Evaluation of Transmission Loss Induced

by Stretched Fabric Treatments
by Dirk Noy

 
Applying stretched fabric in front of loudspeakers is a widely used practice to optically hide the speaker enclosures and therefore increasing the aesthetics of a given space. In some applications, acoustical elements (such as absorbers or diffusers) are also covered up for aesthetical reasons.

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.

 
1. Tested Fabrics

The following fabrics have been used for the test:

 
1.1 Fabric Type 1 - Specifically Labeled "Acoustically Transparent"
 
1.2. Fabric Type 2 - Light Regular Fabric (Not Specifically Labeled)
 
1.3. Fabric Type 3 - Light Fabric with Acrylic Backing


Fig. 1 - Fabric Samples

 

2. Acoustical Measurement Procedures

 
2.1. General
Acoustical measurements were performed to accurately determine the frequency dependent Transmission Loss induced by the fabric samples.
 
2.2. Equipment
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.
 
2.3. Techniques
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)

 

 
3. Measurement Results and Analysis
 
3.1 Transmission Loss versus Frequency Graph
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

 
3.2. Analysis/Comments
It is immediately clear that there is no such thing as "acoustically transparent fabric".

Even the type 1 fabric ("acoustically transparent")* shows some fluctuations in the frequency response and an abation of a few dB above 10kHz.

 
The type 2 fabric (light regular fabric) is very close to the acoustically transparent fabric measurement curve, with a slight mismatch at frequencies above 10kHz, where the abation reaches approximately 5dB max. This is certainly correctable by a standard equalizer that is introduced into the electro-acoustical signal path.
 
The type 3 fabric (light fabric with acrylic backing) shows large fluctuations starting at 1kHz and reaching an abation level of up to 12dB above 10kHz. This fabric is not suitable for covering any acoustical elements or loudspeakers.
 
As a result, it is safe to state that the use of any fabric covering will always decrease the quality of the sound transmission, mainly above the 10kHz frequency band - the sound will lose some of its brilliance and freshness. However - unless we deal with a highly critical environment such as a recording studio, a high end home theater or a broadcast facility - the use of "acoustically transparent" or light regular fabric is certainly acceptable - the modifications to the frequency response can then be compensated by the use of an electro-acoustical equalizer.

 

*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.

 
 
 
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