Psychoacoustics Overview and Loudness Perception

Volunteering for psychoacoustic studies can be frustrating and challenging for participants.
Studies focus on understanding sound perception in terms of intensity, frequency, and loudness.

Absolute Threshold: Minimum detectable level of sound under ideal conditions. Hearing tests aim to measure this threshold, though success can vary.
Human Hearing Sensitivity: The human ear can detect sound that causes the eardrum to move as little as one-tenth of a reference level. Understanding this can help appreciate the sensitivity of human hearing.
Tympanic Membrane: The ear's membrane can withstand very high sound intensity levels without rupture, indicating a vast range of hearing capability.

Decibel (dB) Scale: In psychoacoustics, sound pressure levels are measured in decibels (dBSPL).
- 0 dBSPL corresponds to 20 microPascals, close to the softest sound detectable by humans, but no one can hear 0 dBSPL.
- Human sensitivity varies across different frequencies.
Threshold of Audibility Contour: The graphical representation showing the softest audible sound levels (in dB HL, not SPL) for various frequencies based on the average hearing of individuals with normal hearing.
- Important to note: every point along this contour represents sounds that are equally loud to an average listener.

Equal Loudness Contours (Phones): Represents levels of loudness perceived as equal by listeners at different frequencies.
- A contour at a specific phone level represents sound levels that are perceived as equally loud (e.g., 40 phon contour).
Comparison of different frequency sounds shows that lower frequencies require more dB SPL to reach equal loudness with higher frequencies.
- Doubling loudness typically requires a 10 dB increase at 1000 Hz, but less at lower frequencies (e.g., 20 Hz).

To determine equal loudness, a reference tone (1,000 Hz at a specified dB SPL) is used.
Listeners must adjust the level of other tones until they perceive them as equally loud as the reference tone.
Observations from multiple subjects create a dataset from which average equal loudness contours emerge.

Loudness growth is how loudness perception changes with increasing sound intensity.
- At mid to high intensities, a 10 dB increase typically doubles perceived loudness.
- At lower intensities, the growth function tends to be steeper, and loudness increases more rapidly till it becomes inaudible.

Reduced Dynamic Range: Individuals with hearing loss may experience rapid loudness growth (recruitment) leading to a reduced dynamic range. In these cases, sounds are perceived as loud too quickly compared to normal hearing individuals.
Conductive vs Sensorineural Hearing Loss:
- Conductive hearing loss can affect hearing ability without altering loudness growth characteristics.
- Sensorineural hearing loss typically leads to a reduced dynamic range, with rapid loudness growth, resulting in hypersensitivity to loud sounds.

Just Noticeable Difference (Delta I): Refers to the smallest change in intensity that listeners can detect, averaging around 1 dB across different intensities.
Weber’s Law: Examines the relationship between delta I and intensity, revealing that this relationship is constant across many conditions, although it may not be perfectly linear as suggested in some cases.

Recognizing how varying intensity levels affect perceived loudness is crucial in sound engineering, especially for recording and mixing music.
Adjustments to frequency balance during mixing is essential to ensure music sounds pleasant even when played at lower loudness levels.
Implementation of filters in audio recording systems often based on psychoacoustic principles to maintain sound quality across different loudness levels.