Woofer cone diameter

[JonJT 10+ year member]

Why are drivers designed for low frequency production larger in diameter than drivers designed for high frequency? Anyone got an indepth scientific answer?

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[helotaxi 5,000+ posts]

Why are drivers designed for low frequency production larger in diameter than drivers designed for high frequency? Anyone got an indepth scientific answer?

This might get a little complex but here goes...

The bottom line is that to get audible volume at low frequencies you have to compress/rarify a large volume of air. The human ear is very insensitive to low frequency sounds so you have to get them much louder in unweighted SPL for them to blend in with the high frequency drivers which are more efficient.

Given sufficiently low Le values, large speakers can theoretically play the full range of sound. The problems that you run into that keep that from being the case are as follows:

1) In order to move the amount of air needed to play low frequencies, the voicecoil of a low frequency driver needs to be large and that leads to a relatively high Le which effectively limits the upper frequency range of the driver.

2) There is no such thing as an infinitly stiff or totally weightless cone. As you try to play higher frequencies with a large driver, the cone starts to break-up. If you were to look at a cross section of the sub cone it would be flexing much like a guitar string and would be adding all kinds of harmonics to the sound.

The two really feed off each other. If you make a stiffer cone the larger coil needed to move it will have a lower Le and will limit the upper end of the frequency response. If you make a light cone in order to use a smaller coil, it won't be stiff enough to play the higher frequencies anyway.

Midranges have a natural high end roll-off caused be the voicecoil Le. This rolloff necessitates the use of tweeters that have a very small/light/stiff transducer (the edge driven dome, be it resin impregnated textile or a thin metal film, has proven to be the best trade off between stiffness and weight, but it is only practical for really small drivers.) that allows the use of a tiny voicecoil with a tiny Le. Since the human ear is relatively sensitive to higher frequencies (1-10kHz) the mids and tweets don't need to move a lot of air so a small driver is acceptable.

 

[JonJT 10+ year member]

Thank you sir!