Understanding Noise and Vibration: Key Concepts

Posted on Feb 13, 2025 in Physics

Noise

Noise: A wave that moves through air and water (partially elastic medium). We perceive it as sound pressure.

Sound Pressure and Levels

Sound Pressure Variations: Measured as Sound Pressure Level (SPL) in decibels (dB).

Pressure = Force / Area = Newton / meter2 = Pascal.

  • Audible Field: To produce an auditory sensation, the frequency must be between 20 and 20,000 Hz.
  • Pain Threshold: 120 dB, equivalent to 20 N/m2 = 20 Pa.
  • Perception Threshold: 2 x 10-5 Pa, or 0 dB.

Sound Wave Characteristics

Sound Velocity: Calculated by dividing the wavelength by the frequency.

Frequency: The number of cycles a wave completes in one second, measured in Hertz (Hz).

Wavelength: The distance a wave travels in one complete cycle. It depends on the propagation medium and frequency.

Amplitude (Volume): The maximum distance the wave reaches from its equilibrium position.

Sound Pressure Level (SPL): The variation in pressure.

Power: The acoustic energy emitted by a source per unit of time, measured in Watts (W).

Intensity: The acoustic energy emitted by a source per unit area, measured in Watts/m2.

Types of Noise

  • Stable Noise: SPL fluctuations of less than or equal to 5 dB.
  • Fluctuating Noise: SPL fluctuations greater than 5 dB.
  • Impulsive Noise: Acoustic energy pulses shorter than 1 second at intervals greater than 1 second (peak).

Dose: Relates exposure time to the allowed time. It is calculated when a worker has exposure periods greater than or equal to 80 dB.

Sound Propagation

  • Absorption: Energy remains in the material.
  • Reflection: Sound bounces off a surface.
  • Refraction: Sound transfers to another medium.

Sound Fields

Free Sound Field: Sound propagates without reflection.

Diffuse or Reverberant Field: Sound waves propagate in all directions, and the sound pressure is equal at all points. Reverberation occurs in a diffuse field due to reflective surfaces, and the noise persists after the source has stopped emitting.

Directivity and Absorption

Directivity Factor (Q): Measures the degree to which energy is concentrated in one direction in space.

  • Spherical Radiation (Q1): The source is suspended in space (energy radiates in all directions).
  • Hemispherical Radiation (Q2): The source is on the ground or in the middle of a wall (energy radiates in a hemisphere).
  • Quarter-Spherical Radiation (Q4): The source is located at the intersection of a wall and floor.
  • Eighth-Spherical Radiation (Q8): The source is located in a corner, touching the ceiling or floor.

Absorption Coefficient (α): When a sound wave hits a wall, part of it is reflected, and another part is transmitted through the wall. α = Ratio of absorbed energy to incident energy per unit area.

Room Constant (R): Measures a room’s capacity for sound absorption.

Vibration

The oscillating movement of particles in solid elements is transmitted as sound.

Vibration Measurement

Vibration Length: The vibrational acceleration received by an individual is measured along an orthogonal coordinate system, with the reference point (L) at:

  • Z-axis: Head to toe.
  • X-axis: Back to chest.
  • Y-axis: Right to left.

Vibration Measurement:

  • Triaxial transduction system.
  • Frequency range: 1 Hz to 80 Hz.
  • Maximum frequency for 8 hours of work.

Weighting factors:

  • z (or 63)
  • x (0.45)
  • y (0.45)

Formula: AeqTP = √((1.4 * Aeqx)2 + (1.4 * Aeqy)2 + (Aeqz)2)

Hand-Arm Vibration

Caused by the acceleration of a vibrating tool. It must be measured in three orthogonal directions at the point where the vibration enters the hand. Frequency range: 5 Hz to 1500 Hz. The three axes are measured: zh, xh, yh, in m/s2.