Acoustic

Created: 5/12/2020Last updated: 5/12/2020

Introduction

Acoustic engineering generally related to audio generation, transmission, reception and auditory effect in human hearing in the form of mechanical waves propagating in physical mediums such as air and liquid. The key thing to remember is that acoustic effect is a relative measurement, meaning that the perception of loudness is unique to each individuals.

Background

Decibels: it measures the relative difference between a signal and a reference. Due to large input dynamic range of human hearing, logarithmic scale [dB] is great to quantify large variations.

Sound Pressure level (SPL): it a measure of loudness of human hearing in terms of sound pressure amplitude in decibels.

    • Generally there are couple adjustment weighting curves with weight A and C being the most common.

Audio Range: 20 Hz to 20 KHz are the standard hearing frequency range for human.

Spectrogram: it's time varying graphic visualization of SPL level to frequency spectrum.

Detailed Analysis

  • Audio generation involves an electroacoutic transducer such as speaker driver
    • speaker driver can be full range, subwoofer, woofer, or tweeter covering different range of audio frequency band.
  • Audio transmission involves a wave propagation
    • Normally acoustic wave in far field region of the wave, the SPL level degrades 6 dB per doubling of the distance from the source in an anechoic chamber
    • In practice, the room is reflective and echoic, the roll of loudness level vs distance flattens out in few meters.
  • Audio reception in voltages an electroacoustic transducer such as a microphone.

Q&A

How do sound levels add?

  1. Anti-log back to linear scale
  2. Add or subtract
  3. Log the results.

e.g., two 60 dB noise signals adds up to 63 dB.


Summary & Conclusion

  • Acoustic is a study of pressure wave generation, propagation, reception and hearing effects.
  • Acoustic Transducer transforms pressure wave to electric voltage and vice versa.
  • Hearing range of human is generally 20 Hz to 20 kHz.

Further Reading and Practice:

"Acoustic", https://en.wikipedia.org/wiki/Acoustics#Wave_propagation:_pressure_levels

"Sound Intensities," http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/intens.html