👂Acoustics Unit 11 – Human Hearing and Psychoacoustics

Basics of Sound and Hearing

• Sound is a mechanical wave that propagates through a medium (air, water, solids) by causing particles to oscillate and transfer energy
• Hearing is the perception of sound waves by the auditory system, which converts these mechanical vibrations into electrical signals processed by the brain
• The human auditory system is sensitive to sound frequencies between approximately 20 Hz and 20 kHz, with the most sensitive range being between 2-5 kHz
• Sound waves have three primary characteristics: frequency (pitch), amplitude (loudness), and phase
  • Frequency is the number of oscillations per second, measured in Hertz (Hz)
  • Amplitude is the maximum displacement of particles from their resting position, related to the energy of the sound wave
  • Phase refers to the position of a point on the wave cycle relative to the origin
• The speed of sound varies depending on the medium, with a typical value of 343 m/s in air at room temperature
• The human ear can detect sound pressure levels (SPL) ranging from 0 dB (threshold of hearing) to 120-140 dB (threshold of pain)
• The decibel (dB) scale is a logarithmic unit used to measure sound pressure levels, with a 10 dB increase corresponding to a tenfold increase in sound intensity

Anatomy of the Human Ear

• The human ear consists of three main parts: the outer ear, middle ear, and inner ear
• The outer ear includes the pinna (visible part of the ear) and the ear canal, which guide sound waves to the eardrum (tympanic membrane)
  • The pinna helps in localizing sound sources by filtering and modifying incoming sound waves based on their direction
• The middle ear contains three small bones called ossicles (malleus, incus, and stapes) that transmit vibrations from the eardrum to the inner ear
  • The ossicles act as an impedance matching system, efficiently transferring energy from the air-filled outer ear to the fluid-filled inner ear
• The inner ear consists of the cochlea, a snail-shaped structure filled with fluid and lined with hair cells that convert mechanical vibrations into electrical signals
  • The cochlea is tonotopically organized, with different regions responding to different frequencies (high frequencies at the base, low frequencies at the apex)
• The auditory nerve carries electrical signals from the hair cells to the brainstem and auditory cortex for further processing and perception
• The semicircular canals and vestibular system, also part of the inner ear, are responsible for maintaining balance and spatial orientation

How We Perceive Sound

• Sound perception involves the transduction of mechanical energy (sound waves) into electrical signals that the brain can interpret
• The outer ear funnels sound waves into the ear canal, causing the eardrum to vibrate
• Vibrations from the eardrum are transmitted through the middle ear ossicles, which amplify the signal and convert it into fluid motion in the cochlea
• In the cochlea, the basilar membrane vibrates in response to fluid motion, with different regions resonating at different frequencies
  • This frequency-to-place mapping is called tonotopic organization and is essential for pitch perception
• Hair cells along the basilar membrane bend in response to the vibrations, opening ion channels and generating electrical signals
• The auditory nerve carries these electrical signals to the brainstem, where initial processing occurs, and then to the auditory cortex for higher-level processing and perception
• The brain interprets the patterns of neural activity to extract information about the sound's pitch, loudness, timbre, and spatial location
• Auditory perception is influenced by factors such as attention, memory, and context, which can affect how we interpret and respond to sounds

Loudness and Pitch Perception

• Loudness is the subjective perception of sound intensity, related to the amplitude of the sound wave
• The human auditory system has a wide dynamic range, capable of detecting sounds from the threshold of hearing (0 dB SPL) to the threshold of pain (120-140 dB SPL)
• Loudness perception is not linearly related to sound pressure level; a 10 dB increase in SPL is perceived as approximately a doubling of loudness
• Equal-loudness contours (Fletcher-Munson curves) show the relationship between frequency and SPL required for sounds to be perceived as equally loud
  • The human ear is most sensitive to frequencies around 2-5 kHz, requiring less SPL to achieve the same perceived loudness compared to other frequencies
• Pitch is the subjective perception of sound frequency, related to the repetition rate of the waveform
• The human auditory system can perceive pitch for frequencies between about 20 Hz and 20 kHz, with the most sensitive range being between 2-5 kHz
• Pitch perception is influenced by the fundamental frequency (F0) and the harmonic structure of the sound
  • Complex tones with integer multiples of the F0 are perceived as having the same pitch as the F0, a phenomenon called the missing fundamental
• The just-noticeable difference (JND) for pitch is approximately 0.5% to 1% of the reference frequency, depending on the frequency range and sound duration
• Pitch perception can be affected by factors such as sound duration, intensity, and the presence of other sounds (masking)

Frequency Range and Sensitivity

• The human auditory system is sensitive to sound frequencies between approximately 20 Hz and 20 kHz, a range that varies among individuals and decreases with age
• The most sensitive frequency range for human hearing is between 2-5 kHz, which corresponds to the resonant frequency of the ear canal and the region of greatest sensitivity on the basilar membrane
• Frequency resolution, or the ability to distinguish between two different frequencies, is best at low frequencies and decreases at higher frequencies
  • The just-noticeable difference (JND) for frequency is approximately 0.5% to 1% of the reference frequency, depending on the frequency range and sound duration
• The absolute threshold of hearing (ATH) is the minimum sound pressure level (SPL) required for a sound to be detected in the absence of other sounds
  • The ATH varies with frequency, with the lowest thresholds occurring in the 2-5 kHz range
• The equal-loudness contours (Fletcher-Munson curves) demonstrate that the perceived loudness of a sound depends on both its frequency and SPL
  • Sounds at different frequencies require different SPLs to be perceived as equally loud
• Age-related hearing loss (presbycusis) typically affects high-frequency sensitivity first, gradually progressing to lower frequencies over time
• Exposure to loud sounds can cause temporary or permanent hearing threshold shifts, as well as other hearing disorders such as tinnitus (ringing in the ears)

