All you need is an audio frequency signal generator which is capable of producing a fairly good sine wave over a frequency range of 20Hz to say 1000Hz. Beg, borrow or if absolutely necessary buy one? But only if you have a need for it afterwards. Again, their are kits available from electronics companies. Also you need the sound level meter mentioned earlier.

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The signal generator is connected to an auxiliary, CD or tape input on the HI-FI amplifier and the signal meter is positioned directly in front of the loudspeaker, in line with the centre of the woofer's cone. A distance of a couple of feet is OK. The use of a tripod is recomended. The observer (you) stands at the side of the meter so that you do not cause any direct reflected sound, which could cause an error in the readings.

Procedure

1. Set the signal generator to a high frequency i.e. 1000Hz (1KHz) and take a level reading on the meter. Instructions for using the meter are included with it, so I won't go into details here. You will need to adjust the level of the signal from the generator to your amplifier so that you get a good sound without any distortion. This is very important as distortion causes harmonics (multiples of the original frequency) which will also be detected by the level meter.
2. Now lower the frequency to say 200Hz and take another reading. Keep lowering and measuring in smaller steps as you approach the low frequency cut off point. You will find that the meter needle does not stay at the same point even above the cut off. This is partly due to the fact that no loud speaker system is perfect, but mainly due to room acoustics causing some frequencies to appear louder than others. You could do the test in the garden if you have long leads and understanding neighbors, but this would be taking things a bit too far.
3. Plot the readings on a graph and then draw a best fit line through the points to get as smooth a line as possible. You should see the response drop smoothly below a certain frequency.
4. Note the point where the curve is 3dB lower than the average higher frequency readings. This is where the speaker is producing half the level of sound and is needed to design the Low Pass Filter in your subwoofer.

My Wharfedale speakers are a sealed box design which cause a fall off in level below the cut off frequency of approximately 12dB per octave. This means that every time you half the frequency, the level produced is 12dB lower. This is true for sealed box speakers but not necessarily true for ported speakers. So if yours are ported then you may need to design you filter to rise at a different rate. But having said that, The design here may sound OK to you without any alterations. I just haven't tried it with ported speakers, sorry! Anyone who can help me out with any advice can send me an e-mail to nigel.gough@diamond.co.uk

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