wooden toggles

IS 10420: Scope & Key Specifications

Scope:

• Specifies the method to determine sound absorption coefficient of timber using the standing wave (tube) method at normal incidence.
• Applicable for evaluating timber used in panelling and false ceilings to reduce reverberation and improve audibility.

Key Points:

• Sound absorption coefficient is reported at each test frequency to the nearest 0.01.
• Test conditions include: thickness, moisture content, timber species, and other specimen details.
• The method is simpler and faster than reverberation chamber methods, suitable for routine testing.

Equipment (Clause 3.4):

• Probe tube with a microphone fixed on the axis of the impedance tube.
• Probe tube cross-sectional area ≤ 5% of impedance tube cross-section.
• Probe tube wall thickness ≥ 1/8th of impedance tube outside diameter.

Rounding (Clause 0.5):

• Final results rounded per IS 2-1960 rules, matching the number of significant digits of specified values.

Typical Formula for Sound Absorption Coefficient (α)

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

Where:

• (R) = Reflection coefficient derived from standing wave measurements inside the tube.

Summary Diagram of Equipment Setup

flowchart LR
    A[Audio Oscillator] --> B[Amplifier & Filter]
    B --> C[Impedance Tube]
    C --> D[Probe Tube with Microphone]
    C --> E[Specimen Holder]
    D --> F[Output Indicator]

This standard aligns with IS 707-1976 definitions and international practices for timber acoustics testing.

IS 10420 - Definitions Summary

• Reference Standard: Definitions are primarily as per IS 707 - 1976.
• Key Term: Sound Absorption Coefficient (α) — ratio of absorbed sound energy to incident sound energy on timber.
• Measurement Method: Standing Wave (Tube) Method at normal incidence.

Important Formulae for Impedance Tube Dimensions (Clause 3.2):

• ( f_{min} ), ( f_{max} ) = minimum and maximum test frequencies (Hz)

Reporting (Clause 6.1):

• Report average sound absorption coefficient at each frequency to nearest 0.01.
• Include details: thickness, moisture content, species, test method (stationary wave at normal incidence).

Conceptual Diagram: Standing Wave Method Setup

flowchart LR
    A[Sound Source] --> B[Impedance Tube]
    B --> C[Specimen Mounted at Tube End]
    B --> D[Microphones Measure Pressure Variations]
    D --> E[Calculate Sound Absorption Coefficient]

Summary: IS 10420 defines timber sound absorption measurement referencing IS 707, using the standing wave tube method with specified tube dimensions based on test frequency range. Results must be reported precisely with specimen details.

IS 10420: Apparatus Specifications for Sound Absorption Measurement

Key Formulas (Clause 3.2)

• Minimum length of impedance tube (L_min):
  [ L_{min} = \frac{300}{f_{min}} \quad \text{(in meters)} ]

• Maximum diameter of impedance tube (d_max):
  [ d_{max} = \frac{20,000}{f_{max}} \quad \text{(in cm)} ]

where:

• ( f_{min} ) = lowest frequency of measurement (Hz)
• ( f_{max} ) = highest frequency of measurement (Hz)

Apparatus Components (Clauses 3.1, 3.4)

• Impedance Tube:
  • Rigid, vibration-free, uniform cross-section tube.
  • Length and diameter as per formulas above.
• Probe Tube:
  • Movable microphone on tube axis for detecting standing waves.
  • Cross-sectional area ≤ 5% of impedance tube area.
  • Wall thickness ≥ 1/8th of impedance tube outside diameter.

Equipment Schematic (Fig. 1)

• Variable frequency audio oscillator
• Amplifier and filter
• Output indicator
• Movable microphone/probe tube

flowchart LR
    A[Audio Oscillator] --> B[Amplifier & Filter]
    B --> C[Impedance Tube with Specimen]
    C --> D[Movable Microphone (Probe Tube)]
    D --> E[Output Indicator]

This setup ensures accurate measurement of sound absorption coefficients by analyzing standing wave patterns inside the impedance tube.

IS 10420: Specimen Preparation for Sound Absorption Test (Standing Wave Method)

Key Specifications (Clause 4.1)

• Specimen thickness: 15 mm plank.
• Shape: Square or disc.
• Size: Side/diameter = (Diameter of tube + 50 mm).
• Surface: Smooth, sanded with No. 100 sandpaper.
• Conditioning: Constant weight at 60 ± 5% RH and 27 ± 1℃.
• Quality: Free from cracks, splits, loose/decayed knots, or defects affecting results.

Mounting (Clause 5.1)

• Orientation: Grain vertical.
• Backing: Solid steel/brass plate, ≥ 10 cm thick.
• Fixing: Tight with flying nut screws to prevent airborne transmission.

Impedance Tube Dimensions (Clause 3.2)

• ( f_{min} ), ( f_{max} ) = lowest and highest test frequencies (Hz).

Summary Diagram: Specimen Setup

flowchart LR
    A[Impedance Tube] --> B[Specimen mounted at tube end]
    B --> C[Specimen holder with rigid backing]
    C --> D[Backing: Steel/Brass plate ≥ 10 cm thick]
    B --> E[Grain direction vertical]

This preparation ensures accurate measurement of timber sound absorption by the standing wave method per IS 10420.

