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For a few years I've had a suspicion that you could mount drivers VERY far away from the cabin if you built a proper waveguide.

Looks like this is true.

Danley discusses it in this thread, where he describes routing sound through three feet of tubing. This sounds a little ridiculous, but the reason that it works is that the wavelengths are shorter than what they're passing through, so they come out the other side intact.

You could use this to do some bizarre things, like mounting your tweeters in the glove box. Or putting a subwoofer under the hood of your car and routing the output into the cabin. Or put your midranges under the seat. All kinds of bizarre stuff. Just keep the duct acoustically small.

Very technical discussion here:

PSW Sound Reinforcement Forums: LAB: The Classic Live Audio Board => Mid cone driver slots and holes

Think of a vented box and woofer, one has several things;
First, obviously you have a source that has two phases and one has a 2nd order acoustic low pass filter attached to the rear side of the radiator.
That low pass filter is made from the air in the box, a compliance volume or acoustic spring force (which electrically appears as a series inductance) and the mass of the air in the port (which looks like a Capacitor to ground), hence a 2nd order system.
You can see that relationship if you imagine a ruler clamped to a table, you can see its resonant frequency goes down if you add mass (add a penny at the end / make a longer port or reduce its area) or make the spring weaker (make the ruler longer / or box larger).

When the radiators two opposite phases add, they cancel each other out as they are always 180 degrees apart and of equal magnitude.
The sealed box simply contains one half of the signal allowing the other half to radiate away unhindered.
The low pass filter in a vented box (the vent + box volume) is in the form of a Helmholtz resonator and “at resonance” is an inverter, in other words introduces a phase shift which makes the port radiation “in phase”, additive with the front radiation.
While one doesn’t normally think about this as a “low pass filter” it is. Progressively less comes out of the port as the frequency rises above resonance.

As one lowers the frequency from the resonance (normally the low corner in a vented box), one finds that the phase shift imposed by the L and C reduces towards zero and the sound coming out of the port reaches the same phase angle as the rear radiation.

At this point, this is the “pass” region and the rear radiation increasingly cancels out the front side as the frequency lowers and the vented box has twice the roll off compared to a sealed box.
At a point WAY below Fb, the cancellation is complete.
Above resonance, the output from the port decreases, reverting to a sealed box, which contains the anti-phase rear radiation.

When one has a normal horn, one finds that the radiators rear volume is a sealed box.
At the high frequency corner, one finds that another acoustic “low pass filter” is present although potentially not as obvious.
Here, some portion of the air in the horn throat acts like a lump of mass like a port and some portion of the air between the radiator and throat acts like a compliance or spring, forming a low pass filter.
By sizing the L and C relative to the resistances, one can often extend the hf response by having a suitable low pass filter. As with the vented box, the ultimate roll off is steeper than an unaided alignment.
Here the only output is what comes out of the port (and drives the horn) so to speak.

In the Synergy and Unity horn boxes I designed, I use that “low pass” filter effect to attenuate the distortion products that all drivers produce. The distortion products are 2,3,4,5,6, ect times the fundamental frequency and so to the degree these fall on the rolled off part of the acoustic filter, they are attenuated.
The filter here is made of the volume trapped under the cone and the mass of the air in the port and throat. I don’t use phase plugs here.

Also, when sound is introduced into the horn at some point forward of the apex (such as the cone drivers in our horns), one finds that the upper frequency limit is also set by an additional “low pass” filter effect caused by internal self cancellation.
When the wavelength is short enough (frequency high enough) the sound that went to the pointy end, bounces back and arrives out of phase with the driver pressure.
One finds (as you raise the frequency) that you eventually have a BIG cancellation notch when that “driver to dead end” distance is about 1/4 wl.
The two low pass filter effects strongly attenuate above band energy from the cone drivers and helps make the distortion especially low.
It was that notch, or pondering that notch that made me wonder about and then try what became the Tapped horn.
I thought what happens if I substitute a source of the opposite phase for that reflection? (a source which was present in the back side of the radiator), then they add instead of cancel. Some considerable fiddling in the computer eventually resulted in boxes that work better than similar sized normal bass horns using this new principal.

Anyway, up to now, the only function a phase plug has is to occupy an excess air volume that would have other wise made the acoustic “low pass filter” too low in frequency.

