If I generate an impedance curve for an enclosure while it is placed in the cab of the vehicle will I get an extra peak in the plot? I am thinking that the cab will act as a sealed chamber and produce another peak. I am trying to find a reliable method to measure the cab resonance frequency. Any suggestions?
This what I do to measure cab resonant frequency(and frequency response)
get a pair of good quality subs, they don't need to be huge, I used JL8W-1's, put them in a sealed enclosure with a Q of around .5, mount the drivers on opposite sides of the enclosure (so their mechanical motion canels our)
Using test tones and an spl meter measure the "ground plane" response of the enclosure, enclosure flat on the ground with the mike 1m away, also on the ground (do this outside away from buildings)
I used 20hz to 100hz in 1hz steps, start at 20hz and push the volume up untill the drivers are going fairly hard (but still in linear motion, no amp distortion ect) Measure the voltage output of the amp at this stage so you can calibrate the other tests accuratly.
Measure the db reading at each step and record them.
Position the enclosure in the vehicle in the location you intend to use, repeat the above procedure remembering to calibrate the amp output voltage and record the readings again.
Subtract the ground plane readings from the cab readings and you have the + and sometimes - db response of the cab.
If you have Bassbox or another sub enclosure program that lets you input room response data you can plot the actual response you would get in the cab.
Putting the box in a car boot (trunk) or cabin, room in a house or outdoors makes no difference to the impedance curve results because the acoustical impedance difference that the speaker cone sees to the air outside is negligible between these situations.
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