Time-Alignment for Folded Horns (Patent)

[sfogg]

"I believe there was a dbx digital processor that had up to 96dB/octave

slopes? I forget exactly, but we still had to dial in the time delay."

Possibly, there are units that go higher still. For example the Deqx

can go up to 300db/octave linear phase if desired. But that is a $3k

box.

Shawn

[sfogg]

DM,

There is a lot of information on that site... not neccessarily

organized as well as it might be though. David Griesinger is one of

the formost experts in the world on acoustics, psychoacoustics, reverb,

surround sound and a host of other things. The man is no dummy. Lot of

great info there, if you ever want some 'light' reading check it out.

Shawn

[Deang]

I'm staying in denial. With the Klipschorn, we are talking 3ms, right?

[D-MAN]

Khorn approx 64" horn pathway is slightly over 5 feet -or- approx. 5-6 mSec overall "delay", that's my guess (1 mSec=1 foot of travel). I have read that 3 mS (3 ft) or less "displacement" is undetectable by the human ear; from research resulting from the famous "tap-dancing" problem of early theater speakers.

The frictional forces caused by the internal horn walls and folds presumably would also slow things a bit...

Some British DIY hornbuilders recommend painting the interior horn channels with high gloss paint for this reason.

DM

[Al Klappenberger]

DrWho,

I think you are localizing the subwoofer by the high frequency components of the wavefore that are well above 70 Hz. One example I can think of is a sonic boom from an aircraft. You can easily tell where the jet noise is comming from, but I doubt of you could point to the source of sonic boom without the jet noise! I suspect it's a matter of wavelength of the sound compared to the distance the source is from you and other things in a room.

Dean: Hearing a sound twice is only twice as good if it was only half as good the first time. Everybody knows that! [*-)]

Al k.

[DrWho]

If it was a test tone then yes perhaps (considering that they generally build up standing waves which creates a constant pressure in the room). If nothing else, you can feel the impact of a low frequency signal on your body and can easily tell the direction (just like you can tell the direction of the wind). I really don't think it has anything to do with harmonic material or any of that...

The sonic boom might be a bit hard because it's such a short signal though. Likewise, constant test tones would be hard too because they quickly form standing waves, which results in a constant pressure zone essentially making it impossible. With normal sounds we've still got our two ears (triangulation and all that nonsense)...granted it gets way less accurate at low frequencies, but you can still tell if the bass is coming more from the left or more from the right (I would never claim pinpoint accuracy). I would argue though that the process of moving your head around quickly contributes to the accuracy of hearing (turning one's head quickly reveals the subwoofer location). Not to mention that the bass can be either more or less enveloping, which would also be a product of directivity. I believe there were some recent articles posted discussing this effect.

anyways, gotta go...time for bed

[Al Klappenberger]

DrWho,

I think you will find there is really no final answer to this. It would take a lot of serious research to answer the question. It may even come down to each person's individual talents! Everybody is slightly different.

Al K.

[3dzapper]

Using the standard 1100ft/sec @ sealevel and 70 oF the 5' wave of the bass bin would be 5/1100 or.00455 sec. the delay of the mid on a Khorn would then be 2/1100 or.00182 sec. The net delay, .00273 sec or 2.73mSec, lies under the threshold of human perception.

Adding a sub-woofer 6 feet away from the horn mouth would introduce a more noticeable effect IMO as the effects would then be additive. Like 7.3mSec from the squaker. In the audible range.

Gloss painting of the horns interior seems, to me, counterproductive. The slightly rougher natural wood finish has many air pockets. Although the air in these pockets would be compressed, it is relitively stationary and provides a low friction face to the moving sound wave. Without this air barrier, the smooth painted surface would present to the wave a higher friction surface. This effect is similiar to the ground effect on a high speed race car which utilizes the air barrier attached to the asphalt to reduce air friction, an air hockey puck or a boat which will go faster on a slightly choppy sea than a ruler flat one due to air pockets trapped under the hull.

Again, I am not a "scientist" and my observations are not limited by my lack of emperical data.[]

Rick

PS: I could be wrong but I did have some fun with my calculator.

