Hearing and Perceptual Aspects of Sound

Hearing enhances our lives not only by allowing us to perceive events beyond our visual range but also enriches our existence through music and aids communication through speech.

Key characteristics of sound perception include:
- Loudness: A perception reflecting differences in the magnitude of sounds (Are they audible? How loud are they?).
- Pitch: Relates to the quality of sounds, ranging from low to high (For instance, the notes from left to right on a piano keyboard).
- Timbre: This quality differentiates two tones having the same loudness, pitch, and duration, leading them to sound distinct.

Loudness is directly associated with the amplitude level of an auditory stimulus:
- Figure 11.7 depicts the loudness of a 1,000-Hz tone based on intensity, identified using magnitude estimation.
- Loudness values are relative; for instance, a loudness of 1 is assigned at 40 dB SPL.
- An increase of 10 dB (from 40 to 50 dB) typically results in an almost doubling of perceived sound loudness.
- It is important to note that both the threshold of hearing and perceived loudness can be influenced by frequency.

Audibility Curve: This represents the threshold for hearing in relation to frequency.
Auditory Response Area: Shown in Figure 11.8.
- Humans exhibit heightened sensitivity to frequencies between 2,000 to 4,000 Hz, correlating with the frequency range crucial for speech comprehension.

Determined using a standard 1,000 Hz tone:
- Subjects match the perceived loudness of other tones relative to the 1,000 Hz standard.
- Two critical dB levels are measured: 40 dB and 80 dB SPL.
- At 40 dB, high and low frequencies appear softer relative to mid-range frequencies, whereas at 80 dB, loudness tends to be more uniform.
- Recognizing that at the threshold, different frequencies may have significantly differing levels, above this threshold, frequencies can attain comparable loudness at identical decibel levels.

Pitch: The perceptual quality distinguishing high versus low sounds (e.g., bird chirping vs thunder).
- Defined as:
- “the property of auditory sensation in terms of which sounds may be ordered on a musical scale.” (p. 266)
- “… that aspect of auditory sensation whose variation is associated with musical melodies.” (p. 266)
- Pitch is inherently tied to the fundamental frequency of sound.
- Low frequency correlates with low pitch; high frequency correlates with high pitch.
- Examples:
- The violin, being the smallest string instrument, produces the highest pitches.
- The cello generates deeper, richer sounds.

The piano keyboard illustrates pitch:
- As one moves right on the keyboard, both frequency and tone height increase (measured in Hz).
- Notes that share the same letter (e.g., A1, A2, A7) have the same tone chroma.
- The frequency doubling observed for each octave results in a similar perceptual experience across different octaves.

Timbre: This quality differentiates tones that share the same loudness, pitch, and duration.
- Descriptions include terms like "clear" and "reedy."
Timbre also encompasses all other auditory aspects excluding loudness, pitch, and duration.
- It relates to harmonics, attack, and decay characteristics of sounds.
- Figure 11.10 shows frequency spectra for a guitar, a bassoon, and an alto saxophone, highlighting the differences in harmonic strength and number of harmonics.

Attack: Refers to the initial phase of a sound's life, indicating how quickly it reaches its peak loudness.
Decay: Follows the attack, representing the time taken for sound to fall from peak to sustain level.
These phases affect timbre through:
- Instrument Identification: Different instruments have unique attack and decay rates that facilitate identification by listeners.
- Example comparisons:
  - Plucked guitar string: rapid attack; long decay.
  - Struck cymbal: instant attack; extended decay.
  - Gong: slow attack; rapid decay after being struck.
Sustain & Release: Sustain denotes the maintained volume when a note is held, while release signifies the duration taken for a sound to fade completely.

Timbre incorporates all auditory aspects except loudness, pitch, and duration.
- Its characteristics depend on both harmonic structures and the time course of attacks and decays associated with those harmonics.