The test method used in North America to evaluate the acoustical performance of duct silencers is ASTM E477. The ASTM standard provides an approved method for measuring the aerodynamic pressure drop, dynamic insertion loss, and self-generated noise of duct silencers. Unfortunately, restrictions on the construction of the test duct in the current standard cause erroneous results in the measurement of insertion loss at low frequencies. These errors are due to acoustic resonances that occur naturally in the empty test duct. It is recommended that the standard be modified to require anechoic terminations at both ends of the test duct. It is also recommended that more specific design criteria for the source chamber be established, and that a new section be added to provide for the measurement of the radiated noise from the casing of the test specimen. Specific recommendations relating to all of these issues will be provided.