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Although Car-HiFi follows the same physical laws than Home-HiFi there are some additional points to be remembered:
Interior cruising noise of a VW Sharan 1.9 TD and ideal frequency response of a Car-HiFi system according to autohifi:
You can see very clearly that during cruising pretty high background noise of up to 105 dB(lin) occurs (increasing towards low frequencies with about 25 dB/decade). Even in idle (0 km/h) the interior noise increases by up to 30 dB starting from 100 Hz (maximum at the ignition frequency of about 30 Hz). This background noise may cover the wanted noise (the music to play back). Therefore the German magazine autohifi recommends an ideal frequency response with a 10 dB increase towards lower frequencies. By this the intersection between system frequency response and background noise is shifted by about 2 octaves towards lower frequencies! A portion of this increase is delivered for free by the compression chamber effect of the interior compartment (see below).
"Acoustic fingerprint" of a vehicles interior compartment:
How much the frequency response is depending on the loudspeaker position and the type of vehicle may be demonstrated by the following measurements. For that a FORD Mondeo wagon Ghia (model year 2001) was chosen. Using the principle of acoustic reciprocity a loudspeaker with omni-directional radiation up to 1000 Hz was placed at a head location and its response was measured on 31 different locations with an omni-directional microphone. These response spectra were divided by the response spectrum of the loudspeaker under free field conditions in 20cm distance with identical excitation. By this the frequency behaviour of the loudspeaker alone cancels out and the results shows the response of an ideal loudspeaker:
As you can see the best reproduction of very low bass tones can be achieved by positioning the loudspeaker on the trunk floor. Nevertheless the response drops down above 40 Hz by 15 dB! within 10 Hz! In this case a spectrally balanced reproduction can only be achieved of the front loudspeakers work linear down to 50 Hz. As this usually is not the case (a system like that would rarely need a subwoofer) such a system will sound very unsatisfactory!
On this vehicle bass loudspeakers located in the front area (doors, instrument panel -> thin black curves) show a much more balanced response.
Regarding all curves statistically yields an average frequency response of this vehicle. Even the average shows an increase of more than 10 dB below 60 Hz. Above 100 Hz the curve show an average scatter of +/- 7.5 dB and a maximal scatter of +/- 15 dB.
As the results are relative to a free field radiation in a distance of 20 cm a loudspeaker with a sensitivity of 88 dB/W/m (1 W input, radiation into half space) would generate a sound pressure level (SPL) of 99 dB within the vehicle. Including the 5 dB gain below 50 Hz this will result in 104 dB SPL for 1 W input power (compression chamber effect)! Above 250 Hz the resulting SPL inside the vehicle is similar to the half space free field radiation in 1m.
This is a slightly different presentation of the same results. On the Y-scale the different loudspeaker mounting locations are given (top half trunk, lower half front area). The amount of gain is coded as a color.
In order to make things even more complex this behaviour is completely different for each listening position (front/rear seats).
The logical conclusions of all above results are: