[Daryl]

Hi Russell,

Harabeth is using high loss instead of high rigidity.

They are still claiming that their cabinet walls have next to zero vibration they are just using a different approach to reducing panel vibration.

Daryl

[Russell Dawkins]

Hi Russell,

Harabeth is using high loss instead of high rigidity.

They are still claiming that there cabinet walls have next to zero vibration they are just using a different approach to reducing panel vibration.

Daryl

Daryl,
with respect, I think these are over-simplifications.
Harbeth does not claim their cabinet walls have "next to zero vibration".
Harbeth says "Whatever solution one arrives at, the best one can achieve is to suppress a panel's output and/or to steer it into a frequency band where it is either inaudible or benign: this implies at the bottom end of the audio spectrum."

I interpret this as saying that the panel resonances are purposely shifted lower in frequency than is the norm (because the panel material is thin and floppy and weighted by the damping material) and very damped. i.e. low Q.
Looks to me like their cabinets are functioning somewhat as huge damped passive radiators.

Russell

[Daryl]

Hi Russell,

I did paraphrase what I took from their writings, however.

Using a thin wall and applying a lot of damping still will result in very low panel radiation.

Either way I think it is more semantics I don't believe we disagree.

We are forced to communicate by typing on this forum so we end up leaving a lot out to make communicating less cumbersom.

Looking at each others thoughts in abridged form allows for assumptions and misunderstandings about what the other is trying to say.

Daryl

[JohninCR]

This latest twist in the discussion may bring up a dispute regarding a commonly recommended tweak.  Let's say you have some cheap, thin walled boomy speakers, and you want to improve their sound.  "Add bracing" is a common recommendation, but bracing by itself may be a very bad idea.  This will raise the resonant frequency of the subdivided panels and may make their stimulated vibrations even more obnoxious.  Damping them may be a better recommendation, or even adding some mass to the center of the panels (to lower their Fs) along with damping.

I'm about to build a big box HT sub (a false stage).  It's not big enough to be IB, and I know I need help down in the low teens.  I'm tempted to build only a rigid driver mounting baffle and experiment with an unbraced cab of thin material to see if I can tune the large panels to 10hz, so they act as very large surface passive radiators.  Any suggestions to shortcut the tuning process?

[jules]

Daryl,

I simply diagree with you on this:

quote Daryl:

 "Reflection happens best when a surface is perfectly rigid (we all know there is no such thing).

The surface being deflected and springing back has nothing to do with reflection.

Thus material resonance is not the reason a cabinets bass alignment works or the reason it reflects sound.

A perfectly rigid cabinet if it could be built would work exactly as it should and is the best possible situation (you still would need to damp the interior of the cabinet of course)."

I've stated my case several times now in slightly different ways so there's no point in us having an "is so ... is not ... is so ... is not ....... " exchange.

Russell and JohninCR,

some great input to the discussion. I've learned something from you.

What I will try to do here is get the opinion of an expert in the field of audio physics and present it back to this circle at some time in the future. Clearly, if a cabinet is actually a musical instrument [something the purists will hate] it has big implications for choice of material.
 

Don Quixote

[jules]

just a final thought and I'm not claiming the gods of physics are behind me on much of this one ...

At the beginning of this thread, and when I made my own speaker boxes, I took the view that the best way to go was a strongly designed MDF cabinet with a moderate amount of damping

What I now think, and yes this is intuitive, is that MDF is a an "easy" option that's fairly compromised in that it's less rigid than ply and also more energy absorbent.The next time I build, I'll try ply because I now think that while it might be harder to get right it's going to contribute to a cleaner sound and, if I can controversially call up the gods of physics again, what you do to the column of air behind a driver, you also do to the columg of air in front of the driver. Hence, since a driver is a part of the column and the column is connected to the cabinet walls, the walls count.

jules

[Daryl]

This latest twist in the discussion may bring up a dispute regarding a commonly recommended tweak.  Let's say you have some cheap, thin walled boomy speakers, and you want to improve their sound.  "Add bracing" is a common recommendation, but bracing by itself may be a very bad idea.  This will raise the resonant frequency of the subdivided panels and may make their stimulated vibrations even more obnoxious.  Damping them may be a better recommendation, or even adding some mass to the center of the panels (to lower their Fs) along with damping.

First I would make shure the speaker is worth putting your time into.

