Explaining horn technology to the layman

[binsley8112]

So my day job is pretty brainless and a I have a lot of time to think about things... Unquestionably anyone using BFM cabs has run into questions about them; anything from "what are they" to "why do they sound so good" and everything in between. The laymen tend to have more of an open mind but lack the technical knowledge to understand the "whys" of horn loading. This is the group I typically deal with, though I've dealt with a few know-it-alls as well that only trust commercial back boxes. For the record, I don't think I'm any kind of expert on the topic either, but I've spent a good amount of time reading these forums and I've built/heard some BFM cabs myself, so I feel confident in defending their merits.

I was trying to think of simple analogies to paint a picture in peoples minds about how horns work. It's interesting that, even as children, we know almost instinctively that cupping our hands over our mouths will project our voices further. It's a pretty crude version of a horn, but it follows the same basic principles (correct me if I'm wrong). Most people understand the "how," but not the "why" of this simple trick. So anyways, the best analogy I could come up with is to use a golf driver. Bear with me on this one, because I could be way off-base here...

Comparing the two, the person swinging the club represents the input force, which in this case is the cone of the speaker. The point where the hands meet the grip of the club is the horn throat, the shaft of the club is the horn pathway and the head of the driver is the mouth, or output force. The real magic happens in the shaft/horn pathway. Say you compare two drivers - for the sake of this example driver A will have a completely stiff (non-flexible) shaft, and driver B will have a flexible graphite shaft. How flexible it is is relative to your swing speed, but that's not important to the analogy. Everything else being equal (swing speed/ball type/point of contact on the face, etc etc), driver B consistently hits the ball 40 yards further than driver A. Why is this? Because on the down-swing the graphite shaft in driver B is flexing and storing kinetic energy. By the time the club head makes contact with the ball, the graphite shaft is beginning to flex back to its original state which is adding overall speed to the club head, and therefore hitting the ball further. No extra effort has been made on the part of the person swinging, but instantaneously he/she has added 40 yards to their swing simply by exploiting the characteristics of a graphite shaft.

Ok, so I realize that was very wordy and probably no shorter than the actual answer, but in summation you could say coupling a speaker to the horn is analagous to using a graphite shaft instead of a metal one in golf. You're simply exploiting the flexibility/elasticity of a medium to work with the force applied. In the case of the horn, it's the compressibility of the air itself rather than the flexibility of a graphite shaft. It's just a more efficient method of coupling an applied force to get an output force.

Am I on the right track here?

[Bill Fitzmaurice]

The answer is that the horn is a transformer, and how it works relates to impedance. For maximum energy transfer between devices the driving device should have an output impedance that's much lower than the input impedance of the device being driven. For instance, a mixer might have an output impedance of 600 ohms driving an amp with an impedance of 10k ohms. When there's an impedance mis-match because the input impedance of the driven device is too low a transformer can be used to give the driving device a higher impedance load to work into. Speakers have an output impedance, and while it's fairly low electrically speaking it's still very high compared to the impedance of the device being driven: air. Because of the mis-match the typical operating efficiency of a direct radiator speaker is only about 1% of electrical watts converted into acoustical watts. The air mass of a horn is a higher impedance load than open air, acting as a transformer between the driver and the free air. The length and mouth area of the horn determine to how low a frequency the impedance improvement extends. Efficiency of a horn is still low, maybe 10%, but the difference between 1% efficiency and 10% efficiency is a 10dB gain in sensitivity.

[madmallard]

I love it when an expert introduces their subject~

The answer is that the horn is a transformer, and how it works relates to impedance. For maximum energy transfer between devices the driving device should have an output impedance that's much lower than the input impedance of the device being driven. For instance, a mixer might have an output impedance of 600 ohms driving an amp with an impedance of 10k ohms. When there's an impedance mis-match because the input impedance of the driven device is too low a transformer can be used to give the driving device a higher impedance load to work into. Speakers have an output impedance, and while it's fairly low electrically speaking it's still very high compared to the impedance of the device being driven: air.

that being the free air between the driver and the listener, right?

Because of the mis-match the typical operating efficiency of a direct radiator speaker is only about 1% of electrical watts converted into acoustical watts. The air mass of a horn is a higher impedance load than open air, acting as a transformer between the driver and the free air. The length and mouth area of the horn determine to how low a frequency the impedance improvement extends. Efficiency of a horn is still low, maybe 10%, but the difference between 1% efficiency and 10% efficiency is a 10dB gain in sensitivity.

So if the horn is a transformer of relatively low impedance into higher impedance than before, (and forgive me if this is a super basic engineering question) then where is the difference made when the sound exiting the horn throat still has to travel through the open air to the listener?

thx for the lesson, btw

[binsley8112]

Ok thanks for explaining that, Bill. My analogy doesn't really apply then, but I feel like I'm getting a better understanding of the physics behind it.

I remember as a kid being fascinated with setting the alarm on my (waterproof) watch and then dropping it in my grandparents' pool. When I swam underwater I discovered that no matter where I was in the pool proximity-wise to the watch, it always sounded like it was right beside my head. This aligns with your description of impedance matching, as compared to air, water is much more dense and therefore has a much higher impedance. This stuff is fascinating!

[Bill Fitzmaurice]

The difference is made because the driver no longer works into the lower impedance of the free air but into the higher impedance of the air mass of the horn. The transformation is complete at the horn mouth. At that point you'd see the full increase in sensitivity compared to the near field sensitivity in a direct radiator.

I guess the impedance of water is higher than air, but most of the reason why you hear better underwater is that the speed of sound in water is four times that in air.

[Grant Bunter]

There's a very simple example.

