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IS 10420:1982 is the Indian Standard (BIS) for method of determination of sound absorption coefficient of timber by standing wave method. 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 Note: This standard has been officially withdrawn by BIS.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
! 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.
ISO 10534-1:2024ISO (International Organization for Standardization)
HighCurrent
Acoustics — Determination of sound absorption coefficient and impedance in impedance tubes — Part 1: Standing wave ratio method
Defines the same standing wave ratio method but for general acoustic materials, not just timber.
ASTM C384-21ASTM International (USA)
HighCurrent
Standard Test Method for Impedance and Absorption of Acoustical Materials by Impedance Tube Method
Covers the identical standing wave method for determining acoustic properties of materials in a tube.
ISO 10534-2:2023ISO (International Organization for Standardization)
MediumCurrent
Acoustics — Determination of sound absorption coefficient and impedance in impedance tubes — Part 2: Transfer-function method
Achieves the same objective (absorption in a tube) but uses a more modern two-microphone transfer-function technique instead of the standing wave ratio method.
Key Differences
≠Scope of Material: IS 10420 is specifically for 'timber', whereas ISO 10534-1 and ASTM C384 apply to a broad range of 'acoustical materials'.
≠Sample Conditioning: IS 10420 requires determining the moisture content of timber according to another Indian Standard (IS 287), a material-specific requirement not found in the more general international standards.
≠Test Frequencies: IS 10420 mandates testing at specific discrete frequencies (125, 250, 500, 1000, 2000 Hz), while ISO and ASTM standards recommend testing at standard one-third-octave band center frequencies, offering more flexibility and detail.
≠Sample Thickness: IS 10420 specifies a fixed sample thickness of 25 mm for the test, whereas international standards do not specify a thickness, as it is a variable property of the material being tested.
Key Similarities
≈Fundamental Principle: All standards are based on the same physical principle of measuring the standing wave pattern generated in a rigid tube to determine sound absorption.
≈Core Measurement: The primary measurement in IS 10420, ISO 10534-1, and ASTM C384 is the Standing Wave Ratio (SWR), defined as the ratio of maximum to minimum sound pressure.
≈Apparatus Setup: The basic test apparatus is functionally identical, consisting of a rigid tube, a sound source (loudspeaker), a movable probe microphone, and a rigid termination for the test sample.
≈Calculated Parameter: The final objective of all these standards is to calculate the normal incidence sound absorption coefficient (α) from the measured SWR.
≈Sample Mounting: All standards require the test specimen to be cut to fit the tube diameter precisely and mounted with a tight seal to prevent air leaks around its periphery.
Parameter Comparison
Parameter
IS Value
International
Source
Primary Measured Quantity
Standing wave ratio (SWR)
Standing wave ratio (SWR)
ISO 10534-1:2024
Low Frequency Tube Diameter
10 cm (recommended)
Not specified, but performance-based. A common size is 100 mm.
ASTM C384-21
High Frequency Tube Diameter
3 cm (recommended)
Not specified, but performance-based. A common size is 29 mm.
ASTM C384-21
Test Frequencies
125, 250, 500, 1000, 2000 Hz
Preferably one-third-octave band centre frequencies within the tube's operating range.
ISO 10534-1:2024
Sample Thickness Specification
25 mm
Not specified; thickness is a property of the material under test and must be reported.
ISO 10534-1:2024
Number of Test Specimens
Three specimens shall be prepared.
At least two specimens shall be tested.
ISO 10534-1:2024
Sample Conditioning Requirement
Moisture content to be determined as per IS 287.
Specimens conditioned to equilibrium with the laboratory atmosphere (e.g., 23 °C, 50% RH).
ASTM C384-21
⚠ Verify details from original standards before use
Key Values7
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).