Physics of Sound and Hearing PDF Lecture

Physics of Sound and Hearing Parkkapus Shoowit, MD Department of Otolaryngology Head & Neck Surgery Overview: This lecture will cover the fundamental physics of sound, the physiological mechanisms of hearing, and their clinical relevance. The focus will be on understanding...

Physics of Sound and Hearing Parkkapus Shoowit, MD Department of Otolaryngology Head & Neck Surgery Overview: This lecture will cover the fundamental physics of sound, the physiological mechanisms of hearing, and their clinical relevance. The focus will be on understanding how sound waves are generated, transmitted, and perceived by the auditory system. Learning Objectives: By the end of the lecture, students should be able to Explain the basic properties of sound waves Describe the physiological structures involved in hearing Understand the transmission of sound from the environment to the brain Discuss the clinical relevance of sound physics in audiology and medicine. 1. Introduction to Sound Physics 1.1 Definition of Sound Sound is a mechanical wave that propagates through a medium (air, water, solids) Requires a source, a medium, and a receiver  1.2 Properties of Sound Waves Frequency (Hz): Determines pitch; human hearing range is 20 Hz - 20,000 Hz. (PTA 500 Hz – 4000 Hz) Amplitude (dB): Determines loudness; logarithmic scale used Wavelength (λ) and Speed (v): Related by equation v = fλ shows that the speed of a wave is directly proportional to both its frequency and its wavelength. This means that if the frequency of a wave increases, with the wave speed remaining constant, the wavelength must decrease, and vice versa. Timbre: Quality of sound that distinguishes different sources. 1.3 Types of Sound Waves Longitudinal vs. transverse waves. A longitudinal wave is a wave where particles move parallel to the direction of the wave propagation, while a transverse wave is a wave where particles move perpendicular to the direction of the wave propagation Sound waves are a classic example of a longitudinal wave, while light waves and ripples on water are examples of transverse waves. Pure tones vs. complex sounds. A pure tone is a sound consisting of only one frequency, creating a simple sine wave pattern, while a complex sound is made up of multiple frequencies combined, resulting in a more intricate waveform, often with a recognizable pattern that isn't a simple sine wave A tuning fork produces a pure tone, while a violin playing a note creates a complex sound. 2. The Auditory System: Structure and Function 2.1 Outer Ear Pinna: Gathers and directs sound waves. External Auditory Canal: Amplifies certain frequencies. 2.2 Middle Ear Tympanic Membrane: Converts sound waves into mechanical vibrations. Ossicles (Malleus, Incus, Stapes): Amplify and transmit vibrations. Eustachian Tube: Equalizes pressure. 2.3 Inner Ear Cochlea: Converts mechanical vibrations into neural signals. Organ of Corti: Hair cells detect frequency- specific vibrations. Inner ear cochlear hair cells are sensory cells in the inner ear that detect sound and convert it into electrical signals for the brain. They are the basis of hearing and are located in the organ of Corti. Basilar Membrane: Tonotopic organization (high frequencies at base, low at apex). How they work Sound waves cause vibrations in the inner ear fluids. These vibrations cause the basilar membrane to vibrate. The vibrations deflect the stereocilia of the inner and outer hair cells. The stereocilia respond to the fluid motion and send receptor potentials to the brain. 2.4 Auditory Pathway Cochlear nerve → Brainstem → Thalamus → Auditory Cortex (Temporal Lobe). 3. Sound Perception and Clinical Relevance 3.1 Perception of Pitch and Loudness Place theory of hearing. The "place theory of hearing" states that different frequencies of sound stimulate different locations along the basilar membrane within the cochlea, allowing the brain to perceive different pitches based on where the vibrations occur on this membrane; essentially, different "places" on the cochlea correspond to different sound frequencies, with high frequencies stimulating the base and low frequencies stimulating the apex. Temporal coding for low frequencies. Temporal coding for low frequencies" refers to the process where the auditory system encodes the pitch of low-frequency sounds by the timing of neural firing, essentially "locking" the neuron firing rate to the frequency of the sound wave, which is most effective for low frequencies due to the ability of neurons to phase lock to the sound cycles at slower rates; this is in contrast to high frequencies which are primarily encoded by the location of activation on the basilar membrane ("place coding") within the cochlea. 3.2 Binaural Hearing and Sound Localization Interaural time difference (ITD) and Interaural level difference (ILD). An "Interaural Time Difference (ITD)" refers to the difference in time it takes for a sound to reach each ear, while an "Interaural Level Difference (ILD)" is the difference in sound intensity between the two ears, both of which are crucial cues used by the brain to determine the direction of a sound source in space. 3.3 Hearing Loss and Disorders Conductive Hearing Loss: Outer/middle ear dysfunction. Sensorineural Hearing Loss: Inner ear or nerve damage. Presbycusis: Age-related high-frequency loss. Tinnitus: Perception of sound without external stimulus. 3.4 Medical Applications and Audiology Hearing aids and cochlear implants. Importance of early diagnosis and treatment. Clinical Application Case Study Chief Complaint: Difficulty hearing and persistent ringing in the ear History: 45-year –old Thai male, a construction worker for the past 20 years, presents to the clinic with complaints of difficulty hearing conversations, particularly in noisy environments, and persistent ringing (tinnitus) in both ears. He states that he has worked around heavy machinery, jackhammers, and drills without consistent use of hearing protection. His family has noticed that he often increases the volume of the television and frequently asks others to repeat themselves. Case Study Describe physiology of sound in this case What is the diagnosis? How does prolonged exposure to high-frequency sound waves impact cochlear hair cells? Why do low-frequency sounds travel further than high-frequency sounds in air and water? What preventive measures can be recommended for workers exposed to high-decibel environments? How do hearing aids and cochlear implants function in restoring hearing? Audiogram Questions ?