Sound Absorption Notes

Sound Absorption

  • Sound absorption coefficient (α\alpha) measures how well a material absorbs sound energy.
    • It's the ratio of sound energy absorbed to the sound energy that hits the material.
    • 0 = reflects all sound; 1 = absorbs all sound
    • Higher coefficient = more sound absorption
  • Measured in labs using experimental tests, like in a reverberation chamber or with a two-microphone impedance tube.
  • High sound absorption is important for quiet environments like offices, libraries, and concert halls.
  • Materials with high coefficients include foams, rockwool, glass wool, fibrous polyester felts, and wool & cotton, especially if less smooth, less dense, thicker, porous, or mounted over airspace.

Materials

  • Cellular (closed cell polyurethane foam, cellular rubber foam, cellular polyethylene)
  • Fibrous (parallel fiber bundles, rock wool insulation, fiber glass wool, acoustic felt)
  • Granular (stacked identical spheres)

Noise Reduction Coefficient (NRC)

  • NRC is a single value indicating a material's sound absorption capability.
    • Ranges from 0 (no absorption) to 1 (complete absorption).
    • Calculated by averaging sound absorption coefficients at 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz.
  • Used to compare sound absorption of building materials for acoustic design.

Sound Absorption & Treatment

  • Sound absorption reduces sound energy by trapping waves, converting energy to heat, and minimizing reflections.
  • Sound Treatment manages sound using absorption, diffusion, and bass trapping.
  • Porous materials trap sound waves in air pockets, dissipating energy as heat.
  • Benefits include reduced echoes, improved clarity, and balanced acoustics.

Common Materials for Sound Absorption

  • Acoustic panels (fiberglass, mineral wool, open-cell foam).
  • Curtains and drapes (thicker fabrics).
  • Carpet (effective for high frequencies).
  • Soft furniture.

Aspects of Sound Treatment

  • Absorption reduces reflections.
  • Diffusion distributes sound evenly.
  • Bass trapping absorbs low frequencies.

Applications

  • Recording studios, home theaters, conference rooms, classrooms.

Types of Sound Absorbing Materials

  • Porous materials (fiberboards, mineral wools).
  • Non-perforated/Membrane materials (wood, gypsum boards).
  • Cavity/Helmholtz Resonators (perforated panels).

Additional Materials

  • Hairfelt
    • ≈\approx 0.60 (25mm thick).
  • Acoustic Plaster
    • 0.15-0.30
  • Acoustical Tiles
  • Strawboard
    • ≈\approx 0.3 at 500 cps (13mm thick).
  • Pulpboards
    • ≈\approx 0.17 at 500 cps (13mm thick).
  • Compressed Fibreboard
    • ≈\approx 0.30 - 0.52
  • Compressed Wood Particle Board
    • ≈\approx 0.40 at 500 cps (13mm thick).
  • Perforated Plywood
    • Up to 0.95.
  • Wood Wool Board
    • ≈\approx 0.20 at 500 cps (25mm thick).
  • Quilts and Mats

Relative Efficiency of Sound Absorbers

  • Sound absorption coefficient is key (0 to 1).
  • Thick, porous foams are most efficient, especially at mid-range frequencies.
  • Hard, dense surfaces are poor absorbers.

Material Properties

  • Porous materials (fiberglass, mineral wool, open-cell foam) are better than dense materials.

Frequency Dependence

  • Materials absorb different frequencies differently.

Thickness

  • Increasing thickness improves low-frequency absorption.

Examples of Common Sound Absorbers and Their Relative Efficiency:

  • Highly efficient: Thick, open-cell polyurethane foam, Fiberglass insulation ,Acoustic panels made of mineral wool, Specialized bass traps designed for low frequencies.
  • Moderately efficient: Carpet, Fabric curtains, Loosely packed fabric or textile panels Egg crate foam.
  • Low efficiency: Bare concrete walls, Smooth, hard surfaces like glass or metal, Thin layers of soft materials like fabric without additional backing.