Folded Horn Enclosures: How do they work?

[Gregory East]

afaik air stops being compressible when it's liquified.

Isn't the velocity constant at the speed of sound? This is where the "transducing" effect gets speedwobbles in my brain. I don't comprehend a wavelength in that "piston" of air generated in the horn throat.

[CafSentryGnome]

doesn't sound travel faster in higher air pressure and in denser materials?

sound is areas of high and low pressure. the size of these areas is the wave length.

im guessing this, if the horn throat has high pressures in it, meaning the sound wave will move faster. as the wave moves to lower pressures closer to the mouth it converts the velocity of the wave to amplitude of the wave. so im guessing that the longer the horn the more energy can be converted where it cant be lost.

[SoundInMotionDJ]

afaik air stops being incompressible when it's liquified.

You keep using that word...I do not think it means what you think it means. "Incompressible" means "not able to be compressed." "Compressible" means "able to be compressed."

So, what I think you meant to say was that air "stops being compressible when it's liquefied."

Except that also is not true. For the record, liquids are still compressible...just not very much.

With air, until the velocities get above about Mach 0.3 (about 230mph), air can be treated as being incompressible. I'm not just pulling that out of my hind end...that was the result of both my undergraduate degree in Aerospace Engineering, and the first 20 hours of graduate level work that I have completed. (Yes...I actually am a rocket scientist. )

Isn't the velocity constant at the speed of sound? This is where the "transducing" effect gets speedwobbles in my brain. I don't comprehend a wavelength in that "piston" of air generated in the horn throat.

The velocity of a wave at a given frequency is based on the speed of sound in the working medium...providing the medium is uniform, and at rest. Sound can be transmitted through almost any working medium, including fluids (e.g. air and water) and solids.

doesn't sound travel faster in higher air pressure and in denser materials?

Assuming that you are talking about "air" as the working medium, then the pressure and density are related to each other according to the perfect gas law. In the end, the only variable that alters the speed of sound in air is temperature.

If you are talking about mediums other than air, then yes, the overall structure of the medium does alter the speed of sound. The speed of sound in a steel beam is quite a bit higher than the speed of sound in water, and the speed of sound in water is quite a bit higher than the speed of sound in air.

sound is areas of high and low pressure. the size of these areas is the wave length.

im guessing this, if the horn throat has high pressures in it, meaning the sound wave will move faster. as the wave moves to lower pressures closer to the mouth it converts the velocity of the wave to amplitude of the wave. so im guessing that the longer the horn the more energy can be converted where it cant be lost.

As a sound wave propagates through air, pressure changes in the effected air can be ignored. (There is a very slight change in pressure that accompanies wave propagation, but until you approach Mach 0.7-0.8, the transonic region, NO ONE considers those effects.) Any pressure gradient in free air will create a movement of air to equalize that pressure differential. Sound does not typically create a "flow" of air...your pant legs aside.

To be pedantic, there is also a slight rise in temperature as a wave is propagated through air (the 2nd Law of Thermodynamics tells me so)...and that change in temperature will change the local speed of sound (and the local density and pressure, as told through the Perfect Gas Law). But, all that can be ignored too.

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


EDIT: For the record, there is an over/under pressure inside the horn path. But, in general, it is safe to ignore the compressibility effects in the horn path (the rear chamber is a little different). Remember that to include compressibility, the volume of air inside the horn path would change as a result of the movement of the woofer cone (by up to Vd if air were perfectly compressible). However, the air in the horn path reacts at the speed of sound to any movement of the woofer....and that is fast enough, and the horn is short enough, that any changes in the volume of air in the horn path can be ignored. If you ignore the change in air volume, then you are also ignoring all other compressibility effects.

--Stan Graves

[David Carter]

You keep using that word...I do not think it means what you think it means.

Wait a minute... I thought that word was <lisp>inconceivable</lisp>

[SoundInMotionDJ]

You keep using that word...I do not think it means what you think it means.

Wait a minute... I thought that word was <lisp>inconceivable</lisp>

You fool! You fell victim to one of the classic blunders - The most famous of which is "never get involved in a land war in Asia" - but only slightly less well-known is this:

Never go in against an engineer when an argument is on the line!

hahahahahahahahahahahaha!!!!!

