Help with sub design choices

[johninCR]

I believe ThomasW was referring to anything that would restrict the airflow. This would only come into play with an extremely narrow manifold. The IB experts report great results without push/pull . I just always try to do what is best in theory with whatever components I have.

Unfortunately, IB isn't possible for me, so I have to go with dipole and always use push/pull in my W baffles. The magnet structure prevents an opening width that is narrow enough to restrict air flow, so I've always considered it a non-issue. I use a lot of cheap drivers and believe at least with them the distortion reduction of push/pull alignment is audible. The difference is slight but I don't think it's just "psycho acoustic".

[Micah Wells]

Thank you again, you have been extremely helpful.

[Hornlover]

johninCR

Well, I won't recommend the 1505 for 18Hz use (but its dandy to 28). For 18, I would use the WPU1807 in a 6th order alignment. With the 6th order ckt., the built in subsonic filter would prevent any information below Fb from overdriving the woofer.

[jaybird]

Does it matter where you put the IB sub. I live in a bi-level with cathedral ceilings and was wondering if I could put the manifold in a wall which would be faceing the t.v.
So the IB would be firing behind me. How would this effect the sound? Would this kind of set up sound good for music?
Sorry if this is confusing.

[johninCR]

Below about 100hz, bass sounds aren't directional, so placement isn't critical as far as identifying where it is originating. It will sound like it's all coming from your speakers. Placement does impact the location of the peaks and nulls resulting from room modes. If you got unlucky and your seating position ended up in middle of a peak or null that was bothersome, you could fix the problem with bass traps, or moving something in the room.

[directservo]

This is Brian from Rythmik Audio. I would like to inject some ideas here to see if they make sense. The goal is to see if there is enough interest in building directservo version of IB subwoofers.

First, I think the driver that is perfect for IB (or dipole) is not one with 20hz fs and Q=0.5. It is easily seen from simulation that such a driver will only provide half excursion at 20hz compared to the excursion requirement at much lower frequency at say 5hz. This will become a problem for amp selection because we need to determine what is the maximum allowed excursion. If you use the exursion requirement at 5hz as the maximum excursion, the output at 20hz will be only half of the max excursion. On the other hand, if we based on the assumption that we will use rumble filter to limit the output below 20hz and hence use a larger amp, the chance for a bottoming is still very high. This is because the rumble filter can only filter out the input signal, not the signal from the amp itself. When the amp is into clipping, it actually creates heavy low frequency contents that are not in the input and rumble filter has no effect on them. If the clipping frequency happens at low frequency that cause large excursion, the driver will very likely bottom out. In another words, there are weak spots for over-excursion in this type of configuration. Therefore, even if one installs rumble filter, the protection is good only if the amp does not clips.

If the driver has an fs of higher frequency (close to 40hz) and Q value close to say 0.9, the excursion will be relatively constant below fs. It not only makes the amp selection easier, it also eliminate the weak spots. I do not imply there will be no excursion issue, it is just less. The ultimate defense will be overexcursion protection circuit that either cut down or cut out the input so that user can readjust the volume.

Another advantage of higher fs and higher q driver is the excursion utilization will be better between 20hz up to fs. The price is efficiency below 20hz is lower which can be made up by large amps.

If the above logic is sound, the next thing to consider is how to EQ the frequency response to what we really want, such as 20hz, Q=0.5? We can use LT, but the issue of long term drifting of mechanical parameters is a problem. Another consideration is without the back pressure such as one in sealed box to linearize the suspension, the distortion due to suspension is likely to be larger. Personally I think techniques like directservo will be a good solution to tackle these two problems.

Other minor detail of the driver includes the moving mass should be kept as low as possible so the reactive force on the wall can be lower. 15" is better than 12", the DC resistance is 2ohm or 8ohms as it is assumed they will be used in pairs.

As usual, any feedback and comments are welcome. If there is enough interested, I may be able to make them into kits.

Brian D.
Rythmik Audio

[johninCR]

Hi Brian,

Thanks for that info and I for one am very interested in the kit.

I'm already already ahead of the game with 30 higher Qts & Fs drivers and how to address overexcursion is a big concern.

[donepate]

Sounds great to me.