[andyr]

There is no such thing as too little coloration in a speaker (or any other part of the chain).

Characteristic cabinet resonances are just another of many types of coloration typically added to the program signal by the speaker/box/room system ....

Hi Russell,

While I agree with you wholeheartedly (that there is no such thing as too little colouration in a speaker), am I correct in assuming you agree that minimising the colouration caused by cabinet panels flexing is a good thing (even though you're still left with the other colourations inherent in that speaker?     If so:

Q1:  Does this apply to subs as much as "normal" stand-mounts (like the ones DSK is building)?  I wondered if, since the wavelength of the frequencies produced by the sub (which is, say, 6dB down at 60 or 80Hz, with a 24dB slope) - which are just as much inside the box as out through the driver cone - are so much longer than the cabinet dimensions (like 20'), maybe they don't excite the cabinet sides as much as a 1,100cps tone inside a "normal" speaker's cabinet does?  

Q2:  Making a speaker cabinet out of two boxes glued together, using different materials (ie. a CLD arrangement), is one way of reducing cabinet wall flex.  So is increasing the mass of the box walls by adhering lead sheet to the insides.

Would combining these two ideas be a good thing ... ie.:
* making the inner box out of the first material (say, ply or flooring-grade chipboard)
* using contact adhesive to glue lead sheet over all its sides
* making the outer box of the second material (typically MDF) and glueing this to the lead on all sides?  

Regards,

Andy
 

[Russell Dawkins]

Hi Andy,

As to Q1, small box subs should be considered in a different light. What is important to them, assuming reasonable rigidity (1" mdf and smallish dimensions, plus bracing) is the tuning of the box to the driver, whether the box is sealed or ported. Panel resonances, assuming the above, are not the main issue. This is because the resonant frequency of the panels, if small (<2' and braced) is way higher than the frequencies being reproduced by the sub.

I know of one case where the resonant frequency of the whole driver/box system was purposely raised above the operating range and the sub operated entirely below it (Bag End - quite respected in the industry).

Q2. If wall flex is indeed the issue, the best way to deal with it is MASS and stiffness. Lead sheeting and CLD are about distributing and absorbing resonances.

I haven't used lead sheeting, but have seen it employed very effectively, albeit with a waffle texture, not simple sheet, in a boat motor housing.

If I were to use it, and particularly if had only smooth sheet available I would investigate using thin (3-5mm, 1/8") foam under it to allow it to vibrate and absorb more bass energy. I would bond the foam over its entire surface to the walls and then cover it entirely with contact adhesive or something that will remain flexible and adhere the lead sheet (slightly undersized so the edge doesn't touch the walls) to the foam. Then I would use some fibreglass or long fibre wool if easily available as needed.

[jules]

Daryl,

thanks for your detailed reply ... I agree with most of it 

jules

[andyr]

If I were to use it, and particularly if had only smooth sheet available I would investigate using thin (3-5mm, 1/8") foam under it to allow it to vibrate and absorb more bass energy. I would bond the foam over its entire surface to the walls and then cover it entirely with contact adhesive or something that will remain flexible and adhere the lead sheet (slightly undersized so the edge doesn't touch the walls) to the foam.

Hi Russell,

What a brilliant idea ... very easily done!      I shall implement a foam/lead sheet lining to the inner box of my sub!

Then I would use some fibreglass or long fibre wool if easily available as needed.

OK, here I need some more help from you!     In this sub I am building, the internal volume of the box is indeed tuned to the driver - it needs to be 1.6 cu ft to 2 cu ft (and it's sealed).

The kit supplier says "just line the walls with 2" polybatting" ... elsewhere in this thread, someone posted that fibreglass (being heavier) is certainly more effective at damping than polybatting but will make the internal volume of the box appear larger.

So if I stuffed in lots of fibreglass, surely this would make the driver "see" a volume which is more than 2 cu ft?  Which is a bad thing?   

Regards,

Andy

[DSK]

...So if I stuffed in lots of fibreglass, surely this would make the driver "see" a volume which is more than 2 cu ft?  Which is a bad thing?   

