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Well, What makes a driver High Efficiency?

Hi
I think most of the posts have touched on parts of it.
Efficiency is the issue or how much power radiates how much sound.

A loudspeaker radiates sound power into a resistive load that is the air.
There is a radiation resistance curve that is a 6dB/oct slope up to a frequency where the radiator is 1 WL in circumference and known as "K=1"

This means that above K=1, there is no gain in efficiency making the radiator or Horn larger (although doing so causes the pattern to narrow).

In the olden days, they used to say a 30Hz bass horn had to be 10ft in diameter to work. Well a look at bass horns in commercial sound at work shows that's ridiculous BUT IT IS based on the fact that for 30Hz, K=1 is about 10ft in diameter in full space.

So the radiator area is a key part of all this as that set's where you are on the radiation resistance curve. If you place two identical subwoofers side by side, you get +6dB (4x) the amount of bass energy as you have doubled the radiating area. This gain from mutual coupling goes away when the sources are more than 1/3 Wavelength apart.

One can imagine that as the frequency goes up, the acoustic size of the source doubles each octave you go up and that means with the same velocity at all frequencies, the response will climb +6dB /oct, called a velocity response. IF one had a horn which presents a resistive load to the radiator, THAT is the response one would want for the driver.

On the other hand, to make a direct radiator that is acoustically small "flat", one needs to have an acceleration response instead, rolling velocity off 6dB /oct.

How? Go to the link below, scroll down to "electrical subsystem".
Your woofer / cone driver has a series R and (small) L, both are properties of the gauge and length of wire and it's winding into a coil.
Those are in series with the R,L C parallel circuit in the mechanical mobility analogue lower down. This is what the wire in the magnetic field looks like being free to move, attached to a mass C and spring (L the suspension). IT is the Velocity of the radiator / motor that face the radiation resistance.

You can see the parallel RLC tank circuit is where the sealed box impedance peak comes from and at 0Hz or DC, you can see the series R with an ohm meter. Above resonance, the parallel C dominates and now one has a series R, small L driving


Now the ugly wall of science creeps in, Hoffman's iron law.

The upshot is for flat response that no matter what driver parameters you have, even made with unobtainable , your maximum possible efficiency is set by the box volume and -3dB low corner F.

Lower the corner 1 octave for the same box size and the maximum possible efficiency drops 9dB.

Horns have an upper efficiency limit too although the directivity (DI) can cause a large increase in on axis sensitivity that looks like efficiency in front.

Tom Danley





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