Understanding Sound and Noise: Key Concepts
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1. Definitions
Sound: A wavelength above the atmospheric pressure, with a frequency range between 20 Hz and 20 kHz. The pressure range is between 2.10-5 and 200 Pa for 1 kHz (for the rest of the region bounded by the isophone hearing threshold and the threshold of pain).
Wave: A disturbance that propagates, transporting energy but not matter.
Pressure: Force / Surface
Fletcher and Munson Curves (Isophone Curves): These curves represent the sensitivity of the ear to different frequencies, in addition to indicating the minimum pressure in dB required to start hearing.
Audible Range: The area bounded by the isophone hearing threshold and pain threshold curves, and the frequencies 20 Hz and 20 kHz.
2. Representation: Time and Frequency Domain
Pure Sound: A sinusoidal sound.
Complex Sound: Sound in which the pressure is a function of time. By Fourier's theorem, any complex sound can be expressed as the sum of pure sounds.
Time Domain: Representation of sound over time.
Frequency Domain: Representation of sound frequency (spectrum).
Analysis: Analyzing a sound to identify its frequencies.
Synthesis: Building waveforms from pure tones.
Sound is divided into:
- Low Tones: 20 Hz - 500 Hz
- Mid Tones: 500 Hz - 2 kHz
- High Tones: 2 kHz - 20 kHz
3. Sound Pressure Level: The Decibel
SPL = Sound Pressure Level
Pref = 20.10-5 Pa at 1 kHz.
4. Sound Intensity: NIS
Sound Intensity: The energy passing through a unit area placed at that point, perpendicular to the direction studied, in a unit of time.
NIS = Sound Intensity Level
Iref = 10-12 W/m2
5. NPS Variation with Distance
If you have a point source (spherical waves) and two points at distances r1 and r2:
If we double the distance, the sound pressure level drops by 6 dB.
6. Sum of Decibels
Decibels do not add up linearly. Intensities are added.
If we double the intensity of the source, the sound intensity level increases by 3 dB.
7. Decibel (dBA)
dBA: dB taking into account the differential sensitivity of the human ear. It is a dB weighted with curve A.
Isophones are constructed by joining the points having the same sound sensation (volume).
8. Frequency Bands
Frequency Bands: These are often groups between two ends of double frequency.
Octave Bands: Frequency bands whose ends meet the following relationship (f - 2f).
Third Octave Bands: Frequency bands whose ends satisfy a specific relationship.
Each octave band is divided into three bands (for finer analysis).
9. Types of Noise
White Noise: Noise with the same energy at all frequencies.