Nice planar speakers!
The roll-off characteristic in the bass response of the magnetic planar speakers is shared with the electrostatic speakers, which have a similar construction.
More generally, any open baffle (also called a 'di-pole') speaker arrangement will do the same - a speaker just sitting in open air is the most minimalist example of this. Us tinkerers have probably all done this at one time or another, and noticed how weak the bass is.
And you could put a planar speaker in a baffle, and I just came across a web page of someone who did some experimenting with this. I guess it's not common, as the large size of a panel would require a very large enclosure, and they need to be well braced (heavy, lots of materials) to avoid resonance. But since some audiophiles seem to think more $$$$$$ must be better, I'm a bit surprised that I haven't seen any like that (maybe I missed them).
I guess one gives up something to get the clarity of the higher frequencies. The roll-off in bass would be dependent on the half-width of the speaker as a percentage of the acoustic wavelength of the bass note.
With the speed of sound being 1,100 ft/s and the half-width of the speaker being between 1 or 2 ft, it seems to be the roll-off frequency would be rather high, in the few 100s Hz. Am I missing something here?
I was thinking the effect was much lower, but I can't dispute your calculation. So I was curious, so off to google, and basically, it's more complicated than that (though it's pretty close for a round, open speaker in free air like I described earlier).
Some of the secondary effects are: the panel is much taller than wide, so the bass cancellation is spread out over a range of frequencies; the panel sits on the floor, so the floor blocks the wave on that side; room corners tend to re-enforce the bass, so the two effects could actually produce a smoother frequency response. By the time you add those all up, I guess it just isn't a clear cut drop in bass at 6db/octave as the math might indicate. There was also something about the whole dispersion pattern representing a 'flower petal' shape, which softens some of this I guess.
I've done some 'by ear' frequency testing on mine, using my music synth as a test tone. Maybe I have my notes somewhere, but as I recall there was a very sharp drop at around 30~40~50 hz (I really can't recall exactly), but this isn't a very well controlled test, and it includes room effects. I'm not even sure if the output of my synth is flat - I think I created some test CDs using Audacity sound program to generate the tones, but I prefer my music synth cause it's faster/easier to control the signal than with a remote - and test tones can blow your equipment/speakers if you are not careful. But bottom line, I do get a pretty nice smooth bass, just not that lower octave of room shaking bass. Which is why I've wanted to add a sub, but not motivated enough to actually do it yet. For reference, the low-E of a bass guitar (the common tuning) is 41 hertz.
You might also be amazed at how the brain fills in missing info, if it expects it to be there. I remember ages ago, I was doing a rough test of frequency response in my townhouse, using a good mic and the meters on my tape deck, and a cheap little music synth I had (and still have - a Korg MS-20). I kept thinking something was wrong, because in the space of just a few notes, the meters would drop almost 20 db, then back up again. Then when I listened carefully, I realized that fundamental freq really had been 'sucked out' in that range, but my brain seemed to fill it in based on hearing the harmonics, and apparently expecting it to be there.
A little like, if I said " Knock, Knock ...... there?", your brain will likely fill in the missing "who's"
Here's a decent link:
Designing Loudspeakers - Part 15 Open Baffles and Bass
Now we are getting somewhere, and it starts to all make sense when we look at panel loudspeakers like the Quad Electrostatic where the baffle width clearly isn’t over 2 metres! By increasing just one dimension, say height, and allowing the speaker to couple to the floor and, possibly, the side wall, strong bass output can be achieved to satisfy even the organ enthusiast!
I’m not going to claim that you’ll achieve the same bass power in the room from an open baffle speaker as you can from, say, a transmission line speaker of similar overall dimensions. As my wife pointed out when hearing the reproduction of a Bach organ work on the radio “organ music sounds better in a church where you can feel the power of the low notes”.
I know what she means. That ability to really move the air, so that it has visceral as well as audible impact, is something lacking in most hi-fi systems, box speakers or not. But you do need large speakers to really make it happen.
THE BOX IS MISSING!
So, considering that most people like small speakers in their living rooms, what is the point of pursuing the open baffle? I’ll tell you – it is that the box is missing!
Despite the best intentions of the loudspeaker designer in providing bracing, damping, internal absorption, adding a port, a horn or a quarter wave pipe, there is no getting away from the fact that putting a box behind a speaker just encourages resonance.
Now most of us grow up hearing these box resonances from every loudspeaker we listen to. So we are used to it. In fact I formulated a theory many years ago that we are so used to hearing box resonances that the sound seems ‘wrong’ when you take them away.
That might partially explain why panel speakers are often described as ‘thin’ or ‘lightweight’ by some listeners on first hearing an electrostatic. But when you talk to panel loudspeaker adherents they will be the first to describe box speakers as sounding plummy and coloured.
-ERD50
