Sound Absorption Coefficient Measurement with an Impedance Tube

The impedance tube is the standard laboratory instrument for characterising the acoustic properties of materials at normal incidence. Using the two-microphone transfer function method (ISO 10534-2, ASTM E1050-12), it measures the normal incidence sound absorption coefficient (α) and specific acoustic impedance of a flat material sample. The measurement provides results at each frequency across the tube's operating range, giving a complete frequency-dependent characterisation rather than a single-number metric.

The measurement principle is straightforward: a loudspeaker generates plane waves in the tube, and the standing wave pattern created by the combination of incident and reflected waves is characterised by two microphones at fixed positions. The transfer function H12 between the two microphone signals contains all the information needed to calculate both the complex reflection coefficient and the absorption coefficient at each frequency.

Frequency Range and Tube Size Selection

• Small tube (29 mm diameter): frequency range approximately 500 Hz to 6.3 kHz — suits higher-frequency absorbers, thin materials, and surface coatings
• Large tube (100 mm diameter): frequency range approximately 50 Hz to 1.6 kHz — required for low-frequency absorber characterisation
• Dual-tube measurement: use both tubes to cover 50 Hz to 6.3 kHz combined range
• The upper frequency limit is determined by the condition that only plane waves propagate in the tube — at higher frequencies, cross-modes appear and invalidate the measurement

Sample Preparation Requirements

• Diameter: sample must match the tube inner diameter — typically within ±0.2 mm for a good seal
• Thickness: record accurately; required for specific impedance calculation
• Face: the measurement face must be flat and representative of the actual installation face
• Mounting: the sample must be seated firmly against the rigid backing plate; gaps invalidate the result
• Conditioning: allow hygroscopic materials to reach equilibrium humidity before measurement

The PLACID impedance tube system includes 29 mm and 100 mm tubes, a calibrated loudspeaker driver, mounting hardware, sample holders, and PQ Analyst software with full ISO 10534-2 analysis. Both tubes use PLACID Class 1 measurement microphones, calibrated and supplied with the system.

Calibration of Impedance Tube Microphones

The two microphones in an impedance tube system must be individually calibrated for sensitivity and frequency response. A mismatch between the microphone sensitivities introduces a systematic error in the transfer function H12, which propagates directly into the calculated absorption coefficient. For metrologically valid measurements, the microphones should be calibrated as a matched pair with the tube, and the sensitivity correction applied in the analysis software. PLACID impedance tube systems are supplied with the microphones calibrated and characterised for their installed positions, eliminating this source of error.