An impedance tube (also called a Kundt's tube or standing wave tube) uses the transfer function method to measure the normal incidence sound absorption coefficient and specific acoustic impedance of material samples. The measurement follows ISO 10534-2 (international standard) and ASTM E1050-12 (US standard). This guide explains the key selection decisions for a new impedance tube system.
The tube diameter is the most fundamental selection parameter because it determines the upper frequency limit of valid measurement. Above the cut-on frequency for the first cross-mode, results are invalid. The cut-on frequency is approximately 0.586c/d, where c is the speed of sound and d is the tube diameter. For practical purposes: a 29 mm (inner diameter) tube is valid to approximately 6.3 kHz at standard conditions; a 100 mm tube is valid to approximately 1.6 kHz. To cover the full frequency range of interest, most laboratories use both tube diameters and combine the results.
The impedance tube measurement is sensitive to microphone matching. Both microphones must have closely matched sensitivity and phase response. Unmatched microphones introduce transfer function errors that appear as absorption coefficient errors, particularly at frequencies where the sound pressure magnitude ratio between the two microphone positions is small. Matched pairs should be selected from the same production batch and verified before use. PLACID impedance tube systems include a matched microphone pair selected and verified for the application.
PLACID impedance tube systems are supplied with PLACID PQ Analyst software pre-configured for ISO 10534-2 and ASTM E1050-12 measurements, including the microphone phase correction procedure and standard-compliant result output.