ISO 3744 vs ISO 3745: Choosing the Right Sound Power Standard

ISO 3744 and ISO 3745 are the two principal international standards for determining the sound power level of machinery and equipment. Both define measurement surface methods where microphones are placed at specified positions around the source, but they differ significantly in the measurement environment requirements, the achievable measurement uncertainty, and the applications for which each is appropriate. Choosing the correct standard for a given measurement programme has implications for test facility requirements, instrument specifications, and the declared measurement uncertainty on the test report.

ISO 3744: Engineering Grade

• Environment: free field over a reflecting plane — a suitable production floor, outdoor measurement pad, or test pad qualifies
• Measurement surface: hemisphere or rectangular parallelepiped, radius typically 1 m or 2 m
• Background noise requirement: at least 6 dB below source sound pressure level (K2 correction factor ≤ 1.5 dB)
• Achievable expanded uncertainty: typically U = 1.5 to 3 dB depending on source type and environment
• Application: CE marking compliance, machinery noise declaration, most industrial applications
• Facility requirement: most standard test floors qualify without special construction

ISO 3745: Precision Grade

• Environment: anechoic room (down to cut-off frequency) or reverberant room
• Measurement surface: hemisphere in anechoic room; diffuse field in reverberant room
• Background noise requirement: at least 15 dB below source in anechoic room
• Achievable expanded uncertainty: typically U = 0.5 to 1.5 dB depending on source type
• Application: reference testing, test facility qualification, product development comparisons, research
• Facility requirement: dedicated anechoic or reverberant room required — significant capital investment

Which Standard to Choose

For CE marking under the Machinery Directive, ISO 3744 is the standard of choice for the vast majority of machinery types. ISO 3745 provides lower uncertainty but requires a facility investment that is only justified when the precision is genuinely needed — such as for product development comparisons where 1 dB differences are commercially significant. If your regulatory requirement is CE marking noise declaration, ISO 3744 is almost certainly the right choice. If you are conducting fundamental research into noise source mechanisms, ISO 3745 in an anechoic room gives results that stand up to the closest scrutiny.

Microphone Requirements for Sound Power Measurement

Both ISO 3744 and ISO 3745 require Class 1 measurement microphones calibrated to ISO/IEC 17025 for test reports submitted to notified bodies or included in regulatory declarations. The microphone calibration certificate must be traceable to a national metrology institute and must state the sensitivity at the calibration frequency (typically 250 Hz or 1 kHz) with expanded measurement uncertainty. For hemisphere array systems with 10 or more microphone positions, all microphones in the array must be individually calibrated — a single certificate for the array as a whole is not sufficient.