Not Another Unity Horn

[Patrick Bateman]

I've spent a few years studying horns and waveguides, and I've posted a couple Unity horn projects online. (A Unity horn is an invention by Tom Danley that mimics a "point source", which is basically the Holy Grail of audio. A point source eliminates many of the hurdles to achieving good sound from a speaker.)

One unusual aspect to my Unity horns is that they're in my CAR :P

I haven't published any home designs, because if I wanted a Unity horn in my house, I'd just go and buy one. The commercial offerings from Yorkville, DSL, and SPL are well worth the money.

Having said that, I frequently receive questions in my email that go like this:

#1 - Can't you make it smaller?
#2 - Why is it on the dash? Can't you put it in the kick panels?

IMHO, a big waveguide on the dash is the ultimate solution. But it's not the only solution, so I thought I'd explore these two questions in this thread. Instead of posting on diymobileaudio I'm publishing the project here, because the concepts we'll explore are applicable to home loudspeakers.

Once the project is finished, I'll demonstrate how you can get some of the Unity Horn Magic (tm) without actually building a Unity horn.

[Patrick Bateman]

If anyone wants to review my previous Unity horn experiments, here's the URLs:

My first real stab at a Unity horn, in 2006. Sounded excellent, but was hampered by a poorly engineered midrange solution.
http://audiogroupforum.com/csforum/s...ad.php?t=62789

My second try. In this thread I re-engineered the design from 2006, and fixed the deficient midranges. The designer of the Unity horn also revealed some secrets about the design.
Another Unity Horn

Here's where I actually applied what I learned from the 2nd thread. I've been refining this Unity waveguide for a few months now.
http://www.diymobileaudio.com/forum/...oundstage.html

[Patrick Bateman]

OK, the goal of this thread is to get some "Unity goodness" without building a Unity horn.

The "secret sauce" is a trick that Andy from JBL showed me a couple of weeks ago, a method of controlling directivity WITHOUT using a waveguide.

Here's where Andy showed the trick:

http://www.diymobileaudio.com/forum/...tml#post781746

It's a lengthy read, but it boils down to this:

The polar response of a loudspeaker array narrows at certain frequencies. Normally, this is a very VERY bad thing. BUT we can use it to our advantage, by spacing the elements to narrow the response in a calculated way.

[Patrick Bateman]

Using the math from the JBL article, I created a horizontal array with a waveguide and a midrange. In a Unity horn the midranges are ON the waveguide. In this design, they're NEXT to the waveguide. Normally a horizontal array is a terrible idea. Let's see how it works here...



Here's a pic of our array, head-on. Note the midrange output is coming from a frustrum, just like a synergy horn. This gets us a bandpass response, without using a crossover. It also lets us get the midrange very close to the tweeter. (Always a good thing.)



The back of the baffle. That's my fav compression driver, the BMS 4540ND. The woofer is a Misco JC5RTF, a closed back midrange driver.



Side view.



A detail of the ports. They're 2.25" apart. It would have been nice if I could have spread them out further, but I couldn't do it without creating a second baffle, the woofer is just too small. Another option would be using two woofers, or even four.

2.25" corresponds to 6khz; that will be important for our calculations.

Also note the ports are a frustrum, like in the Synergy horn. In the Synergy horn the frustrum goes the other way (ie, bigger on the inside, not the outside.)

Either way will work; I did it this way because it reduces port turbulence. Smaller holes on the outside are good in a Unity horn, because they reduce reflections off the compression driver. Not a problem here, because they're spaced 5" apart.

[Patrick Bateman]

Here's the measured polar response of the device. Let's start with the midranges:



Red is on axis. Green is 22.5 degrees off axis. Violet is 45 degrees off axis.

What we see is well...

They all look identical. Hmm...

I didn't expect that, I was hoping for some directivity. The response narrows at 6khz and 1400hz, but not at 3khz, where we would expect it to.

Here's my hunch -

Because of the bandpass box, the response is falling off by 1500hz. So the phase response is changing, and that's making our directivity difficult to predict.

The solution?

Space the holes further apart, so the response narrows where the woofer has plenty of output. For instance, space the holes six inches apart, instead of 2.25" apart. That will create a 90 degree lobe at 1200hz instead of 3000hz.



Here's a simulation with the holes 2.25" apart. Note that it's omnidirectional from 2khz and down. Above that point, the on-axis response begins to narrow, with a 90 degree lobe at 3khz.



Same sim as above, but now the holes are six inches apart. Now the response is narrowing dramatically in our passband, with a 90 degree lobe at 1125hz. There's also a big peak off-axis at 2250hz, but that won't affect us much. It's off axis AND it's 18db down. It will be masked by the output from our tweeter.

[Patrick Bateman]

If anyone is interested, here's the polar response of the BMS compression driver on the MCM waveguide. It's nothing to write home about, the MCM waveguide isn't anything special. I used it because it fit.



Here are both on the same graph. The Misco has silly efficiency in a bandpass box. On a baffle it's efficiency is 90db, but it appears to jump to 102db in a bandpass box. The BMS has an efficiency of 114db.

I should get the voltmeter and figure out exactly how efficient it is, but I am too lazy

[Patrick Bateman]

Based on the previous experiment, I'm going to take a second stab at it.

The first one only took about 3 hours to build and measure, so hopefully I'll be able to post results in the next day or two.

Here's how we'll make this better:

#1 - Get rid of that crummy MCM waveguide. The new waveguide will have a coverage angle of 120 degrees wide by 60 degrees tall. (An average of 90 degrees.)

#2 - Use a dome tweeter. I'll use a Vifa XT19 because it's small enough to put it right on top of the Misco midrange. Even the tiny BMS compression driver will be difficult to mount that close.

#3 - Move the holes 6" apart instead of 2.25" apart. That will create a 90 degree lobe at 1200hz, right where we want it.

#4 - Make it coaxial. A coaxial arrangement improves the off-axis response.



Here's an illustration of how it will look. The top is the front view, the bottom is the side view. Basically it's a Misco woofer in a bandpass enclosure, with a dome tweeter mounted on top, and a waveguide controlling directivity on the dome.

[mwmkravchenko]

This I'm keeping tabs on. You are one tenacious dude Patrick.

Mark

[Patrick Bateman]

Originally posted by mwmkravchenko
This I'm keeping tabs on. You are one tenacious dude Patrick.

Mark

Mark, it's Bateman.

Patrick Bateman.

You're my lawyer so I think you should know - I've built a lot of speakers. Some two-ways, in an apartment uptown, some horns, maybe five or ten, a waveguide I made in Tacoma. I left her in a garage, near Lake Oswego. I finished my last subwoofer in black.

I guess I've built 20 speakers, maybe 40.

Tonight I just, well, I had to build a lot of speakers and I'm not
sure I 'm going to get away with it this time - I mean I guess
I'm a pretty sick guy.

So - if you get back tomorrow, I may show
up at Harry's Bar, so, you know, keep your eyes open.

http://www.youtube.com/watch?v=5yflWuv8n6E

[mwmkravchenko]

LOL
To much time in front of the boob tube can kill brain cells!

Making speakers is good therapy. Just stay clear from sharp implements. Although from the looks of some of the holes for the midrange you have been banned from sharp tools already.



Mark

P.S. Not to be taken to seriously obsessive compulsive cabinet maker speaking.