The Search for Standard Sound Insulation Samples in Impedance Tubes
2026-03-19Unlike sound pressure level measurement, there is no calibrator for measuring sound absorption coefficient and sound transmission loss. Impedance tube calibration is usually performed using standard specimens. It has been fifteen years since the launch of Soundwel impedance tubes. At that time, there was no national standard for sound insulation in China, nor were there recommended standard specimens for sound absorption. Referring to foreign products, we adopted a type of sponge with good sound absorption performance as the sound absorption standard specimen.
Under varying factory conditions with different temperatures and humidity at different times, the sound absorption test results of the same material using 10 different test systems are shown in the figure above, with very high consistency. The error is smaller than that specified in JJF 1446-2014 Calibration Specification for Impedance Tubes (Transfer Function Method). After the release of GB/Z 27764-2014 Acoustics — Measurement of sound transmission loss in impedance tubes in 2011, sound transmission loss measurements using impedance tubes became more common.
Generally, we expect the sound transmission loss measured in the impedance tube to represent the actual sound insulation performance of the structure. The sound insulation in the stiffness‑controlled region at low frequencies is not meaningful.
When a sample has very high stiffness, its boundary conditions and high stiffness result in an extremely high first‑order frequency.In this case, the measured sound insulation almost entirely falls in the stiffness‑controlled region and is not accurate. Simply put:reducing the sample size causes large discrepancies between the stiffness‑controlled and damping‑controlled regions measured in the tube,compared with the sound insulation of the same material used in large‑area applications.
Therefore, when selecting samples, we aim to make the first‑order frequency of the sample sufficiently low.We must choose samples with low stiffness and softness,so that the measured sound insulation curve can effectively avoid the stiffness‑controlled region.
As shown in the figure above, other manufacturers also install sound insulation materials by pushing them into the impedance tube, which leads to significant edge effects and poor measurement consistency. Poor sealing will result in even worse data.
The standard requires sealing the sample edges with grease or tape. However, as a standard sample, we do not want users to perform extra operations. Therefore, following the recommendations in Appendix C of the standard, we selected a soft, 1 mm thick, white, odorless silicone sheet as the standard sound insulation specimen. The specimen has a diameter larger than the tube inner diameter and is fixed by edge clamping during testing.
Photo of Specimen Installation
The sound insulation curve close to the theoretical value finally obtained is shown below.
The test results of different materials and different fillings are as follows.
It seems that this is the standard sound insulation test piece we are looking for. It is not easily affected by temperature and humidity, has no unpleasant smell, is soft and easy to install. Later, we will customize standard-sized mechanically processed samples for multiple tests and compare them with the test data of other pipes. According to the standards, we pay more attention to the part above the quality control zone above 300Hz.
