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IntroductionThe sound power level is used for quantitative evaluation of sound energy radiated from electrical and mechanical apparatus. Procedures for measuring the sound power level are categorized into the ISO 3740 series (such as ISO 3745 [1]) on the basis of measurement principle, environment, and accuracy. Among them, practical procedures involve the use of a reference sound source (RSS) with a predetermined sound power level, and an apparatus under test can be easily calibrated in comparison with the RSS. The RSS has stable and broad-band sound power output. Requirements for its performance and calibration procedure are prescribed in ISO 6926 [2]. Thus precise calibration of the RSS is essential for end-users to establish reliable sound power measurement.In ISO 6926, the RSS is calibrated in a hemi-anechoic or reverberation room. From the point of accuracy, we decided to use a hemi-anechoic room. However, we have access to only an anechoic room, but a hemi-free-field is realized by laying down wooden boards on the wire-meshed floor of the anechoic room. It is difficult to lay down boards with high surface density owing to the load capacity of the wire-meshed floor, even though ISO 3745 requires that the absorption coefficient of the reflecting plane should be less than 0.06 for the frequency range of interest.A previous study on calibration of the RSS revealed that there is no significant difference between a solid concrete and a lightweight wooden floor when determining the sound power level [3]. However, no detailed description has been found of the surface density of the floor and the influence of the surface density on the sound power level. Thus, we investigate sound power transmitting through the wooden board floor quantitatively.In this work, we formulate the sound power transmitting through the floor, using the mass law. By using the derived formulae, we examine the dependence of the transmitted sound power on the surface density of the floor. Then experimental results obtained with our calibration system are compared with theoretical values.

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