Frank,
Your concept for construction sounds very well thought out. I don't see any problems in your methods. Another method since you are talking covering the structure with fiberglass is to use what is called bender board, it is a thinner ply material that fairly easily bends a radius that you would attach to the ribbing. You can layer the bender board to make it thicker, just lots of aliphatic glue holds the layers together.
I've built some fairly complex furniture this way in the past when I built set pieces for TV shows. I've even laminated over the bender board with metallic veneers when called for. Not exactly a speaker enclosure but I assure you it can be done this way. The same concept was used of internal bracing to create the curvature.
Your concept for construction sounds very well thought out. I don't see any problems in your methods. Another method since you are talking covering the structure with fiberglass is to use what is called bender board, it is a thinner ply material that fairly easily bends a radius that you would attach to the ribbing. You can layer the bender board to make it thicker, just lots of aliphatic glue holds the layers together.
I've built some fairly complex furniture this way in the past when I built set pieces for TV shows. I've even laminated over the bender board with metallic veneers when called for. Not exactly a speaker enclosure but I assure you it can be done this way. The same concept was used of internal bracing to create the curvature.
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Unfortunately ...
... an acoustic wave when exiting the confines of a horn knows nothing about such considerations, for it, it just 'sees' several abrupt changes in the bounding conditions that (to a lesser degree) constrain its propagation. Alternatively such surfaces can be expressed in one surface formed from curves that are continuous in curvature. Of course, the cost of construction will be much higher, unless the quantities of production are large enough to justify use of a molding process. WHG
... an acoustic wave when exiting the confines of a horn knows nothing about such considerations, for it, it just 'sees' several abrupt changes in the bounding conditions that (to a lesser degree) constrain its propagation. Alternatively such surfaces can be expressed in one surface formed from curves that are continuous in curvature. Of course, the cost of construction will be much higher, unless the quantities of production are large enough to justify use of a molding process. WHG
To me making enclosures has to be practical. Simplicity is key. Sure there are many ways to get larger radi, but they loose out in simplicity. The first Summas were molded fiberglass done in a CNCed mold. The corner radi was 3". But the assembly process was way too complex and not worth the extra cost. That's why I stopped making them. the newer NS-15 measures far better than the older Summa and is far simpler to make - I learned some things along the way.
If your going to use wood for the structure why is it any harder to have a blended radius from the end of your horn profile? You would use the tangent at the end of the horn and could have an increasing or decreasing radius or anything in between. I don't see the difficulty of this.
Earl for serial production fiberglass would be a pain compared to your present method of just casting the horn and I assume the front baffle as an integrated whole. But for those one offs that a skilled woodworker can do anything you can think up can be made.
Earl for serial production fiberglass would be a pain compared to your present method of just casting the horn and I assume the front baffle as an integrated whole. But for those one offs that a skilled woodworker can do anything you can think up can be made.
I don't use wood and I do have a blended radius from the horn mouth all the way through to the enclosure side walls. So I am not sure what you are referring to. And sure if cost and time are not a consideration then anything is possible, but then even for the DIY the value goes away because unique gets expensive. I have always been of the mind to look for the best "value" in a loudspeaker whether I make it of someone else does.
I was referring to what FranWW was proposing. He could create any exit shape he would want with the boat type construction and easily blend the curvature to whatever he wanted. Wood can do so much if that is your media. I wasn't suggesting that you Earl change anything on your own design.
Earl a question for you. On the Beyond the Ariel thread some talk about having the horn separate from the bass enclosure and increasing the distance from the top of the lower enclosure to create a gap between that and the large roundover on the end of a horn. Do you think there is any credence to that or just wishful thinking? I have always wanted to keep center to center distances between devices as small as possible but in that situation they are actually increasing the distance intentionally. It just seems you would change where the reflection of the top of the lower enclosure would be but perhaps I am missing something in their argument.
For all sorts of reasons, then, it wasn't for me a practical way of going about things. "That way lies madness."
These days I'd DIY speakers with foam board of some kind.
Yea, I don't see that as a good idea either. I don't think that the waveguide should be in free space either. The best mounting for a waveguide would be an infinite baffle, but that's not practical. Next would be an enclosure that was smoothed in the mouth flare with as little change in slope as possible - that's what I try and do. Mounting the horn in open air creates the greatest change at the mouths edge, not the least.
If you must have a free standing waveguide then it must have a wrap around of the mouth flare as I see in many of the photos. But I would still prefer an enclosure.
I once worked with a guy on putting the waveguide into a sphere, which would be a good way to mount it. The costs became exorbitant and the project died.
I think foam boards are ideal. Working with them is a dream. They are stiff, light and well damped and they finish beautifully and never show seems. Oh, did I also mention that they cost 10 x as much as MDF!
I'm with you up to here. Just to be sure:
At the mouth edge there is a sudden decrease in acoustic impedance, which leads to a phase-inverted reflection. If this reflection interferes with the direct sound on-axis, a comb filter is formed, starting with a peak at the frequency where the path-length difference is equal to a half wavelength. In the case of a waveguide with a mouth diameter of 15" the first peak is somewhere around 1 khz and there will be a dip at about 2 khz. In my experience the effect is smoothed a lot when the waveguide is built into a rectangular baffle, as a result of time-smeared baffle-edge diffraction. The comb-filtering is most pronounced at lower frequencies.
You lost me here. Where are the sounds in phase exactly?
