Why are low impedance speakers harder to drive than high impedance speakers
I don't understand the electrical reason for this. I look at it from a mechanical point of view. If I have a spring that is of less resistance, and push it with my hand, it takes little effort, and I am not working hard to push it. When I have a stiffer spring (higher resistance) I have to work harder to push it. This is inversely proportional when we are looking at amplifier/speaker values.
So, when I look at a speaker with an 8 ohm rating, it is easier to drive than a speaker with a 4 ohm load. This does not make sense to me, although I know it to be true. I have yet been able to have it explained to me that makes it clear. Can someone explain this to me in a manner that does not require an EE degree?
Thanks
As watts got cheaper with ss, designers could not put as much effort into creating high imp. speakers and could also experiment with low imp. designs. But my question is: %-wise, what contributes to impedance: the overall design (enclosure, drivers, venting, etc.) or the crossover? I've always suspected that the crossover had a big role, and fancy crossover topologies often make for low imp. speakers (I suspect). |
Because of these properties of amplifier and speaker systems:
Let’s simplify this to a couple of statement: Amplifiers are current limited. This limit defines the maximum voltage at any given impedance. As the amplifier output impedance rises, or speaker impedance drops, the electrical frequency response at the speaker inputs goes from flat to tracking the speaker impedance. Want to understand why? Read on. As an introduction, see the first graph on my blog post on speaker impedance for a very easy to drive speaker: https://speakermakersjourney.blogspot.com/2016/12/crossover-basics-impedance.html We’ll ignore phase angle, as it’s hardest to grasp. Let’s assume instead:
At low frequency, say 300 Hz where the panel is 4 Ohms, the voltage at the panel is 4/(4 + 1) = 4/5ths = 80% of the amplifier output. At high frequencies, say 10kHz 0.3 Ohms the voltage stays at the amp, and the speaker gets 0.3 / (0.3+1) = 0.3 / 1.3 = 23% of the amplifier output, or about 25% of the amp output! But look what happens with an amplifier with very low impedance of 0.001 Ohms (aka high Damping Factor): At 4 Ohms: 4/(4 + 0.001) = 4/4.001 = 99.97% At 1/3 Ohms: 0.3 / ( 0.3 + 0.001) = 99.67% So in the range of speaker impedance from 4 to 0.3 Ohms, the amplifier output remains nearly rock-solid. |
Actually ports in ported speakers are perfect examples of why low impedance is harder to drive. Impedance at port frequencies is always low. Check any ported speaker impedance curve and see. After all the port itself has little or no physical resistance/impedance. Air blows right through largely "unimpeded". An amp driving the ported speakers well will result in a stronger air flow. One that cannot drive it well will result in little or no air flow meaning the port is not being utilized well to move air at the lower frequencies that it is intended to enable. |
@mapman Ahem, please see the example I posted, above. :) The drop at resonance is no lower than Re. :) I'm sure @Atmashpere will chime in, but depending on the amp, it's really the difference between the peaks and Re that makes a tube amp misbehave, as the high impedance will track this. If Re is high enough, it will be minor. Best, Erik |