Once one is dealing with a radiator who’s dimensions are approaching the wavelength size, then the other function of a phase plug comes in handy.
The speed of sound governs how a pressure disturbance radiates away from its source.
If one has a radiator that is “large” acoustically and also has a single exit point, one finds that just like in the Synergy and Unity horns, one gets a deep cancellation notch when the difference in the two paths is 1 / 2 wl. The range of coherent summation is limited to the frequencies BELOW the region where cancellation begins.
This is like a pile of subwoofers, when the array is less than about 1/4 wl across, they all add together and feel “mutual radiation pressure” while a significantly larger spacing produces directivity and then lobes.

Here, a phase plug can be shaped so that the acoustic passages all have the same length or have a length appropriate to the desired exit wave front shape.
One big difference in the sound of compression drivers (IMO) after being eq’d flat is that many have a phase plug that produces a converging wavefront at the summation, while what one needs at the throat of a horn is a diverging wavefront. That “clash” can cause diffraction or interference, which produces Higher Order Modes that Earl Geddes describes.

So far as the Paraline as used in the VTC array and GH-60, this is an acoustic device which can be shaped to provide an exit wavefront that can be flat, a line source or diverge or converge, an astigmatic point source with positive or negative focal point..
It works by allowing the sound to expand radially between two plate that are too close together to support any reflected modes between them so only radial expansion takes place.

A correction slot who’s shape defines the exit wave front shape and who’s dimensions are small enough to allow the sound to bend around the corners, is placed in the radial path. The sound passes through the slot and what emerges on the other side is a wave that travels to the center from each side, bends around a corner and exist at a center slot having entered at a center hole at the rear. The VTC site had a nice graphic of the one they are using.

It probably sounds weird to suggest that you can bend sound without ill effect but you can when the acoustic dimensions are small enough. The difference as it is in the examples above is that keeping the difference in path lengths less than about 1/4 wl at the highest frequency of interest..
I used to work with 21KHz levitation sound sources and needed to place a microphone in the levitation furnace to monitor the source sound level.
Well, very very few things are “happy” at 1500 degrees C but I found that a Zirconium / Alumina tube with a 1/16 inch bore passed 20KHz sound out of the furnace to an external microphone with no problem.

Funny, the external microphone’s heatsink wasn’t large enough on the first one and the microphone melted.
Later fooling around (research) showed that a 3 foot long, 1/16 inch bore copper tube could be wound around a small coffee cup and not effect the sound passing to the microphone.

A constantly re-occurring theme in much of what I do is that many things depend on how large X is compared to the wavelength.
Anyway, I hope that makes some sense.

 

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I know of a few folks who have done this by putting drivers in the bottoms of the dash and routed the tubes up through the stock locations in the dash. I'm personally not sure how well it would work.
The one thing I've wondered; if sound originates at the voice coil, is there any crazy benefit to PLD's?
 

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That's exactly what I was wondering; is the zero plane the voice coil or radiation point/mouth ?



Sent from my Rezound using Tapatalk
 

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Discussion Starter #6
That's exactly what I was wondering; is the zero plane the voice coil or radiation point/mouth ?



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When it comes to phase, I am a dope. So take this with a grain of salt. But here's how I *think* this works.


Picture a wave moving in a duct. The wave could be composed of *anything* - water, air, light, whatever. For me, water is the easiest to visualize because we can see waves in a pool.

As long as the duct is larger than the wavelength, there are well defined peaks and troughs in the wave.

But as the dimensions of the duct become smaller than the wave, the peaks and troughs in the waves cease to exist.

As Danley noted in his message, the wave comes out of the duct intact. Intuitively, it seems like the wave would be screwed up. I mean, the whole idea of running your loudspeaker through a duct seems a bit absurd. But if you look at it from a theoretical standpoint, the wavelength should arrive intact, up until a certain frequency. (Hang in here for a second and I'll post the math.)


I've built a few speakers like this, and it's always fun to stick your ear against the port end, like you're putting your ear up against a sea shell. If you have a speaker and some PVC at home, you can try this trick with about 30 minutes worth of work. When you listen to the port end, you'll notice that the entire frequency spectrum is present. There are highs, there is midrange, and there are lows. But the lows are predominant. The sound is also a bit strange, because the high frequencies sound like they're coming from far away, while the low frequencies sound like they're coming from the port's mouth.

