Bass chamber stuffing

[JonM]

One thing I noticed in my newly acquired (24 year old) K-horns was a certain resonant boxiness to the upper bass. A long time K-horn owner suggested I try stuffing polyfill in my K-horns' bass chambers (behind the woofer baskets, accessible via the hatch on the side of the lower cabinet). He said that the same trick got rid of some boominess he experienced when he moved to a house with solid walls (unlike the springy, wooden structure he had lived in before).

Well, I tried it, and it worked! The low end is smoother and less boxy/boomy. My RS meter says I'm still getting strong response to about 40 Hz; my ears say that voices are much more natural.

I'm relatively new at calculating speaker responses, but I think what I did was to make the bass chamber appear acoustically larger, which would (I think) lower the Q of the system, slightly lower the mid bass response peak (before cutoff), and perhaps slightly increase low bass response. I'm not sure what the downside is (slightly lowered efficiency?). I have zero idea how much of a change in these parameters putting stuffing in that chamber would make.

But I have seen *no* other mention of this idea of stuffing the K-horn bass chamber anywhere on the web. That makes me think there's an obvious reason not to do it.

So, what's the consensus? "If it sounds better, it is better"? "PWK designed it without stuffing - take that stuff out!"? Or "Well, you traded X for Y. It's up to you."?

BTW, I am looking for the T-S parameters of the K33E, and the volume of that bass chamber, so I can simulate what I did, and even the effect of using other drivers (not that I'd ever consider...)

Great forum!

[John Warren]

r

[JonM]

Thanks for the reply. Upsetting that "delicate balance" would result in what? More distortion at the horn cutoff frequency? In exchange for what - perhaps greater response below that frequency (where the woofer, I gather, acts as a big driver mounted in a small box)?

I know specifics may be tough (you have no idea how much stuffing I put in, for example), but I'd imagine we could come up with some generalizations that would useful and interesting to the members of the forum. To my ears, in my room, the stuffing seems to have improved the balance of the sound through the lower mids and upper bass (less boominess). I'm curious what (at least theoretical) price I paid for that.

Thaks again for any insights.

[John Warren]

m

[djk]

>>addition of adiabatic material<< Fiberous tangles(fiberglass, Dacron, wool, etc)change the pneumatic action of the enclosure from adibatic to isothermic.This is from frictional losses and can be quite substantial at higher frequencies.Carefull stuffing will cause losses of 1~3dB in the 200hz region where the Klipschorn has a peak.The same stuffing can cause an increase in output in the 400hz region where the Klipschorn has a dip.This is because the first rear air chamber resonance occurs at this frequency because of the average distance from the cone to the chamber surfaces.At the commonly used stuffing densities the frictional losses will be minimal at the reactance annulling point because of the much lower frequency(40hz).Reactance annulling is largely an art anyway.In theory all we have to do is balance the mechanical ohms on both sides of the equation.The problem is the equation is for an infinite horn.For a finite horn, which the Klipschorn is because it is only half the length and half the mouth area even taking into consideration that it was designed for Pi/2 loading, the reactance can be from 1.5 to 2 times that of an infinite horn.And what are you going to use as the throat area? Because of the multiple taper rate of the Klipschorn the real throat area as far as reactance annulling is not the 3" X 13" slot in front of the woofer but rather the area in the horn where the transition from 100hz taper rate to 50hz taper rate takes place.Lastly, the woofer suspension stiffness contributes as much as 15~25% of the total rear air chamber stiffness(reactance). Another thought, seldom discussed, is are we doing the right thing? If done properly reactance annulling will extend the bass about 1/3rd of an octave, in this case from 50hz to 40hz. We have also created a harder "knee" in the roll-off of the low end. With a softer "knee" it is easier to extend the low end with eq.

[jdm56]

Very interesting! - as Col. Klink used to say.

I think that if stuffing sounds better to you, then stuff away. Although, I have tried tweaks and changes that I thought were definitely beneficial at the time, but after awhile, with more listening, I would go back to the original set-up. The lesson being sometimes different is better and sometimes it's just different.

------------------

JDMcCall

[John Warren]

v

[Randy Bey]

>>djk: Carefull stuffing <<

Please define what you mean by careful stuffing?

I do not use EQ of any sort and lowering the frequency with still reasonable response is a good thing for me.

Crikies, where can you get another 10Hz at the bottom end as easily? Assuming the concept of "Carefull stuffing" is easy.

So djk, what is right here?

[WMcD]

The Speakerlab K kits used stuffing. From pictures, it seemed to be about two square feet of 3 inch thick fiberglass.

Also, the plans showed that they used the top and bottom "ramps" to enlarge the back chamber. However, the instructions I've seen said to not make the cut out to communicate with the ramps.

It may be that Klipsch experimented with the same and then backed away. Speakerlab rotely followed the original "improvement" and then backed away too.

I'd agree. Different is sometimes only different.

Gil

[djk]

Randy, the carefull stuffing was to get the loss in the 200hz region and gain in the 400hz region without disturbing the gain at 40hz from reactance annulling.50% foam and 50% fiberglass at a 1lb/cf density would be the best for this.Keep the density of the FG that is close behind the woofer as low as possible.The foam goes on the walls.Souther's Sorghum (from a hobby shop) works well on foam.Another strategy would be to stuff as to destroy the reactance annulling to make it easier to EQ.

[John Warren]

The original Klipschorn was developed around a field coil, stiff paper surround 12" Jensen A12 "Concert Speaker". The prototype horn also contained a front chamber between the woofer and baffle acting as a shunt capacitance. Klipsch's discussions and ballparking assumptions are centered around this configuration. Also, for rear chamber calculations, his throat area was (correctly) based on the area of the transition between the 100Hz and 47Hz flare rates and not on the woofer baffle slot.

Half inch plywood is inadequate for this horn. Mid-bass peaks are attenuated by bracing the "V" panels that make up the rear chamber. It requires drilling, screwing and glueing edgewise a 1/2" thick, 3" wide plywood rib down the center of each panel.

[Dieter]

Hello!

Here are some tweaks for the Klipschhorn-Bass:

http://www.artandaudio.com/constrained.html

Dieter

------------------

Bass: Klipschorn

Mids: Fostex H325 + Fostex D450

High: Fostex T825

X : ALK

Pre : Rotel RC 995

Amp : AudioValve PPP 30

CD : NAD 520

Cine: Yamaha E 1000

Cent: LaSala

Rear: DIY EV

[Randy Bey]

Dieter,

I have read this page before. In it the author appears to imply that the computations of horn throats, mouths, etc. can be disregarded for bass speakers.

More or less that anything you do will have no positive effect, but possibly a negative effect.

Other than running the woofer straight w/o the 2.5mH coil, and adding the furring strips to the rear of the bass chamber, not much apparently can be done.

I am interested in the back chamber discussion however. He implies that larger chambers can lower the 100Hz peak in a khorn. Those graphs depicted remind me of the frequency graphs I made of my own stereo.

The implication is that stuffing the back chamber could be a positive thing.

So, three things: remove the coil, add the reinforcing strips, and stuff the chamber.

Food for thought.

[John Warren]

the last section of the URL given by Dieter discusses the bracing of the rear chamber panels that make up the "V". This modification makes a noticable improvement in the bass response. done properly,

it eliminates the mid-bass hangover. it is also irreversible since it requires glue and screws.