Glossary of terms applicable to acoustics in buildings

IS 9736: Scope & Key Specifications

IS 9736 primarily covers terminology, units, and definitions related to wave phenomena, emphasizing international coordination and alignment with Indian practices.

Scope Highlights:

• Defines wave-related terms (e.g., Wave Length: distance between two wave fronts differing by one period).
• Standardizes units using the International System of Units (SI).
• Ensures consistency with global standards and Indian field practices.

Key SI Units & Symbols (Relevant to IS 9736):

Important Formula:

• Wave Length (λ):
  [ \lambda = \frac{v}{f} ] where:
  ( \lambda ) = wave length (m)
  ( v ) = wave velocity (m/s)
  ( f ) = frequency (Hz)

flowchart LR
    A[Wave Source] --> B[Wave Front 1]
    B --> C[Wave Front 2]
    C --> D[Wave Length λ]
    D --> E[Distance between fronts with phase difference = 1 period]

Summary: IS 9736 sets the framework for wave terminology and units, ensuring uniformity in measurement and communication in engineering and scientific contexts.

IS 9736 provides a glossary of acoustic terms essential for building acoustics. Key definitions include:

• Sound Pressure Level (SPL):
  [ L_p = 20 \log_{10} \left(\frac{p}{p_0}\right) \quad \text{dB} ]
  where (p) = sound pressure, (p_0 = 20 \times 10^{-6} \text{ Pa}) (reference).

• Reverberation Time (RT60):
  Time for sound to decay by 60 dB after source stops.

• Sound Absorption Coefficient (α):
  Ratio of absorbed sound energy to incident sound energy (0 to 1).

• Sound Transmission Loss (TL):
  Reduction in sound intensity through a partition.

Common Acoustic Terms Table (Summary)

These definitions form the basis for acoustic design and measurement in buildings per IS 9736.

flowchart LR
    A[Sound Source] --> B[Sound Pressure Level (Lp)]
    B --> C[Sound Absorbed (α)]
    B --> D[Sound Transmitted (TL)]
    B --> E[Reverberation Time (T60)]

Absorption Coefficient (α)

• Defined as the ratio of sound energy absorbed by a material to the incident sound energy on it.
• Range: 0 (total reflection) to 1 (total absorption).

Key Formulas:

1. Absorption of Surface (A)
   [ A = \alpha \times S ]

• (A) = Absorption in sabins
• (\alpha) = Absorption coefficient (dimensionless)
• (S) = Surface area (m²)

2. Noise Reduction Coefficient (NRC)
   [ NRC = \text{Average of } \alpha_{250}, \alpha_{500}, \alpha_{1000}, \alpha_{2000} \text{ Hz, rounded to nearest 0.05} ]

Units:

• Absorption Unit = Sabin (1 sabin = absorption by 1 sq. ft. of perfectly absorbing surface)

Summary Table (Typical Absorption Coefficients)

flowchart LR
    IncidentSound -->|Energy| Surface
    Surface -->|Reflected| ReflectedSound
    Surface -->|Absorbed (α)| AbsorbedEnergy
    AbsorbedEnergy -->|Energy| ConvertedToHeat

This captures the core concept of absorption coefficient per IS 9736.

IS 9736 primarily provides a glossary of terms related to acoustics in buildings rather than detailed formulas or tables.

Key Highlights:

• It standardizes acoustic terminology for uniform understanding.
• Covers terms like Sound Absorption, Sound Insulation, Reverberation Time, Noise Criteria, etc.
• Does not provide design formulas or detailed specifications.

For practical acoustics design (beyond IS 9736), refer to:

Recommended IS Codes for detailed acoustics design:

• IS 4021: Code of practice for sound insulation of buildings.
• IS 4991: Code of practice for acoustics of buildings.

flowchart LR
    A[IS 9736] --> B[Glossary of Acoustic Terms]
    B --> C[Uniform Terminology]
    D[IS 4021] --> E[Sound Insulation Design]
    F[IS 4991] --> G[Acoustic Design Practices]

Summary: Use IS 9736 for terminology; for formulas and design, refer to IS 4021 and IS 4991.

IS 9736: Attenuation in Acoustical Systems

Key Definitions:

• Attenuation (Clause 2.11):
  Decrease of sound power level (in dB) between two points in an acoustical system.

• Absorption Coefficient (α) (Clause 2.1):
  [ \alpha = \frac{\text{Sound energy absorbed}}{\text{Incident sound energy}} ]

• Transmission Loss (TL) (Clause 2.68):
  Decrease in transmitted sound power (dB) across a barrier or system.

