The best space saver is Cerwin Vega, HED-8.4, 200/400watts, dual coil. The box volume can be compressed at 0.2 cu.ft. or 345.6 cu.inch at box resonance frequency of 50Hz (50 to 60Hz is recommended for moving vehicles).
The next is Rockford Fosgate, P28S4, 200/400watts at 0.2 cu.ft (computed based on its parameters and not 0.65cu.ft. as recommended by the manufacturer) at resonance frequency of 40 Hz using ported box. Though 40 Hz or less is recommended for home use, you will greatly appreciate it if your car is free from outside noise.
Bass frequencies can be categorized as BUM or BOM. BUM is more or less 40Hz., while BOM is 50Hz. and above.





Reply With Quote