There are many materials considered for acoustic and noise treatments, more are in the making with the advancement in material science and are driven by the optimised product design & performance requirements.
Two major characteristics for any given acoustic material are
- Effective noise absorption
- Effective transmission loss
Based on multiple physical and composition specifications, each type of material finds their application across various usage needs. While many products specify their noise reduction parameters, the methods used for determining these numbers needs to be understood for a better determination of their appropriate usage.
Transmission Loss (TL) is the terminology and the procedure that defines on how any acoustic material is tested for its property to attenuate noise. While there are few certified methods to perform these tests, impedance tube based approach is the most relevant, consistent methodology in practice; let us find out more about this:
An impedance tube (also called a Kundt's tube or standing wave tube) is a laboratory grade instrument used to accurately determine the acoustic properties of materials under controlled conditions. It is one of the most widely accepted methods for measuring sound absorption, acoustic impedance and transmission loss of materials according to international standards.
The composition of an impedance tube consists of a rigid cylindrical tube with the following configuration:
- Loudspeaker at one end, terminated and sealed appropriately
- Test specimen mounted inside the tube with proper sealing and fitment
- Precision microphones mounted along the tube wall, with close mounting tolerances
- Qualified Signal generator and frequency analyser, preferably with TL test setup
The term "impedance tube" is coined since this tube determines the acoustic impedance of the subject material, the small equation to note is Z=p/u, wherein "p" is the acoustic pressure and "u" being the particle velocity; thus, Z, the acoustic impedance relates pressure and particle velocity and determines how much the material in consideration resists sound transmission.
One of the most important design criteria in impedance tube making is that only planar waves need to exist inside the tube for it to perform and determine the required TL parameters.
Diameter of the tube, type, size & quality of microphones and their configuration distances, incident sound wave and their quality (white, pink etc) will all influence the frequency limits of the setup and as well the overall performance of the tube itself.
A well-made setup considering ISO guidelines and further validating the tube configuration for possible leakages, structural excitations and other influencing factors will now be ready to determine the following parameters:
- Normal incidence sound absorption coefficient
- Acoustic impedance
- Reflection coefficient
- Transmission loss
- Speed of sound in porous materials
- Complex wave number
- Optionally, Flow resistivity
While a lot of care and design tolerances are to be maintained to establish a qualified impedance tube, the resulting outputs are highly reliable and traceable to established guidelines of international standards thus making it a reference acoustic testing mechanism.
Customised Impedence tube developed by NV Dynamics
NV Dynamics both designs and conducts tests of TL using highly reliable SIEMENS Test Lab TL modules with step-by-step ISO 10534 process. We recently designed a special purpose impedance tube for a automotive testing company and the results are encouraging.