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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

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mapman's avatar
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mapman
15,679 posts
 
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. 
erik_squires
5,268 posts
 
@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
bdp24's avatar
bdp24
4,908 posts
 

While a 16 ohm speaker may seem to "present an enormously easy load for virtually any power amplifier", such is not necessarily the case, as that 16 ohm rating is merely nominal. For instance, the original Quad ESL's nominal impedance was 16 ohms, but it’s impedance rose to 60 ohms at low frequencies, and fell to 1.8 ohms at high frequencies---anything but an easy load! That impedance characteristic is one reason the sound of the Quad ESL is so affected by the amp driving it, and why almost no solid state amp is a good match---it makes for overblown bass and missing highs. Ralph Karsten of Atma-Sphere has already explained this a few times.

Related to my post above, using the lowest impedance tap on a tube power amp not only usually results in the lowest distortion and best sound the amp is capable of, but the amp’s damping factor is also highest, leading to the flattest frequency response it is capable of, irrespective of speaker impedance characteristics.

audioman58's avatar
audioman58
1,219 posts
 
Also remember the complexity of the Crossover is directly proportional 
To ohms,resistance to the drivers and demand affecting  the Amplifiers 
Constant changing ohms ,and Amplifier Load depending on demand. 
A speaker can  go from a 8ohm load to 2 ohms which Is very demanding 
For a conventional driver Loudspeaker. Panel or stats ,can dip down to 1ohm .
This  is where a Amplifier be rock solid .verify that the  amplifier you are using 
Can handle the extreme for a given Loudspeaker before purchasing it.
charles1dad's avatar
charles1dad
6,996 posts
 
Hi Bdp24,
For sake of this discussion I’m referring to speaker load impedance characteristics that remain "relatively flat. So my hypothetical comparison would be a 4 ohm vs a 16 ohm. A speaker such as the Quad that you cite with its extreme fluctuations renders nominal rating meaningless lf both the 4 and 16 ohm speakers are stable, the higher ohm speaker demands less current and work from the driving amplifier as predicted by Oohm’s law.

In reality there are speakers with both reasonably flat impedance characteristics and gentle phase angles. There are also those with widely fluctuating impedance and steep phase angles. It seems that the former would be preferable as it is far less demanding on a power amplifier assuming that either provides comparable sound quality. 
Charles
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