Spatial Hearing and Localization

• Spatial hearing refers to the ability to determine the location and direction of sound sources in three-dimensional space
• Sound localization is achieved through the use of binaural and monaural cues, which the brain processes to estimate the position of the sound source
• Interaural time difference (ITD) is a binaural cue that results from the difference in arrival time of a sound at the two ears
  • ITDs are most effective for localizing low-frequency sounds (below ~1.5 kHz) and sounds located off the midline
• Interaural level difference (ILD) is another binaural cue that arises from the difference in sound pressure level between the two ears due to the acoustic shadow cast by the head
  • ILDs are most effective for localizing high-frequency sounds (above ~1.5 kHz) and sounds located off the midline
• Monaural cues, such as spectral shaping by the pinna, head, and torso, provide information about the elevation and front-back position of a sound source
  • The pinna's complex shape and folds create direction-dependent spectral filtering, which the brain learns to associate with specific sound source locations
• The cone of confusion is a region where multiple sound source locations produce the same ITDs and ILDs, making localization ambiguous
  • Monaural cues and head movements help resolve front-back and elevation ambiguities within the cone of confusion
• The precedence effect (or Haas effect) is a phenomenon where the first arriving sound dominates the perceived location, even when followed by similar sounds from different directions within a short time window (~5-40 ms)
  • This effect helps maintain localization accuracy in reverberant environments

Psychoacoustic Phenomena

• Psychoacoustics is the study of the psychological and physiological responses to sound, bridging the gap between the physical properties of sound and human perception
• Auditory masking occurs when the presence of one sound (the masker) makes another sound (the target) more difficult or impossible to perceive
  • Simultaneous masking happens when the masker and target are present at the same time, while temporal masking can occur when the masker precedes (forward masking) or follows (backward masking) the target
• Critical bands are frequency ranges within which the auditory system integrates sound energy, influencing phenomena such as masking and loudness perception
  • The bandwidth of critical bands varies with center frequency, being narrower at low frequencies and wider at high frequencies
• Auditory scene analysis is the process by which the brain organizes and segregates the complex mixture of sounds in the environment into distinct perceptual streams
  • The brain uses cues such as frequency, timing, and spatial location to group sounds into coherent sources and separate them from background noise
• Auditory illusions demonstrate the complex nature of sound perception and how the brain can be "tricked" into hearing sounds that are not physically present
  • Examples include the missing fundamental, the McGurk effect (visual influence on speech perception), and the Shepard tone (ambiguous pitch)
• Auditory adaptation refers to the decrease in responsiveness to a constant or repeated sound over time, allowing the auditory system to maintain sensitivity to new or changing sounds
• Loudness constancy is the ability to perceive the loudness of a sound as relatively stable, despite changes in distance or acoustic environment
  • This phenomenon helps maintain a consistent perception of sound sources as the listener or the source moves through space

Applications in Audio Engineering

• Psychoacoustic principles are applied in various aspects of audio engineering to optimize the recording, processing, and reproduction of sound
• MP3 and other perceptual audio coding formats use psychoacoustic models to compress audio data by removing or reducing perceptually irrelevant information
  • These codecs exploit phenomena such as auditory masking and frequency sensitivity to achieve high compression ratios while maintaining perceived audio quality
• Equalization (EQ) is used to adjust the balance of frequency components in an audio signal, taking into account the human auditory system's frequency sensitivity and the desired tonal characteristics
  • Parametric EQs allow precise control over the center frequency, gain, and bandwidth of each frequency band, while graphic EQs provide a fixed number of bands with adjustable gain
• Loudness normalization and metering standards, such as ITU-R BS.1770, consider the frequency-dependent nature of loudness perception to ensure consistent perceived loudness across different audio content and playback systems
• Room acoustics and speaker placement can be optimized using psychoacoustic principles to enhance the listening experience and minimize perceptual distortions
  • This includes considering factors such as the room's modal response, early reflections, and reverberation time, as well as the spatial arrangement of speakers for optimal stereo or surround sound imaging
• Binaural recording and spatial audio techniques aim to recreate the natural spatial cues of human hearing, providing an immersive and realistic listening experience over headphones or loudspeakers
  • These techniques often involve using dummy head microphones or virtual surround sound processing to capture or simulate the ITDs, ILDs, and spectral cues essential for accurate sound localization
• Hearing aids and assistive listening devices incorporate psychoacoustic principles to enhance speech intelligibility and improve the listening experience for individuals with hearing impairments
  • This may involve frequency-dependent amplification, noise reduction, and directional microphone technology to optimize the signal-to-noise ratio and target the listener's specific hearing needs