That, and always develop an immunity to iocane power BEFORE you really need it.

--Stan Graves

[sine143]

Never go in against an engineer when an argument is on the line!

--Stan Graves

I think a lawyer might be a little more difficult, but living in an apartment with 4 engineering undergrads (mechanical, aerospace, 2 biomedical), I've come to appreciate how intense arguments can get.

[Chris_Allen]

Princess Bride - Classic!

[David Carter]

Never go in against an engineer when an argument is on the line!

Since I feel like a mere farm boy amidst all you knowledgeable folk, all I have to say is, "As you wish!"

[Gregory East]

Well I edited myself while all that was getting typed so there!

For the record I got a whole degree in civil engineering which included a crapload of fluid dynamics before I got a bad bump on the head which left me not so mathematically able. So yeah, I know that liquids compress, but not very much. My point was I'm not buying into the "solid air" theorem just yet.

So where were we? What's a "sound" wave but a compression going back and forth at a certain frequency and wavelength, speed dependent on the density of the medium? High school physics. iirc.

It's the propogation of those waves that gets me. They start with the motor in the driver going out and back, pushing the woofer cone, mushes up the air in front of the cone and sucks it back. Methinks at less a lot less than the speed of sound in that space.

Then they get stuffed through the throat and pushpull all the way to the exit of the horn. Unleashed on the air in the room with a large portion of the energy they left the woofer with.

That's how I see it anyway, feel free to reeducate me.

[Bill Fitzmaurice]

That's how I see it anyway, feel free to reeducate me.

It's all about impedance. A horn better matches the high impedance of a driver to the low impedance of the air, it's as simple as that.

[sine143]

getting over the fact that impedance rating did not equal resistance took some time. The ohm is just the convenient word I guess?

[Bill Fitzmaurice]

getting over the fact that impedance rating did not equal resistance took some time. The ohm is just the convenient word I guess?

Ohms are used to measure resistance, capacitive reactance and inductive reactance. Impedance is the sum of resistive, capacitive and inductive components. And that's why ohmage is not a word: because you can't tell what is being referred to, resistance, capactive reactance, inductive reactance or a combination any of the three.

[Landl.livesound]

getting over the fact that impedance rating did not equal resistance took some time. The ohm is just the convenient word I guess?

Hey Sine,

Actually Ohms is the correct label. As all resistance, reactance, and impedance are in Ohms.

{Bill beat me too it. I was gonna add the extra stuff he added as well, so that worked well.}

Take Care!

[SoundInMotionDJ]

My point was I'm not buying into the "solid air" theorem just yet.

"Incompressible" is not the same as "solid." I'm saying that within the horn path, a basic hydraulic analogy is a reasonable way to think about what is happening (the rear chamber being a little different).

So where were we? What's a "sound" wave but a compression going back and forth at a certain frequency and wavelength, speed dependent on the density of the medium? High school physics. iirc.

Sometimes "they" leave things out in high school....sorry to be the one to break that to everyone.

The "density" of a gas is not part of the calculation of the speed of sound in that gas. Here is the formula for the speed of sound in a perfect gas (i.e. one with a constant specific heat):

a**2 = R T gamma

Where:
a is the speed of sound
R is the gas constant (286 / m**2 / s**2 / K for air)
T is the temperature on an absolute scale
gamma is the ratio of specific heats (at less than Mach 5-ish, this is a constant...and the value for air is 1.4)

This equation says that until you get to a point where air can not be treated as having a constant specific heat, the speed of sound is only a function of temperature.

It is true that pressure, density and temperature are related through the gas laws. But, it is not the case that "density" is an independent variable for the calculation of the speed of sound in air.

Air stops having a constant specific heat in the hypersonic speed range...which is above Mach 5.

"Sound" is transmitted because of collisions between the molecules in random motion within the gas. This process at low speeds is "isentropic", meaning that there is no change to the conditions of the gas from before to after the collisions pass through.

The speed that the collisions can propagate through the fluid is directly proportional to the rate of the random collisions that are taking place in the gas. The faster the random motion, the more frequently the molecules are colliding anyway, and the faster that "wave" of collisions can be transmitted. The rate of the random collisions in a gas is directly proportion to the amount of energy within that gas, which is a function of the temperature of that gas.

