SP Technology Timepiece 2 & Continuum AD

[noah katz]

Parts Express has a 12" waveguide for $14, it says "useful acoustic loading down to 1200 Hz."

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=270-312

Am I just optically illuded? All these waveguides look like bowls to me, i.e., concave, whereas everything I'm reading says they're conical.

"Differences in directivity control. The more directivity, the longer and larger a "waveguide" or horn mouth will have to be."

So mouth diameter is not necessarily correlated with LF cutoff?

"Geddes' book on acoustics design has all the messy math."

It's looking like I'm going to have to bite the bullet.

How many equations does it take to define the waveguide dimensions once you've chosen coverage angle and LF cutoff?

Thanks

[dwk]

Parts Express has a 12" waveguide for $14, it says "useful acoustic loading down to 1200 Hz."

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=270-312

Am I just optically illuded? All these waveguides look like bowls to me, i.e., concave, whereas everything I'm reading says they're conical.

here is a shot of the profile from the side

http://img521.imageshack.us/img521/1006/rd3wc.jpg

Definately not concave. The 'conical' really refers to the radiation pattern - being conical rather than exponential or tractrix means that it is aiming for 'constant directivity' as the design goal rather than acoustic gain. As Geddes Oblate Sphreoidal design illustrates, curved sides that gently transition from the throat to the mouth are required for uniform directivity - discontinuities cause irregularities in the coverage pattern.
I'm slightly concerned about the abrupt corner near the throat, but I'm guessing/hoping that's only on the outside - the inside pics make it look like it transitions smoothly to the throat.

"Differences in directivity control. The more directivity, the longer and larger a "waveguide" or horn mouth will have to be."

So mouth diameter is not necessarily correlated with LF cutoff?

Yes, but somewhat more indirectly. LF support is determined by both the size of the mouth as well as the depth of the horn. The basic rules of thumb are that the LF cutoff* is where the mouth circumference is ~1 wavelength, OR where the depth is ~ 1/4 wavelength.

* waveguides don't really have a 'cutoff' in the same way that classical horns do. Acoustic gain simply falls back to direct radiator levels.

"Geddes' book on acoustics design has all the messy math."

It's looking like I'm going to have to bite the bullet.

How many equations does it take to define the waveguide dimensions once you've chosen coverage angle and LF cutoff?

Thanks

Building/designing waveguides is a fairly practical/pragmatic activity. I don't think Geddes book is necessary if that is all you want to do. In fact, his famed 'OS waveguide' equation doesn't even appear in the book.

The 'hornresp' program is a good tool for getting the basic dimensional parameters established. The acoustic gain of any conical-like device is generally close enough that you'll get into the ballpark by modelling a conical in hornresp. For safety, I'd allow at least an extra half octave on the low end, as my understanding at the moment is that waveguides begin to store energy as you approach the LF limit - I have yet to create anything that has had a long enough life to actually sit down and measure well, though. Then, once you have the basic dimensions, you can create the appropriate curve for the sides (OS or other)

I have some 8" and 12" PE waveguides on the way to play with. Am I the only one looking at these as a chance for a potentially easier Unity implementation? Pity they don't have a 15" version - the 12" is really too small for a Unity to work properly, but it should serve as a 'good enough' testbed for a POC. It looks like a longer conical extension could be grafted on later.

[noah katz]

"here is a shot of the profile from the side
http://img521.imageshack.us/img521/1006/rd3wc.jpg"

Thanks, dwk.

Sure enough, that's not concave. But that's a compression driver; just to be sure, are the waveguides people are using with domes all convex as well?

"The basic rules of thumb are that the LF cutoff* is where the mouth circumference is ~1 wavelength, OR where the depth is ~ 1/4 wavelength."

For 1 kHz (WL = 13.2"), that comes to 3.3" deep and 4.2" dia.

The depth sounds about right, but that sounds way too small for the mouth, unless you add on 2" on the radius for blending to the baffle.

"In fact, his famed 'OS waveguide' equation doesn't even appear in the book."

What?! Where does one find it?

Thanks very much

[JoshK]

What's a Unity? POC?

I read somewhere that, the freq down to which pattern directivity is controlled is

Fc= 10^6/(theta*X)

where: theta is the included angle of the waveguide;
X is the mouth width of the waveguide [in].

*I haven't double checked this.

[dwk]

What's a Unity? POC?

Unity = Tom Danley's Unity Concept horn system. This is an approach where midrange drivers are placed along the side of the horn/waveguide and fire into it via entry holes, so that you kinda/sorta get multiple horn setups firing through the same mouth. Requires some xover trickery to get it to work right, of course, and from my little personal experience and the anecdotes of other folks, it takes many iterations of experimenting with the specific location of the entry holes, size of entry holes, etc. Having relatively cheap waveguides to experiment on can greatly reduce the labor involved, which is a good thing.
The payoff on a Unity is that if done right, you effectively get a single acoustic source over a very wide bandwidth.

POC = proof of concept. ie prototype

[oneoldude]

I'm slightly concerned about the abrupt corner near the throat, but I'm guessing/hoping that's only on the outside - the inside pics make it look like it transitions smoothly to the throat.

I have a pair of horns that look exactly like the one shown above. I got them from SteelSound. But they seem to be the same series as the PE horns. I can tell you the inside of the horn is not discontinuous. It is smooth and fair from the mouth to the throat. The irregularity is only on the outside.

[noah katz]

How's the waveguide work going?

[EdL]

bump:

"How's the waveguide work going?"