Multiple midrange drivers in a speaker, but why not tweeters? - AVS Forum
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post #1 of 6 Old 02-10-2012, 01:40 PM - Thread Starter
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Looking at the Polk Monitor line (and most other floorstanding speakers), there are floorstanding with one, two, three, and even four 6.5'' midrange drivers, but all of them only has a single tweeter.

Can somebody explain to me why a floorstanding version of a speaker may have 2 or more identical midrange drivers, but only one tweeter, no matter how many midrange drivers a speaker has? Why aren't tweeters added?

Also, I know for every doubling of the number of drivers, there is a 6dB increase in sensitivity. Why can speakers achieve the 6dB increase in sensitivity with only doubling of midrange drivers?

Thanks.
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post #2 of 6 Old 02-10-2012, 02:00 PM
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Quote:
Originally Posted by LowerFE View Post

Looking at the Polk Monitor line (and most other floorstanding speakers), there are floorstanding with one, two, three, and even four 6.5'' midrange drivers, but all of them only has a single tweeter.

Can somebody explain to me why a floorstanding version of a speaker may have 2 or more identical midrange drivers, but only one tweeter, no matter how many midrange drivers a speaker has? Why aren't tweeters added?

Looking at: http://www.polkaudio.com/homeaudio/s...ent/monitor70/

The detailed description says that 2 of the 6.5 inch drivers are Mid-woofer and 2 of them are subwoofers.

The arrangement of a single tweeter with a mid or mid/woofer above and a mid or mid/woofer below is known as MTM, and is highly recommended for a variety of reasons by some authorities. Look up MTM if you want to find out more information - I don't want to waste bandwidth when others have covered it well.

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Also, I know for every doubling of the number of drivers, there is a 6dB increase in sensitivity.

Not really. The laws of conservation of energy are still in effect for woofers, so the overall efficiency remains the same no matter how many drivers you use, all other things being equal.

But all other things don't necessarily remain equal!

What does change is that putting two drivers in parallel doubles the amount of power they take when a given signal is applied because their joint impedance is halved.

The other thing is that putting drivers next to each other does is make them more directional, which increases their output in a narrower angle in front of them.
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post #3 of 6 Old 02-10-2012, 02:22 PM
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Quote:
Originally Posted by LowerFE View Post

Looking at the Polk Monitor line (and most other floorstanding speakers), there are floorstanding with one, two, three, and even four 6.5'' midrange drivers, but all of them only has a single tweeter.

Can somebody explain to me why a floorstanding version of a speaker may have 2 or more identical midrange drivers, but only one tweeter, no matter how many midrange drivers a speaker has? Why aren't tweeters added?

Also, I know for every doubling of the number of drivers, there is a 6dB increase in sensitivity. Why can speakers achieve the 6dB increase in sensitivity with only doubling of midrange drivers?

Besides extra volume, is there any advantage sound quality wise using multiple midrange drivers?

Thanks.

The big issue is interference. Drivers have to be within 1/4 wavelength of the highest freq they are reproducing in order to not cancel each other (at certain freq).

In the case of woofers and midrange-the wave lengths are longer-so therefore more distance can be tolerated without interfering.

Even if you say the upper limit is 10KHz (not real high), which has a wavelength of around 1.3 inches. So the limit for the spacing would be 1/4 of that or around 1/3rd of an inch.

It is physically impossible to get the drivers that close (without the use of special "lenses").

Now granted that distance is at the listeners ears-so a greater distance "could" be tolerated-as long as only a single listening position is considered. Once you start to consider the distances involved over the entire listening area-it gets a bit more complicated.

So it all boils down to sonic reasons-based on wavelength.

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post #4 of 6 Old 02-10-2012, 02:25 PM
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Plus you'd have uncontrollable cancellations between the 2 tweeters. At 10KHz the wavelength is under an inch and a half. IF you could find a place where the 2 tweeters added up properly, move under 3/4 of an inch and you've made them exactly out of phase with each other, causing theoretically complete cancellation of any 10000 Hz content at that point.

Plus it takes a lot less power and a lot less displacement to reproduce the highs, so you can get 'r' done with one tweeter. Which means that those bookshelf versions of tower speakers that use the same tweeter have the tweeters padded down to make them quiet enough to play nicely with the system. Once you get to really high SPLs, though a typical dome tweeter lacks the oomph and that's why you see horns on pro equipment . . .
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post #5 of 6 Old 01-15-2014, 09:04 PM
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post #6 of 6 Old 01-16-2014, 04:49 AM
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Quote:
Originally Posted by LowerFE View Post

Looking at the Polk Monitor line (and most other floorstanding speakers), there are floorstanding with one, two, three, and even four 6.5'' midrange drivers, but all of them only has a single tweeter.


Can somebody explain to me why a floorstanding version of a speaker may have 2 or more identical midrange drivers, but only one tweeter, no matter how many midrange drivers a speaker has? Why aren't tweeters added?


Also, I know for every doubling of the number of drivers, there is a 6dB increase in sensitivity. Why can speakers achieve the 6dB increase in sensitivity with only doubling of midrange drivers?

Here are some graphs of the energy in some typical musical selections:



We can see significant and even dramatic decreases in energy as frequency goes up.

Speakers tend to be stressed more heavily at the low end of their frequency range. Woofers are required to play far louder (20 dB or 100 times more power) than tweeters.\

As frequencies go up, wavelengths go down proportionately. A 50 Hz wave is large architecturally or in the range of dozens of feet. A 3 KHz wave is far from it - its 60 times smaller or in the range of inches. Speakers and speaker arrays are often sized to control directivity, and that job gets done well by smaller drivers as the frequency goes up.
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