Sometimes I get lucky.

Our loudspeaker guru, Chris Brunhaver knows more about the art and science of loudspeaker design than anyone I have ever known (and I’ve known quite a few).

He recently answered a poster’s question about bigger vs. smaller woofers in the same size cabinet. His answer is so well said that it bears repeating here in this post.

“Well, it comes down to “space versus bass” like a lot of speaker design decisions. It might seems counterintuitive but, all things being equal, using a bigger driver in a given enclosure actually gives you LESS bass extension. Larger drivers will give you more output capability (all other things being equal) but require a larger enclosure.

The amount of air displacement required for a given output level quadruples every octave you go down. For instance, to play 20 Hz at the same volume as 40 Hz for a given cone size, the drive needs to move 4 times the amount.

If you can double the excursion of a driver for a given distortion level, you can gain 6 dB of output at low frequencies (where displacement is the limiting factor). There is an IEC standard around the level of distortion for this rating (measuring the limits around the motor force, suspension compliance and inductance vs excursion). If you can optimize these curves (to give you more excursion for a given distortion), you aren’t adding distortion or creating issues.

It’s not so much about low mass or a strong motor – those are related to sensitivity, not excursion and linearity versus stroke, it’s about how linear these parameters are with excursion.

For what it’s worth, your use of transient response in this context is probably better described as “large-signal behavior” or dynamic linearity or something like that. I know that you are referring to reproducing big transients but the term doesn’t mean that in this context.

Here’s a little poster on the causes of these distortions and how speakers are optimized for greater excursion. We used these techniques in our woofers too.

Of course, it would be even better to have a larger driver with a higher level of excursion but the resulting enclosure size would need to be proportionally larger to the cone area difference.

Thanks, Chris.

Paul McGowan / PS AUDIO

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