Folded Horn Enclosures: How do they work?

[sine143]

Alright. I'm sure this has probably been answered on the forum many times, however... I'm asking again

How exactly does the folded horn design work to provide the extra sensitivity and output displayed by the response charts? I thought that I understood, but when I tried to explain it to a friend (who claims a folded horn is like putting a spoiler on a consumer car), I found myself drawing blanks.

With the wavelength of a 50hz sine wave at over 22 feet.... I guess its not some sort of reflection thing. I can give answers like it "matures" the sound and other non technical bs all day, but I'm not happy with those answers.

Like I said, I'm fully confident in the performance of these cabs, As I've heard T36s A/B'd with dual 18s and almost crapped my pants.

Thanks ahead of time!

[SeisTres]

First of all, a folded horn is just a regular horn but bent at sharp turns since it's used for bass and bass is fine with this. So think about it like that - it's a horn, and follows such laws. In my mind, if I think about the sound bouncing and reflecting at weird angles it just jumbles my mind, so I always picture a straight horn.

And the reason for the horn increasing the sensitivity is because the energy transfer between the cone of the driver and the medium(in this case, air) is improved by increasing the resistiveness of the air (or in other words, according to acoustics, making the air "thicker"). And since the cone is quite solid, it can deliver more energy than the air will fully transfer into open air; adding the horn concentrates the energy transfer.

The length of the horn determines the extension of the horn because since sound waves basically repeat every cycle, the horn must be long enough for the wave to repeat (IIRC 1/4 of the wavelength is the cut off point) or it will not work. So it not really that it reflects as in the sense of wall loading, but the whole wave is contained within the horn.

And keeping in mind that folded horns are simply horns, ask him why compression drivers are not left with their 1" output into open air. But then again, those horn are short because the wavelenght is much shorter than bass frequencies. And better yet, if horn are so useless, why in the world do marching band make tuba player carry around that 50 pound monster when (according to your friend) they would only to blow raspberries with their lips


That's as best as I can explain it, but hopefully some of the others with more knowledge on this will chime in add more or correct anything I have not gotten right.

[AntonZ]

Here's what I have understood so far, in plain talk (because I don't understand the more complex background either )

At loud and low frequencies (subwoofer) you can actually feel the air move if you are standing right in front of the mouth. That's the air moving back and forth into and out of the subwoofer. Sound waves are movements in the air. Speaker cones are not a very efficient means to make air move. The surface is rather small which is made up for partly by making the movement fast/far. What would be better is a larger surface, slower movement, but it seems physics dictates that drivers cannot be built to do that. So this is where the horn comes in. It is small at the beginning, where the driver sits. Air at that point in the horn moves fast, and through a very small surface area. Further down the horn, it gets wider and wider. Air movement is spread across a wider area and slows down. This makes the horn act like a transformer, gradually adapting the "little bit of air moving fast" into a "large amount of air moving slowly".

The experts may correct me or add math to taste


Edit: Seistres has already posted while I was writing

[wounded horse]

Which of us, during a walk home and after a few beers, hasn't picked up a traffic cone and yelled down it? You can hear it yards and yards away.

[Chris_Allen]

Which of us, during a walk home and after a few beers, hasn't picked up a traffic cone and yelled down it?

..at attractive ladies only to turn round laughing and looking at a copper standing right next to you.

[Bill Fitzmaurice]

How exactly does the folded horn design work to provide the extra sensitivity and output displayed by the response charts?

It works very well.

I tried to explain it to a friend (who claims a folded horn is like putting a spoiler on a consumer car

Advise said friend to read a book or two about acoustical engineering. A starting place:
http://billfitzmaurice.info/forum/viewforum.php?f=18

[bzb]

Your buddy clearly doesn't understand what a horn is.

Ask him how a trumpet works, since it's just a spoiler on a Civic.

[jcmbowman]

Here's the best analogy I've heard so far to explain horn-loaded subs to lay people:

It's like the way a hydraulic lift works. On the one end of the system you've got a small motor-driven piston that moves fast, but doesn't exert a whole lot of force. On the other end you've got a large piston that moves very slowly, but is capable of exerting massive force.

A horn-loaded sub is the same idea - you have a driver acting as a piston to move as much air as possible (this is why the Vd, or volume displaced, is one of the most important measurements of a driver for horn subs), into a narrow throat, and at the other end you've got a large mouth acting as the large piston - not moving very fast, but exerting some serious force on the air in the room.

And then the added benefit of folding that horn is two-fold - you get something that's much easier to transport, (can you imagine trying to move and set up an unfolded Tuba36???), and you also get the folds acting as a filter to remove the unwanted harmonics and distortion.

[sine143]

Yeah, well Like I said, I myself didn't completely understand, and I already went through building one.

I believe he honestly just didnt understand the concept. This should help me explain it.

Oh, and yes, it does work very well. We built a direct radiator box as well to A/B the driver. Ohhhh yeahhhh.

[Ron K]

The easiest analogy in describing how a horn works is the water hose with a spray nozzle attached.Open up the nozzle to a wide angle and the water doesn't go as far as closing if off a bit where it shoots a stream further in distance but narrower. remove the nozzle and the water simply rolls out the end and only goes a few feet or so.

[sine143]

Yep, that's the analogy I used initially. He said "then theoretically you only need to have a small hole in front of your driver?"

[BrentEvans]

Yep, that's the analogy I used initially. He said "then theoretically you only need to have a small hole in front of your driver?"

That's part of the equation, but not all. The problem with comparing horns to anything else is that you're trying to visualize sound. Sound and light are very different, so metaphors are very difficult, and intelligent people can and will invariably poke holes in them if they're the type to do so. Without getting technical, the best thing to do is point people toward brass horns for comparison purposes.

The basic talking points of horns (as I understand them) are that horns focus the sound coming out of a driver by aiding in the acoustic coupling of the sound waves to the air. Also, folded horns filter out high-frequency distortion from sub drivers becasue the higher frequencies can't traverse the sharp bends in the horn, which helps avoid hot zones in front of the cabinet.

[mloretitsch]

Use your outstretched arm to move the air. Use all of your strength and swing your hand as fast as possible, you still won't move a lot of air. This is how a raw speaker moves air. Now hold something larger like a baking sheet and move some air. You have coupled your strength in a more efficient manner to the air. The cookie sheet gives you much better efficiency.

Also, in theoretical land, a horn would have the exact same movement at the throat and the mouth. If your 15" driver moves 5mm in each direction, the air should move 5mm in each direction at the mouth. Now you should understand why a bigger mouth gives you higher sensitivity.

-Matt

[Gregory East]

Hmmm, for real?

I thought the idea was to convert a small large amplitude high energy wave into a large small amplitude wave with the same energy. Velocity being constant.

[SoundInMotionDJ]

Here's the best analogy I've heard so far to explain horn-loaded subs to lay people:

It's like the way a hydraulic lift works. On the one end of the system you've got a small motor-driven piston that moves fast, but doesn't exert a whole lot of force. On the other end you've got a large piston that moves very slowly, but is capable of exerting massive force.

A horn-loaded sub is the same idea - you have a driver acting as a piston to move as much air as possible (this is why the Vd, or volume displaced, is one of the most important measurements of a driver for horn subs), into a narrow throat, and at the other end you've got a large mouth acting as the large piston - not moving very fast, but exerting some serious force on the air in the room.

+1

At the pressures involved in pro sound applications, air is essentially incompressible, so the hydraulic analogy works pretty well for folded horns.

--Stan Graves