things through further and can happen upon things we hadn't previously considered and factored in. This is all good.

Can we keep the taunts and emotions out of this thread please. It would be sad to see it locked down or moved to Fight Club. If someone has to have the last word or 'win' the argument, then do it by continuing to challenge (without emotion) what others have said and by submitting additional facts and theories.

Please check the emotions at the door ... we are all here for the same thing.

Peace Out!

edited by Moderator with permission of the author to change font size and color

[Daryl]

Jules,

Nobody especially me has changed their position.

Just read the thread.

Besides Jules, you changed the subject again.

Everyone has been trying to explain that absolute rigidity of a surface would not prevent reflection of sound and that reflection actually becomes more efficient as rigidity increases.

This has been your position all along...

A rigid object would have no frequency repsonse and no ability to deflect. Now, if you are right Daryl, this makes it a 100% reflector BUT if an exchange of energy is required the infinitely rigid objst just can't do it in my book. For sound to travel from a speaker and out through a port, it has to bounce of the walls of the cabinet. If it can't bounce, it doesn't come out.

Just like the questions you asked the proffessor we also never discussed whether or not there was transfer of energy between a soundwave and cabinet material because the subject of the thread was about what to do about that energy.

Daryl

[fredgarvin]

The idea of cabinet rigidity has to do with cabinet resonance, not reflection. Reflection,generally,  has to do with wave-form interferance from the exterior cabinet/baffle. I feel one does not want resonant cabinets such as Michael Green advocates. One also wants to reduce exterior reflection either from the baffle or from surfaces. Elasticity of the surface is not required. How in the hell can an argument ensue over such elementary issues?

[Occam]

Well, I hope everyone has gotten in their last licks. I see no good coming from a continuation of this thread. Everyone has made their points, and no one is going to change their postions.

The thread is now locked.