Speaker ratings, how to interpret?

The nominal rating is the impedance the speaker manufacturer wants you to think the speaker is. The minimum impedance is closer to what really matters: what's the toughest load a speaker presents to an amplifier? (Impedance varies with frequency, as you may already know.) But when I see a minimum impedance of 4 ohms, I generally assume that means "3 ohms."

As for amps, it's not so easy. What you'd like to know (among other things) is how much power the amp can produce in a short burst at the frequency at which your speaker's impedance dips to its lowest point. But your power spec is for a continuous tone (by government fiat, by the way), usually at 8 ohms.

As James Boyk once wrote, the only two specs that mean anything are the dimensions and the weight. The dimensions tell you whether it will fit on your shelf, and the weight tells you if your shelf can hold it.

There is no link between the db and the ohm rating.

The dB rating basically tells you how loud the speaker will sound when driven with a certain amount of power.

The ohm rating, the impedance, can tell you how difficult it is for an amplifier to jamn a certain amount of power into the speaker. Neither a very low nor a very high impedance is desirable ... most amps are happiest around 8ohms ... they're designed that way.

Read on if you want to get technical (though it's 10 years since I did any amplifier design).

Amplifiers are limited by the current supply (usually down to size of transformers, capacitors, and duration of the loud passage), and by maximum output voltage, which is an inherent part of the design (usually near the power rail voltage, switched by the power transistors for SS amps).

From Ohm's law, V=I*R, and from the power law P = V * I one can derive that the power delivered to the speakers can be expressed in terms of voltage OR current :

power = V*V/R OR

power = I*I*R

Therefore, given the maximum voltage of the amplifier, or the maxiumum continuous current (or both), and given the worst case R, you could calculate the maximum power which could be driven into the speakers. (This is theoretical and the real value is a little lower).

So power delivery to the speakers when the speakers are higher impedance is usually limited by the maximum voltage of the amplifier (before clipping occurs).

Conversely when speakers have very low impedance the power delivery is limited by the current driving ability of the amplifier.

If you can find out the rail voltage and maximum continuous current of your amplifier and the minimum and maximum impedance of your speakers you could calculate a rough maximum power transfer, then use the dB efficiency to come up with the SPL. Of course, they're all AC signals, so you need to multiply by 1/root PI, or something like that .. it's too long ago for me to remember.

You have to look at ALL of the specs, inch by inch. If they spec "at one watt", the spl levels are directly comparable regardless of impedance. If they spec "at 2.83 volts" and are a relatively lower impedance speaker, they are playing games. This takes into account that they are "averaging" the spl level over the "midband" frequency range and not just picking some arbitrary frequency that may produce a slightly higher peak SPL.

With a speaker that is "nominally" 8 ohms, 2.83 volts is equivalent to one watt of power. Applying the same amount of voltage into a "nominal" 4 ohm speaker is NOT one watt of power. That is because, even though the voltage is the same, current is doubled due to the speaker having half the resistance. The 4 ohm speaker may appear to be more efficient or as efficient, but in reality, it is sucking up measurably more power to obtain those spl levels.

Another little known trick is to spec the speaker at 3 ft instead of at 1 meter. While this is only a matter of 3.5" or so, taking the measurement a little closer and "fudging" the figures can help them out a bit. Combine the distance factor with a higher current level into a low impedance load can make things seem quite a bit "sweeter" than they really are.

So, to compare apples to apples, you have to have the same reference level i.e. X amount of wattage into the speaker and measured at the same distance. Otherwise, you can end up with more power ( same voltage but more current ) measured at a closer distance.

Even with all of that in mind, you need to realize that different types of speakers ( dynamic "cone" drivers, planars / ribbons / e-stat's, horns, etc...) will produce different spl levels at various distances EVEN if they may produce the same spl at 1 meter. This all has to due with how they load into the room, their individual dispersion characteristics and radiation patterns, etc...

Like anything else, there are TONS of ways to create specs, interpret specs and "bamboozle" the unsuspecting. That is why so many "old timers" say things to the effect that specs are relatively useless. Even with the few rules that the FTC has created to "protect the consumer", those rules have some BIG loop-holes in them. Your best bet is to keep your eyes and ears open and try to experience as much for yourself as you can. First hand experience can be a HELLUVA teacher... Sean
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