wooden toggles
1982 Edition

Overview of the Standard's Scope

• Defines the approach for calculating the sound absorption coefficient of timber via the standing wave tube method at normal sound incidence.
• Intended for assessing timber used in applications such as panelling and suspended ceilings to mitigate reverberation and enhance acoustic quality.

Highlights:

• Sound absorption coefficients are reported per test frequency with 0.01 precision.
• Test parameters include specimen thickness, moisture content, timber species, and other relevant properties.
• The protocol provides a simpler and faster alternative compared to reverberation chamber methods, suitable for routine evaluations.

Equipment Requirements (Clause 3.4):

• Probe tube incorporating a microphone aligned on the impedance tube axis.
• Probe tube cross-sectional area should not exceed 5% of the impedance tube's cross-sectional area.
• Probe tube wall thickness must be at least one-eighth of the impedance tube's external diameter.

Rounding Rules (Clause 0.5):

• Final results are rounded following IS 2-1960 conventions, ensuring consistency with specified significant digits.

Formula for Absorption Coefficient (α)

[ \alpha = 1 - |R|^2 ]

Where:

• (R) represents the reflection coefficient deduced from standing wave measurements within the tube.

Equipment Configuration Diagram

flowchart LR
    A[Signal Generator] --> B[Amplifier and Filter]
    B --> C[Impedance Tube]
    C --> D[Probe Tube with Microphone]
    C --> E[Specimen Holder]
    D --> F[Output Display]

This standard corresponds with IS 707-1976 terminology and international acoustic testing practices for wood materials.

Definitions According to IS 10420

• Based primarily on terms from IS 707 - 1976.
• Key Term: Sound Absorption Coefficient (α) — proportion of sound energy absorbed by timber relative to the incident energy.
• Measurement Technique: Standing wave tube method at normal sound incidence.

Important Dimensional Formulas for Impedance Tube (Clause 3.2):

• Where (f_{min}) and (f_{max}) denote the lowest and highest frequencies used in testing (Hz).

Reporting Instructions (Clause 6.1):

• Present average sound absorption coefficient values at each frequency, rounded to two decimal places.
• Include specimen parameters such as thickness, moisture content, species, and testing method (standing wave normal incidence).

Conceptual Diagram: Standing Wave Test Setup

flowchart LR
    A[Sound Source] --> B[Impedance Tube]
    B --> C[Specimen Mounted at Tube Termination]
    B --> D[Microphones Measuring Pressure Variations]
    D --> E[Calculation of Absorption Coefficient]

Summary: IS 10420 details the method for timber sound absorption measurement using the standing wave tube technique, with dimensional requirements based on test frequency range, and precise result reporting protocols.

Apparatus Specifications for Sound Absorption Testing (IS 10420)

Dimensional Requirements (Clause 3.2):

• Minimum impedance tube length: [ L_{min} = \frac{300}{f_{min}} \quad \text{meters} ]
• Maximum impedance tube diameter: [ d_{max} = \frac{20000}{f_{max}} \quad \text{centimeters} ] where (f_{min}) and (f_{max}) are the minimum and maximum test frequencies (Hz).

Apparatus Components (Clauses 3.1, 3.4):

• Impedance Tube: Rigid, uniform cross-section tube, free of vibrations, dimensions per formulas above.
• Probe Tube: Movable microphone mounted on tube axis to detect standing wave pressure variations.
  • Cross-sectional area ≤ 5% of impedance tube area.
  • Wall thickness ≥ one-eighth of the impedance tube's external diameter.

Equipment Layout (Figure 1):

• Variable frequency audio oscillator
• Signal amplifier and filter
• Output display
• Movable microphone attached to probe tube

flowchart LR
    A[Signal Generator] --> B[Amplifier and Filter]
    B --> C[Impedance Tube with Sample]
    C --> D[Movable Microphone (Probe Tube)]
    D --> E[Output Display]

This configuration enables precise measurement of sound absorption by analyzing standing wave patterns inside the impedance tube.

Timber Specimen Preparation per IS 10420

• Specimen thickness: 15 mm from plank material.
• Shape: square or circular with side length or diameter equal to tube diameter plus 50 mm.
• Surface finish: smooth, sanded using No. 100 grit sandpaper.
• Conditioning: specimens stabilized to constant weight at 60 ± 5% relative humidity and 27 ± 1°C.
• Quality criteria: specimens must be free of cracks, splits, loose or decayed knots, and other defects that could influence results.

Mounting Details (Clause 5.1):

• Orientation: grain aligned vertically.
• Backing: solid steel or brass plate with thickness ≥ 10 cm.
• Secure fixing using flying nut screws to prevent airborne sound leakage.

Impedance Tube Dimensions (Clause 3.2):

• (f_{min}) and (f_{max}) denote the minimum and maximum test frequencies in hertz.

Diagram of Sample Mounting

flowchart LR
    A[Impedance Tube] --> B[Specimen Mounted at Tube End]
    B --> C[Specimen Holder with Rigid Backing]
    C --> D[Steel/Brass Plate ≥ 10 cm Thick]
    B --> E[Vertical Grain Orientation]

This preparation ensures reliable and consistent acoustic absorption measurement of timber samples using the standing wave method.