Andy, don't forget that if you are completely lead lining the enclosure (with say 0.25" felt or foam between enclosure walls and lead sheeting) the lead sheeting effectively becomes an artifical internal enclosure that will load the driver and reduce the internal volume.

[andyr]

Andy, don't forget that if you are completely lead lining the enclosure (with say 0.25" felt or foam between enclosure walls and lead sheeting) the lead sheeting effectively becomes an artifical internal enclosure that will load the driver and reduce the internal volume.

Hi Darren,

1/8" of foam and 1/32" of lead all over (except the baffle) reduces the internal volume of 2 cu ft to 1.9 cu ft ... which I would've thought was no big deal?   

But at least that can be calculated - and even allowed for - whereas the amount of effective increase which the fibreglass would cause is unknown?   

Regards,

Andy

Regards,

Andy

[Russell Dawkins]

[/quote]
So if I stuffed in lots of fibreglass, surely this would make the driver "see" a volume which is more than 2 cu ft?  Which is a bad thing?   
Regards,
Andy
[/quote]

Yes, it could be bad. In this delicate dance of trade offs there is an optimum size and damping. All sealed boxes are resonant and the trick is to engineer the appropriate Q and frequency for the driver you are using.

It sounds like there is some slop built in to the specs, since you talk of 1.6 to 2.0 cu ft volume. If the volume is actually 1.9 to 2.0 cu ft you could experiment between 1.6 and 2.5 cu ft by taking the woofer out and putting a couple of bricks or whatever in to reduce volume to experiment with the small end of things and experiment with the effects of various amounts of fiberglass. You might want threaded inserts with the number of times you will be removing and replacing the woofer!

[JohninCR]

I have to question the benefit of lead in a subwoofer cab.  Yes it will make the cab heavier to anchor it in place to help counter the mechanical forces of the driver's operation, however, inside a sub the wavelengths are too long for the pressure wave to convert to wave form.  I believe this means that the primary requirement is structural rigidity and there are many materials that will perform much better.  Damping and rigidity are two different things, and while lead could be useful in a speaker box, in a sub I think steel or aluminum plate or even fiberglass would be better.

Regarding stuffing, my understanding is that the theoretical limit is 30%, with a 20% apparent volume increase being more realistic for a fully stuffed cab.

[Russell Dawkins]

I have to question the benefit of lead in a subwoofer cab.  Yes it will make the cab heavier to anchor it in place to help counter the mechanical forces of the driver's operation, however, inside a sub the wavelengths are too long for the pressure wave to convert to wave form.  I believe this means that the primary requirement is structural rigidity and there are many materials that will perform much better. 

I largely feel the same way and meant to say something similar - all this effort would be more appropriately applied to the mid enclosure. As a former speaker designer I grew really tired of tuning for bass since I so extremely seldom heard it done right - by me or anyone else. I grew to prefer to pay the piper.

In my case I bought a pair of Hsu 10" cylindrical subs in 1994 and haven't looked back. The speakers I have used these with typically need help from 40 down only.

These days if I were building anything it would be a pair of OBs, especially for the bass and probably for the mids (widerange), and maybe a Behringer electronic crossover for experimentation and quickly finding suitable crossover frequencies to be duplicated with discrete components.

With an OB the whole resonant situation is simplified and the difference to my ears is a sense of ease and effortlessness.

[DSK]

1/8" of foam and 1/32" of lead all over (except the baffle) reduces the internal volume of 2 cu ft to 1.9 cu ft ... which I would've thought was no big deal?   

But at least that can be calculated - and even allowed for - whereas the amount of effective increase which the fibreglass would cause is unknown?   

Correct! I had imagined twice that thickness of lead sheeting (some people use 3mm) and twice that thickness of sandwich layer ... this would have resulted in over double the 0.1 cu.ft reduction you calculated ...still no big deal.

From what I can gather, the increase in 'virtual' box size caused by damping materials (poly, wool, f'glass etc etc) is largely guesswork and, in sealed boxes, 10% or so either side of the target doesn't make a difference. Just start with an approximate amount of damping material and flavour to taste.

