What effect does Bi-wiring have?
Biwiring (as opposed to bi-amping) is when two separate speaker cables are connected to the same terminals on the amplifier, but connect to the tweeter circuit and woofer circuit separately at each speaker. This arrangement forces the tweeter currents and the woofer currents to run in separate cables. The first effect of this configuration is that the voltage drops that occur on the woofer cable (when large currents are present) do not affect the tweeter cable. The second effect is that the load impedance that each cable experiences is different, since for typical speakers the woofer is very low impedance and the tweeter is relatively high impedance. Whether or not these affects are audible is debatable and indeed it is debated hotly among academics. Lets avoid this debate for the moment and examine the circuit differences between simply doubling-up the speaker cable and then biwiring it.
A SPICE3 simulation was done to explore the advantages of biwiring. A 4 meter 11AWG ZIP cable was first doubled-up and connected to drive a 3 ohm combined woofer and tweeter load. Then the two ZIP cables were independently wired in a bi-wire configuration, one to the tweeter, which has a 20 ohm load and one to the woofer, which has a 3.5 ohm load. It is typical for the tweeter to be higher in impedance than the woofer in most speakers. The following plot shows the difference in the frequency response of the double-wired speaker case versus the bi-wired speaker case:
The graph above shows that in the bi-wired version, the frequency response to the tweeter is increased from 478 KHz to 1.762 MHz over the double-wired version. The response to the woofer is essentially the same in both cases. If the impedance of the tweeter was identical to the woofer, then the two curves would be on top of each other and there would be no advantage to bi-wiring. The next graph performs the same simulation, but shows the phase response:
In the graph above, the double-wired version has a phase shift of 2.5 degrees at 20KHz. The bi-wired version, however, has a phase shift of only 0.5 degrees at 20KHz at the tweeter. Phase shift at the tweeter frequencies will be more audible than at the woofer frequencies. Also, phase shift may actually be more audible than the high frequency attenuation shown in the previous graph.
Another possible benefit of biwiring is that the cable natural resonances will change and the reflections and standing waves seen by the amplifier may be reduced. Biwiring essentially makes the cable twice as long, which will lower the natural resonance to one-half of a single cable. Also, since the amplifier is now located in the middle of the cable with the tweeter and woofer at each end, it may be at a cancellation node for standing waves. It is unlikely that cable resonances can actually be audible because they are in the range of a few megahertz for typical cable lengths, however there may be affects that become audible as a result of resonances. For instance, if the resonances and reflections at high frequency are significant, the impedance seen by the amplifier could be reduced to less than one ohm. This can possibly cause an amplifier to become non-linear due to saturation of the transistors or introduce distortion by increasing the crossover distortion.
Some speakers may benefit from bi-wiring, particularly those which have a tweeter with a higher impedance than the woofer. Also, IF resonance in cables is audible through some secondary mechanism, biwiring may reduce this by creating an unbalanced transmission-line that will tend to damp the resonance.