• Absorption Unit (Sabin) (Clause 2.2):
  One sabin = 1 sq. ft of perfectly absorbing surface.

Key Formula for Attenuation:

[ \text{Attenuation (dB)} = 10 \log_{10} \left(\frac{P_1}{P_2}\right) ]

• (P_1) = Sound power at the source point
• (P_2) = Sound power at the receiver point

Typical Absorption Coefficient Values (α):

Transmission Loss (TL) Approximation:

[ TL = 20 \log_{10}(f \times m) - 47 ]

• (f) = frequency (Hz)
• (m) = surface mass per unit area (kg/m²)

flowchart LR
    A[Sound Source] -->|Sound Power \(P_1\)| B[Acoustical System]
    B -->|Attenuated Power \(P_2\)| C[Receiver]
    B -- Attenuation (dB) --> D[Decrease in Sound Power]

Summary:
Attenuation quantifies sound power reduction in dB, linked to absorption coefficients and transmission loss. Use the formulas above to calculate sound attenuation and design effective noise control systems per IS 9736.

IS 9736: Cavity Wall Key Points

• Definition (Clause 2.14):
  A cavity wall consists of two separate masonry walls (leaves) with a cavity width of 50 mm to 100 mm or more between them. The leaves are connected by solid or flexible ties.

• Purpose of Cavity:

  • Improves thermal insulation
  • Prevents moisture penetration
  • Enhances sound insulation

Typical Specifications for Cavity Walls:

Key Formula for Tie Spacing (approximate):

[ \text{Tie spacing} \leq \text{max vertical } 600 \text{ mm}, \quad \text{max horizontal } 1200 \text{ mm} ]

Cavity Wall Components Diagram:

graph LR
A[Outer Leaf] ---|Ties| B[Cavity (50-100 mm)]
B ---|Ties| C[Inner Leaf]

Note: For detailed design, refer to IS 2185 (Part 1 & 2) for masonry units and IS 1905 for masonry design.

IS 9736 - Completely Diffuse Sound

According to Clause 2.15 of IS 9736:

• Completely Diffuse Sound is defined as sound with uniform energy density in a region.
• The direction of sound propagation at any point is random and uniformly distributed.

Key Points:

• Energy density ( E ) is constant throughout the space.
• No preferred direction of sound propagation.
• Ideal assumption for reverberant sound fields in room acoustics.

Relevant Formula for Energy Density in Diffuse Field:

[ E = \frac{p^2_{\text{rms}}}{\rho c^2} ] where:

• ( p_{\text{rms}} ) = root mean square sound pressure
• ( \rho ) = density of air (≈ 1.2 kg/m³)
• ( c ) = speed of sound in air (≈ 343 m/s)

Application:

• Used in reverberation time calculations (Sabine’s formula).
• Basis for sound absorption and reflection analysis in enclosed spaces.

flowchart LR
    A[Sound Source] --> B[Room Volume]
    B --> C[Uniform Energy Density]
    C --> D[Random Propagation Directions]
    D --> E[Completely Diffuse Sound Field]

For detailed absorption coefficients and reverberation time, refer to IS 9736 annexures and tables.

IS 9736: Continuous and Impulsive Noise – Key Points & Formulas

1. Noise Types (Clauses 2.16, 2.51)

• Continuous Noise: Constant vibration source (e.g., machinery hum).
• Impulsive Noise: Short bursts of high intensity (e.g., drop forge hammer).
• Random Noise: Fluctuating sound pressure with Gaussian distribution.

2. Damage-Risk Noise Criteria (Clause 2.18)

• Specifies maximum permissible noise levels and exposure durations to prevent hearing damage.
• Typical criteria limit exposure to:
  • 85 dB(A) for 8 hours (continuous noise)
  • Higher levels require shorter exposure times.

3. Octave-Band Noise Levels (Clause 2.42)

• Noise is analyzed in octave bands for frequency-specific assessment:

  Band Number	Frequency Range (Hz)
  1	f₀ to 2f₀
  2	2f₀ to 4f₀
  3	4f₀ to 8f₀
  ...	...

4. Key Formulas

• Equivalent Continuous Noise Level (Leq):

[ L_{eq} = 10 \log_{10} \left( \frac{1}{T} \int_0^T 10^{\frac{L(t)}{10}} dt \right) ]

• Peak Impulsive Noise Level (Lpeak): Measured directly, important for impulsive noise.

