Impedance tube testing
Turnaround and logistics
- Typical turnaround
- About two weeks from receipt of a complete specimen and an agreed scope
- Urgent work
- An urgent route is available — tell us the date you are working to at enquiry, not after booking
- Before you ship
- Confirm specimen size, mounting and packing first — see sample requirements
- Report language
- Reports issued in English; correspondence in Thai or English
Small samples, fast turnaround, low cost. This is the right tool for material development, because you can iterate ten formulations for the price of one reverberation room test.
Tube configurations
- 100 mm tube
- 50–1,600 Hz
- 30 mm tube
- 800–6,300 Hz
- 16 mm tube
- 2,500–10,000 Hz
- Sample form
- Cut discs matching the tube diameter, minimum 3 per configuration
- Reported quantities
- α at normal incidence, normalised impedance, TL (ASTM E2611)
Impedance tube is not a substitute for ISO 354
Normal-incidence absorption from a tube is not the same quantity as random-incidence absorption from a reverberation room, and converting between them is unreliable. Use the tube to develop and compare; use the room to certify and publish. Any laboratory that tells you otherwise is saving itself work.
The tube and the room answer different questions
An impedance tube measures absorption at normal incidence — sound arriving perpendicular to the surface, in a tube a few centimetres across. A reverberation room measures absorption in a diffuse field, with sound arriving from all directions onto a specimen of ten square metres or more. These are different physical quantities and they do not agree, and neither is a cheap approximation of the other.
What the tube is superb at is comparison. Twenty formulations, small discs, a fast turnaround, and a clear ranking of which direction the development is going. What the tube cannot give you is a number for a specification, because building specifications, product declarations and green building credits are written around the diffuse-field result.
How to use it without wasting money
- Screen wide in the tube, then confirm narrow in the room. Take twenty candidates down to two or three in the tube, then put only those into a reverberation room test.
- Cut discs carefully. A disc that fits loosely leaks around the edge and reads low; one forced in compresses the material and reads differently again. Edge fit is the dominant source of scatter in tube results.
- Send at least three specimens per configuration. A single disc tells you nothing about the variability of your own material, which is often larger than the difference you are trying to detect.
- Do not compare a tube result from one supplier with a room result from another. That comparison is meaningless and it is made surprisingly often in procurement.