How do you determine the impedance of a speaker system?

[denjo]

Hi Alan

I have often been intrigued by how you determine the specific impedance of Harbeth speakers? Taking an example, the SHL5 has a 6 ohm officially published impedance. The impedance is very important since it determines how much power is needed from the amplifier to suitably drive the speakers. Generally speaking, a 60 watt amplifier (at 8 ohms) would be 90 watts at 6 ohms and 120 watts at 4 ohms. When I study the impedance chart (or at least try to), the impedance is represented as a curve which rises and dips across the frequency spectrum. Do you take an average across the board or is there any possibility to "massage" the figures in order to get the 'right' numbers? As a speaker designer, I would appreciate your thoughts on this question of impedance and of how manufacturers can determine the imepdance depending on the parameters they consider important.

Best regards
Dennis

[Pluto]

Generally speaking, a 60 watt amplifier (at 8 ohms) would be 90 watts at 6 ohms and 120 watts at 4 ohms

Denjo - what you have written would be precisely accurate if impedance and DC resistance were one and the same thing. I think, before your question is answered, we need a short discourse on the nature of AC and the relationship between AC voltage and current and what happens when the frequency in question is near a crossover point.

What a can of worms :)

[garmtz]

The simplest way to do it, is to use a signal generator on the speaker input that generates a 1 V signal and let the generator sweep the signal from 20 Hz to 20 kHz and measure the resulting current on a large number of points, which you then can calculate back to impedance with Ohm's law (U = I * R, or R = U / I, where U = 1 V, so R = 1 / I).

Well, this is theory, I haven't really tried something like this in practice.

[witwald]

I have often been intrigued by how you determine the specific impedance of Harbeth speakers? Taking an example, the SHL5 has a 6 ohm officially published impedance. The impedance is very important since it determines how much power is needed from the amplifier to suitably drive the speakers. Generally speaking, a 60 watt amplifier (at 8 ohms) would be 90 watts at 6 ohms and 120 watts at 4 ohms. When I study the impedance chart (or at least try to), the impedance is represented as a curve which rises and dips across the frequency spectrum. Do you take an average across the board...

Firstly, the doubling of power output from an amplifier with a halving of the resistive load is a theoretical ideal. It takes a very good amplifier with a strong power supply to even approach that sort of behaviour.

Taking a look at the SHL5 electrical impedance curve, it can be seen that the minimum impedance is about 5.9 ohms in the vicinity of 150 Hz, although it does drop as low as 5.2 ohms at 20 kHz. The minimum at about 150 Hz has a phase response of about 0°. This indicates that the speaker is behaving more or less like a pure resistance around that frequency, and that is a relatively easy load for any competently designed amplifier to drive.

Keep in mind that there is usually quite a bit of sound energy in the 80 Hz to 500 Hz frequency range, so having a relatively moderate impedance of around 6 ohms in that range makes for an easier load for any amplifier. Outside that frequency range, the impedance of the SHL5 is usually greater than 6 ohms, so that means that the amplifier will need to supply relatively less power to the loudspeaker at those frequencies for a given sound pressure level. The maximum phase shift in the electrical impedance of around ±40° also helps to make the SHL5 an easier load to drive as the impedance is not very reactive in nature.