Khorn & Fostex

[WMcD]

James, I really don't have any easy solutions to suggest. But a few more observations. Does anyone mind a long post?

Everyone has a theory. Real scientific evidence is hard to come by. Talk is cheap. Take it all with a grain of salt. I certainly don't want to lead you astray.

I doubt the probem is a simple reflection off the wall behind the Belle. The distance of 18 inches, or 36 inches in path length is just not long enough to create a bass problem.

When there is a problem with the bounce off the wall just behind, it is called an Allison (sp?, the guy who described it) dip. It is usually in the midrange or upper bass. Not 50 Hz. People solve it by moving the speaker a bit away from the wall.

I suppose there is some chance you've formed a cavity resonator in the space behind the Belle. Pulling the Belle out a bit might be an experiment.

The room dimensions sound dangerous. They are close to 1:2:2. That is very suspect. Not as bad as a cube with 1:1:1, but almost as bad. You've got half a cube. I'll send you, and anyone else, a copy of the Dope from Hope on room dimensions if you e-mail me your postal address.

Some folks seem to think you should have 1: 1.6 : (1.6 x 1.6) ratio. Golden mean, progression.

I've read of some software which will predict problems with room modes. Perhaps someone on the forum can suggest a source.

The present problem seems to be that you're not going to be able to move the speaker location, you can't move the walls, equalization doesn't work. What is left to do?

Maybe absorptive material as suggested, but one must be realistic.

I've read that even anechoic test chambers used by the manufacturers don't absorb perfectly well below 200 Hz. Of course what is perfectly without echos is not what is necessary to solve a given problem.

The issue with absorbing bass is that the absorber has be almost as big as the wavelength of the sound in free space. Therefore, even a 4' x 8' panel might not be enough. Measured results of sound absorbing material show that they work well in the treble, and not very much at all in the bass. I think the problem is that even if they are relatively wide and tall, they are not deep. So they don't absorb bass frequencies.

One technique which I've read of, and not experimented with, are "tube traps" placed in corners. These are essentially columns of fiberglass roughtly 1 or 1.5 foot diameter. Usually hollow. The fiberglass batting can be trapped between coaxial cylinders of wire mesh. So you have a maybe a 1.5 foot diameter outside cylinder and a 1 foot inside cylinder, with fiberglass in between. Then these are placed in the corner.

How tall? Maybe 4 feet. Floor to ceiling would be better. This is almost silly because it is like building the Great Wall of China with no guarantee of success. But interesting to talk about.

There is some good reasons to think these would work even in view of the wave mechanics. They are sort of like absorbing K-Horns in the corner (for you the back corners). Just as corner placement of the K-Horn magnifies it's apparent size by 8 (in theory), here the absorbers are 8 times bigger when placed in a corner. If they are up to the ceiling, you're putting in two at either corner of the room.

Clearly only a fanatic would do something like this. Would a couple of over stuffed Lazy Boy recliners in the corners absorb as well? I dunno. Hard data is difficult to come by.

I've had my own interesting failures. I had a three channel set up in a garage. I made sweeps with the mike at four positions using an LMS system, and averaged them. Then I opened the garage door to remove the sixth wall. Ran the sweeps again. This was going to be real dramatic.

The change was almost nothing. Cue the Therimin music and Outer Limits theme. Either my set up was all wrong, my understanding was all wrong, or the garage door had never been substantial enough to serve as a wall for the bass in the first place. Probably a lot of all three. Amateur acoustics is a bear.

Gil

This message has been edited by William F. Gil McDermott on 05-15-2001 at 09:12 PM

[jdm56]

Thanks Again, Guys!

I'm probably showing my ignorance here, but since my problem is nulls, I don't see how bass traps would help me. I've got too much bass trapped already! I'm tryin' to let it out!

The easiest fix would probably be the addition of the belle klipsch in the center. My experiments with adding a full range center showed a marked overall bass increase below 100hz, with substantial output at the frequencies where the cornerhorns are deficient. But they are not in production right now, and I've had no luck finding one anywhere else.

The cheapest, and second easiest fix would be to sell the k-horns, and go with a more flexible system. Maybe RB5's and a reference sub. Again, my experiments with speaker placement (an old pair of Infinity Studio Monitors - think Cerwin-Vega) show that good bass is posible in this room, but not neccesarily(sp) from the corners. I know this seems to run contrary to the conventional wisdom, but my k-horns, the old Infinity SM12's, and a small 12" sub ALL are substantially down at 50hz and 80hz with corner placement, when measured from the listening position. In fact the 'horns are the most down relative to their 200hz output. The best single location for smooth bass seems to be about 1/4 to 1/3 of the wall length out from the corners. Corners and center wall produce the biggest peaks and dips, though at different frequencies.

