Sub excursion vs ESL panel

[Vaughan Odendaa]

Hi guys,

I heard this today from a guy punting ESL's :

"Speed":
Assume all things being equal - ability to play certain Fq range, in-room placement/coupling, etc.; With this in mind, assume that perceived SPL is directly and only related to volume of air displaced.

Assume nearly 1m2 (10000cm2) (Acoustat-M3, perhaps M4 or 2+2, easily) ESL panel, with (for argument sake) max. excursion of +/- 1mm (2mm total movement) (practically, this is "elevator music level" volume on ESLs), Assume a large (12") sub (cone area ~700cm2 - 14 times smaller area than ESL) => For the same volume of air to be displaced, the sub cone has to move +/- 14mm (28mm total movement)

Now, assume +/- 2.5mm average ESL membrane displacement, for a very low listening volume; Total Sub cone movement of 70mm!!! (coil in the magnet gap). In order for coil/magnet flux field to maintain the required motor strength (ability to pull back the coil) the length of the pole piece would have to be ... at least 100mm?

Now, with all this in mind...
if there are subs out there capable of 70mm excursion within same time it takes ESL membrane to travel 2.5mm (the "speed" is the essential requirement for different sound sources to sound at least similar, without altering the original instrument timbre to the point that a bass sounds like organ, and piano like bass...),
.. and within the financial reach of the mere mortals,
.... I'd love to know about them!"

For the technical guys. Deconstruct this please. A lot of it does not make any sense to me. Why would a sub need 70mm of excursion to match 2.5mm ESL excursion? How is "speed" an essential requirement for difference sound sources to sound similar?

Does any of the above make any sense?

 

[Steve81]

I can't claim to be an expert on ESLs, but I'm not really aware of any that are known for being "high output" speakers. As one example:

Quad ESL-989 electrostatic loudspeaker Measurements | Stereophile.com

Although the different kind of interaction between a panel speaker and the room can make it sound louder than its measured sensitivity would suggest, the Quad ESL-989 is still on the low side, at an estimated 83dB(B)/2.83V/m. Given its strict 100W power handling, this puts quite a low ceiling on its maximum loudness capability, as LG found.

Suffice it to say, we're not talking about huge output levels here: plugging 83dB sensitivity and 100 watts of output into an SPL calculator translates into 103dB at 1m.

Now lets run over to an xmax calculator:
Piston Excursion calculator

For a 12" driver to produce 103dB @ 1m @ 40Hz for example requires under 7mm of travel (in a sealed or IB alignment), a far cry from 70mm. Plug in an 18" driver, and you can cut that to 3mm. Start factoring in things like boundary gain, and things aren't looking too bad at all for a subwoofer.

Hope this helps.

 

[GranteedEV]

For the technical guys. Deconstruct this please. A lot of it does not make any sense to me. Why would a sub need 70mm of excursion to match 2.5mm ESL excursion?

On paper, surface area is a great thing compared to a smaller driver.

Of course, what that guy forgot to mention, is that the sub is a monopole and the ESL will be dipolar.

Let's say the ESL begins its gradient roll off (6db/octave) at, i dunno, 250hz based on its dimensions.

That means that if its theoretical monopole output at 125hz is, I dunno, 110db, it's actual output will be 6db lower due to dipole cancellation - so 104db. And at 63hz, if it's theoretical monopole output is 110db, it's actual output will be 12db lower - so 98db. And if at 31hz, its theoretical output is 100db, its actual output will be 18db lower - so only 82db! So if the monopole has a max theoretical monopole output of about 90db, its actual output is... 90db. The monopole can also utilize a resonator, such as a passive radiator, to extend its response down to its tuning frequency. The dipole relies solely on the diaphragm to produce output, while it has a massive rolloff simultaneously.

Below 30hz, the monopole will normally couple to the room and pressurize the room and have rising output thanks to the room, whereas the dipole will NOT pressurize the room and roll off.

