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      12-23-2010, 11:13 PM   #20
montr
Second Lieutenant
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Drives: 2010 335d
Join Date: Sep 2009
Location: Atlanta, GA

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My install is done. I did few more measurements. To get a flat response without recoding, I had to set the switch on the amplifier to #2 (reduce output to woofer under the seat by 4dB) and set bass=-2 and treble=-5.

Figure 1 is the electrical output response of the HU for the front speakers from 10Hz to 20KHz. As you can see, there is strong peaking at 60Hz. This peaking is partly compensated by switch position #2 and bass=-2. The high frequency peaking is compensated by setting treble=-5 and the rollof of the acoustical drivers. Note: The rolloff around 18KHz is not the amplifier. It is my measurement setup, I forgot to set the signal capture to 96KHz sampling rate instead of 44.1KHz.

Next I tried to measure the acoustical performance of the Alpine speakers kit. I have an expensive ($5K) calibrated microphone (Bruel & Kjaer #2238) to do the measurements. First, doing acoustical measurements in a car is tricky. You move the position of the microphone by few inches and you get different results. You open the window or the sunroff and you get totally different results.

In this test (figure 2), I tried to measure the response of the midbass and the tweeter by locating the microphone 2" from one driver and blocking the sound from the other driver by using a blanket on it. Note: The valley around 1.5KHz in the midbass is maybe due from out of phase signal from the tweeter. The distance between the tweeter and the midbass is 20cm. This is about one wavelength at 1.5KHz. These measurements are done with the door open.

I also did more mesurements from the driver seat (figure 3) with all the doors and sunroff closed. The microphone is at ears level without anyone on the seat. As I said before, these are tricky measurements. You should not pay too much attention to the sharp peak and valley, they move in frequencies when you change the position of the microphone by only few inches. What is important is the trend.

Note: The figure 3 red is for switch=1, bass=-2, treble=-5. For switch=2, the peak at 60Hz will be 4dB lower.
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