To determine the closed-field frequency response, the same test-bench setup was used, but the loudspeaker was positioned inside the Sprinter. First, the closed-field response was determined in the cargo area. The loudspeaker was positioned about 5 feet (1.5m) behind the driver and passenger seats, facing in the forward direction (see Figure 3). All doors and windows were completely closed. A small ladder was used to position and support the PAA2 such that the microphone was one meter (3.28 feet) in front of the loudspeaker and on axis to the tweeter. Three measurements were taken using exactly the same technique as previously described.
Second, the closed-field frequency response was determined for the cockpit area at the driver's listening position. For this setup, the loudspeaker was repositioned as far forward as possible in the cargo area in an effort to excite the cockpit area (see Figure 3). The PAA2 was held by hand with the microphone facing forward in a position that approximated the location of a driver's head. Again, three measurements were taken using exactly the same technique as previously described. (See Figure 3)
Figure 4 compares the reduced test data for the free-field and closed-field responses. To determine the vehicle's transfer function, the free-field response was subtracted from the closed-field responses and plotted. The uncertainty associated with each of the measurements was propagated according to standard practices.6 Figure 5 shows the vehicle's transfer function at the driver's listening position and in the cargo area. There was substantial room gain at low frequencies at the driver's listening position, as much as 14dB at 25Hz. The room gain decreased as the frequency increased to about 110Hz, beyond which the room gain was negligible. Higher in the frequency spectrum, there was a peak at 250Hz of 7dB in magnitude followed by a dip at 400Hz of 7dB in magnitude. Another peak was evident at 1,600Hz of 4dB in magnitude. The higher treble frequencies (greater than 5KHz) were rolled off at the driver's seat; however, this was believed to be an artifact of the test setup, where the microphone was not only off-axis from the tweeter but also aimed toward the direction of sound propagation. Obviously, the microphone was measuring reflected, attenuated sound. Clearly, 14dB of room gain at the lowest frequencies will substantially reduce the amplifier power requirements and augment any low-frequency roll-off associated with the loudspeaker enclosures.
The cargo area transfer function, shown in Figure 5, differed dramatically from the transfer function at the listening position. First, the room gain at low frequencies was substantially less. Second, the cargo area exhibited a different set of peaks and dips in the response. It had four substantial peaks at about 31.5, 63, 200 and 630Hz, of magnitudes 4, 7, 11, and 9dB, respectively. It's interesting to note that first three peaks are in approximate agreement with those predicted in Table 2. The cargo area room gain also had a plateau of about 4dB in magnitude from about 800 to 4,000Hz. The data suggest that sound damping and barrier materials should be used in the cockpit and cargo areas to attenuate the peaks at about 250Hz and 1,600Hz that were evident at the listening position. Bass absorbers, tuned to 31.5 and 63Hz, and panel absorbers, tuned to 205 and 630Hz, may also be necessary for the cargo area.