• Noise Dose (D):

[ D = \sum \left( \frac{C_n}{T_n} \right) \times 100% ]

Where:

• (C_n) = actual exposure time at noise level (n)
• (T_n) = permissible exposure time at noise level (n)

flowchart LR
    A[Noise Source] --> B{Type of Noise}
    B -->|Continuous| C[Constant Vibration]
    B -->|Impulsive| D[Short High Intensity Bursts]
    B -->|Random| E[Gaussian Fluctuations]
    C --> F[Measure Leq]
    D

IS 9736: Damage-Risk Noise Criteria (DRNC)

Key Concepts:

• Damage-Risk Noise Criteria (Clause 2.18) define safe noise exposure limits based on maximum noise level and duration to prevent hearing damage.
• Noise exposure is cumulative; higher noise levels require shorter exposure times.

Noise Reduction Coefficient (NRC) (Clause 2.41):

• NRC = Average absorption coefficient at frequencies 250, 500, 1000, 2000 Hz
• Rounded to nearest 0.05
• Indicates material's sound absorption efficiency (0 = no absorption, 1 = total absorption)

Typical Damage-Risk Noise Criteria (General Engineering Practice):

Above 110 dB, exposure should be minimal to avoid hearing damage.

Summary:

• Use DRNC to limit noise exposure duration based on level.
• Use NRC to select materials for noise absorption in buildings.
• Follow IS 9736 for uniform acoustics terminology and design criteria.

flowchart LR
    A[Noise Exposure Level] --> B{Is Level > 85 dB?}
    B -- Yes --> C[Limit Exposure Time]
    B -- No --> D[Safe Exposure]
    C --> E[Use DRNC Table]
    E --> F[Set Max Exposure Duration]
    D --> G[No Hearing Damage Risk]

IS 9736: Dead Sets and Studios – Key Points

• Dead Sets/Studios (Clause 2.20):
  Enclosed spaces lined with highly sound-absorptive materials to minimize sound reflections, creating an acoustically "dead" environment. This is critical for recording or broadcasting to avoid echo and reverberation.

• Materials Used:
  Typically, porous absorbers (e.g., mineral wool, foam), heavy curtains, or acoustic panels with high Noise Reduction Coefficient (NRC > 0.8).

• Dead Spots (Clause 2.21):
  Areas with negligible sound intensity caused by destructive interference. Avoid by proper room geometry and speaker placement.

Key Specifications for Dead Studios:

Basic Formula for Reverberation Time (Sabine’s Formula):

[ RT_{60} = \frac{0.161 V}{A} ]

Where:

• ( V ) = Volume of room (m³)
• ( A = \sum (S_i \times \alpha_i) ) = Total absorption (m² sabins)
• ( S_i ) = Surface area of ith material (m²)
• ( \alpha_i ) = Absorption coefficient of ith material

flowchart LR
    A[Sound Source] --> B[Dead Studio]
    B --> C[Absorptive Materials]
    C --> D[Sound Absorbed]
    B --> E[Minimal Reflections]
    E --> F[Clear Sound Recording]

Summary: Dead studios use high absorption materials and design to minimize reflections and reverberation, ensuring clear sound capture as per IS 9736 definitions.

Dead Spots (IS 9736 - Clause 2.21)
Dead spots are locations in a hall or room where sound intensity is minimal due to destructive interference of sound waves.

Key Concepts

• Dead Spots: Result from sound waves meeting out of phase, causing cancellation.
• Dead Rooms/Sets (Clause 2.20): Enclosed spaces with materials that absorb nearly all sound, eliminating reflections.

Relevant Formula: Wavelength (Clause 2.69)

[ \lambda = \frac{c}{f} ]

• (\lambda) = Wavelength (m)
• (c) = Speed of sound in air (~343 m/s at 20°C)
• (f) = Frequency (Hz)

Practical Notes for Dead Spots

• Dead spots occur at positions where path difference = ((2n+1)\frac{\lambda}{2}), (n=0,1,2...) causing destructive interference.
• To minimize dead spots: use sound diffusers, absorbers, or alter geometry.

Units (SI) for Acoustics (Summary)

flowchart LR
A[Sound Source] --> B[Sound Waves]
B --> C{Interference}
C -->|Constructive| D[High Intensity]
C -->|Destructive| E[Dead Spot (Low Intensity)]

Summary: Dead spots are caused by destructive interference where sound waves cancel out. Understanding wavelength and frequency is key to predicting and mitigating these zones in acoustic design.

Flutter Echo as per IS 9736 is defined as a rapid succession of echoes of even rate within an enclosure, caused by sound reflecting repeatedly between parallel surfaces.

Key Points:

• Flutter Echo occurs mainly between two parallel reflective surfaces.
• It is perceived as a rapid, repetitive sound, disturbing speech clarity and sound quality.
• Typical in rooms with hard, flat, parallel walls and ceilings.

Basic Understanding (No direct formulas in IS 9736):

• Echo delay time ( t = \frac{2d}{c} )
  • ( d ) = distance between parallel surfaces (m)
  • ( c ) = speed of sound (~343 m/s at 20°C)
• Flutter echo frequency ( f = \frac{c}{2d} )

Control Measures:

• Break parallel surfaces with diffusers or absorptive materials.
• Use angled walls or acoustic panels to reduce flutter echo.

flowchart LR
    A[Sound source] --> B[Parallel reflective surfaces]
    B --> C[Multiple rapid reflections]
    C --> D[Flutter Echo perceived]
    D --> E[Use absorbers/diffusers]

Summary Table:

IS 9736 emphasizes understanding flutter echo as a phenomenon and controlling it through architectural acoustics rather than prescribing direct formulas.

IS 9736 Noise Key Points & Formulas

• Noise Definition (2.40): Unwanted sound, typically measured in decibels (dB).

• Random Noise (2.51): Fluctuates with time, amplitudes follow a Gaussian distribution.

• Octave-Band Noise Levels (2.42): Noise measured in frequency bands doubling each time:

  • Example bands: f₀–2f₀, 2f₀–4f₀, 4f₀–8f₀, etc.
  • Used for frequency-specific noise analysis.

• Air-borne Noise (2.8): Transmitted through air via openings like doors, windows, ducts.

Typical Noise Level Measurement Formula:

[ L_p = 20 \log_{10} \left(\frac{p}{p_0}\right) \quad \text{(dB)} ]

• (L_p) = Sound pressure level in dB
• (p) = RMS sound pressure
• (p_0 = 20 \times 10^{-6} \text{ Pa}) (reference sound pressure)

Octave Band Frequency Table Example:

Summary: IS 9736 defines noise types and measurement bands, emphasizing octave-band analysis for sound control design. Use the sound pressure level formula and octave band table for noise quantification and mitigation.

IS 9736 - Public Address System (PA System) Key Points

As per Clause 2.50 of IS 9736, a PA system consists of:

• Microphones (sound input)
• Amplifiers (signal boosting)
• Loudspeakers (sound output)

Key Specifications & Design Considerations:

• Sound Reinforcement: Ensure adequate loudness for comfortable hearing.
• Coverage Area: Loudspeaker placement must cover the entire audience area uniformly.
• Power Rating: Amplifier and speaker power ratings should match to avoid distortion.
• Frequency Response: Typically 100 Hz to 10 kHz for speech clarity.
• Signal-to-Noise Ratio: Should be >60 dB for clear sound.

Typical Formulas:

• Sound Power Level (Lw):
  [ L_w = 10 \log \left(\frac{P}{P_0}\right) ]
  where (P) = power in watts, (P_0 = 10^{-12} W)

• Sound Pressure Level (Lp) at distance r:
  [ L_p = L_w - 20 \log r - 11 ]
  where (r) = distance from speaker in meters

Suggested Loudspeaker Spacing (for even coverage):

flowchart LR
    Microphone --> Amplifier --> Loudspeaker --> Audience
    Audience --> Feedback loop --> Microphone

Summary: Design your PA system by matching power ratings, ensuring uniform coverage, and maintaining clarity per IS 9736 guidelines for effective sound reinforcement.

IS 9736 defines a Sound Level Meter (SLM) as a device to measure sound pressure levels, following international specs (IEC 61672).

Key Specifications for Sound Level Meter (per IS and IEC standards):

• Frequency Range: 20 Hz to 20 kHz (typical)
• Measurement Range: 30 dB to 130 dB SPL (Sound Pressure Level)
• Weighting Networks:
  • A-weighting (dBA): Mimics human ear sensitivity, used for noise measurement
  • C-weighting (dBC): For peak measurements and low-frequency sounds
• Time Weighting: Fast (125 ms), Slow (1 s), Impulse (35 ms)

Important Formulas:

• Sound Pressure Level (SPL):
  [ L_p = 20 \log_{10} \left(\frac{p}{p_0}\right) \quad \text{dB} ]
  Where:
  ( p ) = measured sound pressure (Pa)
  ( p_0 = 20 \times 10^{-6} ) Pa (reference sound pressure)

• Frequency Weighting:
  Weighting filters modify ( p ) to ( p_w ) based on frequency response curves.

Typical Table: Frequency Weighting A (dB adjustments)

Summary Diagram of SLM Operation:

flowchart LR
    A[Sound Wave] --> B[Microphone]
    B --> C[Pre-Amplifier]
    C --> D[Frequency Weighting Filter (A/C)]
    D --> E[Time Weighting (Fast/Slow)]
    E --> F[Display: Sound Level