Identifying Enclosure Resonances

[Mr. Widget]

Earl K and I have been having a discussion about finding, identifying, isolating, and eliminating cabinet resonances that was begun following an ordeal I have been going through while experimenting with a new type of cabinet construction.

After building a pair of prototype speakers, I determined that I was not pleased with the "quality" of the bass response. I ran the usual frequency response measurements and impedance curves (see below) and didn't see any apparent anomalies. I scratched my head and wandered around lost in the desert for a bit.

Once I eliminated outside sources as the problem, my first inclination was to blame standing waves within the cabinet. After making modifications that satisfied me that they were not the problem, I thought about cabinet resonances... these cabs are way over built and I found it difficult to believe that this would be the problem, but with nudging from others I took a much closer look at the LF impedance curves at a very high resolution.... bingo!

Essentially, the idea is that every resonance, including those within the driver itself will show up in the impedance curve. Even something as minor as changing the construction of the basket will affect the impedance curve.

[Mr. Widget]

Here is a plot showing the resonant peak that was in my speakers and the effects on that peak by making changes.

[Mr. Widget]

I added some sandbags at key spots, (identified by trial and error) and ended up with the resonances being reduced to the level show in these curves. I had hoped to do better and will rebuild the cabinets using a different technique, that said, I post these to show the level of objective improvement... this yielded a speaker that is quite listenable and the disturbing, soggy bass has tightened up considerably. If I didn't have the ability to measure this, it would have been a much greater task to "fix" the problem...

In another conversation, Techbot has said that the relatively low cost "Woofer Tester" can acquire these types of impedance plots so others should be able to hunt down and destroy resonances and standing waves that may be affecting the performance of their systems too.


Widget

[duaneage]

It is unfortunate but true that any mechanical movement will affect the impedance. I have always looked at the impedance curves for tuning before using a microphone. It is surprising what varying degrees of fill can do.

Could you feel resonance on the cabinet or horns at the offending frequency?

[Mr. Widget]

It is unfortunate but true that any mechanical movement will affect the impedance.

I see it as a bonus, since without an anechoic chamber it is really difficult to accurately measure lower frequencies. Obviously I agree that damping that mechanical movement is a pain, but at least we have a fairly easy method of looking for some anomalies.




Could you feel resonance on the cabinet or horns at the offending frequency?

It was the cabinet, and oh yeah baby! When I checked out the region with a frequency sweep, nailing the frequency was quite easy!


Widget

[Lancer]

the relatively low cost "Woofer Tester" can acquire these types of impedance plotsJust for fun - here's the 1500AL mounted in a 4.00 cubic foot sealed test cube. One should note the standing wave at ~ 350 Hz.

The standing wave can be seen in the impedance curve. It is the blip at ~ 350 Hz. With the oscillator set at that frequency the most horrid sound emanates from the system as would be expected. The internal dimensions of the enclosure are 484 mm cubed.

Note that 484 mm corresponds to a wavelength of ~ 712 Hz hence the standing wave will have a frequency of ~ 356 Hz.

The addition of 2" thick OC fiberglass on all panels is sufficient to reduce the standing wave ~ 350 Hz. Impedance drops to "normal" and the objectionable sound is greatly attenuated.

This doesn't necessarily have anything to do with the project. I just thought it might be interesting to some that the standing waves generated in a cube are definitely objectionable and are viewable in an impedance run.

Some closed box data:

driver mounted normal
4.0 cu ft gross
no fill
Qtc = 0.56
Fc = 49.8

driver mounted normal
4.0 cu ft gross
2" OC fiberglass on all panels except baffle
Qtc = 0.54
Fc = 47.8

[Earl K]

Nice Info guys .

- I might add that Widgets boxes were built using a method of layering up pre-kerfed mdf over a substantially beefy internal skeleton.

- This approach was adopted so that he could execute curved walls ( 2 of 6 ) .


( multiple fixes ) ,,,,,,, these yielded a speaker that is quite listenable and the disturbing, soggy bass has tightened up considerably. If I didn't have the ability to measure this, it would have been a much greater task to "fix" the problem...

- I find it quite intriguing that "disturbing, soggy bass" translates into a blip that is a fraction of "1 ohm" , on a high resolution impedance run .
- Man, 2 ohms must really suck ! :p

Here's a porduct link to WT2 ( at Parts Express ) (http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=390-802&raid=25&rak=390-802) for those interested in using it to track down a possible cause for "disturbing, soggy bass" .
:)

[Ian Mackenzie]

Just for fun - here's the 1500AL mounted in a 4.00 cubic foot sealed test cube. One should note the standing wave at ~ 350 Hz.

