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Old 6th January 2013, 05:49 PM   #911
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Quote:
Originally Posted by xrk971 View Post
For anyone wishing to try the FH3 quickly and cheap in a slightly smaller format, I just built a 30 in tall one
It is not an FH3. As soon as you changed the dimensions it ceased being an FH3. Nice to see the FH3 insporation for sure.

dabe
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Old 7th January 2013, 05:39 AM   #912
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Originally Posted by xrk971 View Post
For anyone wishing to try the FH3 quickly and cheap in a slightly smaller format, I just built a 30 in tall one in an evening and morning using paper faced foam core boards from the dollar store and the Vifa TC9FD 3.5 inch driver with pretty good results. More info in this thread:
Foam Core Board Speaker Enclosures?
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Originally Posted by planet10 View Post
It is not an FH3. As soon as you changed the dimensions it ceased being an FH3. Nice to see the FH3 insporation for sure.

dabe

it could sound quite fine, and Scott/ Dave should probably be flattered

of course, if it doesn't, well then "Mr Phelps, the Secretary will disavow any knowledge of your team or mission" (did I get that more or less right?)
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Old 8th January 2013, 09:59 PM   #913
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how far does the felt go down the internal board right behind the driver ? equal to side pieces or further down the board? thanks
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Old 8th January 2013, 10:42 PM   #914
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Quote:
Originally Posted by chrisb View Post
it could sound quite fine, and Scott/ Dave should probably be flattered

of course, if it doesn't, well then "Mr Phelps, the Secretary will disavow any knowledge of your team or mission" (did I get that more or less right?)
Thanks for the support Mr. Secretary To me, it sounds really good, so much so that I promptly ordered more drivers to build the other channel. I am playing with varying the stuffing and type of stuffing. The neatest thing is how light weight they are yet still manage to make such a lovely full rich sound with nice bass. The lightness makes me wonder if many speaker designs can be made with 1/4 in or even 3/16 thick plywood and work just fine. As much as this foam core design ceases to be an FH3 the moment dimensions are changed, it owes it's great sound to the design that inspired it. One other thing, if you are not sure if you will like it, the cost and effort to build isn't a big deal. And if you find that you like it in foam, you can go forward with a real wood and pricey driver build with confidence.
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Old 9th January 2013, 10:53 AM   #915
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Does make you wonder about the obsession with void free Russian birch ply and associated loathing of other materials, MDF particularly which I for one rather like. My suspicion is that the stiffness of a board is less important then it's ability to store and return energy, isn't a material that's inherently lossy preferable? I always think, could you make an effective spring out of it? If so, it's probably not good.
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Old 9th January 2013, 12:40 PM   #916
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I look at this problem as being three separate issues.

1. Panel resonance. This is where the panel will contribute sound at specific frequencies.

2. Panel breathing. This is where the panel moves in and out with the pressure wave within the box.

3. Panel transparency. This is where sound simply passes through the panel.

I did some very cursory experiments with a relatively small box and panel. The panel was 10"x17" 18mm TigerPly. The SPL contributed by the panel was better that 40dB down from the SPL of the driver. I found no spikes in the FR that would indicate panel resonance. I should have run a THD, but didn't. I would expect any resonances to appear in the THD.

I plan to run more exhaustive tests this summer, and my current opinion is subject to rapid change but:

If we are not talking about a sub or very large panels (whatever that means), cross bracing in irrelevant. As long as the panel is stiff enough to prevent breathing at the lowest frequency, don't bother.

In small panels, resonant artifacts are sufficiently low in SPL to be ignored.

The major issue in panel sound production is transparency. Wood is a pretty good conductor of sound. The stiffer and harder the wood, the better it conducts sound. This suggest that MDF might really be the better choice for small boxes. I am thinking of even softer materials; polystyrene bead-board or Celotex.

I don't want to restart the MDF/BB flame war and I will not address any response the tries to go there. I will address this later when I have hard data. Anyway, I am beginning to think that bracing is a small box, say 20L or less is a waist of time.

Bob
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Old 9th January 2013, 12:50 PM   #917
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Almost all materials used for cabinets will have "spring" to them, i.e., a Young's modulus value sufficiently high to allow them to be used in structural members. It's the amount of damping that is important. MDF probably has high inherent damping and wood boards used for guitar bodies probably much less. The foam core definitely has spring in it because you can push down on it and see it flexes back. There is however, damping in the foam and the paper as it is flexed. Horns and TL's are like flutes and other woodwind instruments - 1/4-wave aero-acoustic devices where the dominant acoustical prodcction results from the vibration of the air volume contained within the boundaries defined by the walls. The sound emanating from the walls themselves are a smaller secondary acoustic effect. Of course, this is an area of hotly contested debate - for example, the argument of which metal produces a better sound in a concert flute? Silver, nickel, gold? There may be very minor differences but the majority of the sound from a PVC or wooden flute will be indistinguishable from a nickel, silver or gold flute. So in a similar way, I am seeing a similar thing in horn and transmission line speakers. Of course, the vibrations in the walls of the speakers will produce sound and many will say that it colors the sound and you are not getting a true reproduction of the sound as recorded. True. However, people don't buy studio monitors (which are designed to have a flat response) to listen as their main speakers for the *enjoyment* of music. Some coloration is not necessarily bad as it may reinforce/copy the natural harmonics of certain instruments that are aero-acoustic like woodwinds, the human voice, horns, etc. If you ever look at the spectrum of a single note played on a flute or a single note of the human voice, it is not a single peak at one frequency but rather a distribution of peaks very much like the multiple peaks that a horn loaded speaker puts out. Sorry if this hijacked the thread.... I can continue this in another new post or a similar one regarding materials for speaker enclosures.
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Old 9th January 2013, 12:57 PM   #918
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Quote:
Originally Posted by Bob Brines View Post
I did some very cursory experiments with a relatively small box and panel. The panel was 10"x17" 18mm TigerPly. The SPL contributed by the panel was better that 40dB down from the SPL of the driver. I found no spikes in the FR that would indicate panel resonance. I should have run a THD, but didn't. I would expect any resonances to appear in the THD.

The major issue in panel sound production is transparency. Wood is a pretty good conductor of sound. The stiffer and harder the wood, the better it conducts sound. This suggest that MDF might really be the better choice for small boxes. I am thinking of even softer materials; polystyrene bead-board or Celotex.


Bob
I totally agree with Bob Brines. Real measurements - great! 40 dB down is pretty much in the mud. The suggestion of softer materials is interesting - foam core boards are definitely softer!
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Old 9th January 2013, 02:37 PM   #919
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When I get some time I'm definitely going to laminate some corrugated cardboard up to 18mm thickness, maybe some glass tape between sheets all bonded with PVA.
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Old 9th January 2013, 02:42 PM   #920
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Also a very easy way to form curves.
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