Many things must be considered and planned out when building a speaker.

If the cabinet is no good that doesn't say much for the rest of it.

Bracing usually improves matters.

The Harabeth way is excellent also I see no problem with either.

Daryl

[Daryl]

I'm about to build a big box HT sub (a false stage).  It's not big enough to be IB, and I know I need help down in the low teens.  I'm tempted to build only a rigid driver mounting baffle and experiment with an unbraced cab of thin material to see if I can tune the large panels to 10hz, so they act as very large surface passive radiators.  Any suggestions to shortcut the tuning process?
 

Hi Johnnin

If you want to build a passive radiator system I would recomend a passive radiator like this one from Acoustic Visions...



You would not want to use your cabinet walls as a passive radiator for a couple of reasons.

Most important is that the walls will not flex in a linear fashion causing distortion.

The other is that even thin walls will be too stiff as in a passive radiator system you want the PR's suspension many times more compliant than the air inside the cabinet so that it's self resonant frequency is an octave or so below the intended Fb.

The PR pictured above has an extremely compliant suspension, self resonant about 5hz and is capable of displacing 9 litres of air.

Daryl

[andyr]

Hi Jules,

I think the gods of physiks have disowned you, actually!!  

At the beginning of this thread, and when I made my own speaker boxes, I took the view that the best way to go was a strongly designed MDF cabinet with a moderate amount of damping

I think a strongly designed MDF cabinet is certainly a potentially good way to go ... if by this you mean "strongly designed" is the opposite of "flimsy" - ie. the cabinet walls don't flap.  

However, we've had discussion in this thread what IMO are two separate opinions on what "damping" is - damping the movement of the sound waves within the box ... or damping the walls of the box so that they move much less (and therefore colour the sound less).

In terms of stopping the cabinet walls from flexing under the "hammer blow" of the air movement caused by the driver moving backwards ... I suggest the more you can do this, the better the result will be.  A "strongly designed" cabinet is therefore good.  

What I now think, and yes this is intuitive, is that MDF is a an "easy" option that's fairly compromised in that it's less rigid than ply and also more energy absorbent.The next time I build, I'll try ply because I now think that while it might be harder to get right it's going to contribute to a cleaner sound.

I agree MDF is an "easy" option because it's fairly inexpensive, easy to work with and quite strong.  However, it's not a particularly well damped material - which you can prove to yourself by tapping a sheet of it with your knuckles.  There is a substance called "HDF" (high density fibreboard) but I suspect that while this is much denser than MDF, it is similarly not that well damped.

A "well damped" material needs to be made up of fibres or particles embedded in the glue matrix.  These fibres or particles act to stop the material from flexing much more effectively than the wood dust which makes up MDF is able to do.

Actually, plywood meets this criterion so I suspect it is actually a good substance to use.  However the wood that the ply sheet is made from will be crucial to the degree of damping achieved.

I also suspect - but have not been able to prove - that a sheet of 1" thick fibre-cement sheet (typically used by builders for the floor of "wet areas" in place of wooden sheet flooring) would be much more damped than a 1" sheet of MDF.

... what you do to the column of air behind a driver, you also do to the columg of air in front of the driver. Hence, since a driver is a part of the column and the column is connected to the cabinet walls, the walls count.

Absolutely, the walls count!!    Therefore one wants them to move as little as possible, otherwise:
1. they will "interfere with" the pressure wave inside the cabinet - imagine a particular tone that hit the resonant frequency of a cabinet wall.  This would cause the wall to vibrate excessively if it was not damped and so would be modulating the tone produced by the driver.
2. the outside of the cabinet walls would become an additional - and entirely spurious - sound source, colouring the output from the driver.

Now in the above, I have not touched on how to damp/attenuate/remove entirely the sound waves inside the box caused by the driver going backwards.  I must admit I am totally confused about this, after reading through this thread!  

Regards,

Andy

[jules]

Andy,

thanks for your opinion.

As I said in a previous post, I am following up on the physics of sound/box interaction and I'm confident the gods won't abandon me 

I'd like to clarify one point I made. I was not suggesting that MDF is well damped but I believe it has more damping qualities than ply and that the damping qualities it does have are going to be selective but not controllable. Any energy that is absorbed and not reflected by a cabinet wall is lost signal. 

