Impedance |
Impedance graph of reflex (ported) loudspeaker - solid green line (1). Broken lines show - 2 - D.C. resistance, commonly 4 Ohms. This is also the minimum impedance, and an ideal impedance characteristic. 3 - bass resonance of closed box (no port) loudspeaker. 4 - almost ideal impedance, with low peaks. 5 - midband impedance rise caused by bass inductor in crossover. 6 - fall in impedance above 2kHz due to high pass section in crossover and tweeter. WHAT IT TELLS US This graph shows change of impedance with frequency, and is a more detailed look at how the loudspeaker behaves as a load on an amplifier. Because a loudspeaker’s acoustic behaviour reflects back into the load it also shows how the loudspeaker is behaving, especially at low frequencies where the acoustic load is high. This is why we publish the impedance graph, it says a lot, but it does need interpretation. Ideally, the green trace should be smooth and horizontal, and at 4 Ohm on the left vertical scale for a 4 Ohm loudspeaker (see broken line 2), 6 Ohms for a 6 Ohm loudspeaker, etc. If impedance were flat like this the loudspeaker would be ‘ideal’ as far as an amplifier is concerned, so flatness is what we are looking for. In practice most impedance plots look like a series of hills and dales, our solid green trace showing a typical two way reflex design. Compare this with a real two-way like the Q Acoustics 2050 below. The hills and dales can be explained, at least in basic outline. Bass resonance (3) is opposed by the anti-resonant system of the port, which introduces a dip at either side of which lie residual peaks. These sudden and steep changes of impedance indicate high values of reactance (energy storage) and are unwanted. The common rise in impedance above 200Hz is due to the bass inductor and low pass network to the bass unit, not the voice coil, Spice analysis shows The sudden drop in impedance above 2kHz is caused by the high pass section feeding the tweeter.
Q Acoustics 2050 impedance graph. It is possible to flatten the impedance curve of a loudspeaker by using equalising Zobel networks across drive units and cleverly adapted bass loading techniques, like stagger tuned chambers, resistive ports etc, but at present loudspeaker designers don’t appreciate the importance of this, so ignore it. Amplifier feedback networks can be affected by excessive reactance and the V/I phase shift it produces at high frequencies. At low frequencies excessive reactance can upset the sensing systems of protection circuits, causing them to ‘chatter’. So what the impedance graph has to say about the loudspeaker’s design sophistication and efficacy as a load is far ranging.
HOW WE MEASURE IT Our measurement system is calibrated with an 8 Ohm resistor first and a gated sine wave measurement signal is stepped downward in frequency to slowly but accurately capture sharp impedance changes. The result is checked with a slow gliding tone test and, where high rates of change exist, with mls noise. IMPEDANCE VALUE (modulus of impedance; we call it 'overall impedance'.) WHAT IT TELLS US The impedance of a loudspeaker is its rating as a load. Traditionally, the U.K. has used 8 Ohm loudspeakers, the USA 4 Ohms. Nowadays, manufacturers are settling on 6 Ohms. A 4 Ohm loudspeaker draws twice as much power from an amplifier as an 8 Ohm, at any given volume setting, so it will sound appreciably (3dB) louder. Modern transistor amplifiers can drive 4 Ohms with ease, although distortion rises a little in most cases. However, impedance varies with frequency and 6 Ohm loudspeakers actually use 4 Ohm bass units. Since most power is delivered at bass frequencies this is the load the amplifier sees (our impedance graph tells more about this). Loudspeakers with a 4 Ohm impedance value are fine providing their minimum value does not sink much lower. If it does current limiting protection circuits may trigger, causing relay chatter, as volume is turned up. Loudspeakers with an impedance higher than 8 Ohms need a powerful amplifier to go loud. They do not utilise the power available from an amplifier and waste its potential. HOW WE MEASURE IT We inject 2.8V rms of pink noise applied for sensitivity measurement and measure true rms a.c. current flowing into the loudspeaker, calculating an impedance value from the two. This should - and does - compare sensibly with the graph of impedance we also publish. DC RESISTANCE WHAT IT TELLS US This is the minimum impedance of most loudspeakers, at d.c. and is in most cases the minimum value of the impedance graph. This does not hold true with loudspeakers using an input capacitor, as some KEFs in the past, or the few with parallel reactive networks in their crossovers. These apart, d.c. resistance is in most cases a measure of bass unit voice coil resistance, including that of the internal wiring and series low pass inductor. This measurement is a useful cross check and also allows reactance to be calculated at any frequency. HOW WE MEASURE IT Resistance is measured with a normal Fluke hand held voltmeter / ohmeter. |