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Old 22nd October 2009, 04:29 PM   #1
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Default Horn loading for Idiots

I have never studied physics - which will be glaringly obvious, very soon, but I can't help being fascinated with the "popular" explanations of physics and engineering. Eventually, even us idiots can intuit some of the bigger picture with a little help.

So I googled 'second law of thermodynamics" and "horn loading" and very quickly got lost in the physics. This question has long puzzled me. How is it, for instance, that the same splutter and blatting that would be easily lost in crowded room, when delivered through a swiss alpenhorn, could stun everyone to silence? It's the same splutter and blat. The horn itself is not plugged into any source of power to amplifiy the "signal".

Is it that the splutter and blatting actually contains a great deal of wasted energy in the form of air movement, lip vibration, etc? And that the horn is merely an extraordinarily efficient mechanism for converting that wasted energy into sound? This seems intuitive, but still does not seem able explain the enormity of "amplification" taking place. I mean, a raspberry on the lips of someone that would not be heard any further than he could spit, can be broadcast, wtih NO other insertion of energy, literally for miles. Just seems to bend the laws somewhere.

Energy's a strange thing to get your head around for most of us. I sometimes wonder if there weren't at least a few physicists in the crowd at Trinity site who, despite the certainty of their mathmatics, were nonetheless astonished when damn thing actually did what was advertised.
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Old 22nd October 2009, 05:41 PM   #2
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One other.. I do get the idea, i think, of a horn being a 'resonant' device. An amateur's attempts at horn blowing often end up a muffled farting. He hasn't mastered the art of matching his input to the resonance of the horn and acheiving that (feedback?) state where the amplification takes place. But that understanding only seems to make the whole process more difficult to grasp yet. Where is all that energy coming from?

Last edited by peace brainerd; 22nd October 2009 at 05:53 PM.
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Old 22nd October 2009, 06:55 PM   #3
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A horn is a folded up flat baffle, so the extra gain is merely the flat baffle's gain BW concentrated over a narrower arc. WRT to a 'human' horn, our breathing/sound reproduction system is basically a 4th order band-pass alignment, same as a compression horn driver's, so when properly impedance matched to a horn it produces an amplified sound.

HTH,

GM
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Old 22nd October 2009, 07:31 PM   #4
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The masses of ignorant are still blinking determinedly.. breathing shallowly through their mouths.

Is it possible to describe how such 'amplification' is produced, without the introduction of more energy (wall socket for instance) while contemplating the notion that there is no free lunch in physics? Merely focusing, beaming, or narrowing an arc doesn't quite get it since any tapered cone would suffice to induce it, and it doesn't (or not very much anyway). There's something about this resonant dynamic that frustrates the intuition. It seems almost magical that at certain resonance, that 'fourth order bandpass alignment' seems to accumulate energy (from where?) and exit the horn a magnitude more powerful (more energetic) than what was put into the other end.

net energy to device ---> neutral (unpowered) device -----> increased (apparently, but impossible) energy out of device

Certainly something is not so "apparent" here.

Is it not possible to describe what's happening without a physics primer?

Last edited by peace brainerd; 22nd October 2009 at 07:44 PM.
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Old 22nd October 2009, 07:47 PM   #5
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Is the same principle of the wing profile of an airplane . Curved shape on top and flat on the bottom , basically . Evangelism is in the air
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Old 22nd October 2009, 08:13 PM   #6
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Quote:
Originally Posted by svejkovat View Post
net energy to device ---> neutral (unpowered) device -----> increased (apparently, but impossible) energy out of device

Certainly something is not so "apparent" here.

Is it not possible to describe what's happening without a physics primer?
Probably not - but if you think about what comes in and goes out.....

With a normal speaker, you put energy into it, and you spend most of it just moving the diaphragm back and forth. Since it goes nowhere (in the time average) all that energy is just burnt up as heat in the voice coil. A very small amount of air is also moved, and the energy from that gets converted to sound. Between 0.1 and 3 percent of the total.

With a horn (in its passband), you now move not only the cone and voice coil, but an entire column of air. Now the air load is a much larger fraction of the total mass that is moved. A higher percentage (as much as 50%) of the total energy coming in goes into moving air (and therefore making sound). This works from the low end cut off of the horn (about a quater wavelength) up to the frequecny where it quits working efficiency (the "mass corner").

The fact that horns are directional above their passband gives them apparent increase in efficiency , even as the power response is falling as frequency rises.
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Old 23rd October 2009, 12:01 AM   #7
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picowallspeaker, that's another that bugged me for quite a while. There is a small list of very common and physical matters that seem to baffle the "laypersons" in most hobbies despite the fact that they make use of the physics without much problem at all.

Those aerodynamics are pretty well utilized by most kids who modify their paper airplanes, even if their real apprehension of them is limited. I can build a pretty effective version of that Swiss alphorn without having much real calculable understanding of the physics. 15th century Alpine herders certainly knew less than even I do about such. But that didn't stop them.

I was originally trying to rationalize the relative obscurity of, for instance, tapped horns among the Google/Subwoofer weblinks. For all of the purported advantages of them, I wondered if it is the case that they are simply unappreciated. Misunderstood? Still too technically complex to build that they are unattractive to the average guy? Or, (i don't want to start a fight here) maybe over-hyped considering their shortcomings.

