Comprehensive Study Notes on Occupational Audiology: Sound and Sound Measurements

OCAU021 Occupational Audiology: Fundamentals of Sound

• Course Code: $OCAU021$
• Institution: Sefako Makgatho Health Sciences University, Department of Speech-Language Pathology and Audiology
• Lecturer: Mrs. A. Louw
• Date: 23 February 2026

Sound and Sound Propagation

• Definition of Sound: Sound consists of small rhythmic variations in atmospheric pressure caused by specific types of vibration.
• Physiological Processing: These pressure variations act upon the human ear as physical stimuli, which are then interpreted by the brain as sound.
• Medium Requirements: For sound to be created and transmitted, a medium must possess two fundamental characteristics:
  • Mass: The physical substance that can be set into motion.
  • Elasticity: The property that allows the medium to return to its original state after being displaced.
• Molecular Motion: Sound waves are not the continuous forward movement of individual molecules; rather, they are the forward and backward oscillations of molecules, specifically identified as Simple Harmonic Motion.

Theoretical Example: The Shunting Yard Analogy

• The Scenario (Mass without Elasticity): Imagine a long line of uncoupled railway coaches on a straight track. If a locomotive shunts into the first coach and then withdraws, the first coach gains momentum due to its mass and velocity. It moves a short distance ($\text{a few centimeters}$) and collides with the second coach, transferring its momentum. This creates a wave of impacts traveling down the line.
  • Observation: In this system, each coach stops after colliding. The entire system ends up displaced a small distance from the start.
  • Conclusion: This system lacks elasticity. While it transmits power (momentum), it is not a sound wave because sound is a flow of power, not a flow of mass leading to permanent displacement.
• The Enhanced Scenario (Elasticity Introduced): If each coach is anchored to its position with a rubber rope, the rope provides a restoring force after an impact.
  • Mechanical Action: The pulling force is proportional to the degree of displacement. This causes each coach to oscillate back and forth around its anchorage point in Simple Harmonic Motion.
  • Continuous Wave Generation: If the first coach is kept oscillating continuously by the driver, wave after wave of collisions pass down the line. In a sufficiently long line, multiple waves can travel simultaneously.

Wave Characteristics and Representation

• Wave Type: Sound is a longitudinal wave, consisting of alternating areas of high pressure and low pressure.
  • Compression: Areas where molecules are crowded together (high pressure).
  • Rarefaction: Areas where molecules are spread apart (low pressure).
• Visualizing Sound: For clarity, sound is often represented two-dimensionally as a sine (transverse) wave:
  • Crest: Corresponds to the peak of compression in a longitudinal wave.
  • Trough: Corresponds to the center of a rarefaction.
  • Amplitude: In a transverse wave, this is the height of the crest/depth of the trough. In a longitudinal wave, it corresponds to the size or magnitude of the compression.
• Comparison of Wave Forms:
  • Transverse Waves: Example: Water ripples. The movement of the medium is perpendicular to the direction of the wave.
  • Longitudinal Waves: Example: Sound. The movement of the medium is parallel to the direction of wave travel.

Physical Relationships in Sound

• Frequency: Defined as the number of oscillations moving past a specific point in a specific time period. Measured in Hertz ($Hz$).
  • More oscillations = higher frequency (higher pitch).
  • Fewer oscillations = lower frequency (lower pitch).
  • Effect on Amplitude: Amplitude is independent of frequency.
• Intensity and Loudness: Governed by the amplitude of the wave.
  • Greater amplitude = louder sound.
  • Smaller amplitude = softer sound.
• Proportional Relationships:
  • $\text{Increased Pressure} = \text{Increased Displacement}$
  • $\text{Increased Speed} = \text{Increased Displacement}$

Measuring Sound Parameters

• Sound Pressure:
  • Definition: The force per unit area acting on a surface.
  • Dependence: It is dependent on the distance from the source and the specific environment.
  • Unit: Pascal ($Pa$).
• Sound Power:
  • Definition: The total acoustical energy created by a sound source.
  • Dependence: It does not depend on distance from the source. It is an inherent property of the source itself.
  • Unit: Watt ($W$).
• Sound Intensity:
  • Definition: The total acoustical energy per unit area.
  • Dependence: It decreases with distance from the source.
  • Unit: Watts per square meter ($W/m^2$).
• The Heater Analogy:
  • Temperature ($\text{degrees}$) is analogous to Sound Pressure ($Pa$).
  • Heater Power ($Watts$) is analogous to Sound Power ($Watts$).
  • Heat Flow ($Watts/Area$) is analogous to Sound Intensity ($Watts/Area$).