The standing wave can be seen in the impedance curve. It is the blip at ~ 350 Hz. With the oscillator set at that frequency the most horrid sound emanates from the system as would be expected. The internal dimensions of the enclosure are 484 mm cubed.

Note that 484 mm corresponds to a wavelength of ~ 712 Hz hence the standing wave will have a frequency of ~ 356 Hz.

The addition of 2" thick OC fiberglass on all panels is sufficient to reduce the standing wave ~ 350 Hz. Impedance drops to "normal" and the objectionable sound is greatly attenuated.

This doesn't necessarily have anything to do with the project. I just thought it might be interesting to some that the standing waves generated in a cube are definitely objectionable and are viewable in an impedance run.

Some closed box data:

driver mounted normal
4.0 cu ft gross
no fill
Qtc = 0.56
Fc = 49.8

driver mounted normal
4.0 cu ft gross
2" OC fiberglass on all panels except baffle
Qtc = 0.54
Fc = 47.8


Like pointing to a distant Moon gazing at you on a still night there are also the more ponderious issues a lower frequencies:

That of Pressure waves as detailed by the quote below are best attenuated by a thick loosy layer of open cell foam with a laminated thin film plastic layer such open cell carpet under felt to attenuate such pressure resonances.


"Unless an enclosure is spherical or ellipsoidal, all or some of its sides

will consist of plane surfaces clamped at their edges. Such a clamped

panel will have its own acoustic output when forming part of an

energised loudspeaker, derived from sound energy within the enclosure.

The output consists of standing-wave modes at higher frequencies and

pressure modes at those frequencies where the wavelengths exceed the

internal enclosure dimensions. Adjacent panels may be similarly excited

by vibrational energy from the drive unit chassis.




Theoretically a clamped panel has a well defined vibrational series in

both longitudinal (volume stiffness) and bending modes. A further mode

is due to the panel mass resonating with its own and the enclosure's air

volume stiffness. Stevens found that in a typical reflex cabinet this

latter resonance appearedat almost twice the fundamental enclosure

resonance, a condition verified over a range of tuned system frequencies.

[edgewound]

JBL' Enclosure Design Reference Manual states that an enclosure should never be constructed with indentical dimensions, such as a cube, nor dimenesions that can be divided equally into one another...or multiples of one another, due to exciting unacceptable panel resonances. When building a rectangular box, the sides should be odd multiples of one another to reduce the panel resonance and also be well internally damped to thermodynamically eliminate internal turbulence.

[Lancer]

Yeah, we often point people to the JBL Enclosure Design Reference Manual. It's good stuff.

http://www.lansingheritage.org/html/jbl/plans/jbl-plans.htm

http://www.jblpro.com/pub/manuals/enclgde.pdf

For those with BassBox 6 Pro you can look up standing waves in the online help.
Testing Loudspeakers by D'Appolito has some stuff too.
Many other sources out there as well.

The point of the cube was to exacerbate any standing wave and take an impedance measurement to identify it. The cube made it pretty easy to hear and measure as would be expected.

Awhile back someone argued that a cube was perfectly fine. I hope it worked out for them.

[duaneage]

Wow Ian, I got goosebumps reading that very sexy description of a vibrating speaker enclosure.


I am gonna need 20 minutes of quality time after that .....:p

[duaneage]

I see it as a bonus, since without an anechoic chamber it is really difficult to accurately measure lower frequencies. Obviously I agree that damping that mechanical movement is a pain, but at least we have a fairly easy method of looking for some anomalies.




It was the cabinet, and oh yeah baby! When I checked out the region with a frequency sweep, nailing the frequency was quite easy!


Widget

When I first built speakers 15 years ago I had nothing more than a signal generator and meter. I would place sawdust on a cabinet wall and look for the dust to move as I swept the frequencies. Touching the cabinets would sometimes affect the test. The dust would fall off the sides when I hit a magic note. After jamming a brace here or a gusset there I would end up with a reasonably tight cabinet.

One could use piezo elements with wood blocks fastened to measure vibration but the dust trick still works for me.

[Ian Mackenzie]

Wow Ian, I got goosebumps reading that very sexy description of a vibrating speaker enclosure.


I am gonna need 20 minutes of quality time after that .....:p

Yep,

Mrs Lansing loves it when I whisper groovy stuff in her ear..