I agree with most of what you've said except for the one vital detail ... I believe that a cabinet cannot work unless it is live and that the resilience, strength, whatever you like to call it, of the cabinet walls is active and essential.

I also stand completely behind my comment on what happens when a wave hits a wall. It might seem counter-intuitive but I reckon I've got my physics right here.

Good discussion I think and great to see people able to disagree without getting personal.

jules

[jules]

You be the judge:

I posed the following questions to prof Joe Wolfe of physics at the uni of NSW and a specialist in music, speech and the acoustics of musical instruments:

>     * In a bass relex cabinet, is the sound energy that emerges from
> the port a result of and dependant on the "springiness" of the cabinet
> walls?
>
>     * What laws of physics apply here and would it be possible to
> express this process in terms of these laws?
>
>     * If the walls of a speaker cabinet are indeed "live" and
> contribute to the charateristics of the reflected sound, would it
> follow that walls made of different materials would give different
> sonic results both from the port and the drivers [speakers]?


Prof. Wolfe replied:

Let me answer your second question first: yes, indeed physical laws
apply and yes indeed it is possible to quantify the effects. This is
what physics and engineering are all about.

Now to your first question.

Acoustic suspension and ports, as you point out, use the "springiness"
of the air. Technically, one normally talks of the compliance, roughly
the inverse of the springiness. The compliance of the air is in parallel
with that of the walls of the enclosure. This is because, when extra air
goes in and the air is compressed, the walls are bulged out a little, so
the air is less compressed than if the walls were completely rigid.
Hence walls of finite rigidity mean a higher compliance or a less stiff spring.

There is a resonant frequency associated with this combined springiness.
If we are talking about a port, then it is a Helmholtz resonance
(hereafter HR): the mass of air near the port is suspended on the
'spring' of the air & box. One can find this resonance by singing or
playing a note nearby and listening, or putting a microphone inside. HR
is explained in more detail in our web page
http://www.phys.unsw.edu.au/~jw/Helmholtz.html

If you have a loudspeaker in a sealed enclosure, then the mass of the
cone is suspended on the combined spring of its mechanical support (the
ridges around the edges), the air and the walls. So the resonant
frequency is different.

How can you tell whether the wall compliance is important?
Actually quite easily. The obvious experiment might seem to be to pump
air in and measure the deformation but several problems (leaks, thermal,
measurement) would make this difficult. Instead, here is the experiment
I recommend:

1)  Measure the HR of your enclosure in the normal condition

2)  Now bury the enclosure in sand, but keep the sand away from the
port, so that it doesn't block radiation from it (technically : doesn't
much reduce the solid angle of the radiation field). Now measure the HR
of the enclosure. The sand is massive enough that its inertance reduces
the compliance of the enclosure close to zero. You now have the "true"
HR. Without the sand, you have the HR due to the parallel compliance.

Take the ratio of the frequencies and square it. This is (roughly) the
ratio by which you have changed the compliance.

Now to your third question:
Does the finite compliance of the wall mean that different materials
have an effect?

Qualitatively, the answer is yes. However, my prediction is that, for
most MDF enclosures, it will change the HR by less than 10%.
Consequently, the difference in material will be small. However, I'll be
interested to hear any results.

jules

[Daryl]

Jules why would you ask those questions to the proffesor?

We never discussed any of those things.

You should have told the proffesor that you believed that if a surface were perfectly rigid that it would not reflect sound and that you believed that surface must be deflected and spring back in order to reflect sound.

That is what you said.

You also said that if the walls of a cabinet did not flex that the enclosure would not work properly.

Why didn't you pose those questions to the proffesor?

Or maybe let the proffesor read through the thread?

Daryl

[jules]

Daryl,

I did not use the word "flex" at any point. I did use the word "live" and that's the word I used in my questions.

I gave the site details for audiocircle and this topic to prof. Wolfe. and he could have checked this out if he had wanted to.

I think you'll find the answers to all your questions in there including the issue of a rigid wall but at this point I leave it up to you because I think this closes the issue.

As I have said all along, the physics is basic. Have a look at a 1st year uni text and the details of what happens when a sound wave hits a material with "finite rigidity" are clearly laid out.

It's all your Daryl

 

jules

[JohninCR]

Daryl,
It's hopeless.  Jules conveniently forgot to ask what would happen if the
rear wave was completely absorbed inside a sealed box (remember he thinks
no sound will be produced).  He can go bury a speaker in the sand himself
to prove beyond even his doubt that sound will still come out of the port.

