Bass driver position

[davecooper]

Can someone tell me how to calculate the best position for a bass driver in a tall slim sealed enclosure? I have an internal volume to work to but haven’t yet settled on the exact relative dimensions. However, dimensions of 240mm (W), 260mm (D), 750mm (H) are the sort of figures I am veering towards. How would tapering the enclosure by around 50mm from bottom to top affect this? Crossover will be 100Hz max.

[Xoc1]

Hi Dave
I guess that you are still looking to build a pair of subs to go underneath your main speakers.
First point to make is that at 100Hz (which I feel is quite a high frequency for what you want to do) the wavelength is over 3 metres. With such a long wavelength the positioning of the driver becomes less critical.

[head_unit]

In theory, the location of a low frequency driver on the enclosure and the shape of the enclosure have no effect at all because the dimensions are small compared to the wavelength. In actuality, there are small effects but you need a sophisticated program like LEAP to simulate them.

A more significant effect would be how the position on the speaker affects the woofers position in the room. For instance, if you will put the speakers in a certain spot, where you put the bass driver will affect its distance to each room boundary, which will affect the room loading. Consider the speakers placed back against the wall, and the woofer on the side of the enclosure at the bottom. Now the woofer will see a cancellation dip from the floor AND from the back wall at the same frequency-not good. Might not matter for a sub with low crossover, but you could look at a room simulator program to see.

[davecooper]

"I guess that you are still looking to build a pair of subs to go underneath your main speakers".

Yep, still planning. I am more or less happy with the design I have now. It was only when reading another thread that I realised driver positioning could be critical. My drivers will be close to the bottom.
I know 100Hz is probably a bit high but I put a bit of slack in the figure for the sake of this thread.

The idea about a tapered cabinet was a recent one. By necessity, due to the driver size, the cabinet will be a bit larger than the bookshelf speakers. Tapering could give me a similar size at the top with a small trade off in increased height to maintain the volume. The reduction in parallel faces would also appear to be an advantage although I do realise that the distances between the side walls will not be an issue at these frequencies.

I'm not going to worry too much about crossover frequency until I have built them. I can do a bit of experimentation then. As for room interaction, I think it will be a case of "suck it and see".