“Your definition makes me thing of the argument of Peter Walker of Quad about electrostatic loudspeakers:
Imagine I am speaking towards you. Now put a thin sheet of cellophane between us - you will not notice it on the sound. Next remove me and make the cellophane sheet vibrate exactly how I was making it vibrate.
You have perfect sound reproduction!”
Microstrip, It is weirdly curious you say that.
The company I worked for was a NASA contractor, my job was working on sound sources for acoustic levitation. My Boss was an acoustician who had come from England after the war. He had been part of the team at Mullard that developed sonar transducers during the war etc.
Anyway, he was a hifi buff and loved speakers too and after he found I built my own electrostatic speakers, it wasn’t long before he asked me to fix his ESL-63’s. By fix, he meant neutering the spark gap limiters that interrupted his listening.
I took a deep breath and said sure.
These were different than any of my flat panel speakers AND they did a cool thing, it “sounded like” the origin of the sound was right behind the actual speaker.
The confusing part for me was why, with my large panels, the sound was somewhere off in front but this was different and the answer was slowly clear once the grill was removed, THIS was no ordinary speaker.
This speaker produces / radiates a simple spherical segment, a part of a sphere and so your ear hear that as a source somewhere behind the actual driver. Thinking of your clear film experiment, consider that if you were behind the speaker, the sound radiates from your mouth in a forward in all directions, a sphere, spherical radiation. Because of the obstruction of your head, less sound goes the rear.
AS the sound reaches the film, it presses the center first (being a spherical disturbance) and then progressively to the edges.
The ESL-63 uses a flat film that is divided into concentric rings. Pict 1
http://www.audiodesignguide.com/esl/esl63ph.jpg
The signal to each ring is electrically delayed using a “delay line” or multiple all pass filters as shown in picture . Here, the circuit delays each ring a little bit, enough to “act like” the progressing of a spherical wave though the element.
http://mark.rehorst.com/Quad_ESL-63/Quad ESL-63 Simulation Schematic.png
The cool thing about this was that the shape of the sphere was set by the time delay and not the size of the source so the behavior was more or less the same over a broad bandwidth. . A problem with all the large ESS panels I made was the frequency dependant directivity, which in turn makes the frequency response change with distance .
Fast forward 15 years, to 12 years ago, I am tasked with designing a speaker for commercial sound where ALL the acoustic problems are much worse and the needed sound power much higher.
THAT single point source is exactly what I wanted to create but in a speaker that could go 10, 20, 30 dB louder and has constant directivity for much larger spaces.
My approach was to progressively assemble the radiation in frequency and time. Scroll down to Fig#5 here;
http://www.danleysoundlabs.com/pdf/danley_tapped.pdf
The staggered “progressively assembled wavefront” design also compensates for the phase shift in the crossovers such that a speaker like the SH-50 can (like an ESL-63) re-produce a square wave, over a broad band. In the SH-50 doing it from fair to near perfect from about 240Hz spanning the low / mid and Mid / high crossover to about 2700Hz and not just in one position in front.
It radiates the same way as the sheet of film excited by a source at the apex, a segment of a point source.
Best,
Tom Danley
Tim
Well I was trying to make people think about ” How large” the difference the imaging is between the room and headphones. Both kinds of transducers have flaws but a big difference is how much direct sound remains relative to the reflected sound that competes with it.
I used to urge people interested in this stuff to set their stereo up outdoors where there are no walls.
Sure the bass is toned down BUT you will hear imaging like you’re not used with them indoors. Then I realized most people aren’t’ crazy like me and will do that so the headphone approach seemed like an alternative..
The problem is the room sound, sounds just like the direct sound, except all the time information is gone as the reflected sound is a later partial version of the actual thing, but coming from another and wrong direction.