Jules,
Just get yourself off of this idea that the cabinet itself (interior or exterior)
should be a generator of sound.  To the extent that it does, it is just a
coloration/distortion of the signal.  It is the driver's job to convert the electrical
signal to sound waves.  In the case of a bass reflex, a tuned column of air is used
to suplement the bass response that the driver alone is unable to reproduce.  As
the professor stated, the effect of the cabinet construction itself is insignificant
to the overall operation of the port.  He probably threw that 10% number out
there to be conservative, and because he knows that just having the cab
mounted in such a solid massive structure will have some other effects on
response that are not related to panel flex.

[JohnR]

OK, I'll try....

Jules, the Professor said:

Now bury the enclosure in sand, but keep the sand away from the
port, so that it doesn't block radiation from it (technically : doesn't
much reduce the solid angle of the radiation field). Now measure the HR
of the enclosure. The sand is massive enough that its inertance reduces
the compliance of the enclosure close to zero. You now have the "true"
HR.

So, if the cabinet doesn't flex at all, the result you get is the ideal Helmholtz resonance (HR), as predicted by the models (which assume a rigid cabinet...). He did not say you get no output, far from it.

Here's another way to think of it. Conservation of energy is one of the basic laws of physics. Suppose a sound wave impinges on a completely rigid surface. If it doesn't reflect (as you seem to be saying), then where did the energy in the sound wave go. It can't have been converted into heat by friction in the material of the surface, since the surface didn't flex. So where did it go...?

Having said that, I don't subscribe to the idea that one should build cabinets out of MDF and brace it to death either. A DIYer is of course free to do whatever he/she wants

[jules]

ok ... on the "rigid" thing

first, there is no such thing as rigid and even the concept of approaching it is erroneous. A "rigid" material would have a surface that could not deflect and maybe atoms that couldn't move [but ... it doesn't exist]. What happens when a force meets a real fixed object is that there is an exchange of energy. This involves changes in both objects. For instance, if energy in the form of a sound wave hits the wall of a cabinet, the sound energy is taken up by the cabinet wall, converted to various forms of mechanical energy and then spat back out again [minus frictional losses as heat]. 

What has been repeatedly suggested in this thread is that a cabinet is effectively rigid and that rigid would be ideal. If a material existed that did not deflect and had an atomic structure that was fixed [it's an absurd idea] then there would be no exchange of energy and there would be no reflection.

I quite agree with what John has posted but note this ... the wall material of the cabinet will not reflect sound because it is rigid. It will reflect sound because there will still be a transfer of energy into the wall. The sound will be changed to mechanical energy in the form of changes in molecular energy and again, it will be reflected out. A cabinet does not work because of its rigidity but rather because of its elasticity. [That does not mean I'm suggesting flexible walls]

Burying a speaker in sand is not the same as having a "rigid" wall. Just because the wall cannot move does not mean that it is not elastic. A wall backed by sand will still reflect sound only because of exchanges of energy that are still possible.

Johnin: A minor correction ... I did not say that the wall of a cabinet "generated" sound. As John says, conservation of energy is the point.

Go and get your basic physics text books out.

jules



 

[JohninCR]

Jules,

Let us know when you reach earth, because sound waves work differently
on your planet than they do here.

[jules]

you wouldn't want to get up off your arse and present some physics to support your case would you Johnin?

Funnily enough, I think the only situation in which you'd find rigidity would be in a collapsed star. So, if you and Daryl could just get your sailing ship up there then you, the sound and your speaker boxes would be sucked in by the huge gravitational forces never to be seen again.

jules

[Russell Dawkins]

Jules,

you seem not to have any idea how wrong you are in your understanding of the absolute fundamentals involved. I would try to explain, but I think it would be a waste of my time. I could see it taking 1/2 hr for the first post which you would
1. not understand
2. find fault with
3. still think you were right, and
4. not have learned anything from the exercise.

Seems to me 3 people in this thread have patiently explained things and met with the above sequence of events, 1-4, each time.

You are not only wrong, but are completely convinced you are right and stubborn in your conviction, to boot.

Impossible.

[JohninCR]

Russell,

Thank you fellow earthling.  You summed it up perfectly.