Blow Through Test is a valid way to evaluate acoustical transparency.
The “blow through test” is sometimes used as a preliminary way to evaluate acoustic transparency. The person evaluating the fabric blows through the fabric and if they are able to feel their breath on the other side, they consider the fabric acoustically transparent and suitable for acoustic panel use.
The reality is that this methodology has no scientific backing. It can be misleading and may not align with actual outcomes of a true acoustical test. Industry best practice is to use fabrics that have been evaluated by a reputable 3rd party with the industry recognized test ASTM C423 and are NRC rated.
Fabrics for acoustic panels should have an NRC of 1.
Fabrics alone typically have a difficult time reducing NRC in a space on their own. When using acoustical panels, it is the acoustic panel material (the substrate), not the fabric that does the job of absorbing sound.
It is the combination of the fabric when covering the acoustical panel that should have an NRC close to/or over 1.0. The textile's function is to look great and allow sound to pass through to the acoustical substrate which does the work.
When selecting fabrics to be used on acoustic panels, the textile should have minimal impact upon the NRC performance of the panel it is wrapped around. The textile should be acoustically transparent and the finished panel should achieve an NRC rating in the vicinity of .90 - 1.05.
If you are evaluating a fabric on its own to cover a panel, look for an NRC between 0 - .25 (lower the better).