[DSK]

...I think the point might be that the cabinet can act in a way similar to an auto shock absorber on the actual movement of the cone. Hence, if the whole thing is just total "damping" you could actually drain some of the life out of the drivers...

Hmmm, not sure I understand this. If we compare driver movement and the opposing forces (cabinet) to a speaker's relationship to the floor, then any rocking/movement is bad as it means there is energy lost. Surely this is the same with the driver and the box?

Today I found my old roll of roof sealing FlashTac (aluminium backed bitumen), the sticky bitumen layer is approx 1mm thick and the aluminium is far thinner than that. I took 2 identically sized and shaped pieces of 1" thick MDF, hammered a small nail into the end of each one and suspended each from an identical piece of string (so as not to damp the wood with my hand). On one piece I covered one face completely with the FlashTac. I held both by their strings and rapped sharply on the middle of each (opposite face to the FlashTac) with the blunt end of a 0.5" drill bit.
The result .... I could not detect a difference in the pitch of the strike, or in the length of decay.    Pity, it would have been a nice, cheap solution.

[jules]

more hmmmming from me .... 

Is the point here that a speaker box has to be "live", has to have it's own frequency response and is actively part of the process. The other extreme would be to have something so dead that it absorbed all the energy produced by the drivers and basically killed their movement [a cast lead lined cabinet?].

Seems to me the aim is not to loose energy through overdamping or weak cabinet walls but equally, not to gain resonances that weren't there in the first place.

I think it's time for an expert ... that should truly muddy the waters 

jules

[DSK]

...Seems to me the aim is not to loose energy through overdamping or weak cabinet walls but equally, not to gain resonances that weren't there in the first place...

I agree ... a cabinet that is sufficiently rigid and lined with sufficient and appropriate damping material to eliminate any nasty resonance peaks and to attenuate resonance levels across the audible range to a level of inaudibility. Howzat? 

PS. You have an urgent and desperate PM 

[Christof]

I have found that without properly dampened boxes you can have problems with standing waves.  This problem manifests itself as decay.  A well dampened box without standing waves will result in a faster more accurate sounding speaker IMO.  Keep in mind that a standing wave can develope when the distance is 1/4 wavelength so unless you have extremely small cabinets there will be some stored energy inside your boxes that must go somewhere.....out the driver cone, port, cabinet walls, etc.

From the DIYaudio Board:

A standing wave is essentially a resonance with a high Q (low damping factor). As all parts in the box interact (acoustically), the Q of the box resonances have influence on the response of the speaker in it's box.

A completely undamped standing wave inside the box will - once energized - have an easily perceptible influence on how the speaker's cone movement at that very frequency decays. It just takes longer for the cone to stop, once the wave is "standing".

That's why damped boxes sound less "boomey" than before damping them...

The stored energy has to go somewhere. Damping material is there to absorb this energy (and thus probably prevent the standing wave from occuring in the first place) and convert it into heat.   

c-

[Daryl]

Hi Daryl, this is interesting info. I'm no speaker designer but everything I have read suggests using approx 0.5lb of poly/Dacron/etc per cubic foot of enclosure size, generally loosely distributed throughout the enclosure. I've never seen anything recommend 100% fill.

When you say "1lb/ft^3 fiberglass", do you mean to use fibreglass that has a density of 1lb/cu.ft, or do you mean to use 1lb weight of fibreglass per cu.ft. of enclosure size?

Hi DSK,

1lb/ft^3 means using one pound of material per cubic foot of enlcosure volume.

The characteristics of poly and wool are different than fiberglass.

.5lb/ft^3 is recommended for poly and wool because that is the approxamate point where additional material stops increasing complinance and starts reducing it.

Standard fiberglass insulation is still increasing compliance at higher densities and that is why higher densities are recomended for fiberglass.

Again it is true that fiberglass increases the apparent volume of an enclosure more than poly or wool but what is more important is that with 1lb/ft^3 fiberglass you are absorbing the backwave from your drivers to the point where it is a non-issue.

You know your enclosure will be without standing wave issues without even considering goofy enclosure shapes or golden ratios.

Daryl

[Daryl]

more hmmmming from me .... 

