Why are low impedance speakers harder to drive than high impedance speakers

It’s all about the bass, about the bass,. Not the...

well it’s also about size and cost actually.

The reality is low power tube amp lovers have limited speaker choices at least if bass extension matters. Still good ones out there though if a low power tube amp is what one chooses to build around.

Making music especially bass is a lot of work. Somehow combo of amp and speakers must be up to the task else results suffer in comparison. A nice high pressure shower makes for a much better clean. The actual amount of water used might still vary widely though.

Ports in ported designs are always a major factor it seems. 

Because of these properties of amplifier and speaker systems:

1. Amplifiers have a non-zero output impedance
   
2. Voltage is proportional to impedance in a series circuit.
3. Speakers usually present complex impedance, with a range of impedance magnitude and voltage / current phase angles

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:

• Tube amp output with about 1 Ohm impedance,
  
• An ESL such as a Martin Logan electrostatic, which has a panel impedance that varies from 4 Ohms at the bottom of it’s range to 0.3 Ohms at the top end.

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.