Physics of Sound 1 Flashcards
Sound Definition
Propagation of density changes through an elastic medium through compression (higher conc) & rarefaction (lower conc)
Classes of Sounds
- Pure Tones
- Complex Periodic Noise
- Continuous Noise
- Transient Noise
Ingredients for Sound
- Elastic Medium
- Vibrating Source
What kind of waves are sound waves?
Longitudinal Pressure Waves
Classes of Sound
- Pure Tones (Sinosidal waves)
- Complex Periodic Noise
- Continuous Noise
- Transient Noise
How does the air molecules in the sound medium move?
Air molecules oscillate about their equilibrium positions
Sound Features
Pitch: Frequency (Hz)
Loudness: Amplitude (Pressure Level- PA)
Period & Frequency Formulas
Freq= 1/period Period= 1/Freq
Pure Tone
- Simple, naturally occurring wave pattern
- Few natural sources
- Periodic/ repeats regularly
- Spectrum shows one frequency
Complex Waves
- Few pure tones combined together with different amplitudes, frequencies and phases
- Combination of pure tones may result in complex periodic noise or continuous noise
Complex Periodic Noise
- Periodic
- can be analyzed into component sine waves
- spectrum shows frequency of component waves
- all freqs are multiples of fundamental freq, f0, which is their HCF
- made up of Fourier Series (series of sinusoids)
- waveforms must satisfy a harmonic relationship, and belong to a harmonic series
Types of Complex Periodic Waves
- Square Waves
- Sawtooth Waves
Square Waves
Odd integer multiple of f0 and identical starting phases
Sawtooth Waves
Odd and even components with identical starting phases
Continuous Noise
- Not periodic
- Many component frequencies/ not based on common fundamental frequency
- Spectrum shows envelop rather than a line for all frequencies
Transient Noise
- Not periodic
- very short duration
- all frequencies present
- spectrum shown as an envelope
Sound Intensity Definition
- Sound energy passing through a unit area (etc. m^2) per unit time (etc. s)
- unit for measuring the magnitude of vibration in the air
Sound Intensity Unit
Watts per m^2 (W/m^2)
Sound Intensity measurement
Relative measurement, known as relative intensity is preferred
Sound Intensity Formula
N (dB IL) = 10log10 Ix/Ir, where Ir=10^-12W/m^2 Ix being relative, Ir being reference, IL being intensity level
Sound Intensity Chart
\+10dBIL= 10^x x 10 \+3dBIL = double of intensity (Ix) -3dBIL = half of intensity (Ix)
Sound Pressure
-force per unit area (N/m^2 or Pa)
Sound Pressure Formula
I = Prms^2/Po(S), where I=Intensity, Prms=rms pressure, Po=medium density, and S=speed of sound
Air Pressure Standard Condition
Air as the medium,
I = 10^-12W/m^2,
creates a pressure of 20μPa
Sound Pressure Level (SPL)
I = P^2/PoS,
dB SPL= 20log10(Px/Pr),
where Pr = 20μPa
Sound Intensity and Loudness
- Intensity =/ Loudness
- Same intensity may not mean the same loudness
- dependent on frequency
Other dB Scales
- Standard Contour Filter developed to cater to non-uniform hearing responses
- denoted dBA, dBB and dBC, dB HL
dBA Filter
- commonly used to measure musical sound loudness
- most accurate approximation for medium hearing loss
dBB & dBC filter
- appropriate for monitoring loud sounds
- hearing curve is flatter at very high sound intensity levels (from equal loudness curves)
- dBC sometimes used for traffic noise surveys
dB HL scale
- references softest sound the average hearing can hear at a particular frequency
- dB HL =/ 20μPa, and instead is frequency-dependent
- etc. 40 dB HL means the person hears a pure tone 40dB above average
