A laboratory method was developed to evaluate the sound transmission characteristics of road vehicle body seals. Primary bulb seal samples were mounted in a fixture which approximated the geometry of a typical door-gap cavity. The seal fixture was integrated with a rigid panel into the floor of a quiet, low-speed, closed test-section wind tunnel. Flow-excited pressure fluctuations in the door-gap cavity were induced by the air stream instead of by sound waves in a quiescent environment as in standard transmission loss measurements. A soundproof anechoic enclosure located underneath the test-section floor isolated the sound receiver. The sound level reduction between the cavity pressure and the sound pressure into the enclosure, a quantity directly related to the sound transmission loss (TL) in this case, was measured accurately between the 1250 and 5000 Hz one-third octave bands. The effects of cavity geometry, flow velocity, and seal compression, and the influence of the mechanical properties of the rubber compound were investigated. Several different seal designs were compared. The importance of aspiration and cavity amplification were also studied. The results indicated that this methodology gives an accurate indication of the acoustical barrier characteristics of different sealing assemblies. Multiple seal configurations, and configurations with multiple walls were found to yield superior acoustic performance.