Is the point here that a speaker box has to be "live", has to have it's own frequency response and is actively part of the process. The other extreme would be to have something so dead that it absorbed all the energy produced by the drivers and basically killed their movement [a cast lead lined cabinet?].

Seems to me the aim is not to loose energy through overdamping or weak cabinet walls but equally, not to gain resonances that weren't there in the first place.

I think it's time for an expert ... that should truly muddy the waters 

jules

You touch on some important points...

Reproducing a signal as is vs. roproduction with modification,
Science vs. supersticion,
Are experts really experts.

Leaving a cabinet partially resonant has it's effect upon the time/frequency and non-linear characteristics of a speaker.

Therefore it is adding new characteristics to the signal not part of it originally.

I prefer to have the sound of my music designed in the recording studio (they can resonances and distortion if they feel it adds to the art and this is done on many recordings).

If the backwave from your drivers is completely absorbed and no sound is emmited from you cabinet walls you will be left with the original signal and it will be exactly what it is.

If the original signal is lifeless it will be that.

If the original signal is exciting that is what you will get.

It is easy for "experts" to say things like too much damping will suck the life out of your music.

It is easy to prove it to, just take a speaker and jam it full of fiberglass and give it before/after comparison.

The fiberglass will change the bass weight as well as remove resonances which means you will change the design of your filters and or cabinet there is no possible direct comparison.

What is important is that you can measure that the resonances and reflections are gone and you can now design the cabinet and crossover for a even frequency balance.

The "sucking the life out" issue is superticion in the absence of understanding.

These things you must learn to understand yourself.

If you try to believe an "expert" you have no way of knowing if they really are an expert.

I will tell you that in the world of speakers most experts are frauds.

Daryl

[DSK]

Daryl, thanks for the info you have already provided. Thinking about what you have said, I have a couple of further questions...

Say a speaker designer produces a standmount 2-way speaker, using a 20L sealed box with 0.75" panels and 1" baffle, and using 0.4lb of poly stuffing loosely scattered throughout the box. He designs the crossover using this box and gets a flat (+/-1.5db) frequency response.

I then buy the kit, build it with thicker panels (1" panels, 1.25" baffle) and plenty of bracing. I keep the baffle dimensions the same but make the box a bit deeper to maintain the 20L internal volume. If I use 0.75lbs of fibreglass per box (as per your suggested 1lb/cu.ft ratio), instead of the 0.4lbs of poly stuffing used by the designer, what will happen? I presume the driver sees an apparent box size that is larger than the one used to design the xo... is this good, bad or make no difference? What will it do to the frequency response? Does the fact that the designer already had a flat freq response in the original box mean that there were no significant box resonances and that my thicker box and fibreglass stuffing will provide no extra benefit?

Also, am I right that the rear panel is the most important? Would you recommend 2" thick fibreglass on the rear panel and say poly batting on the other panels ....or, do you recommend fibreglass on all panels as long as the woofer is not impeded?

Thanks again, Daryl.

[Daryl]

Hi again DSK,

First off if you are talking about a kit I would recommend you build it exactly as the plans indicate.

Parts Express makes some extremely nice .75ft^3 cabinets.

I would use fiberglass but I am speaking in terms of designing the cabinet with fiberglass in mind and designing the crossover from measurements taken from the actual enclosure with the fiberglass installed.

Using fiberglass instead of poly would give an increase in apparent volume and lower Qm as well as Ftc.

The could result in an apparent lack of bass or shrill sounding speaker.

If you were designing a new crossover from new measurements you could add some series resistance and bring Qm up restoring the bass and resulting in a lower bass extension than before but with a slightly lower system sensitivity.

I have been speaking of stuffed enclosures where the  damping materials will be evenly filling the interior of the cabinet and not attached to enclosure walls.

This method is primarily used for sealed systems and results in the most absorbtion of the backwave possible.

If you use less damping materials or less effective damping materials you will want to build test enclosures and measure them to be shure there is no problem as cavity shape will become more important as the amount and effectiveness of damping materials is decreased.

I remember someone on the MAD board built a fairly large speaker and lined it with Black Hole 5 and was suprised to find it had several standing waves.

Black Hole is intended to isolate panels from sound energy inside the cabinet and I don't doubt that it works (at least at higher frequencies) however many think it will absorb the backwave from their drivers and build speakers with Black Hole and nothing else inside.

