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Old 5th April 2010, 12:36 PM   #21
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by heyday View Post
I threw in the definition of infrasound to establish that ultrasonics includes those frequencies generally considered to be necessay for subwoofer reproduction in audio/HT venues.
The definition you gave of infrasound does not mention ultrasonics.
The webpage you referenced in that regard is clearly clueless regarding acoustics:
Quote:
Infrasound, therefore, is a vibration with frequencies composed of short wavelengths,...
...infrasound possesses the ability to travel further than ultrasound which is composed of longer wavelengths.
Since the author seems unaware that higher frequencies correspond to shorter wavelengths, I wouldn't take seriously anything else she has to say.

Quote:
Originally Posted by heyday View Post
Quote:
Further more, you bring up a "transverse electromagnet wave" in your post to me. What does that have to do with anything listed above, to you?
I explained earlier that I would be starting a new thread in a different forum here at diyAudio to discuss those issues:

The Catt Question- Is science being suppressed
Ivor Catt's "anomaly" has nothing to do with acoustics. Your thread about it doesn't explain any connection you may see between TEM waves and subwoofers.
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Old 5th April 2010, 01:48 PM   #22
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I think he's having a magnet-on-a-capacitor-free-energy moment. It's alright by me. My brain spins cookies once in a while too.
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Old 5th April 2010, 06:35 PM   #23
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Quote:
Originally Posted by heyday View Post
and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna.
More like "allowing the natural impedance to select the correct element". The antenna is the entire assembly; what they're (unclearly) referring to is the driven element (that is, the one that connects into the feedline). It should be noted that this only works well in certain configurations, like a multiband dipole, or a log periodic. You wouldn't do this with, for example, a vertical array, or a curtain, otherwise you're back to needing tuned circuits.


Quote:
there is a fundamental limit relating bandwidth, size and efficiency.
Right. To put that into perspective, and using the broadcast analogy again, those antennas are tuned by the designer/manufacturer for one frequency, period. In the case of FM, you're limited to perhaps a one or two channel deviation (200 to 400 kHz) before reflected power and radiation pattern become a serious issue. On the TV band, a single channel deviation is too much to bear unless you can reduce power and cope with the pattern change. As the article rightfully points out, efficiency is a big deal.
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Old 11th April 2010, 12:02 AM   #24
heyday is offline heyday  United States
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Quote:
Originally Posted by godfrey View Post
The definition you gave of infrasound does not mention ultrasonics.
The webpage you referenced in that regard is clearly clueless regarding acoustics:

Since the author seems unaware that higher frequencies correspond to shorter wavelengths, I wouldn't take seriously anything else she has to say.

Fair enough.

Ivor Catt's "anomaly" has nothing to do with acoustics.

Could be, unless it is an example of how all science is being hindered in general, including the science of acoustics.


Your thread about it doesn't explain any connection you may see between TEM waves and subwoofers.

But, that thread isn't posted here. However, if you check the appendix of my last quoted pdf file the author discusses the electrical model with horn theory and states that a TEM wave has similar properties to a certain kind of plane wave, said plane wave being depicted by scientific characters that I can't display, perhaps similar to this: o.

But, other than that, I can't truthfully say that I see a connection, at least not at this time.

Quote:
5th April 2010 08:48 AMAndrew EckhardtI think he's having a magnet-on-a-capacitor-free-energy moment. It's alright by me. My brain spins cookies once in a while too.
Well, you're right, I guess, my mind was spinning cookies on some of those ideas.

On the other hand, perhaps the central tenant of my question may be onto the right track.

Consider the following calculated graph:

Click the image to open in full size.
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Old 11th April 2010, 12:16 AM   #25
heyday is offline heyday  United States
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Quote:
Originally Posted by letsbangout View Post
More like "allowing the natural impedance to select the correct element". The antenna is the entire assembly; what they're (unclearly) referring to is the driven element (that is, the one that connects into the feedline). It should be noted that this only works well in certain configurations, like a multiband dipole, or a log periodic. You wouldn't do this with, for example, a vertical array, or a curtain, otherwise you're back to needing tuned circuits.
But, you can do it with a simple dipole, a ground plane, a helical, and others, such as a rhombic.




Quote:
Right. To put that into perspective, and using the broadcast analogy again, those antennas are tuned by the designer/manufacturer for one frequency, period. In the case of FM, you're limited to perhaps a one or two channel deviation (200 to 400 kHz) before reflected power and radiation pattern become a serious issue. On the TV band, a single channel deviation is too much to bear unless you can reduce power and cope with the pattern change. As the article rightfully points out, efficiency is a big deal.
My original statement was that an antenna can be designed that operates off of multiple bands. Any length of a dipole antenna also includes it's subharmonic frequencies. It is said that some of the people who live on the east coast (who have enough property) construct large rhombic FM antenna's which allow them reception from european countries.