Its an old trick but a goodie for absorbing the stuff 250hz and below, a very different kind of effect to fibreglass.....that said its basic physics.

[yggdrasil]

Hi Widget.

I just stumbled over this page: http://users.tpg.com.au/users/gradds/curved/curved%20sided%20loudspeakers.htm

Maybe there are a few usable ideas.

[Mr. Widget]

Stacked laminations are certainly one way to do curved cabs... but, boy what a PITA for a 4' tall cab!:banghead:

That is a good link. Someone posted it before on one of the Project May threads.


Widget

[yggdrasil]

You can always go with plywood.

[stevem]

After building a pair of prototype speakers, I determined that I was not pleased with the "quality" of the bass response. I ran the usual frequency response measurements and impedance curves (see below) and didn't see any apparent anomalies. I scratched my head and wandered around lost in the desert for a bit.

Once I eliminated outside sources as the problem, my first inclination was to blame standing waves within the cabinet. After making modifications that satisfied me that they were not the problem, I thought about cabinet resonances... these cabs are way over built and I found it difficult to believe that this would be the problem, but with nudging from others I took a much closer look at the LF impedance curves at a very high resolution.... bingo!


Mr. Widget, I have been thinking about this thread for a while now, mainly because I haven't been particularly happy with the quality of the bass in my system either. Although we use different drivers, I think there may be more similarities than differences between them. Another similarity is that my enclosures, while not curved, are also way overbuilt, being made with braced 1.5" MDF. I have started to wonder if something other than internal standing waves might be contributing to perceived bass quality.

My drivers are mounted to the cabinet securely with T-nuts and steel bolts, using OEM (thin) foam gaskets, which effectively couples the woofer frames to the cabinet. I have been wondering what changes in the bass might result by de-coupling the woofers from the enclosure. The B&W 801 S2 speakers I own also incorporate an overbuilt enclosure (.75" MDF with a honeycomb of MDF bracing inside), and B&W decouples the woofers using rubber grommets and an O-ring gasket. Have you ever done any experimenting with this approach? What effect do you think it might have in the subjective quality of the bass response? Is it possible that it might be resonances of the heavy enclosure walls themselves that are coloring the sound?

[Mr. Widget]

I remember KEF making a big deal with a similar woofer mounting scheme back in the mid 80's with their 105. I expect that it isn't a bad idea, but I have never investigated it. I think it would be a worthwhile issue to look into.

I think this excerpt from Stereophile describes the resonant issue I was experiencing. They simply refer to their issue as rigidity, but I expect it relates to the type of problems I was tracking down.


Widget

[Earl K]

Hi Steve,

- It's always possible your ports aren't functioning properly .
- For Instance : If they are placed too close to the sidewalls you can get a form of "rectification" . This truncation/rectification will hinder the VLF waveforms from properly forming in the ports. This is because the air in the port is in series with the air mass in the cabinet. This coupling is altered when the ports' entrance nears the inside walls .
- I don't know how to really test for this / apart from moving the ports to a more traditional location and retesting ..

:)

[stevem]

Earl, did you delete your first post? Why? I found it very interesting, and probably on target. There have been others that have commented on the unusual bass response of the 1401Nd and their ilk. They are very transparent, and in my view are worth the trade-offs. Are any of the LE14 variants still available from JBL, or am I limited to the used market? Are re-cone kits still available? There is another driver that JBL used in systems along with the pro version of my woofers (the 1400 Pro), which was the 2217HPL. It was used in PA subwoofer applications and had a different (ferrite?) magnet. I wonder if another cone with a foam surround would fit in that basket? Might be interesting.

Your comments about the ports are interesting. My enclosures have slotted ports, which use the enclosure sidewalls as one of the walls of the ports. The ports are 9" high by 1.5" wide, and about 16" long. I have also tried (in different boxes and with fundamentally the same results) triangular vents using the sides and bottom walls, and the traditional round 4" PVC ports on the back of the box. This later arrangement fired the port energy directly into the corners of my listening room, and made the VLF sound overblown, as if I had separate subwoofers in the corners.



What would happen to the perceived response if the enclosure is a little oversized, say 10%, and the ports are trimmed to re-tune them to the original design Fb?

[Earl K]

Steve

- I modified my post but I kept the original contents . Here they are again for you ;



Hi ,

- I should really be placing my comments in a thread over in MarketPlace . (http://audioheritage.csdco.com/vbulletin/showthread.php?p=65513)
- You, Dave & Widget talked some about your system in that thread .