Here's how this works*

The PVC is 7.5cm in diameter. When the dimensions of the duct are much smaller than the wave, peaks and troughs do not form. Smaller than one quarter wavelength, according to Danley's post. 7.5cm is one quarter wavelength of 1133hz, so anything below 1133hz won't form waves. The origin of the waves below 1133hz will seem to originate from the port.

Note that there's a delay introduced. For instance, if that PVC pipe was 340 meters long, and you hit "play", the sound coming out of the cone is still going to lead the sound coming out of the port by one second.

In summary -

Can you run a soundwave through a duct, with impunity? - YES, it looks like you can. To determine the maximum duct size, take the maximum frequency you want to reproduce, then divide it by four. For instance, Danley wanted to reproduce 21khz. 21khz is 1.6cm long. Therefore, he had to use a duct that was 0.4cm or smaller. He used a duct that was 0.0625" in diameter, or 0.15875cm.

Where is the origin of the sound? - The origin of the sound depends on the frequency. If the frequency is more than 4x the size of the duct, the origin will appear at the mouth of the duct. This is because the wave simply cannot form until the wave reaches a space that is large enough to contain it. For instance, with the 0.15875cm duct mentioned above, the wave expands when it reaches the mouth.**

* Please note that I'm still figuring a lot of this out. There are probably some mechanical engineers scoffing at this post. If so, please speak up :D I've studied loudspeakers for almost twenty years, and the more that I do, the more I believe that timing information is incredibly critical, and that most of us spend far too much time obsessing about frequency response, when getting the timing right is more important. A speaker with poor timing can sound very good, but it still sounds like a loudspeaker. Our ears are very well tuned to detect timing problems.

** If you're one step ahead of me, you'll realize that in an expanding duct (aka a horn) the sound can originate from multiple points in the horn, depending on wavelength. Rather mind bending isn't it? The high frequencies can originate from the throat, or even the diaphragm of the compression driver, while the low frequencies appear further down the length of the horn.
 

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Discussion Starter #7
I know of a few folks who have done this by putting drivers in the bottoms of the dash and routed the tubes up through the stock locations in the dash. I'm personally not sure how well it would work.
The one thing I've wondered; if sound originates at the voice coil, is there any crazy benefit to PLD's?
I believe the point of origin is frequency dependent. If the duct is smaller than the frequency, the wave cannot expand, and so it's origin will appear to be at the mouth. Conversely, if the duct is *larger* than the frequency, the wave will expand at a point further up the duct, and that portion of the spectrum will sound farther away.

If you put your ear up against an air condition duct in an office, you'll be able to perceive high frequency sounds, but they sound like they're coming from very far away. But the low frequencies will sound like a dull thud of indistinct origin, because the wave is expanding at the mouth, not further up.

I think :p

 

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Elderidge did this in his 4runner as well as Matt Robert in his Truck
 

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Matt's truck wasn't like this.
ok, apparently I stand corrected, but Elderidge at least did something similar where the drivers were mounted near the headlights and sound funneled in through a wave guide
 

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Damn I wish I was not in Saudi Arabia I would grap the pics for what Mark did showing the detials. This is similiar to what Mark did. The drivers were mounted beyond the firewall but basically horn loaded back into the cab if I remember correctly. So this seems a little different if I am grasping the above concept.
 

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Yeah this is completely different than horn loading.

This is more like a bandpass and putting the port where ever you wanted. Not really bandpass, but an all pass...but it helps illustrate the point.
 

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Discussion Starter #13
ok, apparently I stand corrected, but Elderidge at least did something similar where the drivers were mounted near the headlights and sound funneled in through a wave guide
The thing is, all of these devices are points on the same spectrum. A horn is a waveguide, and a horn can have some features of a waveguide. Even a straight piece of PVC pipe will have directivity at certain frequencies. Put your ear up against the port of a loudspeaker, and you'll hear some high frequencies coming out of it.

The 'trick' is maximizing or minimizing the directivity, depending on what you're trying to do.