I'm not saying the dipole doesn't have some advantages in creating bass in the room, especially above ~120hz where much of bass is still located. It may not excite certain room modes as identically, resulting in a possibly different (but not necessarily better!) frequency response.

But being dipole, its efficiency and max output are severely limited despite its huge radiating surface area.

 

[Vaughan Odendaa]

Hi GranteedEV,

Thank you for replying! The comments concerning excursion, especially the 70mm excursion claim seemed a little "off" to me.

And if at 31hz, its theoretical output is 100db, its actual output will be 18db lower - so only 82db!

Why 18 dB lower?

 

[GranteedEV]

Hi GranteedEV,

Thank you for replying! The comments concerning excursion, especially the 70mm excursion claim seemed a little "off" to me.



Why 18 dB lower?

because a dipole drops 6db in output every octave depending on its dimensions. 30hz is 3 octaves below 250hz, which I assumed as a typical frequency where this gradient loss begins. For a thinner panel this frequency will be higher, and for a larger panel this frequency will be lower. Maybe it's only 12db down at 30hz. the point is that the dipole cancellation is very real.

Also, all speakers roll off 12db/octave naturally. A sealed speaker will roll off 12db/oct below a given frequency. ESLs aren't exempt from that either. So in actuality, an ESL rolls off 18db/oct eventually, while a sealed speaker rolls off 12db/oct eventually. The 6db/oct rolloff begins much higher in frequency though, depending on the dimensions of the panel.

 

[Vaughan Odendaa]

I don't get the claim made about how "speed is an essential requirement for difference sound sources to sound similar".

 

[Vaughan Odendaa]

because it drops 6db in output every octave.

And monopoles drop 12 dB in output for every octave, correct? So the dipole has a shallower roll-off profile?

 

[GranteedEV]

I don't get the claim made about how "speed is an essential requirement for difference sound sources to sound similar".

I would ask that person to define what "speed" is in the first place. Because it sounds like a load.

 

[Vaughan Odendaa]

Does his comment concerning the 70mm excursion for the subwoofer have any basis in truth?

 

[GranteedEV]

Does his comment concerning the 70mm excursion for the subwoofer have any basis in truth?

If the 12" subwoofer were not in a box, maybe. Just a driver in free air.

But as soon as you put it in a box, it gets a big boost and requires much less excursion.

Not that there's any actual "speed" issue with more excursion.

 

[Vaughan Odendaa]

Also, all speakers roll off 12db/octave naturally. A sealed speaker will roll off 12db/oct below a given frequency. ESLs aren't exempt from that either. So in actuality, an ESL rolls off 18db/oct eventually, while a sealed speaker rolls off 12db/oct eventually. The 6db/oct rolloff begins much higher in frequency though, depending on the dimensions of the panel.

Okay, I was wondering about that, because the 6 dB drop every octave sounded like it was a big bonus for the ESL. But you say that at some point the 6 dB per octave will eventually turn into an 18 dB drop. Is that at the point where the wavelengths exceed the baffle size? I'm just reaching here.

 

[Vaughan Odendaa]

Sorry GranteedEV, you've been a great help thus far, but you mentioned that dipoles don't benefit much from room gain. Why is that exactly?

 

[GranteedEV]

By the way, I just calculated it, and apparently that 70mm number was peak to peak, while the 2.5db number was one-way. It means a 12" sub with 35mm excursion will equal the panel in free air. but put it in a simple box and things change.

Okay, I was wondering about that, because the 6 dB drop every octave sounded like it was a big bonus for the ESL. But you say that at some point the 6 dB per octave will eventually turn into an 18 dB drop. Is that at the point where the wavelengths exceed the baffle size? I'm just reaching here.

That's the point where the dipole begins its natural driver rolloff of 12db/oct. The 6db/oct rolloff occurs when the wavelengths exceed the baffle size and the rear wave begins to cancel the forward wave and vice versa.

