Gilmore Audio planars revealed

I've been out of town and/or very busy the past couple of weeks, so just now got back to this thread.

Sellerwithintegrity, I apologize for mistakenly typing "3 mil" as the thickness of the kapton diaphragms; of course it should have been ".3 mil".

Lugnut, just for the record, "Duke" and "Sellerwithintegrity" are different people - "Duke" posts under the Audiogon moniker "AudioKinesis", and over at the Asylum under "Duke". So please don't give me credit for "taking the heat" here - I wasn't even in the kitchen!

The reason for using a weak magnet/low Qts woofer in a dipole was well laid-out by Bob Carver years ago, and goes something like this: Dipole cancellation results in a 6 dB per octave rolloff in the bass region. This is in addition to the inherent shape of the woofer's response around resonance, as predicted by its Qts. If we use a woofer with a carefully chosen very high Qts (weak magnet) and appropriate resonant frequency, taking the baffle size and shape into account, then the natural response "hump" of the high-Qts woofer can compensate for that 6 dB per octave rolloff. The baffle shape and woofer parameters can be optimized together to approximate an ideal target response.

I would expect that at very low frequencies the output of the Gilmores is displacement-limited long before the approximately 2000 to 4000 watt peak thermal limit is reached, as predicted by the spreadsheet on Siegfried Linkwitz's site (mutual coupling between drivers and large vertical path length will give greater bass output than predicted by the driver area and baffle width alone, but by how much I cannot say). Most loudspeakers will displacement-limit at low frequencies well before their thermal limit is reached. For example, I've played around with JBL woofers that have a thermal power handling limit of 600 watts RMS, but are displacement-limited to less than 1/10th of that figure below 25 Hz in a vented enclosure. Nevertheless, JBL is in compliance with AES power rating conventions to claim a power handling capacity of 600 watts RMS (which would imply far greater power handling on peaks, though they don't specify the peak power figure).

The example given above of the "wave" in the football stadium is actually a transverse (or shear) wave, with particle motion perpendicular to the direction of wave propagation. Sound waves are pressure waves, and most importantly are set in motion by physical displacement of the air particles. The mass of the air displaced by the diaphragm movement can be calculated and compared to the mass of the diaphragm itself.

While I wasn't the one sitting in the hot seat this time around, I hope to be there one day. I've dreamed of bringing out my own loudspeaker for some time now, and will probably be disgustingly enthusiastic about it when that day finally comes. In the meantime, I'm looking forward to getting to know the Gilmores first-hand.

Best wishes,

Duke

Planar,

I'll take a shot at some of your questions, though I don't have answers for all of them.

1. horizontal dispersion. you say -3dB at 60 degrees dispersion (plus or minus 30 degrees, for those reading). At what frequency?

a: The horizontal radiation pattern will be the classic dipole figure-8 pattern at low frequencies, and will still be quite wide when we cross over to the ribbon (okay, technically planar magnetic) high frequency driver because its diaphragm is fairly narrow. It looks to me like the ribbon's diaphragm is maybe an inch and a half wide. The radiation pattern will become progressively more narrow with rising frequency, and will be about 60 degrees wide (-3 dB) at roughly 9 kHz. This estimate is based on an assumed diaphragm width of 1.5 inches.

2. vertical dispersion...

a: The vertical coverage at high frequencies will be effectively limited to the height of the diaphragm (with maybe a few degrees of vertical beam widening), and will be oriented perpendicular to the diaphragm plane. So by tilting the speaker back a bit, you improve the high frequency coverage within the listening area. Note that because the bass drivers are in a vertical line array, you won't have the height-dependent tonal balance phenomenon typical of point source woofer/line source planar hybrids, as long as your ear is within the ribbon's vertical "window".

