While Delta provides very good
bass response down to 30Hz or so, which is fine for most music,
further low frequency extension is necessary to
hear the full impact of movie sound effects. Thus, I decided to
design & build Samson, a compact stereo subwoofer system that
will supplement Delta'a low frequency output.
have one very large 90 litre subwoofer, I prefer 2 smaller 45 litre
subs, which has less visual impact and allows more room placement
options. When combined with Delta, the total low frequency output
of the system is then 2 x 7" woofers + 4 x 10" woofers.
That's a total cone area of 1620 cm² which is equivalent to
a typical 20" woofer or 2 x 15" woofers.
Samson features the following
- 2 x 45 litre sealed enclosures (external dimension), 36 litres
- lowfrequency extension to 20Hz (-3dB or -6dB depending on gain)
- adjustable line-level 12dB/octave low pass filter inserted between
amplifier pre-outs/main in. No EQ required.
- driven by the main speaker outputs of my A/V amplifier
- maxiumum output capability complementary to Delta.
design decisions -:
Note: This design detail is for the benefit of DIY speaker builders.
It is not to be used for commercial purposes without my permission.
Sealed or ported? - A sealed
box is smaller, hence stronger & lighter, has better transient
response, integrates better with room acoustics, allows more placement
options, is simpler to build and is inherently protected against
excessive cone excursion
at subsonic frequencies.
Driver - To meet my
form factor and output level requirments, 2 x 10" drivers (1
per channel) would suffice. I chose the Peerless
830843 XXLS 8 ohm subwoofer, which has an excellent reputation,
and is widely available. It also has a rubber surround and well-behave
air-dried nomex cone, which I prefer over aluminium.
Lowpass filter - Lspcad
simulations quickly determined that a 12dB/octave filter produced
the optimum phase characteristics when summed with Delta. Passive
line-level filtering was considered but quickly ruled out due to
it's complexity & loading effects on the pre-amp. Finally, I
decided on the Marchand XM-1 crossover module from Marchand
Electronics, powered by a PS10 +/- 15V power supply.