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All the previous comments about multiple tweeters -- or any multiples whether they be mid-range drivers or even bass drivers -- are all true. Yet -- because of driver manufacturing tolerances, un-even wiring, crossover components' manufacturing tolerances, and such -- they will not and do not behave as one big-ass high performance driver.

Every multiple-driver line-array-based speaker system I have auditioned either privately or at the CAF sounds disjointed -- not of one. And that includes the mighty Infinity IRS system that a Virginia dealer had many years ago. I eagerly could not wait to hear that "State of Art" beast only to walk away from the experience disappointed.

And it is not just the comb filter effect that Tom pointed out -- it's multiple rise times (the signal launch of each individual driver) that gets things messy.

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Walt

An alternative to using an array to boost tweeter power handling capability is using a waveguide or horn to improve efficiency, particularly in the lower portion of the tweeter response. It allows you to use a lower crossover point, once you equalize it back to flat. I have been looking closely at Joe Rasmussen's "Elsinore" speaker design, and that is what he does, for the purpose of keeping current down and lower voice coil temperature (he is really big into low current through the drivers to prevent thermal induced compression of dynamics).

David

You are correct about the SPL (given the same drive voltage) increasing across the whole range not just below resonance when you add multiple drivers. However, we were not talking about running the driver below resonance but rather being able to get more SPL at areas as you go down in frequency but are still above resonance.

The point is that to get the same SPL above resonance with multiple drivers you can drive each one with less voltage and therefore get less excursion for each at each frequency. So as you go down each octave with a four times increase in excursion but still above resonance you can get higher SPLs before any of the drivers exceed XMax. You then have two choices and can either tailor the crossover for a lower frequency but still above resonance and get similar SPLs as with a single driver or keep the crossover point the same and increase the SPL before exceeding XMax. Of course there still is a limit where enough drive will exceed XMax of even the multiple drivers.

Correct about multiple drivers covering the same range. It is especially true where you have bass drivers going down to resonance where slight differences in resonance can cause the drivers to get out of phase resulting in drops in frequency response. The problem with single drivers especially in the treble range is you risk two problems. If you use a larger diaphragm that will reduce the excursion requirements for for a given SPL you lower the frequency where the driver will start to beam, This can be a major problem with bass and mid drivers trying to mate with a higher frequency driver through the crossover point. It is not as much a problem with tweeters as there is no higher crossover unless you are going to a super tweeter.

The other problem occurs where you keep the same diaphragm size but increase the excursion. At some point you will get into the range of doppler distortion which is a bigger problem as frequency goes up.

As Dave said adding a horn concentrates the acoustical power requiring less excursion for the same SPL on axis. That obviously modifies the polar response which can be either good or bad. The good part is that you can run the mid crossover point to the tweeter up in frequency to the point where the beaming of the mid matches the reduced polar pattern of the horn loaded tweeter. That not only makes the transition smoother it further reduces the excursion of the tweeter.

The bad part is the contribution of the room sound now is not uniform as you go up in frequency. Among other things that can have negative effects is a change on perceived image placement. It is always a trade off and the best compromise is to do everything in moderation.

The other problem occurs where you keep the same diaphragm size but increase the excursion. At some point you will get into the range of doppler distortion which is a bigger problem as frequency goes up.

As Dave said adding a horn concentrates the acoustical power requiring less excursion for the same SPL on axis. That obviously modifies the polar response which can be either good or bad. The good part is that you can run the mid crossover point to the tweeter up in frequency to the point where the beaming of the mid matches the reduced polar pattern of the horn loaded tweeter. That not only makes the transition smoother it further reduces the excursion of the tweeter.

The bad part is the contribution of the room sound now is not uniform as you go up in frequency. Among other things that can have negative effects is a change on perceived image placement. It is always a trade off and the best compromise is to do everything in moderation.[/quote]

And people wonder why this hobby has held my interest since the ripe young age of 16.

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Walt

From the IRS Reference III and V days, they used analog active filters without time delay compensation between the servo subs and line arrays. Phase delay can be good, but not the same as pure time delay. I heard them at both the VA audio show and at Excalibur Audio in the day.

The acoustic radiation patterns of the IRS line arrays being a bipole planar and the servo sub being a sealed box style have always been at odds to me in sound into the room. Here I have always preferred the full OB sound to mixing different radiation patterns. Both The Monoliths and The Megaliths are full OB designs for that reason. They both have OB servo subs as well.

The crossover tolerance and time delay problems are corrected by using an excellent sounding DSP crossover. No component tolerance issues to deal with in the XO design. It also makes stereo imaging solid since there is no channel to channel variation in the crossover output. I can use CLIO and see exactly the same frequency and phase responses for all channels on R-L sides.

Using individually powered driver elements allows for the compensation needed to adjust for individual drivers. This can be seen in DBK's last sets of graphs for line arrays mimicking the curved array for radiation patterns into a space. You can also use lower power good sounding amps per driver and get plenty of SPL from the system.

1. Read David’s original post.
2. This quote is self contradicting. You don’t get more SPL only in the lower frequencies by using multiple drivers.

Shashi,

I do not believe it is self contradicting. There IS am implication that using multiple drivers allow more SPL over the the entire driver range, but the issue is the driver headroom now allows you to lower the crossover frequency without risk of overdriving the tweeters.

David

Shashi:

I'll put some numbers to an example that may make what I and Dave are saying clearer. Let's hypothetically assume we have a tweeter with an Fs of 500 Hz and an XMax of 1 mm. All the specs in this example will not represent any particular driver but will explain the mechanism. Let's also assume that we have a crossover to the tweeter at 2,500 Hz with a slope of at least 12dB/octave so that below the crossover point no further increase in excursion occurs with decreasing frequency as long as the drive level is constant. In this example let's say the tweeter excursion will reach 1 mm at 2,500 Hz at a level of 90 dB. Note that I'm not talking about sensitivity, only excursion and SPL. If I wanted either a higher SPL at that frequency or the same SPL at 1,250 Hz, we would exceed XMax and have high distortion.

Now let's add three more tweeters of the same type and look at the same scenario. At 2,500 Hz to get the same SPL we must have the same volume displacement but since there are four tweeters there is four times the radiating area and therefore each tweeter excursion can be 1/4 as much and still get the same volume displacement and SPL. Now if we take the four tweeters and change the crossover to go down to 1,250 Hz the excursion of each tweeter is now four times the excursion at 2,500 Hz but the four tweeters now only required 1 mm each. That is because 4 times the 1/4 mm at 2,500 Hz is 1 mm at the lower 1,250 Hz that is within the linear operation range of the tweeter. We have not changed the frequency response or total SPL of the tweeter array above 2,500 Hz but with the lower crossover point we can now expand the low end of the frequency response by one octave without creating the high distortion that would have occurred with only one tweeter. Note that in either case we are not going anywhere near Fs and whatever roll off occurs mechanically below Fs will remain unchanged. It is all about adding more driver area that can translate into a lower crossover point at a given SPL.

Alternately we could leave the crossover point at 2,500 Hz and add the other three tweeters. Now the combined tweeters can produce 6 db more output at the crossover frequency than before. This will not affect the SPL capabilities above this arbitrary set of conditions because in either case as the frequency goes up the excursion decreases by a factor of four for each octave. In reality, at the lower end of their range most dynamic drivers are excursion limited and as the frequencies go higher they ultimately are limited by power dissipation limits. As a side benefit, the four tweeters can dissipate four times the power so compression or dissipation problems are reduced as well.

Hope this explains the situation.

Thanks Tom
its clear to me and answers my original question.