After our recent tips on speaker impedances we’ve had a few people inquire deeper in to the subject. The following questions (all from one reader) are typical.
“What is the real difference between speakers of different impedances, all other things being equal? I know that higher impedances draw less power and put less strain on the amplifier, resulting in less distortion. It would seem to me that this would be a good reason for higher impedance, not lower, but the trend in audio equipment seems to be lower and lower speaker impedance. Why? What does this really gain you? And is there anything wrong with wanting to stick with 8 ohm speakers?”
If all other things are equal the only difference in speakers with different impedances is, by definition, the impedance. Not to be too glib about it, but in practice that pretty well sums it up as well. There are numerous factors that go into speaker design that we really don’t need to get in to here to understand how things work in practice. Suffice to say that in modern designs you may encounter a variety of impedances among loudspeakers that are otherwise nearly identical from an operational point of view. Further, once multiple speakers are combined in an enclosure with a crossover network, a new net impedance for the system will result. This is the load the amplifier sees, but it may not be the actual impedance of the raw components.
Speakers with higher impedances will result in less current flow from the amplifier. While it is sometimes true that this can result in slightly less distortion in the amplifier, it isn’t always a major consideration. There are other issues, such as damping factor, that are also affected by the load, but those are all considerations that often take a back seat. What you gain by having lower net impedance is more power output from the amplifier. You may not find this to be a huge concern in the studio, but in sound reinforcement applications it is quite often the foremost concern. When outfitting a large PA system you can cut the number of amplifiers you need nearly in half by working with 4-ohm loads instead of 8-ohm loads, and when you’re talking about dozens of amps that can be a big cost savings.
Or, at a more practical level for most musicians look at the following example. You have a bass amp and you need a good speaker for it. You can choose an 8-ohm 15″ loudspeaker or a 4-ohm 15″ loudspeaker. Your amp will produce, say, 250 watts into 8-ohms and 500 watts into 4-ohms. You may or may not decide to get the 4-ohm speaker to maximize the output of your amp. Or maybe you get two 8-ohm 15″ speakers and wire them in parallel with one another to produce a 4-ohm load.
Aside from those practical considerations another reason it is more common to see speakers with lower impedances these days is because modern amplifiers are more capable of remaining stable while delivering large amounts of power into these loads.