Geddes Bandpass Subs and the Multi-sub approach

[seanny]

Originally Posted by gedlee

The easy answer is

I appreciate the easy answer.

I like IB for the easiness of adding multiple drivers. However, I don't like the idea of pressuring the whole attic.

With 4th order BP, I could put multiple boxes in the attic and have ports extending through the ceiling into my room. The bad part is that I don't know much and haven't gotten any good modeling for it.

Most of my friends live in 2 stories houses. If there are boxes the could fit above the ceilings, either in the attic or below the 2nd floor, they might adapt the idea of multiple subs. If I tell them to buy multiple floor subs, their wife would ban me.

[gedlee]

Originally Posted by pjpoes

Dr. Geddes, is it accurate to say though that only a 4th order bandpass and a sealed sub woofer is acting as a monopole bass source. That a ported subwoofer and a 6th order or 8th order, etc. would also act as a dipole radiator below resonance?

Correct

Originally Posted by DougSmith

An IB could do that as well I think.

Correct as well, but true IB is pretty rare and hard to do.

[gedlee]

Originally Posted by seanny

I appreciate the easy answer.

I like IB for the easiness of adding multiple drivers. However, I don't like the idea of pressuring the whole attic.

With 4th order BP, I could put multiple boxes in the attic and have ports extending through the ceiling into my room. The bad part is that I don't know much and haven't gotten any good modeling for it.

Most of my friends live in 2 stories houses. If there are boxes the could fit above the ceilings, either in the attic or below the 2nd floor, they might adapt the idea of multiple subs. If I tell them to buy multiple floor subs, their wife would ban me.

Yes, I agree that IB poses some real problems with noise control - the backside sound being noise to anyone else. The bandpass solves this problem and protects the driver. The ports can be quite large - which is good - when they are not in the room.

SPEAK (on my website) is now free and it does a really good job of modeling bandpass enclosures.

[seanny]

Yes, I've been playing around with speak. Still need to RTFM.

[DougSmith]

Originally Posted by gedlee

Yes, I agree that IB poses some real problems with noise control - the backside sound being noise to anyone else. The bandpass solves this problem and protects the driver. The ports can be quite large - which is good - when they are not in the room.

SPEAK (on my website) is now free and it does a really good job of modeling bandpass enclosures.

So, I thought an interesting alternative to an IB with VLF response would be a large bandpass box located in the "external" space (no need to worry about how big the box is) having an external port (for the front chamber) that comes up to floor level. Just put an 8x8" grill over that and you are good to go... high output down to 10Hz or so (depending on the driver, box, and where you put it). Only problem is that you can't move it - so you have to be pretty sure that you have selected a good location beforehand (check it out with another conventional sub at the same location in the room where the grill will be located to make sure you can get a flat response with the sub there).

I guess the other complication might be the port resonance for the large (6" dia) port required. Perhaps four narrower ones grouped together (so the sound can exit from the same area) would avoid that.

[Taterworks]

I find the claim that a vented box cannot pressurize a room at low frequencies to be ridiculous. How does it make sound, if it does not pressurize the room? Room pressurization occurs when the wavelength is twice the longest dimension of the room or more. The pressures inside a reflex enclosure are several orders of magnitude greater than the pressures in the listening space. If you are talking about infrasound below vent tuning, a reflex box does not become a dipole unless the port is directly opposite the woofer. Otherwise it forms an acoustic short circuit, not a dipole, since the term 'dipole' describes a specific radiation characteristic. But a sealed box (or 4th-order bandpass) cannot pressurize a listening space in any meaningful way at frequencies below 1/2 Fc, either, without EQ or modal reinforcement from the room itself.

I'm sure you've encountered this opposition before, so please educate this unwashed mass. How does a small room interfere with the reflex action of a reflex enclosure, inhibiting its ability to pressurize the room within its passband? (I can think of a simple way to test this, and it involves an impedance sweep.)

[pjpoes]

I believe what was specifically stated was that it couldn't pressurize the room below it's resonant frequency, i.e. the pass band. The reason given was because it's effectively a dipole source at that point, and that only monopoles can pressurize a room. I did not interpret this to be a blanket statement that a vented box is always a dipole, nor that it couldn't pressurize a room above the tuning. It's my understanding that it operates as a monopole above the tuning frequency and thus can "pressurize" a room in that range, but below that range it can not.

[Don Hills]

There is widespread misunderstanding of "room gain" or "pressurisation". For example, it is usually plotted as a curve but in fact is constant, or a "straight line".

Go here for an explanation:
modeling room gain

For a sealed speaker, the "room gain" calculation is straightforward - measure the volume displaced by the cone, measure the room volume, calculate the "pressurisation" SPL.

For a vented enclosure, the air volume from the port has to be added to the air volume displaced by the cone. (Or, below resonance, subtracted from the cone displacement.)

As for the effect of room size on the performance of a vented system, you can model the room as an enclosure attached to the "front" (combined port and driver) of the system. You'll find that any sane size of room is going to be so much larger than the system volumes that the effect on the system tuning will be minimal.

So as a practical rule of thumb, a vented system will "pressurise" a room, but only down to the system's resonant frequency. More specifically:
Any system topology that has one side of the driver sealed off from the room (sealed, 4th order bandpass, infinite baffle) will exhibit "room gain" at frequencies well below the system LF resonance.
Any system topoloy that allows both sides of the driver to couple to the room (vented / reflex, 6th order bandpass, open baffle / dipole) will not exhibit significant "room gain" at frequencies well below the system LF resonance.

[gedlee]

The one factor that is missing in the discussion is the room "leak". Room gain assume a zero leak of pressure or at least a leak that is slower and less than the level of the sound. This is seldom the case, since all rooms leak (or we would suffocate). This leak then becomes the dominate factor in the SPL as the frequency goes to DC.

[Don Hills]

True, most rooms are full of holes. Personally, I assume no room gain and treat any that occurs as a bonus - just roll off the bass drive to suit.

But a well designed home theatre / dedicated listening room shouldn't leak very much. (Concrete slab floor, cinder block or brick walls, prestressed concrete beam ceiling, solid doors with gaskets, HVAC ducts long enough to tune to infrasonic frequencies.)

Even without special construction techniques, room gain should still be taken into consideration in many cases. For example, small rooms usually have fewer openings - often, just one door and a window. In the UK, the room will often have a small window, brick walls and heavy timbered floor and ceiling (the ceiling being the floor of a second storey.) And for a given driver volume displacement, small rooms exhibit more room gain than larger rooms. I believe that room gain is a significant factor in the popularity of bookshelf sized speakers in the UK.