Unique guitar speaker cabinet

Gnobuddy

Member
2016-03-01 4:10 pm
The top reflector eliminates beaming (certainly one of the banes of guitar speakers) and the cylinder eliminates standing waves.
Have you by any chance heard the speaker in question? If so, did you like what you heard?

I ask because I tried *exactly* the same thing decades ago with a pair of full-range drivers in my DIY music system. (Not guitar.) I wasn't at all happy with the results I got.

The reflector idea (which goes back to long before I was born) didn't work at all. If you point a flashlight beam at a flat mirror, the reflected light still forms a beam. It doesn't spread out or disperse.

Exactly the same thing happens with sound. If you bounce tightly beamed treble (sound) off a flat angled reflector, the reflected sound still forms a beam. The laws of reflection say that ("the angle of reflection equals the angle of incidence"), and my ears said the same thing. I heard no useful reduction in "beaming".

Scratch the flat angled reflector idea, then. It just doesn't work. :(

So I tried placing a cardboard cone, point down, walls at roughly 45 degrees to the horizontal, above the upward-firing speaker. This did spread the treble through 360 degrees, seen from above. But if you moved your ear up or down, it was beamed as tightly as ever. :(

Great horizontal dispersion, but lousy vertical dispersion. AND a dull-sounding speaker, because most of the treble energy was being sent away in all the wrong directions, so you only got to hear a little of it - so the speaker sounded dull and lacking in treble.

On to the next claim of the builder:
Glenn Demichele said:
...there's no cabinet vibration because it's a cylinder...
This isn't even remotely true. I had hoped the cylindrical shape would reduce cabinet variation, and maybe it did a bit, but placing my hand on the cardboard tube revealed plenty of vibration happening.

This really isn't a surprise. I think Mr. Demichele is imagining sound waves spreading out like perfectly circular rings on a still pond, so that they all hit the cardboard tube at the same time. But this isn't what happens: first of all, the sound pressure varies dramatically as you go up or down the pipe (organ pipe modes), so the ends of the pipe vibrate much more (or much less) than the middle of the pipe, depending on the frequency of the sound.

Secondly, as the frequency continues to go up, you get different radial acoustic modes too: some of them try to "oval" the tube, sucking in two opposite sides while the bulge out the in-between points, so the tube looks like an oval from above.

Of course this ovalling force is fluctuating at the same frequency as the sound, so the tube is getting sucked and pushed between ovalling one way, and a split second later, ovalling the other way. At one instant, the north-south axis of the oval (say) is smaller than the east-west axis; a split second later, the north-south axis is bigger, and the east-west axis is smaller. There's lots of vibration, in other words. :D

But for e-guitar, cabinet vibration may not be a problem. Plenty of guitarists seem to like thin-walled pine cabinets, which vibrate like crazy.

On the plus side, Demichele's cardboard guitar speaker is certainly light and inexpensive (though definitely not very durable.)

Incidentally, I tried a lot of oddball loudspeaker experiments in my twenties, not just the cardboard-tube, vertical firing speakers. I tried clay flower-pots as enclosures (they ring like steel!), plastic buckets (they sound horrible), and styrofoam boxes (quite nice-sounding, a little bright, but quite nice.) I even made a pair of midrange horns using cardboard for the curved walls of the horn, and Styrofoam for the two flat walls. The horns worked - they made the little 4" driver amazingly louder - but I didn't love the sound, so they went in the trash (there was no recycling back then!)

By the way, Home Depot has cardboard tubes in several sizes (8", 10", 12", maybe others) for pouring concrete columns. Building Demichele's speaker is an easy experiment to try if you're so inclined. That way you can find out if you agree with my experience, or not.

Demichele's idea is not new, by the way: I first saw a vertical-firing speaker in a cylindrical tube enclosure, with an angled reflector on top of it, in Gilbert Brigg's 1948 book "Loudspeakers: The Why and How of Good Reproduction"!


-Gnobuddy
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> midrange horns using cardboard for the curved walls of the horn, and Styrofoam for the two flat walls. The horns worked - they made the little 4" driver amazingly louder

I designed and built a mid-band horn for a special use. Lowest power for long throw and high intelligibility even if it was nasty. It was 2'x4' foamcore panels to a wood throat. The angle was so small (26 degrees) that exponential taper was just a few inches, then 44 inches of pyramid to a 24" mouth.

The 4" cone's chamber against the 1" throat formed a 900Hz lowpass. The $13 cassette recorder "power amp" was modified with a 5KHz 1-pole low-cut. The result was near-flat 700Hz-3kHz falling beyond at 600Hz and 4kHz. However the 250mW amp was expected to clip. This throws-up highs at roughly +6dB/oct. But the -6dB/oct slope of the speaker made it not real nasty. And the pre-emphasis of sibilants over vowels, both well-clipped, gave great intelligibility on loud announcements.

I have a larger version in mind to throw sound a mile through the woods. However the neighbors who inspired it seem to been quiet this year.
 

Gnobuddy

Member
2016-03-01 4:10 pm
...I share walls with my neighbours...
I do too, but have the opposite problem. All the neighbours are as quiet as the grave, which makes me very aware about keeping the loudness down when I play guitar.
...and could only wish I had the same isolation.
Me too. Unfortunately nobody left me a couple of million bucks, which is what you need to buy a free-standing home on a tiny patch of land anywhere near here.

There are parts of Canada where you can get isolation, certainly, but those are also the areas where there are no jobs to be found.


-Gnobuddy