I would think that antenna design would be efficient on my bands.

Quote:
More like "allowing the natural impedance to select the correct element".
Exactly! But, in our subwoofer concept, we need to select the correct elements to match the natural impedance of our targeted frequency range(s).
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Old 11th April 2010, 12:47 AM   #26
heyday is offline heyday  United States
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Quote:
Originally Posted by Andrew Eckhardt View Post
The usfulness of a duct at LF has to do with properties of inertia, ignoring horn ideas, which I don't think you're necessarily entertaining with your main interest. The velocity and mass don't change whether you make the tube straight, spiral, or pretzel. Maybe at very high velocity you'd get some strange effect where there developed higher frictional loss and pressure on the outside of the spiral, due to centrifugal force. Never mind that. It's not happening in your speaker.

My main idea in both of my two threads is whether a spiral could be used in conjunction with a tapped horn to increase output/reduce enclosure size.

Research has shown that sound waves can become focused like a lens and amplify sound. This is accomplished by using multiple heinholtz resonators.

The possibility does exist that using at least a single tuned spiral within each heinholtz cylinder would further add amplification to the sound, i.e, it has been demonstrated so at the upper low frequency range.

It seem's that I have stumbled upon the mathematics involved in the solution to Huygens' Wavelets.

(Omitting the equations here.)

Quote:
There are several interesting things about these equations. First, the equation for velocity u(Rp) is actually a bit more complicated than the Huygens' wavelet for an electromagnetic wave. This is surprising since sound is generally viewed as a simpler process. Second, the waves only become truly spherical in the far-field, and are pretty complicated in the near field. In the far-field the overall shape of the Huygens' wavelet is proportional to 1+cosθ, for both pressure and velocity, exactly the same as the electromagnetic case. This is only true if the ratio p0/Z0=u0. At a boundary between media of different acoustic impedances this is not true, and a reflected wave is created as well as a forward propagating wave.

To me the most startling aspect of these equations is that the velocity component u0 of the source at the origin radiates isotropically, whereas the pressure component p0 has a directional pattern! Physically, higher pressure means a higher molecular density and higher temperature, which is totally lacking in directionality. Conversely the velocity is directional. (This odd directivity could be simply due to the fact that a surface and a normal are specified as well as pressure and velocity.
Quote:
This seemed so counterintuitive that I felt compelled to double check the equations. For the incoming plane wave I considered a spherical surface centered at the origin. I used the values of velocity and pressure on the surface in equation G14, and in the equation derived from the gradient of G12, and numerically computed the sound pressure and velocity inside, and outside, the surface. Inside the surface I got the plane wave, and outside the surface I got zero, exactly as expected. Note that the wavelets in this case are more complicated than the above equations, since the sphere is not an equiphase surface, but the above equations are a special case of the more general wavelets. This numerical test case is also counterintuitive; the superposition of the wavelets create a plane wave inside the sphere, but add up to zero everywhere outside the sphere. Mathematically this is the correct behavior, but a first it was difficult for me to imagine real physical sources resulting in this situation.
Then I considered the following mind experiment: fill all space with small tubes of square cross section and vanishingly thin walls. Excite waves in all tubes such that all space has exactly the same wave values as would exist with a plane wave in free space. Now cut a circular cavity out of the mass of tubes. All boundary conditions are satisfied if the cavity contains a plane wave. Thus the fields at the end of the cut tubes are acting exactly like the sources in the computer model - radiating a plane wave inside the cavity and not radiating at all outside.
With a little more thought the resolution of the directivity "paradox" is also obvious. Consider a point source of pressure. Enclose the source in a small sphere. Each point on the spherical surface radiates a wavelet with a directional pattern, with a maximum in the direction of the surface normal. But integrating over the entire sphere, the normals point equally in all directions, so the sum of the wavelets produce an isotropic pattern. Alternatively, for a point source of velocity directed towards the north pole, each point on the sphere radiates a wavelet with an isotropic pattern. But the north pole wavelet has a positive magnitude, the south pole negative, and the magnitude is zero at the equator. The sum of the wavelets produces a dipole pattern. So the directivity paradox exists only if one wavelet is considered in isolation.