[JohninCR]

Russell,

You and I are definitely on the same page.  I find OBs easier yet more fascinating and challenging at the same time.  I'm going the PC route for XO duties though, as I believe that is the future.

BTW, I believe the baffle of an OB is a great place to use lead, because mass is of great benefit there.  Damping can be useful too.

[jules]

CABINET MATERIALS ... MDF v PLY

ok, since I've been sitting on the tractor for a couple of hours I've managed to generate a challenge to what I feel to be the general feeling that cabinet "rigidity" is everything.

I am an absolute novice in speaker building but I do believe my basic physics is ok and when it come down to it, the physics rules, so ....

When a sound wave hits the wall of a cabinet there's two options. It's either absorbed and coverted to heat or it's reflected. If it's reflected the process involves an active response by the wall of the cabinet. The wall of the cabinet is initially deflected back minutely by the force of the sound wave, it then springs back and transfers into a reflected sound wave. What this amounts to is RESONANCE. [ I am not theorizing here, this is basic physics describing what happens when you apply a force to a mass [surface]. Interestingly, this means an equal and opposite amount of sound is also radiated from the outside of the box?!?!]. In a bass reflex cabinet, the amount of sound energy that comes out of the port is a simple statement of the reality of cabinet wall resonance. In an acoustic suspension cabinet, the energy from the back of the drivers is put into use by the same energy recycling.

MDF and ply, the most commonly used materials in speaker boxes, have somewhat different qualities. I don't have the figures but the point seems to be, that MDF is more of an absorber, has less tensile strength and presumably a lower resonance frequency for a given size. I would expect that for two identical cabinets using the different materials, you'd get more volume from the port of the ply box [and similar equiv. with acoustic suspension].

There's another point about ply ... high quality ply is used in resonant panels in pianos. I believe ply is also the substance used in Salks [?] and certainly other quality speakers.

So, cabinet material resonance is the reason cabinets do what they do and it is a good thing. The choice we make is what frequency we want that resonace to have with higher seeming to be better but I'm inclined to back those who factor cabinet "life" in as part of the equation rather than pretending that it doesn't exist. Put another way, part of the sound of a Salk or any other driver in a non-infinite baffle situation, is partly a product of the sound waves produced by the cabinet wall resonance.

jules 

[JohninCR]

Jules,

You've left me speechless.  Just to get you headed in the right direction
as you study up on the subjects, I'll make a couple of quick points.  First,
you're not considering that the vast majority of the rear wave energy in
a sealed alignment is dissipated as heat.  Second, the frequency range
coming out of a port is based on the internal volume of the cab and the
dimensions of the air column in the port.

[jules]

JohninCR,

I'm certainly not suggesting that "the vast majority of energy in a sealed alignment is dissipated as heat". What I am saying though, is that some energy will be converted from sound energy to heat energy when it comes up against the wall of a cabinet. This is a pretty basic piece of physics.

Yes, I am well aware that the frequency range that comes out of a port is based on the internal volume of the cab but it is erroneous to think of the cabinet as rigid or anything approaching that. There is no such thing as a rigid surface and certainly a piece of 1" MDF/ply is nothing like that. The frequency and other characterics of the cabinet wall will inevitably play a part in the sound that comes out the port. The choice we make is not if a box wall is resonant but at what frequency it is resonant.

quote johninCR:

 "Second, the frequency range coming out of a port is based on the internal volume of the cab and the
dimensions of the air column in the port."

You will not get any sound out of say, a totally damped hollow column of oil [if you could make such a thing] even if it's dimensions were at the same frequency as a sound source at its mouth.

Daryl mentioned science as a basis for some of his earlier comments. What I have noticed is that a percentage of what has been said is science. A larger part seems to be based on common belief and a certain amount simply contradicts some basic laws of physics. As I said, I'm no expert on speakers but the physics here is a combination of a series of relatively simple laws with the main question being the extent to which they apply. The most common assumption that seems to have been made here is that sound is being looked at as though it jsu hits the wall of a cabinet and bounces off without in any way causing a response in the cabinet wall. Law of physcis "to every action, there is an equal and opposite reaction"

Yes, I have probably left a number of people speechless but maybe I'm asking some key quesions here.

jules

[JohninCR]

Jules,

The problem is that you aren't asking questions, but instead you're making
statements based on an incomplete reading of posts, an incomplete
application of related physics, and a lack of empirical evidence.  While I
have built some test cabs that audibly contributed to the sound, that
result was quite offensive to my ears, and proper damping cured it quite
effectively. 

