Measured monopole and dipole room responses

[Vaughan]

I haven't had time to read the entire thread, but correct me if I'm wrong. Dipoles can't pressurise a room like a monopole subwoofer can.

Dipoles have an 18 dB total drop per octave once wavelengths surpass baffle size? I was having an argument with a guy who claims his big ESL's can wipe the floor with monopole subs and how the roll off is not as steep as monopole subs. I don't understand that. I assumed dipolar operation meant there would be more cancellation, hence a more steep roll-off.

 

[Omholt]

All sound pressurizes the room.

 

[Vaughan]

Please explain. So monopoles and dipoles both pressurise the room? By the same amount?

 

[Rudolf]

Dipoles have an 18 dB total drop per octave once wavelengths surpass baffle size?

It is 6 dB/oct below "once wavelengths surpass baffle size". Add to that 12 dB/oct below the Fs of the driver. You always want Fs to be some octaves below the onset of the 6 dB rolloff.

 

[Vaughan]

Any knowledge of Acoustats? How would you calculate the drop in SPL with this :

Acoustat 2+2 loudspeaker Specifications | Stereophile.com

 

[Vaughan]

It is 6 dB/oct below "once wavelengths surpass baffle size". Add to that 12 dB/oct below the Fs of the driver. You always want Fs to be some octaves below the onset of the 6 dB rolloff.

So compared to a monopole sub, the roll-off (in a room) is more shallow/steep?

 

[Vaughan]

Does anyone know what the typical excursion levels are like in most ESLs? 2mm p-p? 5mm? Far less, more?

 

[Rudolf]

So compared to a monopole sub, the roll-off (in a room) is more shallow/steep?

Above Fs the dipole roll-off is steeper. Below Fs it could be shallower, if the monopole sub is of the BR or bandpass type.
Why don`t you just read the thread?

 

[Vaughan]

I read some of John's articles and I won't lie, I don't understand much of it. But he mentioned dipoles not benefiting from room mode pressurisation, if I understand him correctly. Monopole/dipoles don't excite room modes in the same way.

I would love to read the thread but 57 pages? I have long way to go then.

 

[Rudolf]

But he mentioned dipoles not benefiting from room mode pressurisation, if I understand him correctly.

And Earl Geddes says that there is no pressure gain for monopoles also.
But the guy I would trust the least is the one who claims to be an expert in "wiping the floor with ..." .

Rudolf

 

[Vaughan]

Yeah well to give you an idea (monopole sub vs dipolar ) he said that :

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?

Seems like a completely erroneous comparison given that once you place the woofer in a box, you won't need anywhere close to that 70mm p-p figure. That's just crazy.

 

[gedlee]

And Earl Geddes says that there is no pressure gain for monopoles also.

Rudolf

Why would you say that? because I never said what you claim. If the room is completely sealed then there is pressurization. But show me a completly sealed room! Now if it is not completely sealed then how can it contain a static pressure change?

 

[gedlee]

Above Fs the dipole roll-off is steeper. Below Fs it could be shallower, if the monopole sub is of the BR or bandpass type.

And this is not correct either (having a bad day?) A bandpass is shallower below Fs (it is a monopole hence 2nd order), and a BR is the same because it too is a dipole below Fs (hence 4th order).

 

[john k...]

Why would you say that? because I never said what you claim. If the room is completely sealed then there is pressurization. But show me a completly sealed room! Now if it is not completely sealed then how can it contain a static pressure change?

Same way you can fill a bucket with a hole in it.

 

[Rudolf]

Why would you say that? because I never said what you claim.

And this is not correct either (having a bad day?)

Bad day indeed . I apologize.

 

[OllBoll]

One thing I've wondered when you optimize with say 3 subs a la geddes method, is it only the listening area that gets a good response or is the response good all over the room?

 

[markus76]

You'd need to employ a DBA to get smooth response throughout the whole room.

 

[gedlee]

One thing I've wondered when you optimize with say 3 subs a la geddes method, is it only the listening area that gets a good response or is the response good all over the room?

The response will get better everywhere, but maybe not as good as it will get at the location where it is optimized for. I don't have Marcus unfaltering faith in DBA.

 

[markus76]

I don't have Marcus unfaltering faith in DBA.

No reason to doubt active absorption wouldn't work.

 

[gedlee]

No reason to doubt active absorption wouldn't work.

Thats not the point, I don't deny that it works. I deny "You'd need to employ a DBA to get smooth response throughout the whole room." I don't see any one technique as being clearly superior to another. It may end up that one is more advantageous (but the jury is still out on that), but it would be a relatively small difference not a major one.