Power Flow

Next I computed the power flow, which is shown in this strange and fascinating figure [68 kb]. When I originally computed this figure and saw a jet of power flowing out of the origin in the negative z-direction, I immediately concluded that I must have made a sign error someplace. I have now exhaustively checked the equations and I believe the figure is correct.
The solid black lines are parallel to the direction of power flow. The blue line segments are vectors parallel to the power flow, with power flowing from the end with the black dot towards the end with the red dot. For small radii the near-field power flow is dominated by two terms that are inversely proportional to the fourth power of the radius. Even ignoring the singularity at the origin this leads to huge power flows. The length of each vector is proportional to the power flux, except that all vectors have been scaled by a factor of radius to the 4th power, to eliminate these huge variations.
The pattern in 3-D is this figure revolved around the z-axis. Close to the origin the power flows in the form of a toroidal vortex. The power is spit out in a jet in the negative z-direction, and sheds off the vortex to flow predominantly in the positive z-direction. For an incident plane wave propagating in the negative z-direction the sign of u0 is reversed, and the wavelet pattern flips around the x-axis. The power flux at a large radius is proportional to either (1+cosθ)2 or (1-cosθ)2 depending on the sign of p0u0. Far away from the origin the flow becomes radially outward in all directions, in either case.
[ Reminds me of a an acoustical transverse wave, but, it was my understanding that could only propagate within a solid.]

Quote:
There is another pair of Huygens' wavelets generated by the conjugate Green's function (the complex conjugate of Equation G2). Far away from the origin the power flow becomes radially inward in all directions. For an incident plane wave traveling in the positive z-direction the pattern shown in the figure is flipped around the x-axis, and the direction of power flow is reversed everywhere except inside the toroidal vortex. For the top part of the vortex shown in the figure the flow is still clockwise.
So the Huygens' wavelet equations are apparently mathematically valid. Do they have a physical significance? Obviously sound cannot really emanate from a point, so that is one limit to any physical interpretation. But for a one meter wavelength the size of the vortex is roughly 10 cm, so there is no lack of molecules until you get very close to the origin. Huygens' wavelets are a valid solution of the sound wave equations in all space for ρ > 0, and mathematical sources exist for the wavelets. It is hard for me to imagine molecules resulting in this peculiar behavior, but at this point I don't know how to argue with the math.
Extracting just one more point here to conclude the general gist of what might be relevant here:

(Next post)
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Old 11th April 2010, 12:53 AM   #27
heyday is offline heyday  United States
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The concluding quote is:

Quote:
Numerical Experiments

The behavior of Huygens' wavelets varies with the nature of the incident field, and also depends on whether the regular or conjugate Green's function is used to develop the general solution. To study this behavior, a numerical model of the general solution was developed. The integration surfaces are two concentric spheres, centered at the origin, with radii, set to 1.67 and 4.46 wavelengths for the results presented here. The incident field can be a plane wave propagating in the plus or minus z-direction, or an incoming or outgoing spherical wave. The origin of the spherical wave is on the z-axis, and can be set outside the larger sphere or inside the smaller sphere. The inner sphere is empty, except for the source, when it is present. The sound wave pressure and velocity was computed in the volume V between the two spheres. The results were probed along a line parallel to the z-axis, but with arbitrary x- and y-offsets. In all cases the results agreed virtually perfectly with the incident field, which is a very powerful confirmation of both the model and the general solution equations that the model is based on.

The results are totally symmetrical for plane waves propagating in +z and -z directions, and for spherical waves with origins on the positive and negative parts of the z-axis. For simplicity, results are described for plane waves propagating in the +z direction, and spherical waves with origins on the negative part of the z-axis.
Regular Green's function

1) Incident plane wave, or outgoing spherical wave from a point outside larger sphere.
Only the surface of the larger sphere contributes to the results. Huygens' wavelets radiate outward from the -z side of the sphere, and are absorbed by the opposite side. Net result inside V is a wave traveling predominantly in the +z direction.

2) Incoming spherical wave from a point outside larger sphere.
Only the surface of the larger sphere contributes to the results. Huygens' wavelets radiate outward from the +z side of the sphere, and are absorbed by the opposite side. Net result inside V is a wave traveling predominantly in the -z direction.

3) Outgoing spherical wave from a point inside the smaller sphere.
Only the surface of the smaller sphere contributes to the results. Huygens' wavelets radiate outward everywhere on the surface. Net result inside V is an outward propagating spherical wave.