For instance, if you have a waveguide under the dash, you probably want very wide vertical directivity, and very narrow horizontal directivity. For instance, the horn in the pic above has about 45 degrees of horizontal directivity.

But there are situation where you want NO DIRECTIVITY whatsoever. And it looks like you can do that, as long as you size the duct properly.

(The formula is earlier in this thread.)

If I understand how this works properly, the waveform will expand at the point where the duct becomes large enough. So the origin of the loudspeaker will 'appear' to be the point in the duct where it expands to a specific size.

Obivously, you could use this 'trick' to manipulate the apparent origin of the loudspeaker. This would likely be particularly effective below 2khz, because our perception of location is dictated by phase more than by amplitude as frequencies get lower and lower and lower. For instance, all the DSP in the world won't fool you into thinking a midrange that's in the center of the car is *outside* of the car. But if you can move the apparent source of that midrange somewhere else, the midrange enclosure could easily be behind the dash, or even behind the firewall.

Also, it's not obvious in the pic above, but that's exactly what Image Dynamics is doing with the vertical directivity of this horn. If ID wanted ninety degrees of coverage in the vertical plane, they'd need a VERY tall waveguide. For instance, to get down to 1khz, they'd need a waveguide that's 34cm in height! But by making the vertical height VERY small, the waveform doesn't expand vertically until it reaches the mouth of the waveguide. This is one of the reasons that the transition from the mouth of the waveguide to the dash is so critical. Screw up that transition and you screw up the wavefront.

 
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I've been looking at this thread and the creating a perfect soundstage thread and I got an idea about a unity horn design.

If you can get a driver's origin to appear at the end of the pipe/tube, you can do a multiple midrange and compression driver set-up and route the tubing to a waveguide as long as the tubes for each driver and each side are the same length,correct? It sounds like it would be the best thing to do as far as efficiency, frequency and polar response.
 

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simply put, put a 6" driver to mate with a tube, the tube should be smaller than 1/4 of the xover point, put the 6" driver in a suitable cabinet and cover the cone with the tube. if this works then wherever the end of the tube is it will sound like the driver is there. you could then mount a small tweeter on top of the tube and see if you can have the "footprint" somewhere other than the "sound radiation location "... is that the concept? sounds like only an hour or two of work to test that out...6" drivers in the pillars would be awesome... even ported enclosures could be possible this way...
 

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I see what youre saying req. Ive been following this to the best of my ability, but what you described sounded alot like a bandpass enclosure. If I am wrong though, the possibilities are endless with this idea. Especially with drivers that dont necessarily need a large enclosure (type r comes to mind). Some creativity would be needed to create an enclosure and route the pvc, or whatever medium tube to the dash or a-pillar. If it works, then I think car audio has just evolved.
 

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Discussion Starter #19
I see what youre saying req. Ive been following this to the best of my ability, but what you described sounded alot like a bandpass enclosure. If I am wrong though, the possibilities are endless with this idea. Especially with drivers that dont necessarily need a large enclosure (type r comes to mind). Some creativity would be needed to create an enclosure and route the pvc, or whatever medium tube to the dash or a-pillar. If it works, then I think car audio has just evolved.
I took an hour or so and did a "brain dump" of what this is doing here:

Sound Where You Want It. - diyAudio

Some things I noticed, when I played with the 'ducted woofers' in Hornresp:

  • How high you want your woofer to play has a big influence on how big it can be. For instance, to get to 1500hz you need a duct that's less than 6cm in diameter. So you're not going to cram the output of a 30cm woofer through a 6cm duct.
  • Standing waves in the duct are the biggest challenge. The response is NOT flat. But you can flatten it out pretty easy, and getting a couple octaves out of this beast is easy. For instance, let's say you have a 2" dome that will play down to 700hz. You can 'duct in' a woofer and get an F3 of 100hz instead of 700.
  • By far the coolest thing is arraying these. If I had more time, I'd start figuring out how to 'duct in' a bunch of tiny speakers, like the 2.5" woofers that Dayton sells. Since the apparent source of the sound is the end of the duct, you can put the woofers *anywhere* within reason.
Of course, a miniDSP would be killer for this. Gives you the delay you need and some EQ and a crossover.

All of this scales to any frequency band; it's even easier to do this for midbasses or subs, but then the duct size starts getting out of hand.

 
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