Detailed Sound Pressure and the Decibel Scale

• Sound Pressure Formula: Pressure is the result of variations achieved by sound waves. $1 \text{ Newton per square meter} (1 N/m^2) = 1 \text{ Pascal} (1 Pa)$.
• Auditory Thresholds:
  • Threshold of Hearing: For a healthy, normal young person, the softest audible sound pressure is $0.00002 N/m^2$ or $20 \text{ µPa}$ (microPascals).
  • Threshold of Feeling: Sound becomes palpable (felt) at approximately $20 Pa$. This is roughly equivalent to a jet aircraft at maximum power.
  • Threshold of Pain: Slightly higher than $20 Pa$, sound causes physical pain and potential short-term hearing damage.
• The Decibel ($dB$) Scale:
  • The ratio between the softest ($20 \text{ µPa}$) and loudest ($20 Pa$) audible sounds is $1:1,000,000$.
  • To manage this massive range, a logarithmic scale is used: $L_p = 20 \times \text{Log}\frac{p}{P_{ref}}$.
• Environmental Factors: Sound pressure is deceptive because it varies based on reflections and absorption. Reported pressure must always include the measuring distance and acoustic environment description.

Sound Intensity and Power Calculations

• Intensity (I): Movement of energy through a unit area per time unit ($W/m^2$).
  • Relationship: Intensity is directly proportional to the square of the sound pressure ($I ∝ P_a^2$).
• Sound Power (Lw): A constant quantity regardless of measuring distance or environment.
  • Measurement: It cannot be measured directly; it must be calculated using sound pressure measurements.
  • Formula: $L_w = 10 \times \text{Log}\frac{N}{N_0}$.

Frequency Weighting Networks

• Function: Filters applied to measurements to account for the human ear's non-linear sensitivity (the ear is less sensitive at very low and very high frequencies).
• A-Weighting (dB(A)): The most common network. It simulates the ear's response to low sound levels and is used to estimate threats to human hearing.
• B-Weighting: Used for environmental noise; not used for hearing conservation purposes.
• C-Weighting: Used for very loud or very low-frequency sounds. Applied in noise control engineering and peak measurements for impulse noise (e.g., explosives).
• Linear (L) or Z-Weighting: Represents measurement without any weighting filters.

Noise Measurement Instrumentation

• Sound Level Meter (SLM): A handheld device with a microphone housed on an extended point to cut out body reflections.
  • Microphone: Functions as an acoustic-to-electric transducer/sensor, converting air sound into electrical signals.
• Types by Function:
  • Basic SLM: Provides maximum instantaneous sound pressure level ($SPL$).
  • Integrating SLM: Sums noise exposure over a period of time.
  • Data Logging SLM: Records noise levels at set intervals over long periods.
• Standards and Accuracy (IEC 61672 / ANSI S1.11):
  • Type 00: Highest standard; high-precision for scientific laboratories.
  • Type 1: High-precision work (slightly less than 00).
  • Type 2: General use; not suitable for legal matters, research, or calibration.
  • Type 3: Rough survey work or preliminary measurements; least accurate.

Classifications of Noise

• Continuous Noise: Produced by steady machinery (e.g., blowers, pumps).
• Intermittent Noise: Noise that increases and decreases rapidly or starts and stops in cycles (e.g., passing airplanes, cycling machinery).
• Impulsive Noise: Brief, abrupt impacts or explosions (e.g., gunshots, punch presses). Startling effect means they cause more annoyance than their SPL alone suggests.

Mathematics of Sound Levels: Adding and Subtracting

• Adding Sound Levels: Because the $dB$ scale is logarithmic, you cannot add values linearly (e.g., $95 dB + 100 dB \neq 195 dB$).
  • Procedure:
  1. Calculate the difference: $L_{p2} - L_{p1}$.
  2. Use a standardized curve to find the addition value ($L^+$) based on the difference.
  3. Add $L^+$ to the louder source ($L_{p2}$).
  • Multiple Sources: Repeat the process for each additional source using the result of the first two.
• Subtracting Background Noise: Necessary to determine the significance of background noise ($L_{p\text{background}}$) compared to total noise ($L_{p\text{tot}}$).
  • Thresholds:
  • If the difference is $< 3 dB$, the background noise is too high for accurate measurement.
  • If the difference is $> 10 dB$, background noise can be ignored.

Practical Examples of Noise Calculation

• Example 1: Khanyisa Mine (Adding/Subtracting)
  • Winch (Thuso): $81 dB(A)$
  • Blasting Machine (Lerato): $74 dB(A)$
  • Difference: $81 - 74 = 7$.
  • L+ Value (from chart): $1$.
  • Total (Lp2 + L+): $81 + 1 = 82 dB(A)$.
  • Background (Driller): $69 dB(A)$. Calculation: $82 - 69 = 13$. Since $13 > 10$, the background noise can be ignored.
• Example 2: Themba's Band
  • Electric Guitar: $90 dB(A)$
  • Base Drum: $87 dB(A)$
  • Difference: $90 - 87 = 3$.
  • L+ Value: $3$.
  • Band Total: $90 + 3 = 93 dB(A)$.
  • Background Speakers: $91 dB(A)$. Calculation: $93 - 91 = 2$. Since difference is $< 3$, accurate measurement is difficult.
• Example 3: Tau Caterers
  • Boiler: $89 dB(A)$
  • Steamer: $86 dB(A)$
  • Difference: $89 - 86 = 3$.
  • L+ Value: $3$.
  • Canteen Equipment Total: $89 + 3 = 92 dB(A)$.
  • Background Music: $86 dB(A)$. Calculation: $92 - 86 = 6$. Requires correction/adjustment.

Questions & Discussion

• Interaction: The session concluded with an open floor for questions.