4th BP experts?

[danielm]

Cleaning out our storage building and found four of these beast. They are two 15" drivers in a 4th order BP box (JBL MP255 style) I used in a fixed install many yrs ago.

Pulled one out of storage yesterday and checked it outdoors in an open field. Test mic is a rat shack SPL meter placed on ground. The box is 8 ohms so, with 2.83 volts here's what I get at 1m:

30hz 95db
40hz 103db (sounded like I reach xmax here at around 129db, may have been the amp clipping, I failed to check )
50hz 101db
60hz 99.5db
70hz 100db
80hz 102db
90hz 104db
100hz 105db
110hz 105db
120hz 102db
130hz 99.5
140hz 96db
150hz 93db

I'd like to seperate these beast into a single driver per box. Also, it would be nice to get rid of the dip if possible.
And, I don't really plan on operating them above 110hz.

I don't know the driver specs (yet) and the box is about 3 cf sealed and 2 cf vented (per driver). Vent is a 4" slot 6" deep.

I'm wondering if I have any hope at loosing the dip at the cost of upper BW?

Hoping each new box is not larger than what it is now... 4cf would be nice. I can live with the dip if it requires a larger box to smooth it out.

Ideas?

[chris661]

Those figures don't look right.

4th order band pass boxes tend to have a frequency response which (from 20Hz up), rises in amplitude, stays flat for a while, then drops at the same rate as it came in.

Having a dip in the response just seems weird to me...

Still, I'm no expert, just expressing my interest

Chris

[Ron E]

A dip typically means the Q of the sealed chamber is high - that it is too small, or the driver has a high Qts or both. To design this properly you need T/S parameters (preferably real, not published).

Here's a link, with a design spreadsheet (works in open office)
http://www.diysubwoofers.org/bnd/4thorder.htm

[danielm]

I was afraid of that.

My old back just can't deal with anything bigger and was hoping to get by with a smaller box.

What about isobaric loading? Although, doubling up on the drivers would yeild me half as many boxes and I had plans to make a steerable array from many small boxes. Might just have to live with the dip.

I suspect the driver Qts is high. I don't belive they were very expensive. I'll see if I can make some measurements when I pull them apart.

Thanks.

[GM]

My take on it is that the dip is centered somewhere around 66 Hz, so increasing the front chamber's Vb is required to flatten it out, but if used in a high average power app, then it should have an even larger front chamber (higher Q) to account for the response flattening due to VC heating.

Isobaric loading reduces chamber Vbs, but then the vents are much bigger to have a low vent mach, so may have pipe harmonics in the speaker's pass-band that may need damping which in turn affects output, ergo chamber Vb adjustments in a 'chasing your tail' scenario as you try to find an acceptable trade-off. Going isobaric in a BP requires a PR solution IMO if some form of pipe/horn alignment isn't used.

GM