you need a power cap, whats happening is that when your bass kicks in your amp need more power it need 14.4v on its own, the standard battery and alternator cant cope with that sort of demand and starts taking power from other parts of your car IE lights etc. so you need to put a power cap in, a minimum of 1 farrad. it will also give you more volume and a more stable bass note cos the amp is working to its optimum power, but you only put this on your sub amp only.

i would check out for the noise is your earth points, this can also generate noise if you had a bad earth, there needs to be no paint or grease where youve earthed your amps and 1 earthing point per amp as well cos you can get a sort of feed back through the earth cable

 

But once the cap is drained from the long bass notes, won't it draw the same amount of current to charge back up? If anything I'd spend the money on a High output alt. or a second batt.

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Jeff
To the optimist, the glass is half full.
To the pessimist, the glass is half empty.
To the engineer, the glass is twice as big as it needs to be.

 

Regardless where you put a cap the capacitance is electrically the same throughout the system.

What drives inductive loads really well or great for that matter?? Are capacitors.

Since the subwoofer system has a inductive load and the amp itself is a load, this can be figured for continous resistance from a given current, and the supply voltage at RMS. This resistance most times reflect the ohm load at the speaker output. Time constant R*C> 4ohms * 1F is four seconds. Which in turn means that the cap will no longer keep the DC at peak value as soon as it hits its time constant. All the current will be drawn over the amps and speakers, so yeah extended periods of extreme bass will cause the the RC time constant to drain away as the cap slowly drops down in capacitance because less voltage will be able to charge it. The amount of time for a 1F cap to charge is almost instant as the opposition to the battery is minimal.

The voltage drop will not happen all of a sudden, draining the stiffened DC supply to max current capability you will see a gradual slope, draining a unstiffened DC supply at max capability you will notice a drastic curve or even a wave form mimicking the loaded (bass hit) and unloaded phase of operation.

Buying just any car battery will not do the same job as one made specifically for high drain, even if you did get a regular car battery and hook it up which most likely is parallel the capacity of the battery is nearly doubled not necessarily current capability and the weaker battery is a load to a stronger battery. A deep cycle battery is needed one that can deliver huge amounts of amps with out dropping it rated voltage and these batteries are not cheap.

I would still go with caps as this will also filter noise, which most batteries don't do filtering.
A high output alt is the only other real solution, but even then you may still want to uses some caps to stiffen the output some.

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Exactly what Juglenaut said,