Chipamps; Design Considerations for bridging and paralleling.

[PHilgeman]

For an 8 ohm 80dB sensative woofer (After Baffle-Step-Losses of course) to hit 100dB you need 100 Watts.

P=(V^2/R)/1.414
P=(I^2*R)/1.414

(ported box) At Fb impedence is a nice and easy 8 ohms, at say Fb+30Hz there is an impedence peak of say 30 ohms.

Lets figure this out:

At Fb: P=100 R=8

Solving for V and I we get:

V=33 volts peak
I=4.2 amps peak

At Fb+30Hz: P=100 R=30

V= 92 Volts peak
I = 2.17 amps peak

For an 4 ohm load we'll give the benefit of the doubt and say that they are paralleled woofers with greater sensativity of 86dB.

Fb: P=25W R=4

V=11.89 volts peak
I= 2.972 amps peak

Fb+30Hz: P=25W R=20

V=26 V peak
I=1.32 A peak

Because of these things, dosnt it really make a lot more sense to do designs with higher sensativity, lower impedence drivers, using amplifiers with high current output ability? I really dont think that many amplifiers ever will be able to drive many woofers to 100W at one of the impedence peaks. It seems that the lower impedence speaker is easier to drive in every respect because of the better efficency and overall lower power requirement.

-Paul Hilgeman

 

[GregGC]

dosnt it really make a lot more sense to do designs with higher sensativity, lower impedence drivers, using amplifiers with high current output ability? I really dont think that many amplifiers ever will be able to drive many woofers to 100W at one of the impedence peaks. It seems that the lower impedence speaker is easier to drive in every respect because of the better efficency and overall lower power requirement.

-Paul Hilgeman

I think the power requirement is connected only to the sensitivity of the speaker and if you need more voltage because the impedence is higher, that's another story. I think there is an optimum of how low your speacker impedance should be so that you don't have to build monster amps to be able to drive 0.5 ohm speackers.

 

[PHilgeman]

I think the thing that people forget is that as impedence goes down sensativity *generally* goes up, reducing the overall power requirement.

-Paul Hilgeman

 

[macboy]

I really dont think that many amplifiers ever will be able to drive many woofers to 100W at one of the impedence peaks

You don't need to drive the woofer at 100W at an impedance peak. The reason is simple: impedance peaks correspond to resonances. At a resonance, the woofer delivers more SPL with less power. The efficiency rating (SPL @ 1W/1m) is nominal, which means kind of an average over the usual frequency range. The story is very different at resonance frequencies. Also note that when they say "SPL 1 Watt, at 1 metre" they really mean "2.83 Volts" not "1 Watt". The actual power doesn't matter, just the input voltage of 2.83 Volts (which equates to 1 watt into a nominal 8 ohm impedance but of course much less power at a impedance peak).

Also, don't forget about room gain. Below a certain frequency, you will experience a rise of (I think) 6 dB/octave due to room gain. Basically, when the wavelength of the sound is longer than the dimensions of the room, the sound no longer radiates from the woofer as a wave; the woofer pressurizes the room. So you probably will end up needing less than 100 W to achieve 100 dB.