Tuning frequency study

I realised I could do a very easy test to find out what more bass extension would be like. Blanking off one of the ports on the 'Blast Furnace', would lower the port tuning frequency from 28hz to 18.4hz and increase box volume from 130 litres to 150 litres. WinISD gives the following prediction of SPL. (Original in red, one-port in green).

	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 my 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.

Cone excursion comparison showing possible problem with extended base design

Another interesting one was Terminator3 Ch12 - 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. This DVD has an option for DTS sound, which I intend to compare and update here later. I will also do a general article on Dolby 5.1 vs DTS for home theater as there are some issues worth exploring.

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: I have built a pair of Buddy Subs to use this alignment

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