Understanding Synergy Horn Phase

[Patrick Bateman]

I am personally trying to sort out how the phase works in a Synergy Horn. I like making pretty pictures to get a better understanding of how this works, and thought I'd share it with the forum.

At this point, I don't know if I have it figured out, so please take all of this with a grain of salt.

If you have an array of speakers in sealed boxes, you can time align them by arranging the drivers in an arc around your head. The reason that this works is that a well behaved driver in a sealed box has nearly no group delay in it's passband. (This pic is of a car, but the concepts work in a car, a room, a studio or a rock concert.)

An externally hosted image should be here but it was not working when we last tested it.

The Vandersteens that I am listening to work on this principle. So do Dunlavy and Thiel speakers. Phase coherent filters + sealed boxes + physically time aligned drivers.

An externally hosted image should be here but it was not working when we last tested it.

The Danley speakers are phase coherent, but the midrange is closer to the listener than the tweeter, and the woofer is closer still. Lets figure out how this works.

Here's a simulation of two different drivers in bandpass boxes.
The midrange driver is a Fostex FF85WK. The enclosure is tuned to a resonance of 423hz.
The midbass driver is a Dayton ND91. The enclosure is tuned to 163hz. Combined, the two drivers cover about 2.5 octaves of bandwidth, from 125hz to 650hz. In the real world, it will probably be a little bit higher in frequency. (Hornresp exaggerates the high frequency rolloff, as it assumes the drivers are a perfect piston.)

The low frequency rolloff of the midrange is 300hz, and the high frequency rolloff of the midbass is 300hz. This is by design, so that the midrange 'hands off' to the midbass.

Here's the predicted group delay of the two drivers.
At the tuning frequency of the midrange, the group delay is 1.85 milliseconds (25cm)
At the 300hz crossover, both drivers have identical group delay of 2ms (27cm)
At the midbass tuning frequency, the group delay is 3ms (40cm)

We could use DSP to offset the group delay. Or we could physically move them. The animation above shows how far we have to move the midrange and the midbass to compensate for the group delay introduced by the bandpass box. In this example, the midrange is 'pulled' forward by 25 centimeters, and the midbass is 'pulled' forward by 40cm.

Here's a picture comparing the predicted locations of our tweeter, midrange and midbass. Layered on top is the Danley SH-50 Synergy horn.

There are some small differences. The Danley tweeter is closer. I believe this is because the compression driver has a built in group delay. (IE, a dome tweeter would not have a delay, but a compression driver *will*, because a compression driver is basically a bandpass enclosure also.)
The Danley woofers are further away than my sim. I believe this is because it makes the enclosure smaller, and at low frequencies, a difference of six inches won't mean a whole lot. If you look at the Danley SH-25 horn, you'll notice that the woofers are much further down the throat than in the SH-50.

 

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The horn in the last picture and the driver's seat remind me of this thing:

 

[Kris0603]

In this thread Tom Danley provides a lot of insight into how he came about his Synergy horns.

http://www.diyaudio.com/forums/multi-way/208799-western-electric-1928-how-far-have-we-come-last-100-years-73.html

The light went on about 15 years later, it was that expansion rate criteria that was causing the poor loading, at the apex the expansion rate is very rapid, a high frequency horn.
Further down the horn, the expansion was slower and more like a mid horn and a quick estimate and a trip to the garage and the first unity horn was born. A cool thing is that the side mounted drivers connect through the horn though an acoustic low pass filter.
This is the trapped air under the cones and the port mass which are sized so that the low pass corner is somewhat above the low pass electrical crossover. This reduces the inevitable harmonic distortion drivers produce above their operating band when low passed.

You could now use one large CD horn so you have pattern control over a wide band and drive it in sections where it was acoustically suited for the frequency involved, you treat sound according to it’s size.

All the drivers add and combine less than ¼ wl apart in the horn, the result is one source.
Passive crossover normally introduce phase shift, an effect that makes the hf source to speak first and the lf drivers last.

By having the hf driver farthest to the rear and the woofers most forward, some of all of that phase shift can be eliminated using an adapted crossover shape (not ones with names but shapes that do the job).
As the passive xover also deals with the drivers electrical impedance's while shaping the responses, this is the most time consuming part now days.
The design “rules” gradually evolved to the current Synergy horns we make and sell for commercial sound.

One of the fellows here pointed out an interesting thing.
To the degree an impulse response accurately characterizes a loudspeaker, there is a way to hear what the WE, mine and other speaker sound like. It won’t tell squat about stereo imaging but it does seem to capture “what it sounds like”.