Ported Sub Technology

Help e-mail from Bed Audio supplier Mike at Audiocar, London, UK to Kaffe in Mexico City, October 9th 2007:-

“Hiya Kaffe,

Good to hear (no pun intended!) that things are going well.

Ported enclosures will actually produce a higher SPL (sound pressure level) in a given situation, but the downside is that you’ll require bigger boxes for the subwoofer – plus and this is the crucial thing – the subwoofer cone should be placed no closer than half the diameter of the subwoofer to any reflective surface (such as the mattress material). If you put the sub closer than this, what you are effectively doing is to ‘load’ the sub. In other words as the waveform comes out the front of the sub, the reflective wave has a detrimental effect on the sound waves that follow on. The net outcome generally is that the subs cannot play down to such low frequencies – even in ported enclosures.

Also just as an extra tit-bit of information, the distributor of your sick Bed sub that was returned phoned, and said it showed signs of having got reasonably warm – not through overdriving (or distortion), but due to the fact that some of your sub bass frequencies are very, very low (and therefore very, very long wavelength). Due to them being in small enclosures, and moving at very slow speeds the voice coils are getting warm. In a larger enclosure there would be more air to absorb more of the heat. If I remember correctly the Genesis amplifiers you purchased do actually have subsonic filters on them (to prevent the subwoofer from playing frequencies below the capabilities of the system). It could be worth checking that the subsonic filters are actually being used, and are adjusted correctly to a suitable frequency.

Also you need to be cautious of any specs that ‘budget’ speaker / amplifier manufacturers give, quoting you “They do sound better and can produce lower frequencies, but as far as sitting on and feeling…” – if you think of a speaker as a car, and the car manufacturer says “this car can do 200 miles an hour” it sounds very impressive. But, what they have not told you is that if you push the car to 200MPH the engine will blow up! Or another way to look at it is saying “my car does 60MPH” – that’s no big deal unless you put it into context, ie “my car does 60MPH in 3 seconds!”. If the manufacturer of the speaker only tells you the frequency to which the subwoofer can play, but not the level at this frequency then the information is pretty well worthless! “