A question that comes up quite often via email has to do with no bass output even with large car audio subwoofers. Generally this problem occurs with more than one subwoofer but can happen with dual voice coil subwoofers. Almost without fail the email contains the statement "I've wired everything up correctly". But almost with the same frequency the problem is that the installer did not wire everything up correctly. What's happening is the woofers in the system have been wired out of phase (reverse polarity). This reverse polarity condition is what causes the bass cancellation.
In-Phase Subwoofer Wiring
When subwoofers are wired correctly (in-phase) all woofers in the system push out (into the vehicle) on the positive side of the music signal and pull back on the negative half of the music signal. See the diagram below. When one woofer pushes out the other also does. This creates a pressurization of the cabin because both woofers are working together to produce bass in the same way at the same time.
Out-of-Phase Subwoofer Wiring
The opposite of in-phase wiring is out of phase wiring. This is when one or more of the woofers is wired opposite of the others. In this case one woofer is pushing out while the other woofer is pulling back in. It's like two people of equal strength pushing on either side of a swinging door. The push of one is cancelled out by the push of the other in the opposite direction. See the diagram below. When one woofer pushes out the other is pulling back. This creates a cancellation of any bass output that the system would normally have.
Signs of Incorrect Subwoofer Wiring
One of the biggest things you will notice when your wiring is incorrect is that your woofers will appear to be working and there will be sound output. But the bass and sub-bass frequencies will be non-existent or drastically reduced. An easy test to see if reverse wiring is the problem is to disconnect one of the woofers. This will eliminate one of the woofers fighting the other. If doing this increases the bass then you should check the wiring to all of the woofers. Positive from the amp should be going to positive of the woofer and vice versa for the negative terminals.
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