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In Reply to: RE: 105db/meter NON horn loaded speakers posted by horny on April 23, 2017 at 02:58:17
If one driver has 87dB/2.83 volt sensitivity two will have 93dB, four will have 99dB sensitivity, eight will have 105dB sensitivity, so it's as simple as that...if your amp will drive a 1 ohm load. Assuming it won't then you have to use a combination of series and parallel wiring to keep the impedance within a useful range, and that means even more drivers.
OTOH with a large enough horn you can reach 30Hz with 105dB sensitivity with one driver.
I've always been confused by that.
2.83 volts across 8 ohms is one watt.
2.83 volts across 4 ohms is two watts.
2.83 volts across 2 ohms is four watts.
If you have a speaker that will give 87db from one watt and you add another in parallel you get 90db from one watt. Add three (for a total of 4) and you get 93db from one watt, not 99db.
What am I misunderstanding?
I am going to guess that it's because I think in terms of tube amplifiers.
I think of using the 8 ohms tap for one speaker and the 4 ohms tap for two speakers and the 2 ohm tap for four speakers so the amplifier does not output twice the power just because the load impedance has decreased every time the load impedance is halved. In fact it stays the same because the load seen by the output devices stays the same.
Now that leads me to the idea of series/parallel.
The power output of a transistor amplifier only doubles when the load impedance halves so if the speakers are wired series/parallel, to keep the load impedance at 8 ohms, then four speakers vs. one would be 93db vs. 87db even for a SS amp. You would only get to 99db if all four were in parallel.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I am like Tre' in being a bit (a great big bit?) confused on this subject. I accepted the idea that doubling the number of drivers increases efficiency by 6 dB decades ago When Dr. Joe d'Appolito was writing for Speaker Builder. He always stated that doubling the number of drivers increased efficiency by 6dB. I assume he knew what he was talking about. Am I confusing efficiency and sensitivity.If Bill Fitzmaurice or someone else who has a good understanding of this subject could offer a more comprehensive explanation it would be very much appreciated.
Edits: 04/24/17
Hi Don
When two equal sources are about ¼ wavelength apart or less, they "feel" each other's radiation pressure (aka mutual coupling) and add coherently into one new source. In that process, you have doubled power handling (having two drivers now) AND increased radiation efficiency because with coherent addition comes an effectively larger radiator.
BUT when two sources are more than ¼ wl apart, they begin to radiate independently and by ½ wl separation they are independent sources and now when you go from one to two, you go up 3 dB because you no longer have mutual coupling that had raised the efficiency previously.
That increase in sensitivity has a practical upper limit at around 25% efficiency which would be about 106dB 1w1m. O
ne can see a difference between independent radiation and coherent coupling by inverting one of the drivers in each temporarily.
In the close coupled case where they add to become one source, the two sources nearly completely cancel each other out, with more than ¼ wl spacing, only the radiation pattern changes when you invert one because they are two independent sources, not one.
Doubling the driver count parallel wired increases voltage sensitivity by 6dB, not efficiency. It doubles efficiency, which is 3dB of the sensitivity increase. It halves the impedance, giving the other 3dB voltage sensitivity increase. If you double the driver count series wired the double radiation efficiency still gives you an additional 3dB of sensitivity, but the doubled impedance takes that 3dB away, for a net unity gain. But you don't keep getting a doubling of efficiency with every doubling of drivers. 25% efficiency is about it with direct radiators, usually reached with between 16 and 32 drivers.
Here is how I look at it.Doubling the number of drivers (in and of itself) increases the driver sensitivity by 3db.
Now take into account that when the drivers are wired in parallel then the impedance is halved.
With a transistor amplifier (at least in theory) the output power will double when the load impedance is halved.
So with twice an many drivers (wired in parallel) the total increase in system sensitivity is 6db.
To say all of that a different way,
When you double the number of drivers (from one to two) it increases the sensitivity by 6db (twice the power gives 3db and twice the cone area gives another 3db).
BUT if you only provide one watt the net increase is only 3db because each driver is only getting 1/2 watt (twice the cone area gives 3db but we're not providing twice the power so 3db is the net increase).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/24/17 04/24/17
Answers on line array sensitivity can be found in my Near Field Line Array white paper.
SPL calculations are on page 1718.
Jim
"For example, two speakers connected in
parallel and mounted within a wavelength centertocenter spacing would yield up to a 6dB increase in SPL. 3dB from the increase in acoustical energy and 3 dB from the reduction in impedance."That's what I said and what Bill said.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/25/17
Well with two sources a half wl apart, you get +6dB over one on axis only.
Think about a polar plot, the diameter is level vs angle and the area within the polar circle summed is the total acoustic power.
Two sources that far apart produce a figure 8 pattern who's peak value is +6dB on axis but the area enclosed by the polar circle is only 3 dB more than each source individually.
Move those sources to less than 1/4 wl apart and now you have a circle who's diameter is +6dB and the total energy is now 6dB greater than one was.
acoustic power
1+1=2
or
1+1=4
depends on the spacing and location
nt
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