Pressure to Pressure Airborne Transfer Functions

These are the simplest form of airborne ATF, where an artificial sound source and a microphone are placed close to the true noise source of interest, and a second response microphone is placed at the location of the receiver, often the driver or passengers ear position. The difference between the two pressure levels is the ATF. This measurement is most often made across a wide range of frequencies. It is also common to make this type of measurement reciprocally, with the artificial sound source placed at the receiver location and the response microphone placed near to the true noise source.

Volume Velocity to Pressure Airborne Transfer Functions

The volume velocity of a source better represents the energy of a sound source, than a local pressure measurement. Although it's more complicated to measure the source’s volume velocity, when this value is combined with a volume velocity to pressure ATF, a more accurate assessment of the sound level at the receiver can be made. 

The ATF is measured a similar way to the P/P transfer function, except that the artificial sound source is calibrated to a known level of volume velocity. Often a small reference microphone is incorporated into the nozzle of the artificial sound source to give a direct feedback of the source level. Again it is common to measure this type of transfer function reciprocally.