Chapter 11: Hearing

Main Questions Addressed in Chapter 11:
- How can we describe the pressure changes in the air that serve as the stimulus for hearing?
- How is the stimulus measured?
- What perceptions does it cause?
- What is the anatomy of the ear, and how do pressure changes make their way through the structures of the ear to stimulate the receptors for hearing?
- What is pitch?
- What is known about the physiological mechanism for our perception of pitch? What remains unknown?
- What is the pathway from the ear to the auditory cortex?

Concept of Sound:
- The word "sound" can be used as both a physical stimulus and as a perceptual response.
Definitions of Sound:
- Physical Definition: Pressure changes in the air or other medium (e.g., a falling tree causes pressure changes).
- Perceptual Definition: The experience we have when we hear; thus, if no one is present to hear it, there will be no experience.

Sound Stimulus:
- Occurs when the movements or vibrations of an object cause pressure changes in a medium, such as air or water.
Loudspeaker Operation:
- The diaphragm of the speaker moves outward, pushing air molecules together (condensation) and inward, pulling air molecules apart (rarefaction).
- This cycle creates alternating high- and low-pressure regions, known as sound waves.
Sound Wave Characteristics:
- Sound waves move outward as pressure changes occur in the surrounding medium.
- Air molecules move back and forth but do not travel with the wave; they remain in about the same place, transmitting pressure changes.

Definition of a Pure Tone:
- A simple kind of sound wave occurring in a pattern described by a mathematical function known as a sine wave.
- Components:
- Frequency: The number of cycles per second that pressure changes repeat.
  - Measured in Hertz (Hz); 1 Hz = 1 cycle per second.
- Amplitude: The size of the pressure change.
Example Frequencies:
- 500-Hz tone, 800-Hz tone, 200-Hz tone.
- Human hearing range: approximately 20 Hz to 20,000 Hz (20 kHz).

Amplitude and Its Effect:
- Larger amplitudes correspond to the perception of greater loudness.
Environmental Amplitude Range:
- The range of amplitudes varies greatly; for example, a whisper can be represented by a ½ inch sine wave, while a loud rock concert might require a representation several miles high.
Use of Decibels (dB):
- To manage the wide range of sounds, auditory researchers utilize decibels. This unit converts large ranges into a manageable scale using logarithms.
Logarithm Definition:
- A logarithm converts numbers into exponents (or powers).
- The logarithm of a number is the exponent to which the base must be raised to produce that number.

Equation for Decibels:
- dB = 20 imes ext{log}( rac{p}{p_0})
- Where:
  - p = pressure of the sound considered.
  - p_0 = reference pressure (usually set at 20 micrpascals).
- Example Calculation:
- For p = 2000 micropascals:
  - dB = 20 imes ext{log}( rac{2000}{20}) = 20 imes ext{log}(100)
  - Since ext{log}(100) = 2, then dB = 20 imes 2 = 40.
- For p = 20,000 micropascals:
  - dB = 20 imes ext{log}( rac{20000}{20}) = 20 imes ext{log}(1000)
  - Since ext{log}(1000) = 3, then dB = 20 imes 3 = 60.

Characteristics of Complex Tones:
- Complex tones consist of multiple pure tone components added together. Each component is termed a harmonic.
Fundamental Frequency:
- The repetition rate of a complex tone, which can be indicated through its fundamental frequency (e.g., a tone repeating 200 times per second is 200 Hz).
- Harmonics:
- First Harmonic: Equal to the fundamental frequency (e.g., 200 Hz).
- Higher Harmonics: Frequencies that are whole-number multiples of the fundamental frequency. Examples:
  - Second Harmonic: 200 imes 2 = 400 Hz.
  - Third Harmonic: 200 imes 3 = 600 Hz.
  - Fourth Harmonic: 200 imes 4 = 800 Hz.
Frequency Spectra:
- Provide a way to illustrate the fundamental frequency and harmonics that make up a complex tone's waveform.
Waveform Characteristics:
- Not all harmonics may be necessary for the repetition rate to remain unchanged; removing a harmonic may alter the waveform but not its frequency.