Dipole Woofer Project.

Dipole Woofer Array I got hold of 10 inexpensive 12 inch woofers with high Q (1.2-1.5), a decent Xmax (8-9 mm) and a resonant frequency of 24 Hz. Time for a dipole woofer project.

I didn't have to think much about the dimensions of the "baffle". I needed to construct two towers and had only one partial sheet of 3/4 inch MDF available measuring 49" wide by 65 inch high.

I decided to go fot a 14" wide front with side extensions at both sides. The extensions were trapezoidal in size, about 2 inch at the top, approximately 9.5 inch at the bottom. As such the width of the baffle varies from 19" at the top to 34" at the bottom. Spacing between the woofers was 12.625" so that everything would fit within the 65" height. A 14" x 18" x 1" thick base completed the "design".

I was initially planning of connecting all woofers in parallel but this would result in a fairly low impedance load. Instead I got the idea to use a tapered array approach and figure out if there were any merits to this type of set-up. Literature claims advantages inherent to this topology.

One day after I build one tower I had a chance to measure this one out in a large gym. This allowed me to make pseudo-anechoic measurements with a data-window of 50 milliseconds, good for a frequency resolution down to 20 Hz.

The following plot represents 5 measurements. Observe the dipole cancellation/reinforcement behavior. There is a nice example 6dB/octave slope visible. Intriguing is the dipole reinforcement peak that is coming down in frequency as the distance to the dipole driver increases.

Tapered dipole array frequency magnitude plot

It is fairly obvious that with some electronic assistance the response from 20-300 Hz could be linearized. This would require not too much boost since I'm planning to use this array from 40 Hz to 150 Hz and integrate this with a planar magnetic ribbon. Observe how the response from 20-60 Hz doesn't follow the 6dB/octave rule as the high Q of the driver does it's job.

Continued.