The inside of Black Hole has 1" of acoustic foam which is an excellent damping material however just as you want the ratio of panel thickness to span length to fairly high to insure solidity you also will want the ratio of damping material to cabinet volume to be fairly high to insure absorbtion of you drivers backwave.

Acoustic foam is fine but 1" is not enough material for a large cabinet which will have resonances at lower frequencies.

If your enclosure is not stuffed you will want your damping materials attach to the cabinet walls.

The rear wall will be the most important and the baffle the least because the signal originates at the baffle and will not be reflected from it back to you until it is reflected from the rear wall twice.

Daryl

[jules]

Raises more questions than it answers for me ....

Leaving a cabinet partially resonant has it's effect upon the time/frequency and non-linear characteristics of a speaker.

Therefore it is adding new characteristics to the signal not part of it originally.

Surely there is no such thing as a non-resonant cabinet and the nature of that resonance is the question here.

If the backwave from your drivers is completely absorbed and no sound is emmited from you cabinet walls you will be left with the original signal and it will be exactly what it is.

There's an implication here that you could simply build a cabinet out of damping material alone without the MDF/ply box.



Quote Christoff:

"A completely undamped standing wave inside the box will - once energized - have an easily perceptible influence on how the speaker's cone movement at that very frequency decays. It just takes longer for the cone to stop, once the wave is "standing".

That's why damped boxes sound less "boomey" than before damping them...

The stored energy has to go somewhere. Damping material is there to absorb this energy (and thus probably prevent the standing wave from occuring in the first place) and convert it into heat." 

end quote.



In terms of the physics involved ... Damping is damping but the walls of a cabinet are like springs.

This is a balancing act of non-absolutes: there's no such thing as absolute absorbtion of energy by damping material and there's no such thing as absolute rigidity of a cabinet. Given this situation I don't find it the least bit surprising that focal and others take into account the idea that a cabinet can be overly rigid.

jules





.

[Daryl]

Hi Jules,

I had no intention of implying that fiberglass completely absorbes all energy or that any cabinet wall could be perfectly rigid.

I assume we all know about anything being perfect.

My already long posts would become much longer if I included every possible disclaimer so you must stay with me or at least clarify things in question.

My points are that 100% fiberglass fill is so effective reflections and standing waves are not an issue and that if the backwave from your drivers is absorbed completely and no sound is emitted from cabinet walls (I did not say it could be done I said if) then you simply are left with what comes from the front of your drivers which is not a problem.

A cabinet wall can NEVER be too rigid the less they move the better.

They can be rigid enough though so going completely overboard is not necessary.

I believe mine are an excellent blend of quality and common sense.

Daryl

[JohninCR]

This discussion has been pretty good.  Of course you don't want
to stimulate resonances in your cab walls and cause them to be
a sound source.  Where I see the focus typically being lost regarding
damping inside a cab is how easily unabsorbed sound exits right back
out through the cone itself.  We're worried about thick multi-layered
walls of dense material but ignore the fact that the cone is an ultra
thin material made to create sound.  This is patently absurd.

Sure an undamped cab will sound "lively", but part of that sound is
reflected waves exiting through the cone creating an inaccurate
reproduction of sound .  I vote for stuffing a sealed cab fully (not only
loosely) with fiberglass.  Only an interior cab shape that inherently
eliminates reflections to a large extent, such as the baffle being the
base of a cone or pyramid, can reduce the required absorbant material
inside your cab.

[jules]

good point J...Cr,

I think this is recognized in the acceptance that a low Qts driver works best with a ported box whereas a high Qts driver can take the problem you mention better in a sealed cabinet. Either way of course it's still a factor.

It's all about juggling different elements isn't it and that's why the "science" is blurry and experience matters.

jules

[DSK]

...a low Qts driver works best with a ported box whereas a high Qts driver can take the problem you mention better in a sealed cabinet....

I don't want to divert the thread, but isn't EBP (FS/Qes) a better indicator of whether a driver is better suited to a ported or sealed box ... <50=sealed, 50-100 either, >100 ported ... ?

