Hi Leon
In an earlier post, in all caps as I recall, you asked “how do they do that?” and now commented “Understanding how speakers work is valuable” and the perceived sound vs what your used to.
I should have replied to your first question but I was very busy on something and sometimes it’s hard to shift gears. I will take a shot at that now.
We hear in stereo, in fact more than that, we can hear height and depth as well as left and right, we hear in 3d.
When we take one microphone and record or measure, we record the pressure information from one point in space.
If you put a speaker on a stick outdoors and play a voice through it and close your eyes, you can normally easily hear not just what direction it is but how far away it is also. Some speakers radiate “something” which makes them easy to identify in depth, others less so.
In the last 12 year I have been working on these horns, I found that the closer they became to one single source, the more difficult it was to hear how far away the speaker was with your eyes closed.
The aural clues that identify the “depth” have to do with how the sources radiate in space and NOT what you measure with a single microphone from one point.
So one could say my goal has been to make a multi-way speaker which radiated as a single source in time and space.
To answer “how do they do that”, it’s simple looking back on it.
All you have to do is make sure that the sources are about a quarter wavelength or less apart where they are to combine with each other or an adjacent frequency range. This means, like closely spaced subwoofers, they will combine coherently into an acoustic point source and radiate equally in all directions.
Then, if you don’t radiation in all directions but you also want the frequency response NOT to change with distance, then you want a constant directivity radiation. I chose the CD horn as being only one of very few ways to accomplish the desired CD effect.
A horn has a “high pass” filter effect related to how fast the horn path is expanding, for example a 30 hz horn has to expand at a rate where it doubles it’s area every two feet or so.
If you look at a conical horn, one can see that the area expands very quickly at the throat and slows as you progress towards the mouth. This means the horn has a different “low corner” depending where one taps into it. Thus, the horn is divided up into regions where the highs, mids and lows couple into it.
If you look at the picture Ivan posted, you can see how this is done.
The high frequency driver is at the center and radiates an expanding spherical segment wave front into the horn. As the frequency falls, it crosses over into the mid driver who’s energy is introduced into the horn through the four smallest holes at the rear. The four sources add together into one point source and at the frequency they combine with the high driver are all less than a quarter wavelength apart. As you can see, the hf driver is behind the mid driver. The low frequency drivers are next and occupy a larger dimensions.
In time, the signal emerges first from a high pass filter, then from the low pass filter. By using a non-traditional crossover and the physical offset in that position, one can produce a passive crossover that doesn’t have the phase shift normally present.
The combination of driving the horn at a small dimension makes the beam width constant through crossover, no lobes or nulls anywhere and fixing the time, means that most of the Synergy horns will reproduce a square wave anywhere form good to fair, over a broad band, not particularly dependant on position in the pattern.
If you look at the CLF file for the SM-60, you will see it has a very consistent pattern. As a result of being “CD” its spectral balance doesn’t change with distance or position while in the pattern.
So how does that sound?
Imagine you set off a large fire cracker, it is a short energetic impulsive signal that has a wide frequency spectrum. Imagine you recorded it and were able to spread that out by spectrum, divided into octaves and each octave delayed by a different amount, with the lows delayed the longest.
This is what speakers generally do, they have an “all pass” response in time and occupy different places in time depending on frequency.
The drivers on most speakers are also too far apart to combine coherently so if you look at a simple polar plot, one finds plenty of lobes and nulls in many directions and I believe these are in part what give the speaker a depth identity as well as excite he reverberant field (reflected sounds that compete with the direct sound). The goal with the directivity is to direct the sound to where the people are to minimize the reverberant field.
Reproduce, that firecracker with these issues and it will still sound like a fire cracker but now it is dynamically different, much subdued.
Now, if you look up the work by a hero of mine Dick Heyser and Time Delay Spectrometry, you will find the path that lead to your SM-60.
What you hear is one of my latest attempts to make a real wide band constant directivity acoustic point source. Fwiw, if you are inclined to try this, set the sm-60’s up in your living room and try your favorite recordings and old ones. That lack of depth identity makes for a much stronger stereo image than most hifi speakers can produce and the pattern control indoors is helpful.
For fun, try the fireworks recording at the bottom of the download page. This was from a couple years ago, I have a new one to put up from this year but it’s not there yet.
Anyway, I hope our pricing also makes them attractive relative to what they do, the mantle “you just can’t pay more, for sound this good” is taken so we went a different way. I am glad you’re enjoying the SM-60’s and subs, have fun and good music.
Best,
Tom Danley
