Effect of nine different mono-frequency sound stimuli on two gram-negative bacteria (Pseudomonas aeruginosa and Serratia marcescens) was investigated. Frequency of the test sound ranged from 100 Hz to 2,000 Hz. Both the test bacteria responded differently to sonic stimulation. Sound corresponding to 600 HZ caused a notable reduction in quorum sensing (QS) regulated production of the pigment pyoverdine by P. aeruginosa. 400 Hz sound affected prodigiosin production by S. marcescens the most. 500 Hz sound could enhance prodigiosin production without affecting growth of the producing bacterium, suggesting the effect purely to be QS modulatory. This study has demonstrated the capacity of the sound waves of affecting bacterial growth and quorum sensing regulated metabolite production.