[jules]

 , yes, you're quite right about the statement/question thing.

Yes, I am not dealing with all the physics here but empirical evidence isn't required to make the point about forces on the wall of a cabinet.

What I have focused on are several fundamentals which are pivotal.

There is a sort of idealism here based on the idea that a speaker cabinet doesn't interact with the air inside it, so I'll ask a couple of questions ....

  * If a speaker cabinet had no frequency response [something like super lead] would any sound come out of the port?

  * If the material that we use to make speaker boxes does indeed interact actively with the air column, how will this affect the sound that comes out of the port?


jules

 

[JohninCR]

There is a sort of idealism here based on the idea that a speaker cabinet doesn't interact with the air inside it,

The vast majority of us are not instrument builders, so ideally we want to build cabs that do not contribute to the overall sound.  That is the job of the drivers.

  * If a speaker cabinet had no frequency response [something like super lead] would any sound come out of the port?

Yes

  * If the material that we use to make speaker boxes does indeed interact actively with the air column, how will this affect the sound that comes out of the port?

To the extent that panel resonances occur in the same frequency range as port tuning, I guess that they would affect port output.  The net effect would depend upon the phase relationship of those resonances and how well they are damped inside the cab.  Plus you'd have to consider the resonances on the exterior, which would be 180 degrees out of phase with the interior.  In a well built cab, panel resonances are likely to be outside of the range of port tuning.  With cheap store bought cabs, the manufactures often use thin material with no bracing and the audible output of panel vibrations outside of the cab contributes to the total bass output (boom, boom, boom).   This is also a common bit of trickery used to audition car audio woofers.  Most car audio stores down here have a big box made of thin plywood with interchangeable wood cutouts to use as a baffle, and if you plop almost any cheap woofer into the rig it puts out lots of bass with the box vibrations a major contributor.

There are also some high end speaker manufactures who claim to incorporate cabinet resonances in a positive way to the overall sound.  They use thin materials and special wood to tune their enclosures like musical instruments.  I've never heard one, but I imagine they probably sound quite nice playing the violin or acoustic guitar music.

[jules]

I pretty much agree with your last paragraph.  A natural resonance, where the frequency of the cabinet wood corresponds with the frequency of a note is clearly going to be a big problem. For the most part though, what I'm talking about is forced resonance [and standing by my claim that a cabinet simply wouldn't work unless it's "live"].

My answer to the Q about a lead cabinet is, of course that I don't think sound would come out of the port [with the proviso that lead, if that was used, does have a frequency response somewhere so it couldn't be totally dead].

As for your point about not being an instrument builder ... are you sure? That's the core of my case I guess. I played the clarinet for many years and I'd say that the wooden tube used to make an instrument like this would be considerably stronger cm for cm than a flat piece of MDF. Still, if you made a clarinet out of MDF it would sound like shit [even though it would have good stiffeness] and even slight differences in wood quality make a huge difference to the instruments sound quality.

jules

[Daryl]

Hi Jules,

Nobody is trying to contradict basic physics but you do have some misunderstandings about the physics of sound.

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).

When cabinet walls vibrate it is always bad.

They do not flex in a linear fashion so harmonic distortion is always an issue and that is why when you hear cabinets vibrate it makes a rattling or buzzing sound.

Also cabinets don't flex much in forced vibration mode (most of the spectrum), it is just at the resonances where amplitude becomes high so panel resonances are not smooth or linear but peaky and distorted always.

Daryl

[Russell Dawkins]

When cabinet walls vibrate it is always bad.

Daryl

Except, of course, if it's a Harbeth!  Then it's designed to flex.   http://www.harbeth.co.uk/faq/index.php#14

Russell