If a much smaller driver can be made to produce superior levels of accurate sound from a well designed, if slightly larger, cabinet.. then why are they not the rage? I started to wonder if perhaps I was missing something really obvious, but my lack of very basic understanding of how they were accomplishing their 'magic' made me ask the original question.

With really basic aerodynamics, I'm comfortable enough with profile, air density, laminar flow, to play around with them on model aircraft. That's probably because of the interest that the subject generates among writers of popular science and the descriptions they've provided me. (tho I've also suspected that some like to overemphasize the "vacuum creating lift" part of the pressure differential just to point out something counter-intuitive to the layperson and insert a little mystery. Big deal.)

But this making something big out of something small by running it through a resonant horn just flies right over my head. I can't find a coherent description of it anywhere.

Last edited by peace brainerd; 23rd October 2009 at 12:10 AM.
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Old 23rd October 2009, 01:17 AM   #8
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tho I've also suspected that some like to overemphasize the "vacuum creating lift" part of the pressure differential
Five minutes with Langewiesche will cure anyone of that temptation.
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Old 23rd October 2009, 01:47 AM   #9
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Hi

Personally I find most idiots are loud enough without horn loading, in fact too often that seems to be a defining feature.

Seriously I do know what your asking, just not sure how to answer the most understandably.

A piston vibrating in air feels a resistance to it’s motion (it's velocity), this is related to the sound which radiates away and does not return.
For a fixed frequency, the larger the piston is the more resistance it will feel (up to a point).

Since mass and spring forces are not dissipative (not resistive, no work is done) sound power is not radiated here.
I mentioned ‘to a point” what I mean is that when you look at a Radiation resistance curve for a piston (fig 3 in this link), you see that in the left most region, the radiation resistance increases as the size of the radiator approaches a dimension where it is approaching a half wavelength across.

http://www.aikenamps.com/speaker.pdf

Above that frequency, the resistance is essentially unchanging with respect to piston size.
For a woofer, one has a radiator which is much smaller than the wavelength so it operates far down on the left side of the graph.
AS you might guess, a larger piston can be more efficient as it feels more load from that radiation resistance. This is also why two separate woofers placed less than ¼ wl apart, become twice as efficient as one alone, it has in effect doubled it’s area and so has twice the load and so does twice the “work” radiating compared to one.

A “horn” is used because it provides a way to couple the large area at the big end of the horn to a comparatively tiny driver at the small end or in other words, to couple the high resistance part of the curve to the tiny driver which operates at the far left of the curve, thus greatly increasing the radiation efficiency (produces more SPL for a given electrical power input). In reality, a horns radiation resistance curve is somewhat different, the ideal or flat portion of the curve is reached at about 1 wavelength in circumference.

Some like to use the analogy of an impedance matching transformer which is vaguely correct in some cases.
A more accurate analogy is a three terminal “T” resonant impedance matcher if you deal with antennas.
About half way down on this link is such a circuit.
Impedance Matching Network Designer

The ideal is you make the impedance point at the center, the square root of the input times the output impedance.

With sound, resistance is related to area and if you examine a simple exponential horn, one finds the center resistance between the ends and actually at any points you examine, is the square root of the small end times the large end.

An electrical circuit like these in the link has one set of values and works only at one frequency BUT a horn is still different. Consider a large proper horn.

The electrical circuit is obviously resonant and has a narrow bandwidth BUT a horn is a variable device. At it’s low cutoff, the entire horn is working but as the frequency goes up, the horn becomes acoustically larger and longer and one finds the operation portion of the horn moves towards the driver and the mouth end is no longer involved with the impedance transformation as it is already on the flat part of the resistance curve.
If a horn has the area’s consistent with the source and load resistances, the fact that it is resonant is fully masked and in that case really does appear to be a transformer or like a gear ratio and not resonant.

I real life it is nearly impossible to make a horn large enough to act like this so bass horns operate in a strange in between world.

An additional complication is horns have a “high pass” filter effect related to how fast the area expands along it’s length. For a simple exponential horn, if you want a 30Hz horn, the area needs to double about every 24 inches, for a 60Hz horn its every 12 inches and so on.
Why is this?

In a horn one has a momentum transfer from one air molecule to another.
Picture a huge airhocky table with a zillion pucks all touching ( a 2 dimensional analogue)
If you hit the middle of this array of pucks you will see that like sound the pressure spreads in all directions. Only a small amount of the energy will be released in the same direction as the initiating puck.
The goal of the horn is to prevent the pressure from escaping to the sides like a duct would do except that by allowing the area to expand slowly, one can couple the energy to a large number of pucks and have them all go in one direction, like a single large piston would do.
Now, keep in mind that the wavelength comes into this too, the wavelength describes the natural change in pressure at different parts of the wave as it progresses down the horn.
Thus, one finds the expansion rate ties into how low in frequency this momentum transfer can be accomplished.

I have skipped over a great deal here for simplification and have used some descriptions, which are analogies I like, which I hope make things easier to envision.
These are not part of any text book explanation if you can find such a thing but is based on how I have come to see a horns operation as of today.
Hope this helps.

Tom Danley
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Old 23rd October 2009, 03:39 AM   #10
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wg_ski and Tom, thank you very, very much. Tom that was especially generous. I'll have to go over that a few more times but it does light things up a bit and point me in a better direction.
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