[DrWho]

well don't forget to include the group delay on the woofer too... []

[D-MAN]

Traditionally, the audio media has poo-pooed the Khorn because it suffers from time delay, at least that's the excuse they use. I'm sure that you guys have read that, too.

Technically, it does. It is in the "hearable" range, over 3 ft of distance determined by soundpath between the various diaphragms. That's pretty inarguable.

As to whether it actually MATTERS, that I cannot answer. I only thought that this patent was an interesting approach to eliminating one of the last remaining "objections" to folded horns, especially the Khorn.

Frankly, I think that the sheer dynamism (and its twin- efficiency) of folded horns certainly makes up for any other sonic "deficiencies" that the designs may present. As stated by our eminent colleague's sign-off "it's all about compromise".

The extreme slope crossovers DO eliminate the tendency to produce comb-filtering effects caused by different sound paths. Time-delay is still technically present of course, but to the ear it remains indistinguishable as the frequency overlap between drivers is eliminated to a great degree.

Personally, since switching to other bass horn drivers from the K33E, I have a sense of a faster and "tighter" response with increased punch and "slam", which increases the sense of "timeliness". The K33E is part of the time-lag perception problem, in my opinion. I find that it's noticably "slow and sloppy" compared to some other drivers, all other componentry remaining the same.

As far as painting the interior horn walls, that seems to be a good idea to me; painted surfaces present a reduced frictional component to the passage of soundwaves though a conduit compared to an unfinished surface. Whether that would add up to an improvement, I can't say either, but it seems like too much work for too little gain for me. But that's just me.

(attached document(s) in pdf format are available for download from www.volvotreter.de - highly recommended website!).

DM

[sunnysal]

Richard Heyser in his seminal measurements of the Klipschorn in 1980 determined "that the first sound from the woofer arrives about 8.4ms after the sound from the tweeter". I have seen other reports that claim only 7.4ms between tweeter and bass but I tend to trust Heyser, he was a giant in the field. Regards, tony

[D-MAN]

That would seem a little excessive to me, the differences between the respective diaphragms based on the length of pathway travel is not enough to account for that long of a delay.

However, the difference in pathways (the Khorn mouth(s) are side-pointing and therefore "not direct") MIGHT account for the variance between the theoretical and the practical - but THAT depends on where the test microphone was placed. Although I can't figure how on earth one could come up with that great of delay...it cannot be just distance alone = that value would imply that the Khorn bass horn has an overall path length of 8 feet, and that just ain't so. Even the splay angles of the horn mouth(s) doesn't account for that much of a difference EXCEPT if the test microphone was placed center far-field. So now we are measuring the room?

It could also be that the folds slow things down quite a bit, and the interior channel frictional issues has a far greater effect than I first considered (I gave it a +1 mSec value). If so, then this really bolsters the idea of painting the interior horn channels!

Pretty confusing results, but I'm leaning in the direction of Tony and think that the data reported is factual (I'd love to see the article). This leads us towards the painting recommendation, doesn't it?!

One more consideration, I've always wondered about the Khorn tailboard as being a 180 degree obstruction to waveflow certain to cause turbulence. I am making the correlation that turbulence=delay resulting from interrupted waveflow. Perhaps as revisiting of the tailboard could "speed things up" a bit. You will note that this particular issue was "corrected" in the PWK design for the Jubilee... I would presume that the Jubilee has a "faster" response time due to reduced turbulence because of its design.

Brings us back to the fact that it would be far easier to add a tube of a certain length to the top-end drivers as shown in the previous patent than to attempt to modify the bass horn for enhanced speed! The inventor has a point!

DM

[DrWho]

I've always wondered about the effects of the bass horn shooting off to the sides...seems to me like an awfully lot of nasty diffraction would be occuring along the front face of the cabinet. Perhaps there is a factor of time delay there...as the sound wraps around the front, collides into the waveform on the other side and then continues to propogate out.

All I know is that every khorn I've heard so far has a tubby sound to it and other smarter folks attribute it to issues of time delay (and there's prob a factor of cabinet vibration in there too).