[jules]

Darren,

running out of day a bit here and I can't immediately lay my hands on some useful references but just looking at what you've written I would have thought that Qts as total quality factor and including both Qms and Qes is more relevant. I had some thiele small info stored that I can't pick up at the moment though.

edit: I don't think it's exactly OT though as it's part of the issue of what a speaker cone might do in relation to interference from internal waves of one sort or another.

I freely admit this is at the edge of my limited understanding of the topic though 

jules

[DSK]

First off if you are talking about a kit I would recommend you build it exactly as the plans indicate.

Parts Express makes some extremely nice .75ft^3 cabinets.

I would use fiberglass but I am speaking in terms of designing the cabinet with fiberglass in mind and designing the crossover from measurements taken from the actual enclosure with the fiberglass installed.

Using fiberglass instead of poly would give an increase in apparent volume and lower Qm as well as Ftc.

The could result in an apparent lack of bass or shrill sounding speaker.

If you were designing a new crossover from new measurements you could add some series resistance and bring Qm up restoring the bass and resulting in a lower bass extension than before but with a slightly lower system sensitivity.

I have been speaking of stuffed enclosures where the  damping materials will be evenly filling the interior of the cabinet and not attached to enclosure walls.

This method is primarily used for sealed systems and results in the most absorbtion of the backwave possible.

The speaker designer had suggested that thicker boxes were fine as long as I kept the baffle height and width the same and the internal volume the same. So, I followed this criteria, built the boxes and am currently painting them. They are glued, screwed and braced, very solid, rigid and heavy. I estimate approx 40lbs per empty box, not bad for standmounts.

Given your recommendation of how great fibreglass is at absorbing reflections and minimising standing waves (and also from http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damping), I am very tempted to put a 2" thick fibreglass batt on the back panel and use poly batting everywhere else. However, the crossover was designed with approx 0.5lb poly fill scattered throughout the box and I am concerned about increasing the box capacity seen by the midwoofer and perhaps leaning out the bass as you mentioned. I have no crossover measurement or design equipment or skills, so I couldn't use fibreglass then modify the crossover. I also don't want to reduce sensitivity further as it is 83db with baffle step comp. I guess I should just do what the designer did and stick with polyfill. Perhaps in a 20L sealed standmounter, the polyfill is sufficient to minimise standing waves and reflections.

To re-ask a question that I don't think you answered ... if the designer's measurements in the thinner box with 0.5lb polyfill were flat (+/- 1.5db), does this mean that there were no troublesome standing waves or reflections? If so, does it mean that a thicker box, more bracing and fibreglass (instead of polyfill) will not provide much benefit?

Finally, are you suggesting that in a sealed box, 0.5lb scattered polyfill is more effective than an equivalent amount of poly batting (0.75" thick polyfill sheets) against the sides and stapled only in each corner? I was considering using 3 sheets thick of the 0.75" poly batting on all walls, stapled just at the corners so that it is 2.25" thick everywhere except at the corners.

Many thanks again for your help Daryl.

[jules]

Daryl,

I have to challenge some of what you've said:

quote Daryl:

"If the backwave from your drivers is completely absorbed and no sound is emmited from you cabinet walls you will be left with the original signal and it will be exactly what it is."

If the backwave from a driver is completely absorbed, then, in the case of a ported system, no sound would  come out of the port. If you were literally able to absorb all the energy off the back of a driver, that driver would be unable to move at all. [edit: just to clarify this a little ... if you were to attach a small shock absorber to the back of a driver cone, it could absorb the cone energy, convert it to heat, bring the cone near to a dead stop and burn out the coil.]

The space within a cabinet is akin to a tuned spring set to a particular frequency to work in concert with the various Thiele-Small characteristics of the driver [including the drivers own Fs, frequency]. It is unavoidable that  cabinets and drivers have particular resonance frequency but to suggest that...

quote Daryl:

"Leaving a cabinet partially resonant has it's effect upon the time/frequency and non-linear characteristics of a speaker.

Therefore it is adding new characteristics to the signal not part of it originally."

... is to deny the role of box tuning! A box is a lot more than partially resonant.

The design of a speaker cabinet requires the balancing [not denial of] a number of unavoidable frequency resonances in various components. A driver has an Fs. A box has a frequency Fb [related to the Fs and Qt for a given driver]. A port, if used, is tuned in relation to the box Fb.

quote Daryl, opposing the view that damping "will suck the life out of music" :

"It is easy to prove it to, just take a speaker and jam it full of fiberglass and give it before/after comparison.

