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A comparative study was undertaken to investigate engine noise in sub-compact vehicles under typical highway operating conditions. Baseline acoustical testing was performed. Typical sound quality metrics indicated that passenger cabin noise levels and sound quality were similar among all vehicles tested. However, poorer subjective sound quality in a vehicle equipped with a 3-speed automatic transmission was related to a higher degree of perceived periodicity than in other models. Further study of the 3-speed vehicle indicated that installing a 4-speed transmission would reduce noise more effectively than any substantial changes to the vehicle structure or engine mounting system. Damping or barrier treatments applied to the firewall or toe pan did not provide significant benefit.