Subwoofers and ESL's

[Vaughan]

Hi there,

Just a question concerning excursions and SPL re ESL's vs subwoofer.

I heard this today, from someone :

"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!"

Does any of this sound ridiculous or absurd?

[mullardel34]

Interesting analysis of subwoofer requirements for matching electrostatic loudspeaker "speed"...

I've lived for 20+ years with a pair of the original MartinLogan CLS (Curvilinear Line Source) full-range electostatic loudspeaker (MartinLogan | CLS) and the best subwoofer complement that I found was the Janis W1 + Janis Interphase crossover+power-amplifier. A large part of the reason why the Janis W1 was able to present bass frequencies with a similar sonic "speed" characteristic to that of the MartinLogan CLS was the fact that the Janis W1 was a slot-loaded woofer (see attached diagram), which magnifies the effective velocity of the woofer output by the ratio of the woofer surface area divided by the area of the subwoofer slot (see Nelson Pass's slot-loaded subwoofer project The Slot Loaded Open Baffle Project Article By Nelson Pass). Nelson Pass's slot-loaded open-baffle subwoofer design has the additional advantage of eliminating the inherent resonances of the enclosed cabinet of the Janis W1. Wicked cool work by Mr. Pass...

The Nelson Pass slot-loaded subwoofer is very amenable to DIY construction (some MDF and simple power-tools), so you could experiment with tailoring an implementation to your specific needs.

Just my $0.02's worth...

[Vaughan]

So you don't agree with the logic in the above? I don't understand how "speed" is an important factor for different sound sources to sound the same. I thought frequency response was a more significant metric.

As far as excursion requirements go, what are your thoughts in that post? Does the math add up? Why would a sub need to have a 70mm excursion to match 2.5mm excursion on the ESL? Does the whole ESL panel act like a subwoofer? I just don't understand what he was saying.

[KaffiMann]

As far as I can tell mullardel34 is probably agreeing that the speed of the air is the triggering factor for matching the sound of a sub to ESL. So if you do not have the same area in woofers it does not matter much if you can just increase the speed of the air it moves. The sensitivity will of course be lower than if you had the same area as the ESL in woofers, but how much space and money are you willing to sacrifice? And how loud do you really need to play?

Edit:
clarification

Edit2:
A 12" with 700cm2 area sounds like a bit of a stretch, I know it's not part of the discussion, but a 12" will have at most ~550cm2 area, less area the more excursion it has.

[ßart West-VL.]

That looks a lot like the DeathBox from Decware. It's even 'tunable'.

I'm using one right now until my other big ones are done.
Amazing what it puts out, have been using it for some years now.
It is "fast" but SQ isn't high in my opinion. But this can be because of the low budget drivers (that I've put in isobaric configuration).
Apparently the DBII has a lot more SQ to it. (no experience with it)

Originally Posted by mullardel34

Interesting analysis of subwoofer requirements for matching electrostatic loudspeaker "speed"...

I've lived for 20+ years with a pair of the original MartinLogan CLS (Curvilinear Line Source) full-range electostatic loudspeaker (MartinLogan | CLS) and the best subwoofer complement that I found was the Janis W1 + Janis Interphase crossover+power-amplifier. A large part of the reason why the Janis W1 was able to present bass frequencies with a similar sonic "speed" characteristic to that of the MartinLogan CLS was the fact that the Janis W1 was a slot-loaded woofer (see attached diagram), which magnifies the effective velocity of the woofer output by the ratio of the woofer surface area divided by the area of the subwoofer slot (see Nelson Pass's slot-loaded subwoofer project The Slot Loaded Open Baffle Project Article By Nelson Pass). Nelson Pass's slot-loaded open-baffle subwoofer design has the additional advantage of eliminating the inherent resonances of the enclosed cabinet of the Janis W1. Wicked cool work by Mr. Pass...

The Nelson Pass slot-loaded subwoofer is very amenable to DIY construction (some MDF and simple power-tools), so you could experiment with tailoring an implementation to your specific needs.

Just my $0.02's worth...

[Vaughan]

Sorry, can someone please explain the whole SPL/excursion of ESL and subwoofer, in that example? I'm not getting it.

[weltersys]

Originally Posted by Vaughan

Sorry, can someone please explain the whole SPL/excursion of ESL and subwoofer, in that example? I'm not getting it.

The explanation is based on an inflated excursion of the electrostatic panel.
If the very large panel could indeed move +/- 2.5 mm, it could move a lot more air than a 12" speaker, but electrostatic panels don't have near that much Xmax, which is exactly why they can't produce low frequencies at loud levels.

Specifications on electrostatic panels are often sketchy, if "someone" can show any that have actual sound pressure levels supporting a 2.5 mm Xmax I'd love to see them.

As far as the "speed" aspect, sound waves travel at 1130 feet per second regardless of what speaker they come from.
An argument can be made about lagging group delay of a particular sub design making it sound "slow", but if a speaker can't make enough low bass to satisfy one's needs, who cares how "fast" it sounds?

[Vaughan]

Originally Posted by weltersys

The explanation is based on an inflated excursion of the electrostatic panel.
If the very large panel could indeed move +/- 2.5 mm, it could move a lot more air than a 12" speaker, but electrostatic panels don't have near that much Xmax, which is exactly why they can't produce low frequencies at loud levels.

So you are saying that ESL's typically don't have anywhere close to 2.5mm excursion? That 70mm figure, I don't know if that is the woofer operating in free air or not. Wouldn't putting it in a box, with room gain changing the numbers considerably?

Another thing as well which I thought I would ask is the roll-off of ESL's. Is it true that dipoles have a 6 dB roll-off per octave but this eventually becomes a 12 dB drop, and it gets worse as frequencies decrease?

[Vaughan]

I read somewhere that dipoles don't benefit from room gain compared to monopoles. I don't know the science behind this, or whether it's legit. Perhaps someone more knowledgeable can explain.

[weltersys]

Originally Posted by Vaughan

So you are saying that ESL's typically don't have anywhere close to 2.5mm excursion? That 70mm figure, I don't know if that is the woofer operating in free air or not. Wouldn't putting it in a box, with room gain changing the numbers considerably?

Another thing as well which I thought I would ask is the roll-off of ESL's. Is it true that dipoles have a 6 dB roll-off per octave but this eventually becomes a 12 dB drop, and it gets worse as frequencies decrease?

I'm saying that to my knowledge, even a large (one meter square) electrostatic panel could not keep up at low frequencies below about 50 Hz to a real world 12" with around 13mm displacement in a sealed box, ergo, the displacement is likely far less than 2.5 mm.
There is no replacement for displacement, whether in the form of a large diaphragm moving little or little diaphragms moving a lot, but the type of box the driver is put in makes a difference in sensitivity, that being how much sound level comes out per voltage in.
Open backed speakers are the least sensitive regarding low bass, the roll off rate is determined by baffle or membrane area. As frequencies are lowered the baffle becomes small relative to wavelength and the front and back waves cancel as they become progressively more out of phase.