The fiberglass will change the bass weight as well as remove resonances which means you will change the design of your filters and or cabinet there is no possible direct comparison.

What is important is that you can measure that the resonances and reflections are gone and you can now design the cabinet and crossover for a even frequency balance."

Sorry, I don't see your point. What you seem to be claiming here is that damping [fibreglass] is selectively able to remove nasty resonances and reflections while apparently not having any effect on desirable box frequencies. 

and again to quote Daryl:

"I remember someone on the MAD board built a fairly large speaker and lined it with Black Hole 5 and was suprised to find it had several standing waves."

If the volume of a cabinet is wrong the solution is not damping. If, for instance, a cabinet is designed with an Fb of ~120, then the result will be poor bass response and problems with speech intelligibility. I would guess that the problem you mention from the MAD board relates not to the Black Hole material but to the cabinet resonance frequency.

The [Focal] idea that a box can be too rigid might make some sense if you consider that the resonant frequency of the walls of a cabinet is just another "frequency" in the basket that can be used either to advantage or disadvantage. The difficulty I see with this is that, unlike the other characteristics that can be quantified, this one can't.

jules

footnote: Daryl, thanks for the info from the cookbook on relative properties of various fillers ... appreciated.

[Occam]

FWIW, Moray James has started a (IMO interesting) thread about using pealite as a damping material -
http://www.audiocircle.com/index.php?topic=30441.0

[Daryl]

The speaker designer had suggested that thicker boxes were fine as long as I kept the baffle height and width the same and the internal volume the same. So, I followed this criteria, built the boxes and am currently painting them. They are glued, screwed and braced, very solid, rigid and heavy. I estimate approx 40lbs per empty box, not bad for standmounts.

Thise sound nice DSK.

To re-ask a question that I don't think you answered ... if the designer's measurements in the thinner box with 0.5lb polyfill were flat (+/- 1.5db), does this mean that there were no troublesome standing waves or reflections? If so, does it mean that a thicker box, more bracing and fibreglass (instead of polyfill) will not provide much benefit?

+/-1.5db is excellent so long as it is honest.

A CSD plot will show details which you might miss in a frequency response chart (they are two way to show the same thing but you will get a different perspective of each).

Which kit are you getting DSK?

Finally, are you suggesting that in a sealed box, 0.5lb scattered polyfill is more effective than an equivalent amount of poly batting (0.75" thick polyfill sheets) against the sides and stapled only in each corner? I was considering using 3 sheets thick of the 0.75" poly batting on all walls, stapled just at the corners so that it is 2.25" thick everywhere except at the corners.

Damping materials are more effective near boundries.

You would not want to crush your poly too much but what you describe sounds fine.

Daryl

[Russell Dawkins]

 According to Kam Leung, the designer of the FS-888, Focus Audio had experimented with additional damping materials, including lead sheet and bituminous compounds, but found that these deadened the box so much that the resulting sound was sterile. Instead, he preferred to keep the cabinet just slightly resonant, which, he believes, helps retain the emotion and expressiveness that have become Focus Audio hallmarks...

I think it is important to understand that this line of reasoning is fundametally wrong and, if pursued, could waste your time and effort.

I will be charitable and call it misinformed or ill thought out.

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.
The most obvious of these is perhaps the "one note bass" syndrome.

However, any strong speaker or room characteristic will sooner or later wear thin and discourage you from turning your system on, or keep you on an  treadmill of "upgrades", searching for the magic cure.

Yet another reason to start with the speaker/room and work backwards.

[Daryl]

quote Daryl:

"If the backwave from your drivers is completely absorbed and no sound is emmited from you cabinet walls you will be left with the original signal and it will be exactly what it is."

If the backwave from a driver is completely absorbed, then, in the case of a ported system, no sound would  come out of the port. If you were literally able to absorb all the energy off the back of a driver, that driver would be unable to move at all. [edit: just to clarify this a little ... if you were to attach a small shock absorber to the back of a driver cone, it could absorb the cone energy, convert it to heat, bring the cone near to a dead stop and burn out the coil.]

That depends upon how the backwave was absorbed Jules.

