Geddes Bandpass Subs and the Multi-sub approach

[MethMan]

I am not familiar with Karlson coupler. Don't know.

[Pallas]

Any pics of the updated subs? About how big in diameter are the triple ports?

[gedlee]

Originally Posted by Pallas

Any pics of the updated subs? About how big in diameter are the triple ports?

Sorry, missed this thread altogether.

The thre ports are twice the diameter so that three time greater area and hence 1/3 the max velocity. It was a big improvement in "chuffing".

I do want to get to osting my sub data at some point but I have so many things going on at the moment.

I do not have any pics of the new subs, but its just a change in the port size and locations, nothing else.

[pjpoes]

The ports are 2" and flared, they are placed toward the outside edge of three corners of the box. These are located around the left, top, and right side by design. However, Dr. Geddes had suggested I put wheels or legs on the bottom, and then mount it so that the ports are around three of the sides instead, which keeps stuff from falling down the ports.

I have placed small 1" slugs of open cell foam in each port to even further reduce distortion, slightly reduce the Q of the tuning, and reduce high frequency noise from coming from the ports. Dr. Geddes had warned earlier that if the foam created too much resistance, they would unload the ports, which is why I went with such a small slug.

At some point in the future, possibly by early summer, I will have time to take measurements and pictures. I will say this for now though, these subwoofers have helped me move my system up into a shockingly good level. What I had setup before was a mess by comparison, and the bass was just no comparison.

As I mentioned before though, having some form of external mixing device to balance out the subs is a requirement. Hooking a bunch of amps up through y-adapters from the subout and hoping all works out well will net you pretty bad results.

[gedlee]

Matt

I quite agree

[seanny]

I read that ported sub...a sub with direct radiator and a port... could not pressurize a small room. Is that true?
If it is, how does a bandpass sub with multi ports affects a small room?

[pjpoes]

A port is a resonance device which adds additional poles to the subwoofer circuit, similar to adding more parts to a crossover. It's purpose in a subwoofer is to extend the response by resonating in such a way that it creates minimum motion of the cone, with maximum output at that specific frequency. Normally, say in a sealed box, the woofer would be incapable of producing that level of output at such a low frequency because it would run out of excursion, and thus have very high distortion. This is over simplistic, but explains enough what a port can do. Nothing about this would have any impact, negatively, on a woofer's ability to "pressurize" a room. There are pluses and minuses to using the different approaches.

Sealed boxes roll of at a more shallow amount than ported boxes, which means that while the response around the tuning point of the ported box may be higher than in the sealed box (all else being equal), the ported box will eventually surpass this relative output at an even lower frequency. Due to the gain we can get from a decent room, sealed boxes can often integrate into a room with a flatter response than a ported box, but at really low frequencies, this often isn't much of an issue.

Now as for how a bandpass box works, the resonant port in the front chamber is tuned around the center frequency of the sub's response. Picture a bandpass filter, a box is a filter, normally it's just a high pass filter, but in the case of a bandpass, it has both a high pass and low pass filter. It adds two more poles to the circuit, so to speak. In a 4th order bandpass, like Geddes uses, the rear chamber is sealed, thus allowing the woofer to roll off in a fashion similar to that of a sealed box for the high pass filter. However, with the port, we now have a low pass filter as well, and since this is an acoustic low pass filter, it has an ability that no other design can do. It can filter out the noise produced by the woofer which is higher in frequency than the fundamentals we are interested in, i.e. distortion, and produce a cleaner output. Another important factor related to the bandpass design is the superior efficiency compared with other designs. By changing the relationship of the port tuning and front chamber volume, as well as it's dampening, you can change the response Q and amplitude about the center frequency. This means you trade off bandwidth for output.

In terms of actually answering your question, the bandpass design gives far superior efficiency to most subwoofers. Geddes design is around 90 db's or so at their center frequency. by comparison, when I built my previous designs using TC Sounds woofers and Dayton brand woofers, I have had subwoofers with a sensitivity of closer to the mid to low 80's. This is important since, while watts are cheap, they aren't endless. If you want to really fill a room with bass, having 2 or 3 fairly efficient subs can be a much better way to go than say 1 really high excursion and high powered sub requiring 1000's of watts.

Matt

[gedlee]

Originally Posted by seanny

I read that ported sub...a sub with direct radiator and a port... could not pressurize a small room. Is that true?
If it is, how does a bandpass sub with multi ports affects a small room?

The easy answer is that below resonance the ported sub is a dipole and the bandpass is a monopole. Only monopoles can presurize a room at LFs.

[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?

[DougSmith]

An IB could do that as well I think.