The Blast Furnace

Since building this subwoofer, my house has begun cracking up. I blame the clay soil and climate change, but secretly fear that the sub is slowly pulverising the sandstone piers. The price we pay for our passions....

"Blast Furnace"


For this creation I wanted something a little different. The Blast Furnace is so named because it is reminiscent of blast furnace at the steelworks I worked at a some years ago

Consisting of a powerhead , housing two drivers, and linked via a transfer port to a lower chamber which makes up the required volume, this speaker will give an extra 7db over the Sidewinder

Two external ports exit the powerhead at the sides and run through the lower chamber to exit at the front. These ports do not open into the lower chamber but are supported by it. The amplifier is mounted at the back of the lower chamber, with the controls and infra-red receiver at the front

Jaycar's 350W amplifier delivers 240W into 8ohms as presented by the two drivers in series
It was hoped to disable the 6db boost on this amp as it seemed similar to the Parts Express models from the USA. When the back of the amp was opened and compared it to the photos from the net, it was too different! Since then, these instructions have become available, but I didn't bother.

Graph - SPL


Two 160mm ports mean absolutely no port noise. Both ends of the ports were flared by 10mm using a rollover bit in the router.

Graph - Vent Velocity


Flare-it screenshot shows that there is actually a bit of port headroom in this design

Flare-it screenshot


Because the amp has +6db boost at 35Hz, power is not as much in the excursion critical range of 25Hz and lower. No hi-pass filter is required for this unit

Graph - Excursion


So how does the Blast Furnace compare to some commercial subs?

Data as published by Widescreen Review Issues 36 & 49 I found this comparison on SPL performance of selected subwoofers on the web in table format. I have graphed it to see how DIY fares in comparison to "store bought" The source article is unavailable unless you subscribe to Widescreen Review, but from researching each of the suppliers, it seems that these speakers are about 1999 vintage

Subwoofer comparison


Brand and ModelCost $USComments
Earthquake Supernova 15inch$2,999Passive Radiator tuned to 17Hz 580W RMS
B&W 400-ASW 15inch$3,000Obsolete no details available
Velodyne F1800R 18inch$1,999Obsolete. Their recommended replacement is DD15 or DD18, sealed, with 1250W RMS (3000W Peak)
Triad Platinum 18 18inch$2,200Sealed 500W RMS
Bag End Infra18 18inch$1,495Sealed 400W RMS - Note: The Bag End Infra series are intended to be driven with an equaliser that boosts the bottom end. This one was obviously tested without it!

As you can see the 300w twin-10inch ported Blast furnace holds up fairly well!


Tuning Frequency Study

A very easy test was done to find out what more bass extension would be like. Blanking off one of the ports lowers the port tuning frequency from 28hz to 18.4hz and increase box volume from 130 litres to 150 litres.

Blast Furnace with Blanking plug fitted

WinISD gives the following prediction of SPL. (Original in red, one-port in green)

SPL comparison
10dB better @18hz, equal @23hz and 5dB worse @30hz

A quick check of other changes which could affect the sound reveals that Port air velocity rises from 8 m/sec to 11 m/sec. Group delay actually falls in the working range although it peaks further down the range at 62 msec @ 16hz as opposed to the original value of 36 msec @ 27hz. These changes can be ignored


So how did it sound?

As expected, testing with the normal suite of DVDs, the study produced mixed results depending on the frequencies involved.
The standout performer was Star Wars II Chapter3, where the arrival of the starship produced sounds I had never heard before. This title gives a glimpse of how awesome a sub 20hz mix can be!
However all the rest of my DVDs were less impressive, indicating that their LFE content was 25hz upwards.

An interesting one was the first Lord of the Rings movie, The Fellowship of the Ring. The loudest sequence is in chapter 29 where the Troll first appears. With the modifications in place, the result was a little lower in SPL, but there were problems with cone excursion. Looking at the comparison below, I would guess that this sequence is around the area of highest cone excursion ie 32hz

excursion graph
Cone excursion comparison showing possible problem with extended bass design

Another interesting one was Terminator3 - the crane chase sequence. With the mods, there was virtually no subwoofer action apart from the main crash which was present but not as loud.

In conclusion, the additional bass extension is very desirable for those few DVDs that support it. It seems that most DVDs are mixed with the content optimised for the mass market. With not much below 25hz, the tradeoff in SPL above 25hz is too high a price to pay at the moment
However, that said, I could do some equalisation if I had 6dB up my sleeve - hmm, a second one would do it, although I do have some concerns about the seismic resilience of my 100yr old house. Its already been through two earthquakes.....

Postscript: A pair of Buddy Subs have been built to use this alignment


Construction Gallery


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Page modified: Sunday, 21-Oct-2018 02:16:09 EDT