Proper termination of the wave behind the cone would create an infinite baffle situation and the cone would move as freely as if it were in an infinitely large enclosure.

Practical damping materials introduce mechanical loss to the compliance of the air in the cabinet which is reactive.

The moving mass of a driver is reactive also and 180 degrees out of phase with the reactance of air compliance and they will cancel allowing the cone to travel freely at the system resonance.

The mechanical loss introduced by damping materials shifts the compliance of the enclosure from reactive toward resistive.

Now the compliance of the enclosure since it is no longer completely reactive will not completely cancel the reactance of the drivers moving mass and the cone will not travel as freely.

Understand that the enclosure was already restricting the motion of the cone before the damping material was introduced.

It is only that moving mass was able to nullify the airs restriction by resonance.

The damping material actually increases the cabinets compliance causing less restriction to the cones movement.

However due to phase change of the enclosures compliance moving mass can no longer nullify it completely at system resonance giving the illusion that compliance has been decreased.

I do believe however Jules that I it was a poor way of making my piont which was that damping materials will aborb standing waves and reduce system mechanical Q.

You will compensate the reduced mechanical Q in the design of the system.

There is no reason to think there wil be any detrimental effect.

When you change the type or amount of damping material in a system you would then want to redesign for the change in mechanical loss.

The space within a cabinet is akin to a tuned spring set to a particular frequency to work in concert with the various Thiele-Small characteristics of the driver [including the drivers own Fs, frequency]. It is unavoidable that  cabinets and drivers have particular resonance frequency but to suggest that...

quote Daryl:

"Leaving a cabinet partially resonant has it's effect upon the time/frequency and non-linear characteristics of a speaker.

Therefore it is adding new characteristics to the signal not part of it originally."

... is to deny the role of box tuning! A box is a lot more than partially resonant.

The design of a speaker cabinet requires the balancing [not denial of] a number of unavoidable frequency resonances in various components. A driver has an Fs. A box has a frequency Fb [related to the Fs and Qt for a given driver]. A port, if used, is tuned in relation to the box Fb.

Jules, you are talking of system bass resonance(s) which occur at the system low frequency cutoff.

I was speaking of panel resonances which occur in the 100's of hz range.

I was simply saying that panels never can be too rigid and you never want them to resonate.

Panel resonances are peaky and generate harmonics.

If someone is saying anything different they are foolish or lying or both, nevermind that they may have been selling speakers that cost more than cars for decades.

quote Daryl, opposing the view that damping "will suck the life out of music" :

"It is easy to prove it to, just take a speaker and jam it full of fiberglass and give it before/after comparison.

The fiberglass will change the bass weight as well as remove resonances which means you will change the design of your filters and or cabinet there is no possible direct comparison.

What is important is that you can measure that the resonances and reflections are gone and you can now design the cabinet and crossover for a even frequency balance."

Sorry, I don't see your point. What you seem to be claiming here is that damping [fibreglass] is selectively able to remove nasty resonances and reflections while apparently not having any effect on desirable box frequencies. 

The amplitude of sound travelling through damping materials is falling, the greater the distance the greater the attenuation.

The idea is to have enough loss that by time a signal travels panel to panel and back that it is reduced to insignifigance thus avoiding reflections and standing waves.

The sound coming from the front of the cone does not have to pass through any damping material.

You want what comes from the back of the cone to disappear never to be heard again.

and again to quote Daryl:

"I remember someone on the MAD board built a fairly large speaker and lined it with Black Hole 5 and was suprised to find it had several standing waves."

If the volume of a cabinet is wrong the solution is not damping. If, for instance, a cabinet is designed with an Fb of ~120, then the result will be poor bass response and problems with speech intelligibility. I would guess that the problem you mention from the MAD board relates not to the Black Hole material but to the cabinet resonance frequency.

You really took a left turn with that one Jules.

I said the cabinet was large, not incorrectly sized.

You are talking of bass alignment and I am talking about standing waves and reflections which are much higher in frequency.

I believe it was for a 10" driver.

The point is that the larger a cabinet is the lower in frequency it's standing waves will be.

Low freqeuncies require more damping material than high frequencies so you will want to be shure you have a decent ratio of damping material to cabinet volume and 1" of acoustic foam isn't very much in a large cabinet.

Daryl