Procedure for Determining Sound Absorption Coefficient of Timber Using Standing Wave Method (IS 10420)

Specimen Preparation (Clause 4.1):

• Thickness: 15 mm plank.
• Shape: square or disc with side or diameter 50 mm greater than tube diameter.
• Surface: smooth, sanded with No. 100 grit sandpaper.
• Conditioning: stabilized at 60 ± 5% relative humidity and 27 ± 1°C until constant mass.
• Defect-free: no cracks, splits, or loose/decayed knots.

Impedance Tube Sizing (Clause 3.2):

• Minimum length (meters): [ L_{min} = \frac{300}{f_{min}} ]
• Maximum diameter (cm): [ d_{max} = \frac{20000}{f_{max}} ] Where (f_{min}) and (f_{max}) are the minimum and maximum test frequencies in hertz.

Test Frequencies (Clause 5.3):

• Frequencies used for creating stationary waves:
  • 125 Hz
  • 1000 Hz
  • 2000 Hz
  • 4000 Hz

Calculation of Absorption Coefficient (Clause 5.3):

[ \alpha_n = \frac{4(M - N)}{(M + N)^2} ] Where:

• (M) = relative maximum pressure amplitude
• (N) = relative minimum pressure amplitude
• (\alpha_n) = sound absorption coefficient at normal incidence for each frequency

Summary Table for Tube Dimensions

This method provides a straightforward and rapid approach to assess the acoustic absorption properties of timber materials using the standing wave tube technique.

Guidelines for Reporting Sound Absorption Coefficients (IS 10420)

Reporting Criteria (Clause 6.1):

• Present the mean sound absorption coefficient (αn) at each test frequency.
• Round values to the nearest 0.01.
• Include details such as:
  • Measurement technique: standing wave method at normal incidence.
  • Specimen specifics: thickness, moisture content, timber species, and other relevant information.

Test Frequencies (Clause 5.3):

• Conduct tests at 125 Hz, 1000 Hz, 2000 Hz, and 4000 Hz.

Calculation Formula for αn:

[ \alpha_n = \frac{4(M - N)}{(M - N)^2 + 4} ] Where:

• (M) is the relative maximum pressure.
• (N) is the relative minimum pressure.

Additional Notes:

• Follow rounding conventions as per IS 2:1960.
• The method utilizes a simple standing wave tube technique with normal incidence.
• Suitable for acoustic characterization of timber species.

Reporting Process Flow

flowchart TD
    A[Prepare Specimen] --> B[Measure Pressure Amplitudes M & N]
    B --> C[Calculate αn Using Formula]
    C --> D[Round αn to Two Decimal Places]
    D --> E[Compile Report Including Method, Specimen Details, and Results]

This structured reporting ensures compliance with IS 10420 and clarity in timber sound absorption evaluation.

Accuracy and Precision as Defined in IS 10420

• Precision is maintained through the use of a standardized impedance tube with:

  • Minimum length (L_{min} = \frac{300}{f_{min}}) meters.
  • Maximum diameter (d_{max} = \frac{20000}{f_{max}}) centimeters.

• Accuracy depends on:

  • Properly securing the specimen at the tube’s end.
  • Employing a probe tube with a cross-sectional area no greater than 5% of the main tube’s cross-section.
  • Using a movable microphone to capture the standing wave pattern precisely.

• Results must be reported to two decimal places, including specimen thickness, moisture content, species, and test approach (standing wave method at normal incidence).

Key Dimensional Formulas:

Example Tube Dimensions for Frequency Range 100–4000 Hz:

Simplified Equipment Setup

flowchart LR
    A[Signal Generator] --> B[Amplifier & Filter]
    B --> C[Speaker at Tube Entry]
    C --> D[Impedance Tube]
    D --> E[Mounted Timber Specimen]
    D --> F[Probe Tube with Microphone]
    F --> G[Signal Analyzer]

Strict adherence to these parameters is crucial for ensuring reliable and repeatable sound absorption measurements.

Important Considerations for Applying IS 10420

• Purpose: Defines the standing wave (tube) technique for determining timber’s sound absorption coefficient at normal sound incidence.

• Test Frequencies (Clause 5.3): 125 Hz, 1000 Hz, 2000 Hz, 4000 Hz.

• Calculation of Absorption Coefficient (αn): [ \alpha_n = \frac{4(M - N)}{(M + N)^2} ] Where M and N are the relative maximum and minimum sound pressure amplitudes, respectively.

• Reporting Requirements:

  • Values of αn reported to two decimal places.
  • Include specimen characteristics like thickness, moisture content, and species.

• Limitations:

  • The tube method is straightforward, fast, and requires small specimens.
  • Reverberation chamber method better simulates real-world conditions but is more complex and expensive.
  • The standing wave method results apply strictly to normal incidence sound waves.

• Rounding: Per IS 2:1960 standards.

Process Flow

flowchart LR
    A[Prepare Specimen] --> B[Set Up Standing Wave Equipment]
    B --> C[Measure Max (M) & Min (N) Pressure Levels]
    C --> D[Compute αn Using Formula]
    D --> E[Document Results with Specimen Details]

Summary: IS 10420 standardizes timber sound absorption measurement using the standing wave tube method, stressing accuracy, detailed specimen description, and recognizing the tube method’s inherent limitations.