Sorry GranteedEV, you've been a great help thus far, but you mentioned that dipoles don't benefit much from room gain. Why is that exactly?

I dunno. John K outlines these things on his website (music and design) though if you're really interested.

http://www.musicanddesign.com/roomgain.html

 

[Vaughan Odendaa]

By the way, I just calculated it, and apparently that 70mm number was peak to peak, while the 2.5db number was one-way. It means a 12" sub with 35mm excursion will equal the panel in free air. but put it in a simple box and things change.

So the monopole figure of 70mm excursion was peak to peak and the 2.5 dB number was one-way? I'm not thinking very clearly here, what do you mean exactly?

 

[GranteedEV]

So the monopole figure of 70mm excursion was peak to peak and the 2.5 dB number was one-way? I'm not thinking very clearly here, what do you mean exactly?

The monopole figure would be much less. I'm guessing around +/- 13mm (or 26mm peak-to-peak). The dipole figure was stated to be 70mm in a sly way when it would be +/- 35mm. Still a high number, of course. And none of this factors in actual driver efficiency.

This TC Sounds driver FWIW has 29mm xmax (or 58mm peak to peak)

Of course, if you're using a vent, like most 12" subwoofers do, then you need even less xmax!

 

[Steve81]

This TC Sounds driver FWIW has 29mm xmax (or 58mm peak to peak)

Of course, if you're using a vent, like most 12" subwoofers do, then you need even less xmax!

And here's what that driver can do in sealed box:

Data-Bass

Edit: I think you'd set the Quad I mentioned earlier on fire if you tried to get it to keep up with this thing...

 

[Steve81]

So the monopole figure of 70mm excursion was peak to peak and the 2.5 dB number was one-way? I'm not thinking very clearly here, what do you mean exactly?

If you're asking 1 way vs peak to peak, a driver travels in AND out. Peak to peak measures the full stroke; one way measures the center of the stroke to the peak.

 

[Vaughan Odendaa]

The dipole figure was stated to be 70mm in a sly way when it would be +/- 35mm. Still a high number, of course. And none of this factors in actual driver efficiency.

Looking at the claim he is saying the sub needs 70mm excursion, not the dipole.

"For the same volume of air to be displaced, the sub cone has to move +/- 14mm (28mm total movement)

Now, assume +/- 2.5mm average ESL membrane displacement, for a very low listening volume; Total Sub cone movement of 70mm!!!"

The dipole he mentions moving at 2-2.5mm and to match this, you need 70mm excursion from the sub.

 

[GranteedEV]

Looking at the claim he is saying the sub needs 70mm excursion, not the dipole. The dipole he mentions moving at 2-2.5mm and to match this, you need 70mm excursion from the sub. Unless I misinterpreted what he said.

He said the following

- A 12" with 28mm of peak-to-peak xmax has about the same displacement as a large ESL with 2mm of peak to peak xmax.

later he said

- A 12" with 70mm of peak to peak xmax has about the same displacement as a given LARGE ESL with 5mm of peak to peak xmax.

What he forgot to say or intentionally did NOT say was the

a monopole sub requires much less displacement than a dipole for the same SPL. The 12" driver is not a dipole when in a sealed box.

That's all there is to it.

 

[Vaughan Odendaa]

So if I'm understanding things correctly (correct me if I'm wrong), you take that 12" woofer, put it in a box, in a room, and that 70mm excursion figure disappears and is significantly less. That would make sense.

You then add in room gain at low frequencies and that 5mm peak-peak excursion ESL panel is starting to look worse for wear? Plus, as you mentioned, the roll-off profile becomes very steep at very low frequencies for the panel, vs the monopole sub, so the monopole sub starts to gain ground at lower frequencies.

So in reality, you take that 28mm total movement, add in all the factors above and it's much closer than he made it out to be? Is that correct?