Folded Horn Enclosures: How do they work?

[SoundInMotionDJ]

Hey Stan,

If air is "basically incompressible" with what we are dealing with, isn't it that sound happens because of the compression and rarefaction of the air molecules?

Take Care!
Phil

What do you mean by "compression" and "rarefaction"?

It is true that sound waves in air are more like a compression wave than like a wave on the surface of a body of water. It is also true that sound waves are generally better thought of as a "vibration" than a "pressure wave".

At a molecular level, the molecules are constantly and randomly colliding with each other. At this level, the molecules are not "uniformly distributed" and there is not really a concept of "density" as you are likely thinking of it - there are discrete particles in close proximity. As such, there is no compression or rarefaction (both of which are effectively changes in local uniform density).

At a "10,000 ft" level, still air is an incompressible, uniform, continuous fluid. At this level, the "sound wave" will pass through the local area without altering the local conditions before or after the wave....including the local density. Given that, it is not generally helpful to think of the propagation of a sound wave to be because of compression or rarefaction.

Once you get into "compressible aerodynamics" then compression and rarefaction are factors that need to be considered. You might have heard this kind of aerodynamics referred to as "sonic booms." Until the local speed is more than Mach 0.3...compression is not a serious factor.

If there were one wave, one up and one down....then it would be helpful to think of the compression and rarefaction that surrounds that wave. But, since sound is generally composed of a continuous series of waves of various wavelength, amplitude, frequency, and direction...I find that the "vibration" model is generally yields a better overall understanding.

[Landl.livesound]

Hey Stan,

If air is "basically incompressible" with what we are dealing with, isn't it that sound happens because of the compression and rarefaction of the air molecules?

Take Care!
Phil

What do you mean by "compression" and "rarefaction"?

Hey Stan,

On the topic of compression and rarefaction, I can't state it any better as someone said to me: "When the molecules are pushed closer together it is called compression; when they are pulled apart, it is called rarefaction. The back and forth oscillation of pressure produces a sound waves."

If there were one wave, one up and one down....then it would be helpful to think of the compression and rarefaction that surrounds that wave. But, since sound is generally composed of a continuous series of waves of various wavelength, amplitude, frequency, and direction...I find that the "vibration" model is generally yields a better overall understanding.

We may be saying the same thing, or I am just misunderstanding what you are saying in this statement, but from my basic understanding and from the most basic understanding sound ends up being many basic waves combined together to create one complex signal that is produced by the transducer.


I actually also have a "lampi" friend who is in just his third year in aerospace engineering at FIT and we have been able to have some very in-depth discussions and he has some very good understanding of the physics of sound. Both he and you are much more into the physics on many different levels than I probably ever will be. But I'm more interested/focused on sound, physics of sound, and how it applies in the real world.

Take Care!
Phil

[pwfirst]

It all sounds too complicated for mear mortals to understand, so we need to go back to the basics. Take this horn and wrap it around yourself with the horn end to your ear and you can blow your own brains out of their container. Phil

[SoundInMotionDJ]

On the topic of compression and rarefaction, I can't state it any better as someone said to me: "When the molecules are pushed closer together it is called compression; when they are pulled apart, it is called rarefaction. The back and forth oscillation of pressure produces a sound waves."

All that is true. However, that mental model leads to some unfortunate mis-understandings.

Among other things, people tend to think of a wave as it travels across the surface of a pond...this creates a misunderstanding that there is a "flow" of something associated with the propagation of a sound wave. This mental model often leads to the conclusion that the air is moving "back and forth" or "up and down" in order to propagate the sound wave. The "pushed" and "pulled" ideas lead to some of the same mis-understandings. The issue is that the individual molecules of are not not moving with the wave. The molecules are staying more or less in place.