Tell whoever it's like a trumpet. A small volume noise at the mouthpiece is really loud at the mouth of the trumpet.
So horn loaded cab works the same...

[commander_dan]

Or tell them it's like a ... Tuba! Ha ha ha.

[AcousticScience]

A REALLY crude analogy which doesn't go into the lengths, shapes and frequency response of a horn, but serves to illustrate impedance matching.

A small direct radiator loudspeaker is the equivalent to trying to fan myself cool with a 2 by 4. Most of the energy is spent waving the plank of wood around and not much air is moved and I'll probably feel hotter from the effort.
A speaker cone has a lot of momentum compared to the air in front of it and so most of the energy is spent moving the heavy cone and because the air is so light and easy to push out the way, it can't compress much because the force needed to move it out of the way is so small.

A horn restricts the air to a tunnel shape. Because of the walls, the air cannot move "out of the way" so easily, the mass of air in the tunnel means the air isn't shoved forwards so easily as the small amount of air in front of a direct radiator. The speaker cone can compress it more, building up a bigger pressure wave, a bit like the air inside a bicycle pump.

The expanding size of the tunnel helps stop the air simply "moving out of the way" at the end of the tube and helps move more of the "easy to move air".
A speaker cone is best at compressing a small amount of air by a larger amount (think bicycle pump) air is best at sending sound power with a lots of air compressed by a small amount (think air bed or bouncy castle pump).

The types of speaker used for different horns: A big, light speaker with a weak magnet power is best for direct radiator or only a very large horn with low compression. A small dense cone speaker with a big magnet (often seen in car audio, think Sundown, DC Audio etc, or even an Eminence LAB 12) is wasted in any direct radiator system. The magnet can produce so much force that it may as well be compressing lots of air rather than being like Mike Tyson taking a swing at Casper the friendly ghost.

[nick mineau]

I like to use the cell phone theory. most people understand that if they cup there hand under there cell phone , it gets louder. its super simple and people/musicians don't get the advanced theory. but they can understand that. have them do it , and then you just showed them how even the simplest horn loading is effective .
also there are these little boxes you can buy that amplify you cell phones speaker.Something like this will help you sell people horns and principles In my experience

Also I find a lot of old timers use to the old "w horns", and "CV" horns don't get what a real m modern horn sub can do. I find with those people its best to call it a HYBRID HORN design.
then they go wow! that's cool. you have to break them free of there past experience w inferior designs.

https://www.amazon.com/ARCHEER-Amplifie ... +amplifier

[ACUA]

If we take a room, say 20’x20’x20’, and place a subwoofer in it. Lets say a sealed 10” subwoofer in it. Suspend it exactly in the middle of the room and play the thing. The subwoofer will not build very much pressure. The driver has to pressurize the entire spherical space around it simultaneously. A lot or most of the energy is expended at a really high rate and not very much pressure is created. The air impedance with the driver in the middle of the room is really low. So, what mechanical influence the 10" driver has on the surrounding air becomes distributed very vastly almost instantly. Let’s move the sub to a wall exactly in the middle. By doing so you effectively half the spherical space immediately around the subwoofer unit. Now more pressure is forcefully focused onto a lower mass of air. So more pressure to be built or the pressure level will be higher at the source as a result.

Introduce inverse square law, as I understand it. I'll be damned if this is completely wrong. By halving the immediate volume of air that the subwoofer has to contend with then a higher level of pressure is achieved. And with the physics of sound calculated by the inverse square law shown below you extend the distance the sound travels as a result of the higher pressure level initially achieved. So, even if the pressure does not reach the opposite wall in the room, more area in the room is pressurized higher as a result of the subwoofer placement.

Let’s move the sub along the wall to meet another wall. Now play the thing, the initial mass of air the sub has to pressurize is less by half again. With less mass of air to pressurize, more pressure is effectively put into the air. The decay rate is still the same according to inverse square law but because more pressure is initially created the sound will be louder and travel farther before dying off. Finally place the driver to the absolute corner and repeat the process and you again half the immediate space the driver must pressurize and it will be able to concentrate more energy on the smaller space increasing the pressure and as it travels out the fixed decay rate will show the most pressure possible in that room with that set up. That philosophy not including reflections and such.


“Snew = Sref + (20 * Log (Dref / Dnew))

Where:
Dref = Reference Distance
Dnew = New Distance
Sref = Reference Sound Level
Snew = New Sound Level

This calculation will give you the amount of attenuation, in decibels, you can expect with a change in receiver distance, in a free field (outdoors).

For example if you were standing 20 feet from a loudspeaker, and were to move to 40 feet away from that loudspeaker, you would expect to see a drop in level of 6 dB. Sound that is radiated from a point source drops in level at 6 dB per doubling of distance.”

This is from the Crown Audio website not my words

[Tom Smit]

If we take a big room.....

Dude, please use capitalization and punctuation. That post was a hard read.

[ACUA]

I apologize I was on my phone and got really excited to post what I did. I have amended it somewhat.

[jimbo7]

I just tell people it's like a snail shell and the sound wave gets bigger and bigger as it travels to the mouth...like a tuba.
Most are amazed that it's ~7ft of speaker folded up and coming out of a 8"/10" speaker. (Please don't flame me. I just hate explaining things to people and get bored easily)

[CoronaOperator]

Read my post #39 from my old build thread ... viewtopic.php?f=30&t=22720&start=30

If they are truly laymen then why bother to explain it right, you'll just bore them. Have some fun with your explanation!

[Bill Fitzmaurice]

The tuba analogy is the most accurate, with the driver acting like the mouthpiece.