4) Incoming spherical wave from a point inside the smaller sphere.
This is where things get interesting. Both surfaces contribute to the results. The Huygens wavelets from the outer surface combine to produce standing waves. The Huygens' wavelets from the inner surface propagate outward. Net result inside V is an inward propagating spherical wave.
Conjugate Green's function

Wavelets are incoming instead of outgoing, but the superposition of all of the wavelets produce results that are identical to the regular Green's function for situations 1) and 2).

3) Outgoing spherical wave from a point inside the smaller sphere.
Also interesting. Both surfaces contribute to the results. The Huygens' wavelets from the outer surface combine to produce standing waves. The Huygens' wavelets from the inner surface propagate inward. Net result inside V is an outward propagating spherical wave.

4) Incoming spherical wave from a point inside the smaller sphere.
Only the surface of the smaller sphere contributes to the results. Surface sucks in incoming wavelets.
It seems that there is some sound technology that is not being applied to loud speaker enhancement here.

Think of the possibilities.

Click the image to open in full size.
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Old 11th April 2010, 01:10 AM   #28
heyday is offline heyday  United States
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Quote:
The integration surfaces are two concentric spheres, centered at the origin, with radii, set to 1.67 and 4.46 wavelengths for the results presented here.
1.67 = 1 5/8's?
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Old 11th April 2010, 01:56 AM   #29
heyday is offline heyday  United States
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Quote:
Originally Posted by Don Hills View Post
No reference is required to refute it, just a little common sense and logic.

A straight wire exhibits inductance.
A straight pipe exhibits "inductance".

Coiling a wire enables coupling between the coils and concentrates the magnetic flux, allowing a given inductance to be obtained in a smaller space.

Coiling a pipe does not enable coupling between the coils. No space advantage.

----------------------------------------------
- Great Big Billy Goat Gruff.
So, the question becomes how do we generate a toroidal vortex sound wave so that we may concentrate the power flux of said wave?

Quote:
The power flux at a large radius is proportional to either (1+cosθ)2 or (1-cosθ)2 depending on the sign of p0u0. Far away from the origin the flow becomes radially outward in all directions, in either case.
Remembering the lengths of the radii involved in the equations, though at first glance prohibitive, it might non-the-less be feasable with the proper design in a tapped horn yet.

Quote:
So the Huygens' wavelet equations are apparently mathematically valid. Do they have a physical significance? Obviously sound cannot really emanate from a point, so that is one limit to any physical interpretation. But for a one meter wavelength the size of the vortex is roughly 10 cm, so there is no lack of molecules until you get very close to the origin. Huygens' wavelets are a valid solution of the sound wave equations in all space for ρ > 0, and mathematical sources exist for the wavelets.
So, if one placed a radii of tubes at the compression area of the radiating driver, with coiled spirals of lengths corresponding likewise, as the permitted reduction above so states, (which, the added spirals would also contribute to the compression ratio of the driver) couldn't one achieve an acoustical induction and at minimum a velocity increase/power amplification of the wave?

How about a probable constructive effect to the phase delay common to tapped horn designs?
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Old 11th April 2010, 03:02 AM   #30
heyday is offline heyday  United States
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Quote:
Originally Posted by godfrey View Post



[...]


Ivor Catt's "anomaly" has nothing to do with acoustics. Your thread about it doesn't explain any connection you may see between TEM waves and subwoofers.

On second thought, I invite you to read Igor Catt's theory regarding charge radiating as a TEM between the two electrical conductors, whereby he discusses a TEM wave as consisting of two spirals.

Then, observing the that the two spirals in the above posted image were the result of equations applied to a manipulated sound wave showing results described as a power flux.


Quote:
To me the most startling aspect of these equations is that the velocity component u0 of the source at the origin radiates isotropically, whereas the pressure component p0 has a directional pattern! Physically, higher pressure means a higher molecular density and higher temperature, which is totally lacking in directionality. Conversely the velocity is directional. (This odd directivity could be simply due to the fact that a surface and a normal are specified as well as pressure and velocity.
Huygens' Wavelets

See General_solution


But most especially factor in the following appendixed analogy from the following pdf:

Quote:

A.4 Guided Waves
In unbounded space far away from any supporting structures, all electromagnetic
wave propagation approaches that of a plane wave, or TEM type.
z

[Note given:
Quote:

z
A TEM (Transverse Electromagnetic) wave is a wave in which the electric and magnetic
eld intensities are in a direction transverse to, or normal to, the axis of propagation.

The page go's on to explain that this condition can not exist inside a horn or hollow waveguide blah blah blah. Apparantly the writer never considered using a horn as means to generate said condition, with the electrical equational componant useful as an analogy after all.
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