The physical location of a sound can be distinguished based on the activation of receptors near the base of the cochlea and near the apex. Treble sounds vibrate at the base of the cochlea while basss sounds at the apex. Coplex sounds activate multiple areas of the cochlea with simultanious peaks of vibration along the basilar membrane. 

Place Coding

Directly encoding the auditory stimuli at the rate of auditory neurons. If there is a sound at 500 Hz, there will be neurons firing 500 action potentials per second. The firing is phase-locked to the stimuli, and the representation is averaged across ceveral cells. High-frequency sounds closer to the frequency of the action potential instead of the encoded auditory frequency.

Encoding multiple frequency allows the system to have higher than the maximal neural firing rate. 

Temporal coding

Tonality refers to two types of sounds:

* *Tonal sounds*, which are more musical in nature, including melody, motif, and harmony
* *Atonal sounds* (also called non-harmonic sound), which resemble everyday sounds or noise – they don’t conform to traditional musical compositions

Sounds can be designed using tonal or atonal elements, and sometimes a sound will be comprised of both. Tonal sounds work best to communicate personality, emotion, and state changes, whereas atonal sounds better support motion transitions and express a sense of haptic feedback.

Tonality

Audible frequency range is the span of sound frequencies a species can hear, and this range differs across species. In the textbook examples, chickens hear roughly $$125$$ to $$2000$$ Hz, mice hear roughly $$1000$$ to $$91000$$ Hz, beluga whales hear roughly $$1000$$ to $$123000$$ Hz, dogs hear about $$70$$ to $$45000$$ Hz, and cats hear about $$45$$ to $$64000$$ Hz.

Species Differences in Audible Frequency Range

People can discriminate changes in frequency between two tones by about 2 Hz for sounds up to 2000 Hz. Pitch relies on the subjective expereince of sound, which means frequency of sound is not the only determinant of pitch. There are two complementary ways that pitch information is encoded.

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Auditory signals 

Audition, or hearing, is the process by which the auditory system converts pressure waves from the environment into meaningful sounds. This capability is fundamental for human experience, enabling communication through spoken language, the appreciation of music, and the perception of environmental sounds.

Audition (Hearing)

In human audition, a sound wave's physical characteristics directly correspond to specific psychological perceptions. The frequency of a sound wave is experienced as its pitch, whereas the amplitude of the wave is experienced as its loudness.

Relationship Between Physical Properties of Sound Waves and Auditory Perception

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

Cranial Nerve VIII, the vestibulocochlear nerve.

Nerves making up the auditory system

The auditory pathway in the human brain represents the sequence of structures and processes that transmit auditory information. It begins with the mechanical activation of hair cells, which generates neural impulses. These impulses travel along the auditory nerve to the brain. The auditory information is then shuttled sequentially to the inferior colliculus, the medial geniculate nucleus of the thalamus, and finally to the auditory cortex in the temporal lobe for higher-level processing.

Auditory Pathways in the human brain

Encoding pitch information

Auditory signals are able to integrate information from both ears in order to identify the sourse of a sound. This helps with allerting danger, potential mates, or other situational information that is necessary for survival and normal activities throughout the day. Humans have an accuracy of about 1 degree horizontally around their head, and may animals demonstrate even higher accuracy. There are two types of binaural (two-ear)cues that help with localization on the horizontal plane; intensity and latency.

Auditory systems for localizing sound

Refers to the purity of a sound and is influenced by the timing, amplitude, and frequency of the sound waves combined. For example, the timbre of a sound is what makes different musical instruments playing the same note and loudness sound distinct.

Timbre

The anatomy of the human auditory system comprises several anatomical structures that capture sound waves and transmit them inward. Sound waves initially enter the auditory canal. The visible external ear includes the pinna, which leads to the tympanic membrane (eardrum). Beyond this are the ossicles, which consist of the malleus, incus, and stapes. Further inward, a close-up of the inner ear reveals additional structures, including the semicircular canals, utricle, saccule, oval window, cochlea, basilar membrane, and hair cells.

Anatomy of the Auditory System

Sound transduction is the continuous process where sound waves funneled into the auditory canal cause eardrum vibrations that move the ossicles. The stapes then presses the cochlea's oval window, moving fluid that enlarges hair cells on the basilar membrane, which ultimately send neural impulses to the brain via the auditory nerve.

Sound Transduction in the Ear

Based on the fundamental principle that hearing involves converting pressure waves from the environment into sound, explain why the person's hearing was temporarily affected and what happened when they yawned.

Explaining an Everyday Auditory Phenomenon

A musician plays a note on a guitar. They then tighten a string and pluck it more gently, producing a second note that is perceived as higher in pitch and quieter. What physical changes in the sound waves correspond to this perceptual change?

Sound localization is the auditory ability to identify the origin of a sound within an environment, which is a crucial aspect of hearing. This ability is analogous to depth perception in the visual system. Just as depth perception relies on monocular and binocular visual cues, the auditory system utilizes both monaural (one-eared) and binaural (two-eared) cues to pinpoint where a sound is coming from.

Sound Localization

The perceived loudness of a sound is directly related to the amplitude of its sound wave. Waves with higher amplitudes are interpreted as louder sounds, while those with lower amplitudes are perceived as quieter. This perceptual quality is measured using a logarithmic unit of sound intensity called decibels (dB).

Loudness as a Perception of Sound Wave Amplitude

Decibels (dB) are a logarithmic unit used to measure sound intensity, which corresponds to perceived loudness. The scale is used to represent a wide range of sounds, from the low end of human hearing, such as a whisper or rustling leaves, to extremely loud noises like a jet plane taking off, which is near the threshold for pain at approximately 130 dB.

Decibels (dB) as a Measure of Sound Intensity

Timbre is the perceptual quality that allows listeners to distinguish between different sounds, even when those sounds have the same pitch (frequency) and loudness (amplitude). Described as a sound's purity or character, timbre is determined by the complex interplay of a sound wave's frequency, amplitude, and timing. This quality explains why different musical instruments playing an identical note at the same volume still sound distinct from one another.

Timbre as a Perceptual Quality of Sound

An audio engineer is editing a recording of a flute. They determine that the sound needs to be perceived as both higher in pitch and louder. Which of the following adjustments to the sound wave's physical properties would accomplish this?

An audio engineer is comparing two distinct sound waves. Sound Wave 1 has a high frequency and a low amplitude. Sound Wave 2 has a low frequency and a high amplitude. How would a listener most likely perceive the difference between the sound produced by Wave 1 and the sound produced by Wave 2?

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