It's the propogation of those waves that gets me. They start with the motor in the driver going out and back, pushing the woofer cone, mushes up the air in front of the cone and sucks it back. Methinks at less a lot less than the speed of sound in that space.

Then they get stuffed through the throat and pushpull all the way to the exit of the horn. Unleashed on the air in the room with a large portion of the energy they left the woofer with.

It is the "back and forth" movement of the air with the cone that makes what we ultimately perceive as "sound." But it is not "movement" of the air that transmits the energy, it is the collisions.

Even the 3015LF only has a Vd of 864cc. By the time that gets to the mouth of a T48, which is almost 4500cc for the 24" wide version, the air is moving less than 2mm! And that is the amount of air movement when the woofer is going to Xmax on every peak!

By the time you are thinking of a "wave" within air, you have already gotten away from the idea that air is actually composed of individual molecules of different elements and compounds that are randomly colliding with each other all the time. This is not a bad thing...but understand that you have already moved to the "10,000 foot view" of the problem.

Speakers and sound does not create a "flow" of air...or really any movement of air at all (Memorex driven sound systems aside). As long as the motion of the individual molecules is much less than the speed of sound, then air can be treated as incompressible (also a "10,000 foot view of the problem).

--Stan Graves

[Landl.livesound]

My point was I'm not buying into the "solid air" theorem just yet.

"Incompressible" is not the same as "solid." I'm saying that within the horn path, a basic hydraulic analogy is a reasonable way to think about what is happening (the rear chamber being a little different).

So where were we? What's a "sound" wave but a compression going back and forth at a certain frequency and wavelength, speed dependent on the density of the medium? High school physics. iirc.

Sometimes "they" leave things out in high school....sorry to be the one to break that to everyone.

The "density" of a gas is not part of the calculation of the speed of sound in that gas. Here is the formula for the speed of sound in a perfect gas (i.e. one with a constant specific heat):

a**2 = R T gamma

Where:
a is the speed of sound
R is the gas constant (286 / m**2 / s**2 / K for air)
T is the temperature on an absolute scale
gamma is the ratio of specific heats (at less than Mach 5-ish, this is a constant...and the value for air is 1.4)

This equation says that until you get to a point where air can not be treated as having a constant specific heat, the speed of sound is only a function of temperature.

It is true that pressure, density and temperature are related through the gas laws. But, it is not the case that "density" is an independent variable for the calculation of the speed of sound in air.

Air stops having a constant specific heat in the hypersonic speed range...which is above Mach 5.

"Sound" is transmitted because of collisions between the molecules in random motion within the gas. This process at low speeds is "isentropic", meaning that there is no change to the conditions of the gas from before to after the collisions pass through.

The speed that the collisions can propagate through the fluid is directly proportional to the rate of the random collisions that are taking place in the gas. The faster the random motion, the more frequently the molecules are colliding anyway, and the faster that "wave" of collisions can be transmitted. The rate of the random collisions in a gas is directly proportion to the amount of energy within that gas, which is a function of the temperature of that gas.

It's the propogation of those waves that gets me. They start with the motor in the driver going out and back, pushing the woofer cone, mushes up the air in front of the cone and sucks it back. Methinks at less a lot less than the speed of sound in that space.

Then they get stuffed through the throat and pushpull all the way to the exit of the horn. Unleashed on the air in the room with a large portion of the energy they left the woofer with.

It is the "back and forth" movement of the air with the cone that makes what we ultimately perceive as "sound." But it is not "movement" of the air that transmits the energy, it is the collisions.

Even the 3015LF only has a Vd of 864cc. By the time that gets to the mouth of a T48, which is almost 4500cc for the 24" wide version, the air is moving less than 2mm! And that is the amount of air movement when the woofer is going to Xmax on every peak!

By the time you are thinking of a "wave" within air, you have already gotten away from the idea that air is actually composed of individual molecules of different elements and compounds that are randomly colliding with each other all the time. This is not a bad thing...but understand that you have already moved to the "10,000 foot view" of the problem.

Speakers and sound does not create a "flow" of air...or really any movement of air at all (Memorex driven sound systems aside). As long as the motion of the individual molecules is much less than the speed of sound, then air can be treated as incompressible (also a "10,000 foot view of the problem).

--Stan Graves

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