Why so many subwoofers vs. just one tweeter?

[Robert394]

I watched this review of the Kii 3 Audio BTX

It got me thinking about some other really high end systems I've seen, like the Hedd Tower Mains

In both of these systems you have at least 4x as many subwoofers vs. tweeters and likely thousands of watts more power going to the subs than the tweeters.

It seems surprising to me that given the subs only output say 20-100hz or so, and the other drivers output 100-20,000hz, that high end systems like this emphasize a ton of subwoofers.

How does this contribute to optimal and balance frequency response?

Why not use more tweeter/midrange drivers instead of just stacking a ton of subs?

Both of these companies make some of the best pro speakers out there, so I'm wondering why they take this approach.

 

[DonH56]

https://en.wikipedia.org/wiki/Equal-loudness_contour

Look at the rise in SPL as frequency goes down. Note 3 dB louder requires twice the power, 6 dB four times, 10 dB ten times, 20 dB 100 times, 30 dB 100 times more power to sound as loud.

 

[HemiRick]

Systems that make real low bass are systems that sound really good. Bass is a huge element, that IMHO, most people underestimate how much they need or want....More is almost never bad.

 

[andreasmaaan]

Why not use more tweeter/midrange drivers instead of just stacking a ton of subs?

In addition to the point Don mentioned, the other issue is destructive interference between sound sources located at different points in space.

To oversimplify it somewhat, for a pair or group of drivers to sum constructively they must be located no further apart than roughly 1/4 of a wavelength at the highest frequency they will be asked to reproduce.

At 100Hz, 1/4 wavelength is about 82cm. At 20kHz, it's about 4mm.

The woofers on the Kii Three are easily located within 82cm of each other. OTOH, tweeters small enough to be located within 4mm of each other and capable of producing undistorted output down to a usefully low frequency are physically impossible.

There have been a few patents describing ways of coupling tweeters using ports that converge such that they exit into the throat of a horn/waveguide. JBL and Danley both utilise such systems IIRC. But in general, it's difficult and impractical.

 

[Robert394]

Systems that make real low bass are systems that sound really good. Bass is a huge element, that IMHO, most people underestimate how much they need or want....More is almost never bad.

Interesting, thanks for the replies. I agree, bass is actually really important in my personal experience. Not having a sub, or having a sub tuned too quietly or with too low a cutoff, has a significant effect on how enjoyable and "right" a system can sound. Even small differences in volume or cutoff filter can make a surprising difference.

 

[HammerSandwich]

Why not use more tweeter/midrange drivers instead of just stacking a ton of subs?

Because the design goals for real-world speakers don't work with that approach.

Producing a given SPL requires different amounts of air displacement at different frequencies. To make the same SPL an octave lower (1/2 the frequency), your driver's excursion quadruples. Or you use 4 identical drivers at the original excursion, which might look familiar.

(This is equivalent to 12dB/octave and also implies that SPL is a function of driver acceleration. Holding the same acceleration for twice as long gives 4x the distance, because time is squared, so everything matches up. Fundamentally, this all goes back to current in the coil, producing a force, accelerating the driver.)

Now, instead of cutting the frequency in half, imagine going from 5kHz to 50. Square that 100x difference, and where are you going to put 10,000 tweeters? What will this speaker cost, especially after you pay a team to test all of the drivers & connect the wires? Never mind shipping it... Okay, I am playing a little loose with back-of-the-napkin numbers. Your project might work with only ~500 tweeters per speaker, but woofers suddenly look really worthwhile, even if you need a few of them.

There are serious speakers which use only one size of driver: full-range line arrays. They don't use 1" tweeters, but 2-3" drivers that balance these design issues. And even with 25x 3" drivers, subs are needed for strong output in the bottom octave or so.