3. comb filtering. you said, "Whatever comb filtering effect is created by our topology, it is less than anyone elses -- not more because of it." the accuracy of that statement is dubious, but that's beside the point. a full range line source has its element(s) aligned without timing anomalies. remember the old beveridge electrostat? before my time and it's been quite a while since hearing a pair, but that's a full range line source. some people might consider the gilmore to be planar.

a: I presume you are referring to horizontal comb-filtering caused by the vertical side-by-side driver configuration, in the crossover region. At the crossover frequency of 200 Hz, the woofers and ribbon are less than 1/4 wavelength apart. Any comb filter effects due to their lateral spacing will be of negligible significance. The Gilmores' approximation of a full range line source is pretty close, and in theory compares favorably with other commercial multi-driver line source approximations that I'm familiar with (Magnepan, Pipedreams, Wisdom Audio, Newform Research, Martin Logan hybrids). The Beveridge and Sound Lab single-driver line-source speakers of course have an inherent advantage in coherence over a multidriver speaker, but in practice a less than 1/4 wavelength offset is unlikely to be audible.

4. will post a followup on doppler distortion.

a: I do not know what the audibility thresholds for doppler distortion are at different frequencies. Paul Klipsch maintained that doppler distortion was quite audibly significant, and I have heard full-range single-driver loudspeakers lose articulation on complex passages at high volume levels. Note that for a given sound pressure level air volume displacement must be quadrupled each time we go down one octave, so doppler distortion can be minimized by using very very large full range diaphragms (Sound Lab and Beveridge) or by using a multi-way system and choosing crossover frequencies that free the midrange and/or treble driver from long excursions at the anticipated loudness levels (accomplished by many, many manufacturers to a greater or lesser extent).

I have no comments on the binding post/crossover topic.

Of course I'm neither an engineer nor a Gilmore owner (at this point), so don't take my answers here as gospel or even attempted gospel. They're just my $.02 worth.

Best wishes,

Duke

Duke,

The example given above of the "wave" in the football stadium is actually a transverse (or shear) wave, with particle motion perpendicular to the direction of wave propagation.

Of course it is. I noted that waves could be classified into three types. My point was to indicate that the particles do not move from their equilibrium position, but vibrate about it.

Sound waves are pressure waves, and most importantly are set in motion by physical displacement of the air particles.

That's close enough.

The mass of the air displaced by the diaphragm movement can be calculated and compared to the mass of the diaphragm itself.

On the face of it, this seems like a reasonable statement, but please describe how we should do this.

We surely count all the molecules of air in contact with the diaphragm, since they are certainly disturbed. They in turn disturb their neighbours - do we count those? They have neighbours too. Where do we stop?

If we only count the molecules in contact with the diaphragm, is that the mass we seek? It surely would be lighter than the membrane.

Do we reason like this - we consider the maximum excursion of the diaphragm (this would be for the loudest possible volume at the lowest frequency the membrane can produce) and multiply this by the area of the membrane and come up with a volume. Now we consider the density of the air (depends on temperature and humidity) and thus we have a mass. Is this it?

But the speaker hardly ever plays at this volume or at that low frequency, so does the statement "the mass of the membrane is lighter than the air it moves" have to be qualified with volume and frequency specifications? This was not done.

The problem I see is that the phrase is colourful and exotic, but not reasonable. It's a turn of phrase, an advertising slogan. That I don't mind, as the world of hifi is filled with such slogans. But it's not truth.

Regards,

Duke & Sellerwithintegrity,

Sorry for addressing my last post to you Duke. It was, in fact meant for Sellerwithintegrity.

Hello Metralla -

Thanks for taking the time to write back and clarify your stadium wave post. I'm a bit slow sometimes, and didn't understand that the point you were making was about the particles returning to equilibrium position, which of course the familiar stadium wave illustrates perfectly.

On the matter of truth versus good ad copy, that's a tougher issue. I agree that if Gilmore Audio's claim of a diaphragm lighter than the air it displaces is true, then it can only be true at certain frequencies and volume levels. I'm not engineer enough to know the measurement and calculation protocols behind such a claim - maybe they're straightforward, and maybe not. The ideal would be a complete set of data qualifying all such claims, though in the end we'd still have to rely on our ears to tell us which set of compromises sounds the most natural under our anticipated listening conditions.

Obviously, you think things through pretty thoroughly. If I should ever come out with my own speaker, I'll run my claims by you first!

Hello Lugnut,

Again thanks for taking the time to write back. No problem at all with the name thing; I just didn't want people to think I was posting under two names, nor to take undue credit for doing hard time in the hotseat.

Best wishes to both of you.

Duke