Evaluation of Transmission Loss Induced
by Stretched Fabric Treatments
by Dirk Noy
However - no test data have yet been available to document the influences of such a procedure on the acoustical performance of the loudspeaker / acoustical system. WSDG has performed acoustical measurements to investigate these effects.
The following fabrics have been used for the test:
Fig. 1 - Fabric Samples
2. Acoustical Measurement Procedures
Acoustical measurements were performed to accurately determine the frequency dependent Transmission Loss induced by the fabric samples.
Measurements were made using an Earthworks model M30 omnidirectional ¼" measurement microphone, a Stewart routing mixer, a SonicPort A/D converter and SIA-Smaart audio measurement software. Noise was generated by a Neutrik Minirator Pink Noise Generator feeding a Meyer HD-1 active reference studio monitor loudspeaker.
As a first step, the Frequency Response of the loudspeaker - room system has been measured without the addition of a fabric sample. Second, the fabric samples were (one after the other) introduced into the sound's travel path, in close proximity to the loudspeaker - identical to the installed case. The Frequency Responses were measured again. The only change in the system is the introduction of the fabric sample - so it is safe to assume that all changes in the Frequency Response have been induced by the introduction of the fabric sample (and a small measurement system fluctuation). The difference between the Frequency Responses before and after the fabric sample was introduced has then been calculated.
Fig. 2 - Schematic Measurement Setup
Fig. 3 - Measurement Setup
(Loudspeaker: Right, Measurement Microphone: Left, Measurement System: Front Center)
The graph below shows the Transmission Loss values plotted versus the frequency axis.
Fig. 3 - Measurement Results
Fabric Type 1 - Specifically Labeled "Acoustically Transparent"
Fabric Type 2 - Light Regular Fabric (Not Specifically Labeled)
Fabric Type 3 - Light Fabric with Acrylic Backing
Even the type 1 fabric ("acoustically transparent")* shows some fluctuations in the frequency response and an abation of a few dB above 10kHz.
*WSDG ran a number of tests - in fact the best fabric (other than speaker grill cloths) that we found was Knoll Transparency W867/1. We welcome dialogue with any fabric manufacture to explore the transparency characteristics of their fabrics.