Are you saying there is a resonance across the mouth at a frequency where half a wavelength is equal to the mouth diameter? In the case of a 15" waveguide, that would be at about 500 hz. Right? And how is resonance across the mouth going to reach the on-axis listener?
You see, I don't fully understand. The New Summa has its axial hole centered at around about 5 khz. In my view it is more likely to be caused by stuff going on in the throat, closer to the compression driver.
I'd love to hear what you think.
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At one of the speaker design competitions back in Ohio one of the guys added large roundovers to his cabinet by putting large radius pvc pipes at the corners and bridging the tangent lines at their outsides with mdf and plumbers putty. They looked very nice and he said it was easy to do. I assume he had holes in internal walls and pipe surfaces to utilize the volumes in pipes and between panels. This was with full cylindrical lengths, not sliced sections of the pvc pipe (which i had earlier found out the hard way was difficult and downright dangerous to do).
First, the diffracted wave is out of phase of the incoming wave, so the first phase/antiphase in the far field should be a null. And it is a diffracted wave, not a reflected wave. The exact location of the diffraction would be easy to locate in the case of a sharp mouth edge, but when it is radiused then this is much harder to locate and likely moves with frequency. So I think that calculations of locations of frequencies will be tough. In my speakers there are no sharp edge so only the very lower frequency diffractions exist. That's why we don't see many peaks and dips, basically just the one. It is possible that there is a peak below the first dip, but there is also a peak in the driver in this same region so it would be hard to sort out.
It cannot be the throat because the throat is identical in the Nathan, Abbey and Summa, but yet the hole moves around. This means that only the mouth could be the source of this hole.
The resonance across the mouth is clearly seen in measurements and is more like a full wavelength. The Summa waveguide is more like 18 " and the abbey more like 15", but the fact is how do you determine the radius of a flared mouth? I think that you are trying to be too precise with something that has too many variables. I did not know about the resonance across the mouth until I developed my new measurement technique, which allows me to propagate the reconstruction point in the mouth (as well as the far field). When one does this for the Abbey, it is clear that the mouth does not have a flat wavefront but does in fact have a wave that is circumferential in the mouth. This is only evident at a single frequency and corresponds exactly to the region of the hole and a mild disruption of the whole polar pattern.
I don't think that waves on a drum correspond to waves along a tube. The solutions are Bessel functions and the resonances are not integer multiples as they are in a duct. So a simple analysis like you are doing is not going to be accurate here. The cross modes in a duct are also Bessel functions, but of order zero since the walls are rigid. Here (the mouth) the Bessel functions would be of order 1 because the outer boundary condition is free. You can't simply look a linear wavelength situation like you do along a straight line like a duct.
The standing wave across the mouth does not in itself have to reach the listener, it just has to influence the diffraction wave, which does reach the listener.
The standing wave is a second order effect, not the primary one (mouth diffraction). I only see it in the Abbey waveguide, nothing else, and I spent a lot of time studying it. I see far field effects that are not consistent with the major effect of the mouth diffraction, and I can measure a "standing wave" in the mouth of the waveguide. I have not taken the time to see if there is a theory that can be established here - like you seem to want determine - but the evidence is clear and my understand is now sufficient to avoid these issue in future designs. As I said, because of the mouth flare an theory would be difficult to establish. It might be possible with a sharp edge, but that not a situation that interests me.
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I personally have never understood the thinking of waves as simple flat planer waves, it makes little sense to me to look at a waveguide as producing flat wave-fronts? I have always considered that the actual wave-fronts would always be of a curved surface or more realistically a spherical wave shape that is simply distorted by the shape or containment of the waveguide until it exits the horn if larger than the mouth or forms earlier in the horn when the wavelength is smaller than the mouth diameter. I see that it is easier to conceive of waves using a planar wavefront but I don't think it is truly correct to do that.
The way I see it, the diffracted sound arrives one half period after the direct sound. Half a perioid of delay corresponds with 180 degrees, a phase-inversion also corresponds with 180 degrees. That means the direct sound and the diffracted sound are in phase and thus the comb starts with a peak, not a dip.
Semantics? When there is an edge, there is a change in impedance. When there is a change in impedance, there will be a reflection. So diffraction and a reflection caused by an impedance mismatch go hand in hand.
I understand. My intention was to do a thought experiment.
That makes sense.
I don't fully understand the rest of your post, but I'll read it again later. Thanks for your comments!
That depends on the dimensions of the waveguide. The smaller the difference in arrival times at the listening position, the higher in frequency the comb-filter starts.
I think that we are all in agreement.
I suppose that one could say that the portion of the wave that moves back towards the source is "reflected" while the portion that move away from the source if "diffracted", I'm OK with that if we are using two different words to describe the effect. But I would not be comfortable with the forward moving wave being called "reflected". That doesn't sit well with the definition of "reflection" (IMO.)
As I said, there may be a subtle rise from this edge diffraction prior to the dip, it just wouldn't stand out and so not be so noticeable. But it clearly is a spatial comb filter.
I suppose that one could say that the portion of the wave that moves back towards the source is "reflected" while the portion that move away from the source if "diffracted", I'm OK with that if we are using two different words to describe the effect. But I would not be comfortable with the forward moving wave being called "reflected". That doesn't sit well with the definition of "reflection" (IMO.)
As I said, there may be a subtle rise from this edge diffraction prior to the dip, it just wouldn't stand out and so not be so noticeable. But it clearly is a spatial comb filter.
Which is clearly what we see.