Understanding Sound and Hearing: Amplitude, Frequency, and Decibels, Slides of Advanced Physics

Download Understanding Sound and Hearing: Amplitude, Frequency, and Decibels and more Slides Advanced Physics in PDF only on Docsity! Sound and Hearing Nature of the Sound Stimulus “Sound” is the rhythmic compression and decompression of the air around us caused by a vibrating object. dBSPL The transducers (microphones) on sound level meters measure sound pressure (i.e., N/m2 or Pascals). Pressure needs to be converted to power prior to calculation of the decibel equivalent….i.e., acoustic power = pressure2 Finally, we need to agree upon a Reference value. By convention, we use 20 microPa (i.e., the hearing threshold) Thus: dB = 10 log (Observed Pressure2 / 20 microPa2) However…….. dBSPL (continued) Prior to the advent of hand-held calculators and computers (circa 1970), performing a squaring operation was computationally expensive and prone to error. To reduce computational demands, hearing science adopted a somewhat confusing convention in the specification of the dBSPL unit: dBSPL = 20 log (Observed Sound Pressure / 20 microPa) +6 dBSPL = doubling sound pressure +20 dBSPL = 10x pressure +3 dBSIL = doubling acoustic power +10 dBSIL = 10x acoustic power Some Typical Sound Amplitude Values Pressure Level Pressure Ratio Intensity Ratio TTC (pascals) (ida) (PAP) Ola Minimal audible sound 0.00002 (P,) 1 1 0 Soft whisper 0.0002 10 100 20 Quiet office 0.002 10° 10* 40 Average conversation 0.02 10° 10° 60 Vacuum cleaner 0.2 104 10° 80 Subway train 2 10° 10!° 100 Loud thunder 20 10° 101? 120 Jet engine at takeoff 200 10’ 10'4 140 (pain threshold) Wind tunnel 2,000 108 1016 160 Space shuttle 20 10° 1018 180 A “Better” Sound Amplitude Table? 130 Loud hand clapping at 1 m distance 110 Siren at 10 m distance 95 Hand (circular) power saw at 1 m 80 Very loud expressway traffic at 25 m 60 Lawn mower at 10 m 50 Refrigerator at 1 m 40 Talking; Talk radio level at 2 m 35 Very quiet room fan at low speed at 1 m 25 Normal breathing at 1 m 0 Absolute threshold dBSPL Most Sound Stimuli are Complex Complex Sound = Sum of Sines (Fourier Theorem Revisited) J.B.J. Fourier (1768-1830) Fourier Sound Applet Flow of Acoustic Energy (The “Impedance Problem”) Scala tympani Incus Scala vestibuli Oval Malleus Tympanic membrane Stapes Sound waves : Round window {) Outer ear [Mj Middle ear | Inner ear Cochlea Basilar membrane The “Impedance Problem” 99.9% of sound energy in the air is reflected at the air:water boundary (10 log(0.1/100)) = -30 dB loss) (1/1000x) How does the ear compensate for this loss as sound energy is transmitted from the air to the fluid that filled the cochlea? 2 dB gain via ossicular leverage (1.6x) 25 dB gain via surface area condensation (eardrum  stapes) (316x) ~5 dB gain at mid-frequencies (3x) due to pinna and auditory canal resonance The Cochlea Vestibular organ S Vestibulocochlear Nerve Stapes Cochlea Helicotrema Reissner’s membrane Stapes Oval window Round —— window Auditory Transduction Basilar Membrane Modulation Effects upon Sensory Hair Cells Note: K+ ion concentration gradient across sensory hair cells (see pink cavities) IHC Stereocilia “Tip Links” “tip link” connects gate to adjacent cilia. Shearing motion forces gate to open. Mechanical open-and-close of gate modulates influx of potassium ions (much FASTER than slow chemical cascade in visual transduction). K+ depolarization of IHC triggers release of glutamate at cochlear nerve fiber synapse. Sound Amplitude Coding (“Divide and Conquer”) Multiple nerve fibers for each IHC. Each nerve fiber tuned to a different 40 dB “range” of stimulus intensity. Intensity-level multiplexing Tuning Specificity of Cochlear Nerve Fibers “Broadens” with Increased