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Old 16th February 2009, 11:52 PM   #1
pjpoes is offline pjpoes  United States
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Default Driver Choice and Maximum Output

I know it seems like my posts are beginning to have a theme here. What are the factors specifically that dictate the maximum output ability of a driver. Both short term and long term for that matter.

Also, how is this related to size of the driver. For instance, do all compression drivers with 3" diaphragms play louder than all 1" drivers with 1.5" diaphragms. I suspect it's not so simple, but is related to displacement some. In what ways are this related to frequency though. I mean, a 15" woofer will always displace more than a 3" compression driver diaphragm, but we don't use those for high frequencies, and I don't imagine it's simply because of the directivity. It would be a much higher mass, probably harder to accelerate to that speed, no?

What about voice coil diameter, how does that play a role in a speakers efficiency, output ability, and thermal behavior? Due larger diameter voice coils always handle more power, or is it more complicated than that?

Speaking again in terms of high frequency drivers (I wouldn't mind answers relevant to all drivers as well though), what advantages and disadvantages are there in using the different sized compression drivers. As I recall with Dome tweeters, directivity becomes more narrow at higher frequencies with larger domes, but with compression drivers, I imagine that's largely dictated by the waveguide or horn, no? It seem's that larger sized compression drivers have greater low end, but a greatly reduced top end. Does this quality make them less desirable to use as the high frequency driver.

And finally, since I asked Dr. Geddes this same question in an email, I have one last question targeted at him. In the Summa line, what was the thinking behind the 1" driver over other choices. As I understand the white paper and what topics I have read of yours, the goal was a ~1khz crossover point high efficiency 2-way design. On one hand, all other things being equal, the larger driver would have more headroom in that crossover area, since those larger drivers seem to have more low end extension. On the negative, the response doesn't seem as even, and would require more crossover work I suspect. They also cost more, but if the benefits outweighed the costs, I don't imagine a 1" driver would have been used instead of a 2".
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Old 17th February 2009, 01:09 AM   #2
HK26147 is offline HK26147  United States
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The wire in voice coils is among the narrowest paths that current flows through ( post amplification ). The small gage wire has limited ampacity.

DC resistance is slightly less than impedance due to the wire in the voice coil.
A typical 8-ohm four layer woofer voice coil contains about 120 feet of number 28 solid copper wire,
AWG gauge 28 wire has .0649 ohms per foot ( x 100' = 6.49 DCR )

A mid range speaker can have about 30 feet of number 33 solid
copper wire: 33 wire/ 0.2069 ohms per foot ( x30' = 6.138 DCR )

A tweeter can have 20 feet of number 35 solid copper wire.
35 wire .329 ohms per foot ( x 20' = 6.59 DCR )

The lighter gauge necessary for tweeters ( less mass to move ) has less ampacity and therefore less power handling capability.
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Old 17th February 2009, 02:53 AM   #3
gedlee is offline gedlee  United States
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Complex questions and its too late for adequite explainations.

In the Summa line the 1" driver was dictated by HF response. Larger diaphragms will not go high enough, so 1" was the limit without a second waveguide, which I seriously wanted to avoid. In a system that needs MORE output a second waveguide is required, but output is not the issue in a speaker like the Abbey or the Summa, so 1" is the ideal choice.

More on this tomorrow. Bed time - long day.
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Old 4th March 2009, 12:03 AM   #4
MartinQ is offline MartinQ  Canada
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Old 4th March 2009, 03:05 AM   #5
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Old 4th March 2009, 12:40 PM   #6
gedlee is offline gedlee  United States
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Forgot all about this.

The larger the compression driver the more power it can handle, but since it is larger it will have a lower HF capability, but also a lower LF capability. It all scales downward in frequency and upward in power. The larger the throat the more limited the HF control the waveguide can provide. It really makes no sense to use a compression driver that is larger than what you need as this just limits the HF capabilities. And for home use any compression driver is going to have enough headroom. In a large venue like a concert, power limitations are a serious matter and the larger format drivers are a requirement. But not for our usage.
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Old 4th March 2009, 12:43 PM   #7
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To make sound you have to move air. So ignoring lots of other factors like directivity, whatever speaker moves the most air will create the loudest SPL at a given frequency. So volume displacement: cone area * displacement. I suppose the next layer of complexity would be factoring in power handling and efficiency. Ie, you would need to look at the thermal limits and see if you were mechanically limited or thermally limited both in the short and long term depending on what you cared about. In general, for the same BL limited displacement, a larger coil will handle more power. Next you could look at effiency versus frequency - if you have a large woofer versus a 1" tweeter as in your example, the woofer will not reproduce high frequencies as well due to it's higher coil inductance, cone breakup, directivity, cone and coil mass and impedance mismatch between coil and cone, etc. Going from there, you could start to consider directivity and breakup. This would effect your comparisons at high frequencies. Larger diaphragms will break up at lower frequencies, all else equal (which it usually isn't). Also, larger diaphragms will have narrower directivity at a lower frequency than a smaller diaphragm, so you would need to specify your design goals and see if they were met by a specific case. For horns, generally a similar compression ratio is used in compression drivers, so larger diaphragms will go with larger driver exits which leads to the same situation - increasing directivity at a lower frequency.
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Old 4th March 2009, 02:02 PM   #8
MartinQ is offline MartinQ  Canada
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Originally posted by John Sheerin
To make sound you have to move air.
I was tempted to make a comment about moving versus compressing air to produce sound. And how, curiously, that is different in other materials like water which cannot be compressed... but then, that would be a prickish thing to do, wouldn't it?

(sorry I guess I couldn't resist after all)

The point-of-comedy I reference is in this thread:
Compression of water (split from Waveguides)
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