[sunnysal]

D-man, if you like I can make a PDF of the article and send it to you..pm me about it.

re: painting the inside...I think I have mentioned before that painting the inside may give you the opposite result...current wisdom is that you want to create a boundary layer of air for the moving air to ride on (this was mentioned in an earlier post), golf balls, racing cars, swimsuits, race ware for bikes and aircraft are all using this technique to reduce drag from air. certain textures will reduce air drag, below what a completely smooth surface achieves...regards, tony

[D-MAN]

Tony, I sent you a PM!

DM

[D-MAN]

On the topic of painting internal horn channels to reduce friction, I have come up with this:

Remember the motto of Jules Verne's Nautilus? "Mobilus in Mobilae"

Movement in a moving medium.

Consider fluidics (air is a fluidic medium), such as river flow studys, wherein to paddle "upstream" one stays close to the shore, to take advantage of the eddy effect. Anyone who has paddled upstream against the current understands this fact in a practical manner, that's for sure. Closest to shore is where the flow curls around in the opposite direction along the flow path boundaries, forming "eddies". This effect occurs as a natural response to channeled pathways when flow is present. It also applies to ship hulls as they move through static bodies of water. Smooth boundaries are more efficiently travelled. It should be noted that as far as I know, nobody is producing boat hulls that include surface "dimpling" and/or "pebbling" as a means to achieve a faster through-the-water speed.

The same physics apply to the internal horn channels. A smooth surface without discontinuities is definitely preferable to enable efficient fluidic motion through a conduit. Any reduction of obstruction and frictional forces is a good thing for efficient waveform travel. The smoother the channel boundaries, the smaller the eddies produced (always will be there, but less is better), which is less diverted energy in opposition to center channel flow.

It has been brought to my attention that the use of "pebbling" and/or "dimpling" in some new speakers (B&W for instance) ports as perhaps increasing the efficiency of flow through the port. However, I believe that these are there to CAUSE some discontinuity to prevent "chuffing" which ports are known to do from time to time. Golf balls do not fly through the air in a stabilized manner if smooth, the dimples are employed to "grip" the air, i.e., slow things down and prevent inadvertant ballistic path changes due to the vagaries of imparted spin forces. Adding discontinuities to a flow channel is not going to improve efficiency of waveform transit, it will slow things down, increasing inefficiency. Moving a fluidic medium through a flow channel will not be enhanced by increasing channel wall frictional effects. This is contrary to physics. Air cannot be used as a friction-reducing lubricant for other air molocules, just as water cannot be used as a lubricant for other water molocules. All of the laws that apply to fluids apply here. You need elements that are unlike in density for a lubrication effect to take place.

I discount the value of the air column boundary "ground effect" in horn channels, and consider the issue to be one of frictional resistance at the channel boundaries, best addressed by smooth internal horn walls. High gloss paint would do it, I recon.

DM

[Deang]

All I know is that every khorn I've heard so far has a tubby sound to it and other smarter folks attribute it to issues of time delay (and there's prob a factor of cabinet vibration in there too).

"Other smarter folks" are often wrong. Get in your car and come see me this weekend -- I'll pay your gas bill. I'd like to show you how smart other smarter folks are.

[Deang]

Hee hee. 350 miles. O.K. -- I'll split the gas.[]

[D-MAN]

When I last heard a 2004 stock Khorn the bass-to-midrange crossover was absolutely seamless (and flawless) sounding. I thought it was the most impressive thing about the response, overall.

I NEVER would regard that sound as "tubby". So I am confused by the term "tubby". Tubby as in too much low end (bass bloat)? Can you explain the term so we can get a grip on what you are saying.

Do you mean "tube-y"? Like sound in a tube? that is typical of coloration.

But timing issues are not coloration, IMO. They are a smearing sort of thing that make thumps instead of whacks, that sort of thing. Not a coloration or "sounds like your listening to a tube, pipe or conduit".

DM

[Deang]

The AK-4 low pass section has a trap and is EQ'd -- they did a very good job with it. I can closely simulate the performance using the AK-3 low pass filter and my Behringer.

By "tubby" he means like the sound you get when you bang on the side of your tub while your head is under water. I know what he's hearing, but it doesn't have anything to do with 8/1000ths of a second of delay. If I say what it is -- I'll be banned.