SC

Exam 2 Review

How Sound Travels

  • Every medium is made up of molecules (or particles)

  • False - molecules are always in motion, even at rest (they vibrate)

  • True - molecules of air are usually equally spaced when at rest

  • This is called elasticity or elastic restoring force

  • When a material is more elastic, it is more stiff

  • When a material is more elastic, it has less compliance

  • This is called inertia or mass

  • Sound is the propagation of a disturbance (or pressure wave) through a medium

  • This is called atmospheric pressure or ambient pressure

  • Negative pressure (or rarefaction)

  • Positive pressure (or compression)

  • When graphing movement of a particle in time, if the particle is closer to another particle, the pressure should be positive (or higher)

  • When graphing movement of a particle in time, if the particle is at rest position, the pressure should be zero (or atmospheric)

  • False - Sound is a longitudinal wave (particles move parallel to the direction of wave travel)

  • Frequency = 1/Period (f = 1/T)

  • Frequency = Speed/Wavelength (f = c/λ)

  • Frequency is the number of cycles per second

  • The unit of measure of period is seconds (or milliseconds)

  • Period and wavelength identification: One period is the time for one complete cycle; one wavelength is the distance of one complete cycle

  • Period (T) = 1/frequency, Wavelength (λ) = speed/frequency = 33,600/f cm

  • Frequency and period are inversely related

  • Frequency and wavelength are inversely related

  • High frequency waves have cycles closer together (shorter wavelength); low frequency waves have cycles farther apart (longer wavelength)

  • Diffraction is the bending of sound waves around obstacles. Wavelength is important because longer wavelengths (lower frequencies) diffract more easily around obstacles than shorter wavelengths

Simple Waves

  • Air particles move around their rest position due to the forces of elasticity and inertia

  • Particles go back to rest because of elasticity (restoring force)

  • Particles keep moving when they reach rest position because of inertia (momentum)

  • This motion is called simple harmonic motion

  • Simple harmonic motion looks like a sine wave

  • The five parts of one cycle: rest → maximum displacement in one direction → back to rest → maximum displacement in opposite direction → back to rest

  • The 3 measurements are: frequency, period, and amplitude

  • Period is measured in seconds, frequently reported in milliseconds (ms). Convert by: 1 second = 1000 milliseconds

  • The unit of measure for amplitude of speech is decibels (dB)

  • Sound Intensity Level (SIL):

    • Unit: decibels (dB)

    • Formula: 10 log(I/I₀)

    • Referent: 10⁻¹² watts/m² (threshold of hearing intensity)

  • Sound Pressure Level (SPL):

    • Unit: decibels (dB)

    • Formula: 20 log(P/P₀)

    • Referent: 20 μPa (threshold of hearing pressure)

  • True - The period of a sine wave is the same on each repetition

  • False - A sine wave has only ONE frequency component

  • A sound with only one frequency can be called a sine wave, a simple wave, or a pure tone

  • False - majority of sounds we hear are complex, not pure tones

  • False - periodic waves can be simple OR complex

Complex Waves

  • A complex wave has multiple frequency components

  • True - The fundamental frequency is the lowest component frequency

  • When looking at a spectrum: frequency is on the x-axis and amplitude is on the y-axis. This view shows one moment in time (no time dimension)

  • When looking at a waveform: time is on the x-axis and amplitude is on the y-axis

  • You can identify them by the axes: spectrum shows frequency vs amplitude; waveform shows time vs amplitude

  • Spectrum gives us information about frequency components

  • For two complex waves with different waveforms but similar sound:

    • Yes, they can both have the same fundamental frequency

    • Yes, if both are periodic with same fundamental, harmonics will be at the same frequencies (integer multiples of fundamental)

    • What's different is the relative amplitudes (or phases) of the harmonics

  • A simple wave spectrum has only one frequency component (one vertical line)

  • To determine period of each component: Period = 1/frequency for each frequency shown in the spectrum

  • To determine frequencies from period: Frequency = 1/period for each component

  • Match spectrum to simple waves by identifying the frequencies and amplitudes of each component

  • Harmonic frequencies are integer multiples of the fundamental frequency (F₀, 2F₀, 3F₀, etc.)

  • Formula: Fn = n × F₀ (where n is the harmonic number and F₀ is the fundamental)

    • To find a specific harmonic: multiply fundamental by harmonic number

    • To find fundamental from a harmonic: divide harmonic frequency by its number

  • Aperiodic continuous waves show irregular, non-repeating patterns in waveform view, and continuous energy across many frequencies in spectrum view (not discrete lines)

  • Two main types of aperiodic waves: transient (brief, like a click) and continuous (ongoing, like white noise)

Resonance and Filtering

  • Resonance is the tendency of a system to vibrate with greater amplitude at certain frequencies (its natural/resonant frequencies)

  • Helmholtz resonator:

    • Lower resonant frequency: increase volume (V) or increase neck length (L)

    • Higher resonant frequency: decrease volume (V) or decrease neck length (L) or increase neck area (A)

    • The formula shows frequency is proportional to √(A/VL)