Speakers, Drivers and Physics (1 Viewer)

Question for you speaker design experts...

What are the benefits of adding a second driver of the same size to a speaker? Specifically, consider a small satellite speaker with a 3" mid-bass driver. What would be gained by adding a second 3" driver? (Consider the tweeter as the same in all of the scenarios below.)

SURFACE AREA: A single 3" driver has a surface area of Pi x (1.5**2) = 7.07 sq. inches. By adding a second 3" driver, the total area would be 14.14 sq. inches. More surface areas means I can now move more air. So I can play the speaker louder, right?

Comparatively, a single 4.25" driver would have a surface area of 14.19 sq. inches -- virtually the same. So with all other design elements being equal, are there any benefits of having two smaller drivers vs. a single larger one? How would their sounds differ?

POWER HANDLING: I assume that a second driver would increase the overall power handling capacity of the speaker, right?

FREQUENCY RESPONSE: No change here, I think. A second driver of identical specs adds no extension to frequency response, right?

WHAT ELSE?: What am I missing? Why else do designers add the second, same-sized driver?

THE BOTTOM LINE: Most importantly, what would any/all of the above mean in terms of sound quality? What difference would I hear? Fuller? More dynamic?

Thanks!

I'm no expert but I'll try. If you parallel a second identical driver and make the enclosure twice as big, impedance will be halved and power handling will be doubled. Sensitivity (and also output capability) will go up by 6db. If the crossover is changed to maintain the same response, the frequency response of the drivers themselves shouldn't be any different. However baffle diffraction due to the larger enclosure will be different, which could cause some deviations in frequency response at higher frequencies.

You're on the right track. Using multiple drivers just helps to increase a speaker's bass and output capability. But, in terms of bass and output, larger surface area is most important (depending on their linear excursion and power handling, a 10" woofer is probably more powerful than two 6" woofers). The reason multiple smaller drivers are used as opposed to single big drivers is because of size and frequency response constraints. Larger drivers don't extend as high so they're not used much in 2-way speakers. And no one likes 14 inch wide boxes.

Your observations are essentially correct.

However, as drivers get larger, their high frequency response suffers for various reasons... cone breakup modes, narrowing polar response (beaming), cone mass, voice coil inductance, etc. It is often necessary to "double up" the woofer to achieve the desired SPL output and still "reach" up to the frequency where the tweeter can take over.

As an example, 3-4 KHz is about the upper limit for most 6" drivers. And it takes a pretty stout tweeter to pick up from there. So if the designer needs more output, his choices are to use a bigger woofer and insert a midrange, find a 6" driver with equivalent HF response and much greater linear excursion (could be expensive and may introduce unacceptable modulation distortion) or use multiple woofers. The latter is often the best choice.

Smaller drivers also allow a narrower cabinet to minimize diffraction and change baffle step compensation frequency.

As a practical matter, woofers are typically less efficient than tweeters, so double woofer arrangements solve the efficiency mismatch problem.

Finally, the popular double-woofer D'Appolito (MTM or WTW) configuration, combined with the correct crossover, provides very controlled vertical polar response to minimize room interaction.

Thanks guys for your excellent responses! While my question was about theory, the application is practical. I have decided, due to some space constraints, to go with a sub/sat system for a new surround sound setup. I have not been totally satisfied with any of the sub/sats I've auditioned. The smaller sat drivers usually don't fill out the mid-bass, and just don't provide as full of a sound as bookshelf based systems. Also, there is often that audible gap between sub and sub. I've been trying to go with sats with as large of a mid-bass driver as possible to help "fill out" the sound. 4.5" seem to be about the max I can accommodate. However I am intrigued by the new Pinnacle Quantum (with a bigger sub) system. But instead of a 4.5" driver, each satellite has two 3" drivers. I have not been satisfied with any single driver satellites I've heard with drivers this small.

Given your responses, I am not so much worried about the higher frequencies. Do you guys think the second driver would help fill out the sound and give more the "feel" of a larger, single mid-bass driver speaker?

Thanks again for your help.

Denis,
Flat frequency response and seamless integration with sub is technically possible with either 4.5" or dual-3" sattelites. However, don't expect such a setup to go real loud. To keep the sub "invisible", you must keep the crossover below 100Hz. And you just can't move enough air with 3-5" drivers to get much above 100dB at 100Hz.

I haven't heard the Pinnacle system... but as you pointed out the cone area is about the same as a single 4.5, so the performance should be similar.

What specifically are your space constraints? Could you go with in-walls?