Delta 3 - Design

Though I've been very happy with the 3.5-way Delta 4, I've decided to build a smaller and simpler 3-way version. This involves a compromise in terms of sensitivity but this will be offset with an easier amplifier load and a smaller, more decor-friendly, enclosure. Delta 3 will use the same satellite enclosure as delta 4, but with the a variovent added and an improved crossover. The bass section will still use the 26w8861 but the 18w8531, which was used for baffle-step compensation, will be omitted and BSC will instead be achieved via the crossover.


Bass extension and power handling - how much ?

In an average listening room, with normal music, we typically listen to music at about 87 dB average levels. This is fairly loud, but not uncomfortably so.

The average loudspeaker sensitivity is 87dB/W, therefore the amplifier is typically producing about 1 watt.

Peaks however can be up to 20dB higher than the average level, so then the amplifier is producing 100W for the speaker to make those107dB peaks.

Woofer excursion increases 4 fold for every halving of frequency. Studies show that for music, the peak spectral content at 40Hz is approximately 10dB below the overall peak levels. Therefore, the bass power handling needs to be 107-10 = 97dB at 40Hz at which point the amp will be at 10W. Ideally we'd want the bass driver to be operating within +/- 1/2 Xmax excursion at this level to keep harmonic distortion and compression in check.

The 10" Scanspeak 26w8861, which has an Xmax of 9mm, is able to produce 96dB at 40Hz within 1/2 Xmax with the amp at 16 watts. When at Xmax at 40Hz, it will be producing 6dB more, or 102dB, with the amp at 60W.

So it appears that a good long throw 10" woofer with a 100W amplifier is nicely adequate for normal music listening. Greater low frequency and extension requires much larger cabinets or much more more amplifier power, both of which go against my goal for a compact and simple system. For movies, which have greater low frequency content below 40Hz, the driver will exceed Xmax/2 and therefore distortion will increase.However, I think a bit more distortion during sound effects is acceptable.

16W at 40Hz produces cone displacement of 1/2 Xmax (4.5mm)
60W at 40Hz produces cone displacement of Xmax (9mm)
Filter response
Measured driver response with filter
Filter response
Measured driver response with filter
Filter response
Measured driver response with filter
Overall frequency response & impedance predicted by LspCAD.
Measured impedance.
Predicted low frequency response in room.
Group delay.