Errors and Omissions in Test Standards for Measuring Transmission Loss using Sound Intensity

Transmission Loss can be measured using a reverberation room as the source room and a sound intensity measurement in a semi-anechoic receiving room. ASTM E2249 and ISO 15186-1 are the test standards typically used for this type of measurement. This paper will show that in both test standards, the equation for calculating the transmission loss neglects to include the difference between the reference quantities in the subtraction of sound pressure decibel level and the sound intensity decibel level. The difference between the dB reference quantities can be described by the characteristic acoustic impedance of air. The equation given in both test standards is only valid for a single characteristic acoustic impedance value and is thus oversimplified. This paper will propose a change to both of these test standards to include a term to account for the atmospheric pressure and temperature.