Other than the 50hz and 80hz dips, bass is actually quite good, with no bothersome resonances, although a full range center speaker does excite a nasty one at about 63hz; but this would be easy to fix. And that full range center definitely fills in those 50 and 80hz notches.

This is my room dimensions with their axial modes below 300hz:

length width height

18.5' 16.0' 8.67'

30.5hz 35.3hz 65.2hz

61.0hz 70.6hz 130.4hz

91.5hz 105.9hz 195.6hz

122.0hz 141.2hz 260.8hz

152.5hz 176.5hz

183.0hz 211.8hz

213.5hz 247.1hz

244.0hz 282.4hz

274.5hz

...you'll notice; nothing even close to 50 or 80hz, but not too bad, really.

I'd have to think long and hard before giving up my beloved horns, cause they'd be a long time gone. But still for the kind of $ I've got invested, I think getting bass performance of +/- 3dB from 35 to 200hz should not be asking to much.

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JDMcCall

[jdm56]

quote:

---

Originally posted by Randy Bey:

JD,

But, since I don't know anyone with a perfect room, or perfectly flat speakers in that perfect room, perhaps it is best to imbibe mass quantities of malt beverages until the unpleasant memory of peaks and troughs go away....

---

You,ve got a point there, Randy, but I'm afraid my next problem might run along the lines of that recent "puke on my speakers" thread.

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JDMcCall

[Aspenmarie]

Have you considered moving your listening position around? Your speakers have to be in the corners (more or less) so that is pretty well given. However, you haven't mentioned moving around where you listen. This can have a significant effect on the bass. Try moving your listening position to the rear wall. I think you'll notice a significant change.

If that doesn't work, try putting the speakers on the long wall instead of the short one. Again, you may have to experiment with your listening position.

Good Luck!

Jeff Englert

[J M O N]

James,

Let's see if I can shed a little light on your problem with the nulls that you are experiencing by telling you a story of what happened to me once upon a time. Back when I was in college, I had put together a car stereo system that utilized a dual-woofer sub. It sounded pretty good and put out quite a bit of bass in a car environment. I then went out of the country for a year, and when I returned I reinstalled my sub. A year had gone by, but it sure didn't seem like I was getting as much or as good bass as I did before. I thought maybe it had to do with being gone so long, and remembering the system to be better than it actually was. One day while waiting for someone in my car, listening to music, I was goofing around with the stereo controls. I adjusted the balance and the sound from one side disappeared, but the bass actually increased and improved in quality. What the ??? How did that happen? After some tinkering, it turned out that I had connected one of the woofer connections backwards. So the two woofers were out of phase with each other. What happens in this situation is when one woofer moves out, the other moves in, and in essence, they are acting to cancel each other out. When you remove one woofer (as what happened when I adjusted the balance all the way to one side), you no longer have the cancelation effect happening, and have the bass from one woofer. Bass from one woofer is significantly greater than bass from two woofers that are out of phase. By eliminating one woofer, you essentially have "addition by subraction". I connected the sub correctly, and bam, the bass was there like it had been before!!

The nulls that you are experiencing could be a result of bass waves canceling each other out in certain parts of your room. The bass traps that were mentioned would help increase your bass by eliminating secondary bass waves (typically these would be waves reflected off your walls) that would act in the cancelating the primary bass waves (those coming directly from the speaker) -- "addition by subtraction". This problem is going to be more a result of your room and the placement of your speakers (nothing you can do about that with Khorns), than it is with the speakers themselves. If you replaced your Khorns with other speakers, you may very well still have the same problem (although being able to move them around the room to an optimal location may give you some improvement). It is my understanding that this problem can be fixed using room treatments, but depending on the severity of the problem, it could be a tedius/expensive process.

I am not an expert in this area by any means, so you may want to do some research on this subject or solicit input from some of the other folks on the BB. I don't claim to be 100% correct on what I said, so I welcome corrections by anyone here. There have been several suggestions for finding information on room acoustics from others on the board. Anyway, I hope I was able to provide a little bit of explanation of what could be happening.

[jdm56]

Aspenmarie,

I've moved my couch back and forth so much, I oughta put a license plate on it. I've even took spl readings at six-inch increments front to back, midway between the speakers. And yes, I've tried listening within the "boundary effect" zone of the rear wall, but this places me too far off the horns axis - poor imaging AND anctually a little too much bass! Oh, I do have the horns on the long wall. I tried the short wall, but didn't like it as well.

