That would be additional distortions in the music that are not included in the THD ratios, as I understand it additional back emf etc(Back emf is generated in the subwoofer via the electrical voltage travelling threw the coil along with magnetic reaction, and travels back up the speaker wire to the amplifier, disturbing the electrical flow. this is greater the lower the load impedance). will result in the woofer moving in opposite directions to the music at times, which means inaccurate reproduction rather than distortion. You should get better bass and better woofer control and better distortion numbers at an 8 ohm load, vs 4 ohm and 2 ohm. Also as stated before there is such a huge drop in gain after achieving 1000 watts, that low ohm loading offers zero benefits vs the cost on the amplifier itself, and only offers further distortions do to low resistance. Resistance is always good not bad. This is why i purchased dual 4 ohm subwoofers as I want the full benefits of a normal resistive 8 ohm load.
Low ohm loading has always been a shortcut to getting more power at a huge cost. It is used a lot in car audio as you are restricted to low voltage so there is a purpose here, as you can not achieve big power without it. Car audio also requires class d, as you cant fit anything else. In home audio you do not need to take this shortcut. Best to bridge an amplifier at 8 ohms on each sub to get high capacitance, and twice the power at half the voltage at 8 ohms than go to low ohm loading. As per the lenard quote below this is the proper way to get more power rather than low impedance load, while lowering distortion, increasing headroom, and therefore dynamic range, for nice deep unrestrained bass.
The main issue is Zero gain for nothing but higher distortions, and amplifier wear. The only reason one would want to bridge into a 4 ohm load or run stereo two ohm load would be the same as the reason for car audio. to get more power out of a small amplifier. Ie, Get 600 watts out of a 200 watt rms amp at 8 ohms to power a 4 ohm 600 watt subwoofer. Not much reason to do this unless you work at Mcdonalds and its the only way to blast your face off. I Always over power and bridge for better results. Not for mains just for subwoofers.
Quote lenard audio
The advantage of bridge is that it delivers the same power as a single ended amplifier with only half the rail Voltage. (40V RMS into 8R = 200 Watt) 40V RMS from a single ended amplifier requires + – 60V rails, whereas 40V RMS from bridged amps only requires + – 30V rails. With bridged amps the speaker is powered from both + – V rail supplies at the same time, instead of alternate between supply rails as with a single ended amp. Therefore Bridge amps make more efficient use of the rail supplies. Also the maximum Voltage across the transistors is half by comparison to single ended amp. Bridge is the most effective method to drive a speaker. The only disadvantage is higher cost.
See Also Lenard Audio Speaker impedance Z
With a speaker load of 8Ω this amplifier will only be capable of 50 Watt. With a speaker load of 16Ω this amplifier will only be capable of 25 Watt and so on. The higher the load impedance, the cooler and more reliable the amplifier will be, and the lower the internal distortion but less power.
As can also be seen the advantage of bridging is actually lost when dealing with 4 ohm loads. See the same first link for more info.
Quote Lenard audio...
By paying close attention we can see that 4 times the power is achieved from bridging 2 amplifiers delivering 40V RMS into a 8Ω speaker (200 Watt) if we are comparing it to a single ended amplifier delivering 20V RMS into the same 8Ω speaker (50 Watts). However when comparing a bridge amplifier delivering 40V RMS into a 8Ω speaker (200 Watt) to a single ended amplifier delivering 20V RMS into a 4Ω speaker (100 Watt) then bridge only appears twice as powerful.Edited by johnplayerson - 12/15/13 at 12:11am