Watts an Ohmn all about?

[seyles]

The other thing is, if you want to get more power to go louder, you should look at the sensitivity of the drive unit. This is listed as dBs for 1 watt of input power at 1 metre away. The higher the dB the louder it will be for the same input power. 3dB is a doubling of power so a driver with 93dB sensitivity will bejust as loud with 1 watt driving it as a 90dB one with 2 watts driving it.
Sensitivity isn't a measure of quality though! Enclosure design will also make a huge difference in SPL levels, for out and out SPL without quality or extension, huge peaks in the response are easily acheivable.
In my experience in building subwoofers there are 3 things to aim for;
quality
extension
SPL
Any 2 of the above 3 are easily acheivable. All 3 costs money.

I agree about the speaker being an reactive load, whereas a resistor is purely resistive. 4 ohms for a speaker impedence is an average at best. It will probably go down as far as 1.5 ohms and up to about 16 ohms at different frequencies. So 2 ohm stable isn't such a bad thing when driving a '4 ohm' speaker.

[Colin]

thanks for all your help everyone it's been interesting reading.
I guess i'll be simply going for the single sub set up, but if Seyles comes back on or anyone else knows what he was talking about in message 2 and the Isobaric set up, i believe it's two speakers run face to face but a laymans term breakdown would be interesting.
In regards to RMS power, i still can't understand why manufacturers still quote other "mickey mouse terminology" (peak, mpo, pmpo, etc)apart from making a sh!te product sound more impressive in a brochure. If your dealer can't quote an RMS figure then go and buy something else, or you'll only be disappointed.
If anyone wants a fair product for fair money, check out the mutant range at halfords. or try www.mutant.uk.com.

[seyles]

Isobaric is 2 drivers face to face.
Vas is one of Thiele Small parameters which are used to work out the ideal box volume and port lengths etc. It is the volume of air which has the same compliance (springyness) as the cone of the driver. This value is directly related to the ideal volume the drive unit needs for the flatest reponse. If you put two drive units together in a clambshell the result in effect is a single drive unit with half the Vas value and this means the ideal volume is halved.
To work out the ideal volume and port length you only need 3 Thiele small parameters for the drive unit
Vas-as above.
Fs the resonant frequency.
Qts the total damping.
The maths can seem a bit difficult but is pretty straight forward.
If you want, tell me what the parameters are, and I'll tell you what size enclosure to use and what length ports to use for the best response.
The best response in this case means the flatest.