This standard details the laboratory method for determining the normal incidence sound absorption coefficient of timber using a standing wave apparatus (impedance tube). It covers the required apparatus, test specimen preparation, procedure for measuring standing wave ratio, and the formula for calculating the coefficient. Note: This standard has been officially withdrawn by BIS.
Method of determination of sound absorption coefficient of timber by standing wave method
Quick Reference Values
Maximum permissible moisture content12 %
Type 1 Toggle Length100 mm
Type 1 Toggle Diameter25 mm
Type 1 Toggle Hole Diameter8 mm
Type 2 Toggle Length150 mm
Type 2 Toggle Diameter38 mm
Type 2 Toggle Hole Diameter12 mm
Key Formulas
α = 1 - [(n-1)/(n+1)]² — Where α is the sound absorption coefficient and n is the standing wave ratio (SWR).
Practical Notes
Ensure the timber is well-seasoned to a moisture content not exceeding 12% to prevent dimensional changes and cracking after manufacture.
Carefully inspect for prohibited defects like decay, live insect attack, and loose or dead knots, as these can significantly reduce the toggle's strength.
Verify the finish is smooth and free from splinters, and optionally treated with a preservative like linseed oil for enhanced durability.