IS 10420: Test Procedure for Sound Absorption Coefficient of Timber by Standing Wave Method

Specimen Preparation (Clause 4.1)

• Thickness: 15 mm plank
• Shape: Square or disc, side/diameter = 50 mm + tube diameter
• Surface: Smooth, sanded with No. 100 sandpaper
• Conditioning: At 60 ± 5% RH and 27 ± 1°C until constant weight
• Defects: Specimen must be free from cracks, splits, loose/decayed knots

Impedance Tube Dimensions (Clause 3.2)

• Minimum length, ( l_{min} ) (m):
  [ l_{min} = \frac{F_{min}}{300} ]
• Maximum diameter, ( d_{max} ) (cm):
  [ d_{max} = \frac{20,000}{F_{max}} ] Where ( F_{min} ) and ( F_{max} ) are the minimum and maximum test frequencies (Hz).

Test Frequencies (Clause 5.3)

• Frequencies to produce stationary wave pattern:
  125 Hz, 1000 Hz, 2000 Hz, 4000 Hz

Sound Absorption Coefficient Calculation (Clause 5.3)

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

• ( M ): Relative maximum pressure amplitude
• ( N ): Relative minimum pressure amplitude
• ( \alpha_n ): Normal incident sound absorption coefficient at each frequency

Summary Table for Tube Dimensions

This method offers a simple, rapid test for timber's acoustical effectiveness using the standing wave (tube) method under normal incidence.

flowchart LR
    A[Specimen Preparation] --> B[Conditioning at 60±5% RH, 27±1°C

IS 10420 - Key Formulas and Specifications for Report on Sound Absorption Coefficient

1. Reporting Requirements (Clause 6.1)

• Average sound absorption coefficient (αn) must be reported at each test frequency.
• Values rounded to the nearest 0.01.
• Report must specify:
  • Measurement method: Stationary wave method at normal incidence.
  • Specimen details: Thickness, moisture content, species, and other relevant data.

2. Test Frequencies (Clause 5.3)

• Stationary wave pattern tested at frequencies:
  • 125 Hz, 1000 Hz, 2000 Hz, 4000 Hz

3. Calculation Formula for Sound Absorption Coefficient (αn)

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

• ( M ) = Relative maximum pressure amplitude
• ( N ) = Relative minimum pressure amplitude

4. Additional Notes

• Use IS 2:1960 rounding rules.
• The method is a simple tube (standing wave) technique under normal incidence.
• Useful for timber species acoustic evaluation.

flowchart TD
    A[Specimen Preparation] --> B[Measure M and N at test frequencies]
    B --> C[Calculate αn using formula]
    C --> D[Round αn to 0.01]
    D --> E[Prepare report with method, specimen details, αn values]

This concise framework ensures compliance with IS 10420 for timber sound absorption reporting.

IS 10420: Precision and Accuracy in Sound Absorption Coefficient Determination

Key Points from IS 10420:

• Precision is ensured by using a standardized Impedance Tube with:

  • Minimum length ( l_{min} = \frac{300}{f_{min}} ) (in meters)
  • Maximum diameter ( d_{max} = \frac{20000}{f_{max}} ) (in cm)

  where ( f_{min} ) and ( f_{max} ) are the lowest and highest test frequencies in Hz.

• Accuracy depends on:

  • Proper specimen mounting at one end of the tube.
  • Use of a probe tube with a cross-sectional area ≤ 5% of the impedance tube's cross-section.
  • Measurement of standing wave pattern using a movable microphone.

• Reporting:

  • Sound absorption coefficient (\alpha) must be reported to the nearest 0.01.
  • Include specimen thickness, moisture content, species, and test method (standing wave method at normal incidence).

Important Formulae:

Summary Table for Tube Dimensions:

Conceptual Diagram (Simplified):

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

Note: Precision and accuracy rely on strict adherence

IS 10420: Notes on Application and Limitations - Key Points

• Purpose: Specifies the standing wave method (tube method) for determining the sound absorption coefficient of timber under normal incidence.

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

• Sound Absorption Coefficient (αn):
  Calculated from stationary wave pattern using relative max (M) and min (N) pressure amplitudes:

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

• Reporting:

  • Report αn to nearest 0.01 at each frequency (Clause 6.1)
  • Include specimen details: thickness, moisture content, species, test method (stationary wave method at normal incidence).

• Limitations:

  • Tube method is simple, rapid, and requires small specimens.
  • Reverberation chamber method simulates field conditions better but is costly and complex.
  • Results apply strictly for normal incidence sound waves.

• Rounding: Follow IS 2:1960 rules for rounding final values.

flowchart LR
    A[Specimen Preparation] --> B[Stationary Wave Setup]
    B --> C[Measure Max (M) and Min (N) Pressure]
    C --> D[Calculate αn using formula]
    D --> E[Report αn with specimen details]

Summary: IS 10420 standardizes timber sound absorption testing via the standing wave method at specified frequencies, emphasizing accuracy, specimen characterization, and limitations of the tube method.