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How to find the resultant SENSITIVITY & IMPEDANCE of a 2 way or 3 way speaker

Naveenbnc

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#1
Dear all,

In a 2 way or 3 way, we combine two drivers with different sensitivities & different impedance.
In such case, how to find out the resultant sensitivity and impedance of the overall speaker?
Does it have to do anything with the crossover?

For example, lets say, I build a 2 way with the following drivers:

1. Full range driver with 97dB sensitivity and 8 Ohms impedance.
2. Sub driver with 89 dB sensitivity and 4 Ohms impedance.

What will be the final sensitivity and impedance of the overall speaker?

Any suggestions or formulae please ....
(I am aware that impedance changes with parallel/series connections, but not sure about sensitivity)

Regards,
Naveen
 
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Kannan

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#2
It depends on the type of crossover, if passive or active.
In an active setup, it is quite straight forward as amps see only one pair of drivers that it is connected to, and there is no combined effect.

In passive it is more complex and depends on the design of the crossover, that is why it is the most critical part of the design.
The best crossover is one which crossed and sloped (capacitors and conductors), and attenuated (resistors and zobel networks) to get the linear targeted average impedance and spl across the entire bandwidth.
For example, if the tweeter n woofer are both 4 ohms, the crossover should be designed in such a way that the amp sees a uniform load of 4 ohms (or the average targeted impedance) anywhere in the music bandwidth.

It gets even more complex with 3-way and 4-way speakers as there will be high pass, low pass and bandpass section to take care of.

Example of a simple singe order design:
Tweeter f/s 1500 hz, nominal 4 ohms, spl 89db
woofer f/s 55 hz, nominal 8 ohms, spl 86db....cone break up 5000hz

In the above example the safe HPF for the tweeter is around 4.5 khz at 6 db offloaded by a capacitor.
The LPF for the woofer should be around 2000hz at 6db offloaded by an inductor.

Upto around 900hz, the tweeter will offer high resistance, so the voltage will flow across the woofer, and with an inductor in front it will shelve off the high frequencies.
Similarly from around 5khz, the woofer will have high impedance forcing voltage to flow across the tweeter and the capacitor will cur off low frequencies.
The bandwidth from 900hz to 5khz is where the current will flow across both the woofer and tweeters, and the crossover design will have to take care of the right crossover point and attenuation with the slope being single order to compensate for the combined impedance of both the drivers (which is in effect in parallel) and their spl levels.

Hope this clarifies to some extent.
 

Naveenbnc

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#3
It depends on the type of crossover, if passive or active.
In an active setup, it is quite straight forward as amps see only one pair of drivers that it is connected to, and there is no combined effect.

In passive it is more complex and depends on the design of the crossover, that is why it is the most critical part of the design.
The best crossover is one which crossed and sloped (capacitors and conductors), and attenuated (resistors and zobel networks) to get the linear targeted average impedance and spl across the entire bandwidth.
For example, if the tweeter n woofer are both 4 ohms, the crossover should be designed in such a way that the amp sees a uniform load of 4 ohms (or the average targeted impedance) anywhere in the music bandwidth.

It gets even more complex with 3-way and 4-way speakers as there will be high pass, low pass and bandpass section to take care of.

Example of a simple singe order design:
Tweeter f/s 1500 hz, nominal 4 ohms, spl 89db
woofer f/s 55 hz, nominal 8 ohms, spl 86db....cone break up 5000hz

In the above example the safe HPF for the tweeter is around 4.5 khz at 6 db offloaded by a capacitor.
The LPF for the woofer should be around 2000hz at 6db offloaded by an inductor.

Upto around 900hz, the tweeter will offer high resistance, so the voltage will flow across the woofer, and with an inductor in front it will shelve off the high frequencies.
Similarly from around 5khz, the woofer will have high impedance forcing voltage to flow across the tweeter and the capacitor will cur off low frequencies.
The bandwidth from 900hz to 5khz is where the current will flow across both the woofer and tweeters, and the crossover design will have to take care of the right crossover point and attenuation with the slope being single order to compensate for the combined impedance of both the drivers (which is in effect in parallel) and their spl levels.

Hope this clarifies to some extent.
Dear Kannan,

Thanks a lot for a very detailed explanation. I understood this at the periphery, but I have to take some more time, and study it thoroughly.

But surely I understood it is not as straight forward as I thought. And crossover design is also critical !!!
 

Naveenbnc

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#5
Go for active, PLLXO, its DIY friendly
Thank you.. :) sooner or later I have to try active Xo.

But now I started realising crossover design is not something that I can really do on my own any time soon. Any way, I m so happy to have our friend Aniket help for all my crossover needs. :D

I can limit my self to enclosure design for now.
 
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