My mother is a physics teacher. She gets to play with a bunch of cool little gadgets and such at her job. Yesterday she was telling me about a gadget she has that will detect audio waves and give a readout in volts. She can then convert audio(volt) readings she gets to db. Here was her story. She had a kid in her class that wanted to find out the db level his car audio system was putting out. They took out the little gadget to his car and did a readout in his trunk. He then complained that it didn't work right because the db reading was nothing near the level it should have been. I told my mom this was the reason: If he took the readout in the trunk then most likely whatever he was using to detect audio waves was not detecting the full wave from the sub. Say the sub was putting out frequencies between 25 and 40mhz. He might have picked up 35-40mhz frequencies but nothing lower b/c there wasn't enouph room for the entire wave to complete itself within the room he had in his trunk. Think of it like this... when the sub makes a sound it puts out a wave but not in it's full length. It puts out a wave that completes itself, and therefore becomes audible only 5 feet from the sub. *(is this right?) The sub is simultaniously putting out sounds in the frequency range of 25-40mhz. This means that if the mike was put 4 feet from the sub then the higher frequencies were being detected but the lower frequencies were not being detected. Ok, now if your following me here, if they had some type of detection device that was detecting the volts from the audio waves it recieved then this meant that the detection device was determining the length of the wave upon detection. Then it calculated the amount of volts it takes to create that wave length. (highter frequencies requiring less power and lower frequencies requireing more power) I was telling my mother that his reading were askew most likely because he didn't put the microphone far enouph away from the sub to allow it to detect all the waves the sub was putting out. If he wasn't detecting the lower wave then he was missing half the volt readings on his readout so the db level they get when converting the volts to db would be much lower than what it should be. I also understand the device used to detect DB levels in car audio competitions is rediculously expensive. (7k and up) I'm wondering if my theory was correct. I really have no idea about this stuff.