Keep in mind that the actual molecules in air only take up about 0.1% of the space they occupy...the other 99.9% is empty - a "vacuum." The average air molecule is moving about 1.8 times the speed of sound (sound can never travel faster than the speed of the individual air molecules). On average, a molecule of air travels about 1000 times its radius before colliding with another molecule. The mean distance between collisions is about 10**(-8) meters. This is roughly the same as a softball traveling the length of a football field before colliding with another softball. It is nonsense to think of of terms like "density" or "compression" or "rarefaction" at this length scale.

A cubic centimeter of air has about 3 * 10**19 molecules of air in it. By the time air is "uniform" and concepts like "density", and "compression" and "rarefaction" start to make sense....the length scale is on the order of 10**(-5) meters...which equates to about 10**10 molecules. At that length scale, thinking about the interaction of individual molecules is equally nonsensical.

When people think of "vibration" an implicit view that the effect is happening "in place" is almost universal. Since the average molecule during a sound wave propagation would move on the order of 10**(-5) meters - between 1000 and 10,000 times the average distances between individual collisions - "in place" is a much better model for understanding what is happening with sound in air.

I actually also have a "lampi" friend who is in just his third year in aerospace engineering at FIT

I'm sorry. I can't promise that he will ever return to "normal"...but given 10 to 15 years, he should be able to reach a level of normalcy that will allow him to reintegrate with the rest of society.

But I'm more interested/focused on sound, physics of sound, and how it applies in the real world.

Those are very different topics. As I have said, in the "real world" there are lots of things that can be safely ignored. In the "physics of sound" area, it is not as safe to ignore those same things.

Add psychoacoustics (the study of how humans perceive sound) to the puzzle, and then things really get to be fun. I am just starting to get comfortable with the first levels of psychoacoustics and it is changing a lot about how I think about "real world" pro-audio applications.

But that doesn't mean I'm not having fun with all of this...

--Stan Graves

[SoundInMotionDJ]

It all sounds too complicated for mear mortals to understand, so we need to go back to the basics.

In my defense....this is actually rocket science.



--Stan Graves

[SeisTres]

It all sounds too complicated for mear mortals to understand, so we need to go back to the basics. Take this horn and wrap it around yourself with the horn end to your ear and you can blow your own brains out of their container. Phil

RRRRRICCOLLLLLAAAAAAAA!!!!

LOL.

wow, I thought this thread was going to only a couple more responses, but guess I was wrong. Any more pages and I'd swear this was a discussion about kick bins

To sine143: since your friend is the one that's asking the question and then questioning your answers(or at least picking at them from what you've told us), simply tell him to the best of your knowledge how they work. If he questions any more, tell him to grab some books and learn theory himself. And you simply continue to use horns and let him use what he wants.

You'd be suprised just how well this method actually works. And not just in sound reinforcement stuff, but it can be used across almost all disciplines. Go figure, it even works for religious debates, as well. But then everyone will know you as a blasphemer(oh, nevermind, that's my case)

[BrentEvans]

wow, I thought this thread was going to only a couple more responses, but guess I was wrong. Any more pages and I'd swear this was a discussion about kick bins

No, it would be locked by now.

[sine143]

Hey, all I can say is that I love it. Only expected like 2 or 3 responses by myself, but this gives me plenty of stuff to draw out on my whiteboard and think about

[jcmbowman]

To sine143: since your friend is the one that's asking the question and then questioning your answers(or at least picking at them from what you've told us), simply tell him to the best of your knowledge how they work. If he questions any more, tell him to grab some books and learn theory himself. And you simply continue to use horns and let him use what he wants.

It's even better when they actually -hear- your horns. I've got a couple of naysayers who swore up and down that there was no way I was going to build my own subs and have them come anywhere near anything commercially available, let alone -their- speakers. Now that they've come to several of my shows I've got them asking me if I'll rent mine out.

[sine143]

Yeah, He definitely falls into that category. Honestly, now that I have it working better (air leaks, and the amp was finicky for the first couple hours idk why), it sound reallllly nice. I'm very ready to have 2. Then we will get to bring the pain

[SeisTres]

Yeah, He definitely falls into that category. Honestly, now that I have it working better (air leaks, and the amp was finicky for the first couple hours idk why), it sound reallllly nice. I'm very ready to have 2. Then we will get to bring the pain

Don't forget to mention that with the recommend driver (3012lf), it would almost be like putting two of those together.

