Question Translating specs

[FizzyPickles]

I'm on a bit of a mission to educate myself. I often find myself looking at Hi-Fi product spec sheets and wondering what the hell this or that actually means.
This week it's speaker specs.
What is nominal impedance, minimum impedance and crossover frequency actually telling me about how that speaker behaves and what bearing that has on system 'synergy' if any?

 

[Beobloke]

Nominal Impedance

This is the impedance rating of the loudspeaker averaged across its frequency range in some manner. Sometimes it's just literally an average of the whole lot; sometimes it's an average of the values at specified points. Either way, it's what you think of when you hear talk of a "4 Ohm" or an "8 Ohm" loudspeaker.

Minimum Impedance

Exactly what it says – the minimum value of the impedance For a conventional speaker this is usually just below the nominal impedance, so a nominal 4 Ohm speaker might have a minimum value of, say 3.2 Ohms. How low this figure is and where it happens in the frequency range are important for amplifier matching. Old Apogees used to go down to about 1 Ohm at the very low bass IIRC, which upset many amplifiers! Equally, electrostatics tend to hit their minimum impedance at high frequencies, which some amplifiers don’t like. In general, the lower the minimum impedance value, the more of a tricky load the speaker can be to drive.

Crossover frequency

Again, fairly self explanatory. If you have a conventional two way speaker with a crossover frequency of 500Hz, then the bass/mid driver is handling everything below 500Hz, and the tweeter is handling everything above. Obviously it's more complex than that, depending on the slope of the crossover, which is a measure of how quickly the circuit is stopping frequencies the 'wrong' side of the crossover point from reaching the driver.
It's something that sounds simple to work out. Trust me - it isn't!

 

[Nopiano]

As to your point about ‘synergy’, really only the minimum impedance matters in terms of matching with a suitable amplifier. It has virtually no bearing upon the sound you hear, and you can do nothing about these design choices in any case (aside from something silly like tinkering with the crossover).

Typically, I’d say that ‘popular’ speakers are engineered for an impedance above 4 ohms, to suit the less capable amplifiers they might be paired with. Once you get above, say, £1000 to £1500 a pair then the designs may become more ‘demanding’ (that is lower impedance) because matching amps are unlikely to flinch.

The crossover frequency can be interesting if you’re curious about the drive units themselves, but again there’s not really one defining thing you might hear. If there were then all manufacturers would strive towards that ‘thing’ - but, trust me, they don’t! Indeed, as speakers get more advanced the more they diverge in design. This proves there’s no universal aim, which is actually quite interesting in itself.