SLHS 305 Exam 2

Acoustics - -Properties of sounds as they travel in air and how they are affected by objects in their environment Waveform (time domain) - -When the pattern of movement is displayed with amplitude as a function of time Condensation - -Increased density of air molecules, corresponds with an increase in sound pressure Rarefaction - -Decreased density of air molecules, corresponds with a decrease in sound pressure Simple vibrations - -Also called "pure tones" Characterized by their physical (acoustic) dimensions of frequency, amplitude, and starting phase. Pure-tone (sine wave) - -"sinusoids" "Regular repetitive movements" Relative to phase angle (sin θ) Sin θ = x/r Cycle - -The pattern of movement as the object goes through its full range of motion one time One cycle represents the movement of an object from its starting point, to maximum peak, to negative peak, back to its starting point Frequency (and units) - -How many cycles occur per second Units: Hertz (Hz)/ kilohertz (kHz) 1kHz = 1000 Hz Ex: vibration that repeats itself 100 cycles per second is called 100 Hz pure tone. Frequency range of audibility for humans is 20-20,000 Hz Period (and units; also converting s and ms) - -Time that it takes to complete one cycle Units: Seconds (s), Milliseconds (ms) 1 ms = .001s T = 1/f or f = 1/T Period may be converted from seconds (s) to milliseconds (ms): s * 1000 = ms SLHS340 SLHS340 Starting phase (and position re cycle/circle) - -Where a vibration starts in it's cycle is called starting phase, e.g., 90, 180, 270 degree starting phases One complete cycle can be represented as angles round a circle Sound can be cancelled out if the condensation points were met by an equal rarefaction point (displacement would be 0) Amplitude (peak, peak to peak, rms) - -Amount of displacement How far an object moves back and forth and/or the amount of maximum and minimum air pressure created. magnitude of a sound (larger the magnitude, higher the amplitude) Amount of displacement (expressed in units of displacement, intensity, or pressure) rms= .707 x Peak RMS (root-mean-squared) -Square each of the instantaneous amplitudes to eliminate any negative values -Average the squared values -Take the square root of the average Peak-to-Peak Amplitude (Ap-p) -Take the amplitude change that occurs between the positive peak and the negative peak Peak Amplitude (Ap) -Measure the amplitude from baseline (zero) to one of the peaks Periodic (regular) vibrations - -the vibratory pattern repeats at regular intervals. Complex vibrations (periodic, nonperiodic) - -two or more pure tones are combined into a non sinusoidal pattern "Tonal" or "buzzing" quality Aperiodic (random, irregular) vibrations - -the pattern of vibration does not regularly repeat itself over time, there is no periodicity in the pattern Typically produces noise Noise - -Produced by combination of many pure tones with random starting phases SLHS340 SLHS340 Generally produced by aperiodic vibration FFT - -Fast Fourier Transform Ex: electronic instruments Propagate - -(travel), the medium that it is traveling through must also be capable of vibrating (can be air, fluids, or solids). Threshold of audibility curve - -Threshold sensitivity varies across frequencies (most sensitive range Hz) Wavelength (and units) - -Distance that a pure-tone travels in one cycle (distance between 2 points of condensation or 2 points of rarefaction). Units are in meters, centimeters, or feet. Greek symbol lambda (λ) Scientific notation - - Logarithms (base 10) - -Log10 n = to what power must 10 be raised to equal n In scientific notation, the log is the exponent e.g., log 100 = 2 log (1 x 102) = 2 Rules of Logs 1. log ab = log a + log b 2. log a/b = log a - log b 3. log an = n log a 4. log 1/n = -log a Pressure (and units) - -Measure of force distributed over an area Units: (µPa), (dynes/cm2), (newton/m2) Pressure (p) = force/area (p = F/A) Pressure, therefore, has units of N/m2 = Pascals (or µPa) Intensity (and units) - -A measure of power that is distributed over an area Units: (watts/cm2) or (watts/m2) Intensity (I) = power/area (I = Pwr/A) Reference level for intensity (and units) - -Lowest audible (@ 1000 Hz): SLHS340 SLHS340 Intensity = . w/m2 = 1.0 x 10-12w/m2 Reference level for pressure (and units) - -Lowest audible (@ 1000 Hz): Pressure = 20 µPa = 2.0 x 10^1 µPa =.000020 N/m2 = 2.0 x 10-5 N/m Pain (upper) level for intensity (and units) - -Upper limit (pain): Intensity = 100 w/m2 = 1.0 x 102 w/m2 Pain (upper) level for pressure (and units) - -Upper limit (pain): Pressure = 200,000,000 µPa = 2.0 x 108 µPa = 20 x 107 µPa Decibel - -Ratio scale in which a measured value is related to a specified reference value dB dB IL (and range) - -dB Intensity Level (dB IL) If asked to calculate dB IL, the reference level is assumed to be 10^-12 w/m2 "Decibel of intensity level" (dB IL) How much more intense is this higher intensity level of sound above the lowest audible intensity? dB SPL (and range) - -dB Sound Pressure Level Level (dB SPL): (20 µPa) dB pressure = 10 log (p2meas/p2ref) dB pressure = 10 log (pmeas/pref)2 dB pressure = 20 log (pmeas/pref) Inverse square law - -Sound waves travel outward in a spherical distribution The farther away from the sound source = less intensity and less pressure (softer level of sound) The inverse square law describes the relation between the distance sound travels and the expected decrease in the level of the sound. I = 1/d2 (d = change in distance) p2 = 1/d2; p = 1/d Reflection - -If flat surface with high density (greater impedance), the sound wave is directly reflected (bounced-back) toward the source with no change in speed of propagation. Angle of incident and reflected waves are the same