OH, and that with the same 12", if you had built the titan 39, it would be even louder still. But then again, that wold be just bragging


OH, and just wait till you try to Eq with the tops, you'll be pleasantly surprised just how loud they need to be in order for everything to sound "natural". If you run dance music through the system, it becomes apparent just how little bass you actually need. Anything above a certain level and while you do feel the bass, the beater sound of the kick becomes lost and all you have left is the trailing bass which makes it completely and utterly undanceable. (yay, according to spell check, I just made a word). And if the tops sound cleaner than ever, it will be because they will be, since folded bass horn have this tendency to clean up the sound without the added harmonics.

[Landl.livesound]

On the topic of compression and rarefaction, I can't state it any better as someone said to me: "When the molecules are pushed closer together it is called compression; when they are pulled apart, it is called rarefaction. The back and forth oscillation of pressure produces a sound waves."

All that is true. However, that mental model leads to some unfortunate mis-understandings.

Among other things, people tend to think of a wave as it travels across the surface of a pond...this creates a misunderstanding that there is a "flow" of something associated with the propagation of a sound wave. This mental model often leads to the conclusion that the air is moving "back and forth" or "up and down" in order to propagate the sound wave. The "pushed" and "pulled" ideas lead to some of the same mis-understandings. The issue is that the individual molecules of are not not moving with the wave. The molecules are staying more or less in place.

Keep in mind that the actual molecules in air only take up about 0.1% of the space they occupy...the other 99.9% is empty - a "vacuum." The average air molecule is moving about 1.8 times the speed of sound (sound can never travel faster than the speed of the individual air molecules). On average, a molecule of air travels about 1000 times its radius before colliding with another molecule. The mean distance between collisions is about 10**(-8) meters. This is roughly the same as a softball traveling the length of a football field before colliding with another softball. It is nonsense to think of of terms like "density" or "compression" or "rarefaction" at this length scale.

A cubic centimeter of air has about 3 * 10**19 molecules of air in it. By the time air is "uniform" and concepts like "density", and "compression" and "rarefaction" start to make sense....the length scale is on the order of 10**(-5) meters...which equates to about 10**10 molecules. At that length scale, thinking about the interaction of individual molecules is equally nonsensical.

When people think of "vibration" an implicit view that the effect is happening "in place" is almost universal. Since the average molecule during a sound wave propagation would move on the order of 10**(-5) meters - between 1000 and 10,000 times the average distances between individual collisions - "in place" is a much better model for understanding what is happening with sound in air.

I actually also have a "lampi" friend who is in just his third year in aerospace engineering at FIT

I'm sorry. I can't promise that he will ever return to "normal"...but given 10 to 15 years, he should be able to reach a level of normalcy that will allow him to reintegrate with the rest of society.

But I'm more interested/focused on sound, physics of sound, and how it applies in the real world.

Those are very different topics. As I have said, in the "real world" there are lots of things that can be safely ignored. In the "physics of sound" area, it is not as safe to ignore those same things.

Add psychoacoustics (the study of how humans perceive sound) to the puzzle, and then things really get to be fun. I am just starting to get comfortable with the first levels of psychoacoustics and it is changing a lot about how I think about "real world" pro-audio applications.

But that doesn't mean I'm not having fun with all of this...

--Stan Graves

Hey Stan,

All very true about the individual molecules not actually moving "with the wave" so to say, which I don't know if anyone actually alluded to that they do, but I don't think so.


It would seem that the physics of sound, the psycho-acoustics, and how it all applies to the real world is important all together and not necessarily totally different topics. It seems it would be good to understand all of them, because one could reinforce the other and help you understand the whys and hows. And since we will never know it all might as well keep learning! That has helped a lot on PSW when people go about explaining very in-depth audio topics that I find very fascinating.


Take Care!
Phil