you can do it by a cheap radio shack meter, using a sealed box graph each frequency usually 30-70hz but keep the meter at the same place and keep the volume the same as well, once that is accomplished take the sealed box out of the car and raise it off the ground a few inches, place the cheap meter about 3-4ft away and repeat the process but keep the volume the same as you did when it was in the car, go thru the tones again and graph them. When you have the results from both tests, compare them and notice the db jump from outside to inside and note that frequency that is usually your resonant frequency of the vehicle or cabin gain. Note with using the cheap meter will give you a great starting point for which tone is the loudest, could vary in frequency 3-6hz either way when getting metered with the termlab. Hope that helps.
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