Jmon,

Yeah, the polarity is correct. But it never hurts to check.

I see what you mean about how bass traps correctly placed might actually increase output at the listening position. Thanks a lot. That had eluded me until you spelled it out for me. I will check into bulding a couple.

Thanks a bunch, guys, for the help.

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JDMcCall

[J M O N]

James,

I didn't mean to imply that you had you speakers connected wrong. I only told that story to try and explain the cancellation effect. But as you say, it never hurts to double check. In fact, I couldn't believe that I would have connected those speakers backwards. I did double check, but didn't find my mistake right away. I later triple checked, this time much more closely and realized that I did indeed have them connected wrong. It turns out that it was hard to read polarity on the cheap (very thin) wire I used. The white line I saw on the black insulation jacket wasn't a white line afterall, it was the reflection of the light that looked just like the (smaller, less obvious) white line on the other wire. Boy did I feel dumb when I realized what happened.

[J M O N]

P.S.

Just as reflected waves can cancel each other out, they can also can also add upon each other and yield as much as double the amplitude depending on what phase they are in when they reach each other (this depends on room dimensions, speaker positions, and location in the room where they meet). This could be the reason why in some spots you have too much bass, and in others you have nulls. It does sound to me like the problem is with your room, and not the speakers. Again, I'm not an expert in this area but I would suggest that you go out and do some research on this before buying anything. I believe someone posted a website where you can download an "Acoustics 101". You might want to do a search on this BB.

Good luck.

[Randy Bey]

Regarding bass traps...

I have only one free corner in my listening room and I have stuffed it floor to ceiling with bass traps as Gil has described. Large cylinders with a trapped air space inside, wire mesh and fibreglass, with an "attractive" burlap covering.

I also have three sound absorbtion panels, 2x4', placed at the walls where a flashlight aimed the same as the speaker would shine. And the third midway between the two speakers on the front wall. They all hang about 8" from the wall they are near.

Plus as much furniture and bookshelves, etc. as possible around the walls and in the room. This is in an effort to reduce the amount of blank, flat wall space available for reflections. The only reflections I want are the ones to either side of the speaker where the dispersion characteristics of the tweeter/squawker naturally put them.

Regarding the bass traps, yes, oh yes, they do affect the bass. This is readily apparent as they are quite easy to move in and out of place. Well, fairly easy, they weigh about 20 lbs each.

There is a Zen like quality about they way they work. Yes, they work by sucking up bass modes in the corners of the room. But, by so doing, they clean up the muddiness and "one-note" aspects of the lower frequency responses in the room. What is left is purer, more articulate bass that is more pleasing than what you started with. And without the room resonance there to overwhelm other frequencies, your lower frequency response will show through.

Sure, if you listen to Aerosmith (no digs on the group but you know the type of music I refer to) the loss of the 50-60Hz resonance will reduce the apparent bass response and make it sound less gut-thumping good. But, given a bit of time, you may appreciate hearing individual notes as opposed to long unnaturally sustained decays.

Yes, even bass sounds have attack, sustain and decay. Not so obvious when your room wants to hold on to each sound as long as it can.

It took me about a weekend to build two bass traps (and about $50 in raw parts).

Ditto the sound absorbtion panels.

I would highly recommend them to anyone, even those that don't feel they would be effective. You may be surprized. Room acoustics is far beyond my pea brain, but all is not hopeless. I no longer believe you need to build a new house to hold your stereo.

[jdm56]

Thanks a ton for all the advice. And Gil, thanks for the "Dope From Hope".

I really should count my audio blessings, I guess. With my current listening position, the sound is very, very good. Sure those nagging bass cancellations are still there, though minimized to the best of my limited ability. If I ever can aquire a belle center, I am confident that problem will be largely solved, too. But still, I plan to set about finding the cause of these nulls and how to eliminate them. The bass trap option sounds promising! But, all in all, I've still got the best sounding set-up I've ever had.

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JDMcCall

[Pete C]

Dieter,

How did you fit the new drivers into the cabinet, especially the T825? Any photos to share with us?

[Dieter]

Hey Pete!

I have not the original Klipschorn, it is DIY.

The H325 is lying on the top of the basscabinet.

On EBAY is such a horn with a bigger horn and another tweeter.

http://cgi.ebay.de/aw-cgi/eBayISAPI.dll?ViewItem&item=1239136991

regards

Dieter

[johns02]

I have the fix!!!!!

K-43 woofer , McIntosh tube amp or similar

No tone controls and it rocks

With Bryston 4B it hurts!