Intensity Q: Why the broadening and asymmetry? A: Look to the Basilar membrane’s response Temporal $ereten T(t) Thalamus lobe Midbrain Medulla Ascending Pathways Left side Right side tT ree CeNc nucleus rE geniculate nucleus Te Tig colliculus colliculus Seale Superior olive olive eee ners Cochlear nucleus nucleus: Cochlea Auditory cortex Medial geniculate nucleus Inferior colliculus ‘SENSATION & PERCEPTION 4e, Figure 9.20 (© 2015 Sinauer Associates Inc. Brainstem. Superior olive Cochlea Frequency Mechanism versus Place Mechanism Georg von Békésy (1899-1972) Ernest Rutherford (1871-1937) Frequency Theory Place Theory Frequency Theory (Rutherford) • Basilar membrane analogy to microphone diaphragm • Each oscillation yields nerve pulse • Problem: Max. neural response approx. 500 Hz • Solution: Time division multiplexing (aka “Volley Principle” ) Supported by “cochlear microphonic” (Wever & Bray; but consider Botox results) von Békésy Place Theory: Focus on Basilar Membrane Dynamics High-Frequency Tone Low-Frequency Tone Von Békésy’s “Place Mechanism” as Biological Fourier Analyzer Basilar Membrane Dynamic Simulation (animation) Functional Aspects of Hearing Species-Specific Frequency Range 100000 E- Whale Mouse bal Cat Locust Dog Sea lion r Horse Cow i Human Elephant 10000 - 1000 = 100 Frequency (Hz) Clinical Audiogram (dBHL) dB-HL (Hearing Level) uses a different reference level for each test frequency. That reference level represents the average threshold (18 yr-olds) demonstrated at that frequency. Hence, a value of 0 dB-HL means “average” hearing level at the frequency under test. Normal vs. Noise-Induced Hearing Loss Source: http://mustelid.physiol.ox.ac.uk/drupal/?q=acoustics/clinical_audiograms Note “notch” At 4 KHz. Age-related Hearing Loss (Presbycusis) AUDIOGRAM LeftEar x Right Ear O Inevitable or preventable? a z= 2 3 g & = = i) . c s 3 = 500 1000 2000 4000 8000 Frequency in Hertz (Hz) Loudness Using magnitude estimation techniques, S.S. Stevens has quantified this nonlinear relationship as: L = k * P0.6 = k * I0.3 L=loudness; P=sound pressure (µPa) I=sound intensity (pW/m2) Stevens’ Power Law; Linear in log-log plot; slope ≈ exponent log(L)=log(k)+0.3 log(I) straight line log(L)≈0.3 log(I) Hence, a log unit increase (10dB) of intensity yields 0.3 log (100.3 or 2-fold) increase in loudness. Note: Binaural presentation perceived as approx. 2x more loud than monaural equivalent. Sone Scale Landmarks Normal conversation 1-4 Automobile @ 10m 4-16 Vacuum cleaner 16 Major roadway @ 10 m 16-32 Long-term hearing damage dosage 32+ Jackhammer @ 1m 64 Brief-exposure hearing damage 256 Pain threshold 676 Pitch = f(Frequency) MEL Scale Reference unit of perceived PITCH: 1000 Hz = 1000 Mels Perceived pitch increases “linearly” with stimulus frequency below 4KHz; but grows at a much slower rate at 4KHz and above. Semi-Log Plot Linear Plot Sound Localization Localization Accuracy vs. Frequency NO & T x = oo = nN Error of localization (degrees) o>) T ol ! poi 4 1 a ! ~ 50 100 200300 500 1000 2000 3000 5000 10,000 Frequency (Hz) Signature of a dual-mechanism process? Localization Accuracy vs. Frequency: Low Freq – Interaural Time Difference High Freq – Interaural Intensity Difference ΔIΔT ITD versus Location Straight Ahead Right Ear (Perpendicular) Straight Behind ΔT Delay Line Theory (How to Build a Cell tuned to delta-T Signals) Delta-T = 200 microsec “Active” Localization (Continuous Sound Sources) Cross-Section of a Head-Related Transfer Function (Spectral Coloration by Head, Torso & Pinnae) Auditory/Visual Integration What you hear is what you see Ventriloquism Effect Visual capture of sound localization McGurk Effect “Compromise” between conflicting sound and visual cues in speech understanding