The system sounds substantial in the driveway and thin at 70 mph. Nothing has broken. Road noise is masking exactly the octave your subwoofer works hardest in, and there are three fixes, two of which are free.
What masking actually does
Tyre and wind noise in a moving car sits mostly between 30 and 200 Hz, and it rises roughly with the square of speed. At 70 mph the noise floor in an ordinary hatchback can be 20 dB higher in that band than it is stationary. Your subwoofer has not got quieter. The background has got louder, and it is loudest exactly where the bass lives.
Fix one: stop boosting, start high-passing
The instinct is to add bass boost. That asks the driver for more excursion at one frequency and is how subwoofers die. The better move is to high-pass your door speakers at 80 Hz so they stop attempting bass they cannot produce, which frees their excursion for midbass, the band that actually cuts through road noise.
Try this before spending anything
- High-pass the front doors at 80 Hz, 24 dB per octave
- Low-pass the subwoofer to match at 80 Hz
- Turn any bass boost to zero and leave it there
- Raise the 60 to 100 Hz region by 2 to 3 dB, no more
Fix two: lower the noise floor
Two hours with constrained layer damping tiles on the door skins and the boot floor will do more for perceived bass at speed than 200 extra watts. You are not adding output, you are removing the thing that was covering it. Coverage of about 25% of a panel captures most of the available benefit; going to 100% adds far less than the internet suggests.
Adding power to beat road noise is fighting the symptom. Every decibel you take off the noise floor is a decibel you did not have to buy an amplifier to produce.
Marcus Reed, founder and editor-in-chief
Fix three: a loudness contour that tracks speed
Some processors offer speed-dependent volume, and used gently it is the correct tool for this problem. Set it to add no more than 4 or 5 dB at motorway speed. Beyond that it becomes obvious and unpleasant when you slow down. Our DSP guide covers where this sits among the other processing worth having.
