Acoustic Worx provides at test and measurement facility to local Industry.
Sound Power testing in accordance with ISO 3744 and ISO 3745
Materials testing in accordance with ISO 354 and ISO 10534-2
Enquiries are welcome for other form of acoustic test and measurement
We have our own semi reveberant test chamber but also utilise the reberant chamber at SABS Pretoria, the hemi anechoic chamber at SABS Pretoria and the full anechoic chamber at the University of Kwa Zulu Natal.
Materials Testing - a short description
The process by means of which the organised motion of sound is converted into the disorganised motion of heat is of major importance in acoustic design. Material testing allows for accurate prediction of how a material will perform in a given space or application. We do 2 types of material testing
1. Absorption coefficients of porous materials
2. Transmission loss
This is done in accordance to the INTERNATIONAL STANDARDS ORGANISATION ISO 10534 - part 2and ISO 354
The ISO 354 testing is done in a reverberant chamber located at the
ISO 10534-2 testing is carried out at Acoutic Worx in a purpose built
set of impedance tubes.
The test method uses an impedance tube, two microphone locations and a digital frequency analysis system for the determination of the sound absorption coefficient of sound absorbers for normal sound incidence. It can also be applied for the determination of the acoustical surface impedance or surface admittance of sound absorbing materials. Since the impedance ratios of a sound absorptive material are related to its physical properties, such as airflow resistance, porosity, elasticity and density, measurements described in the test method are useful in basis research and product development.
The test method is similar to the test method specified in ISO 10534 -1 in that it uses an impedance tube with a sound source connected to one end and the test sample mounted in the tube at the other end. However, the measurement technique is different. In this test method, plane waves are generated in a tube by a noise source, and the decomposition of the interference field is achieved by the measurement of acoustic pressures at two fixed locations using wall-mounted microphones or an in-tube traversing microphone, and subsequent calculation of the complex acoustic transfer function, the normal incidence absorption and the impedance ratios of the acoustic materials.
This describes the process used to determine the STC of a sample under test. Sound transmission loss (STL) measurements involving a modified standing wave tube have been developed in recent years. The techniques were originally developed for the measurement of TL in muffler systems, but have been adapted to measure the STL of acoustic materials.
The technique utilizes a four microphone setup and an impedance tube. The standard two microphone setup as described in ISO 10534 part 2 – Transfer function measurement of absorption coefficients in an impedance tube, is modified by placing 2 additional microphone holders on the receiver side of the sample under test. Complex acoustic data is recorded at all four microphone positions. STL is computed by solution of the transmission matrix.
The basis for the algorithm lies in the definition of the transmission loss matrix which is unique for all acoustic materials. There are four acoustic waves relating to the material. The forward traveling wave is defined by its incident, A1, and transmitted, B2 pieces. The backward traveling wave is also defined by its incident, B1, and transmitted A2 pieces.
The transmission loss matrix is then defined as the matrix relating to the forward and backward traveling acoustic waves.
The sound pressure at specific points in space and time are related to pressure measurements at other points in space and time through phase relationships. These relationships are gathered through the acquisition of complex ensemble averaged cross spectra. A measure of strict sound pressure magnitude at a point is acquired as an ensemble averaged auto power spectra.
By acquiring the complex cross spectra from microphones 2, 3 and 4 using microphone 1 as reference and the auto power spectra of microphone 1 with the impedance tube in two different end conditions, the sound transmission coefficient can be solved using the determinant method.
The fit into the tube is crucial and hence the need to accurately CNC sample to ensure a perfect fit. The sample is also decoupled from the tube with rubber gaskets.