- I'll start with my conclusion to your dilemma :

"Retire one pair of your 14001nds and replace them with either le14h-3s or le14h-1(s)" .
- Put these replacements into the 4.1 cu ft boxes you have. You'll be happier with their VLF performance

( Now some Blabbing ) FWIW :
- I don't see your dissatisfaction as being enclosure related whatsoever . I see it as unfulfilled expectations derived from listening to JBLs' newer series of deep-gapped woofers. Simply put ; these 1/2" deep-gappers just won't easily produce the VLF in a manner that most everyone has come to expect .
- This forum has only a few contributors who have experienced this phenomena. I'm one of them, with my ME150h woofs .
- Yes the newer deep-gappers have outstanding articulation in the musical fundamentals ( in fact, I've never heard better ) / but it's this same great articulation that seems to leave the "Western" ear dissatisfied with the woofers performance in the lowest octave & a half.
- Simply put; apparently, most here in the western civilizations prefer at least marginally "looser" bass than what these types can deliver in the typical Ts tunings .
- Hence the movement to augment these newer types with insanely heavy coned subwoofers ( think 1500 subs ) to reclaim that VLF .
- Just to simulate similar dynamics ( when using a 1/2" deep gap ) , 80 to 100 grams of extra cone weight ( ontop of the existing 140 to 155 grams ) must be added to these woofer types .

- JBL sells the le14h-3 in the high $ 200.(s) range . It's a part for the PS1400 sub ( Performance Series ) .
- This is the 14" I would recommend trying out . It's gap is only marginally deeper than the original le14h-1 so it won't be suffering from this "no VLF" syndrome ( all other things being equal ) .
- Zilch has a pair that I believe he bought direct from Harman. Give him a PM for the details.
- Right now the Harman web site indicates any order placed will go onto back order. This is most likely because this is the same 14" used in the new "ARRAY Series" .
- From my perspective, a pair of these represents the KISS solution to your situation since you have the boxes already made. All other possible solutions are much more intrusive .
- NOTE : The 250ti series used the same box tuning for its' le14h-1 .

<> :)

[Earl K]

What would happen to the perceived response if the enclosure is a little oversized, say 10%, and the ports are trimmed to re-tune them to the original design Fb?

- Simple answer ; I don't know how oversized a box would need to be to regain a sense of VLF. I don't know if it's really even possible to accomplish this goal by just using a larger enclosure .
- Hopefully you have some tuning software. Tune up the 4.1 cu' size . This enclosure size is already larger than the size for critically damped enclosure (.5 Qtc) . If I remember correctly , this woofer/box combo gives a low .4 Qtc range. Few people use system tunings lower than .5 Qtc .
- I don't know if there's a system tuning number ( magic Qtc # ) where this woofer starts to come slightly unglued ( electromechanically ) and begin to act like the woofers we grew up with .

- Back to my first point ,,, try out the le14h-3 if you can .


:)

[stevem]

-- Back to my first point ,,, try out the le14h-3 if you can .

:)

I currently operate both 1401Nds in each speaker as a single driver (covering the same frequency range from 300 Hz down). Do you suggest leaving it that way after adding an LE14H-3, or crossing it over (at about 60Hz) so it just handles the VLF? Thanks for all your advice!

[Earl K]

.....I currently operate both 1401Nds in each speaker as a single driver (covering the same frequency range from 300 Hz down). Do you suggest leaving it that way after adding an LE14H-3, or crossing it over (at about 60Hz) so it just handles the VLF? ....

Hi Steve,,

Here's my very abridged answer ; ( for the time being ,, disregard my advice to buy any more woofers )

(A) Primarily , you need to be certain that the ports of those enclosures are performing as you expect they should . That means ;
(i) Owning BB6 Pro so you have an idea of what to expect from a simulated VLF curve. Buy it , if you don't own it .
(ii) Something to actually measure the VLF in room response.
- I'd recommend buying an RTA of sorts if you don't already have one . A Behringer DSP 8000 / 8024 / DEQ2496 all are good enough. Get TrueAudios' TrueRTA software solution if that turns your crank more. You'll need a reasonable test mic for either approach . The ehringer mic is good enough .
(iii) Get Woofer Tester Pro so that you can verify that your ports are actually set to the correct frequencies.

(B) A person needs the functionality of all three of these categories if one expects even half decent results in the DIY realm .

(C) If you have all this test stuff; then take some measurements / create screen shots of the results ( & turn them into jpgs ) & then post them here so we can move on .

(D) There's just no point assigning blame to these woofers when it could be the execution of your design .


:)