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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

128x128crazyeddy
ghosthouse's avatar
ghosthouse
4,312 posts
 
I ain’t Al but one of the few things I sort of understand about electronics is Ohm’s law.

From Wikipedia
Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:

I (current; amps) = Voltage divided by Resistance (ohms)

At the risk of embarrassing myself, as resistance decreases the current required for a given voltage goes up.

As an example:
1 volt/8 ohms = 0.125 amps;
1 volt/6 ohms = 0.167 amps;
1 volt/4 ohms = 0.25 amps
1 volt/2 ohms = 0.5 amps

So, for a constant voltage, current requirements double going from 8 ohms to 4 ohms and quadruple going to 2 ohms. Current production is a more useful indicator of amp "grunt" than rated wattage (or so I think).

The other thing to remember is that for a given speaker design, resistance might not be constant across the frequency range. Low resistance dips occurring at different points can result in differences in how things sound between two amps with similar wattage ratings but differing in current output capability.

That’s what I think I know.
rcprince
3,073 posts
 

I'll embarrass myself by saying I have always thought of it as a hose with water going through it.  If you have a narrow opening at the end (say, using a nozzle), which equates with high impedance in my analogy, you can get a powerful jet of water from the hose with your normal water pressure or even less, but if you take the nozzle off (low impedance) you won't get the same powerful jet of water unless you significantly increase the water pressure (in the analogy, a much more powerful amp). 

I'm sure Al will give the best technical explanation.

jafant's avatar
jafant
19,737 posts
 
Maybe unsound will see this and chime in;
dlcockrum's avatar
dlcockrum
2,190 posts
 
"Current production is a more useful indicator of amp "grunt" than rated wattage (or so I think)."

Nice job ghosthouse.


Best to you,
Dave
gdnrbob
4,447 posts
 
And, remember most of the amps power is going to the lower frequencies. So, if the impedance dips at low frequecies, your amp will have a hard time keeping up with the demands of the speaker.
Like the water in the hose instead of being restricted by a nozzle, is now having to provide the same pressure in a larger diameter hose.
HTH
Bob
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