Samson subwoofer
Fall seven times, get up eight - Japanese proverb

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.

Rather than 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 internally.
- 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.

Major 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.

Marchand XM-1 manual (pdf)
Marchand PS-10 manual (pdf)
My proposed system setup is shown below.
Note that my main Delta speakers are driven by a dedicated stereo amplifier, and the stereo subwoofers are driven by the main outputs of the A/V amplifier.
The A/V amp is configured with mains set to large, no subwoofer output. The mono subwoofer output of the amp isn't actually used, and low pass filtering occurs with the active filter inserted between the main pre-out and pre-in terminals.

Frequency response simulations.

Below is the Unibox simulation of the Peerless 830843 in a stuffed 36 litre enclosure. It can reach 93dB at 20Hz. This is 3dB more than Delta can produce. So, if the Peerless is given 3dB of gain, both Samson and Delta will then reach their limits simultaneously.

Delta consists of the 10" Scanspeak 26W8861, 7" Scanspeak 18w8531, 5" Audiotechnology 15h5206, and Morel Supreme 110 tweeter.

The response below shows how the Scanspeak 18w8531 (red), 26W8861 (blue), Audiotechnology 15H52 (green) and Peerless 830843 (yellow) combine to produce a total (black) when the Peerless has a 2nd order Butterworth lowpass at 40Hz and a gain of 3dB relative to Delta. This is calculated into a 2pi space, which is a good approximation to the in-room response. Note that the response of the Peerless is over 24dB down at 200 Hz. Note the flat phase response, one of the major benefits of not using the AV amp subwoofer outputs.

Below shows how Samson results in a 6dB boost in the system response at 20Hz. Subjectively, this is about a doubling in volume.
As Delta can produce a maximum 90dB per channel at 20Hz within it's Xmax (see Delta design), then from above, the total maximum SPL of Delta + Samson at 20hz is 6dB higher ,or 96dB SPL per channel at 20Hz into half-space. Combined output from both channels is therefore around 102dB, roughly equivalent to 2 x typical 12" subwoofers, or 1 x 12" Adire Tumult (33mm Xmax pk).
Samson will be driven by the main speaker ouputs of my Marantz SR-18 AV amplifer. It is rated at 140W RMS per channel, 5 channels driver simultaneously (700W transformer). Vpeak is about 48V and Ipeak about 6 amps. Plugging these figures into Sigfried Linkwitz's Closed-box spreadsheet, shows an excellent match of enclosure volume, driver output, and power amplifier capacity. As desired the amplifier hits it's limit (yellow curve) when the driver reaches it's Xmax (blue curve). The 2nd order lowpass filter at 40Hz will ensure the power requirements fall above 40Hz, rather than increasing as shown in the pink curve.

Below compares the total output using different lowpass cutoff frequencies, and differnet gains on Samson, which allows for some compensation for room modes and boundary conditions. As I have some sort of room mode at 50Hz, choosing a 30-35 Hz Fc may help to suppress it.

Here's the expected input impedance and group delay.
Construction plans
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