Introduction to Sound and Hearing

What You’ll Learn

Basics of sound: amplitude, frequency, phase, and harmonics
How the ear processes sound
Introduction to psychoacoustics: How we perceive sound
Practical implications for audio production

Sound areas

The basics of sound
The characteristics of the ear
How a sound stimulates the ear
The psychoacoustics of hearing

The Transducer

device -> energy conversion -> resulting energy

https://commons.wikimedia.org/wiki/File:Django_Reinhardt_(Gottlieb_07301).jpg

The basics of sound

Understanding Amplitude

Key Concepts

Amplitude measures the strength or intensity of a sound wave.
It directly relates to perceived loudness.

Amplitude

Amplitude in the Real World

Example: Think about the difference in loudness between a whisper and a shout.
Interactive Example amplitude
Tip: When recording, always monitor amplitude levels to avoid distortion.

Understanding Frequency

Key Concepts

Frequency refers to the number of cycles per second in a sound wave.
It determines the pitch of the sound.

Frequency in Action

Example: A bass note versus a high-pitched whistle.
Interactive Example frequency
Tip: Different frequencies interact in complex ways in a mix.

Phase Relationships

Key Concept: Phase refers to the timing relationship between two sound waves.
Impact: Out-of-phase waves can cancel each other out, leading to a reduction in sound.
Sound Demo: Phase

Random phases - square and sawtooth

Harmonic Content

Key Concept: Harmonics are additional frequencies in a sound that define its timbre.
Real-World Example: Why a guitar and piano sound different, even when playing the same note.
Sound Clips: Fourier Series 3D interactive demonstration

The Decibel

The Ear

Introduction to Psychoacoustics

What Is It? Psychoacoustics explores how our brain interprets sound.
Key Concepts: Pitch perception, loudness perception, and spatial hearing.

Psychoacoustic Masking

Key Concept: One sound can make another harder to hear, especially if they are close in frequency.
Example: Kick drum and bass guitar often mask each other.
Why It Matters: MP3 and AAC compression rely on masking to discard “inaudible” sounds.

Interactive Demo: Auditory Masking Experiment (Music Perception Lab)

Binaural Beats

Key Concept: Two slightly different frequencies, one to each ear, produce a perceived “beat” at the difference frequency.
Example: 440 Hz in the left ear and 446 Hz in the right ear produces a 6 Hz beat.
Why It Matters: Shows how the brain constructs sound beyond the ear.

Interactive Demo: Binaural Beats Generator

Auditory Perception

This is the equal-loudness contour graph (also called Fletcher–Munson curves).

What it shows:

X-axis (Hz): frequency, from bass (20 Hz) to treble (20,000 Hz).
Y-axis (dB SPL): physical loudness level.
Curves: each line shows tones judged as equally loud by listeners.
The numbers on the curves are in phons, matched to the loudness of a 1,000 Hz tone at that level.

Why multiple lines?

Each curve starts with a different reference loudness at 1,000 Hz.
Example: a 40-phon contour = 1,000 Hz tone at 40 dB SPL used as the reference.
An 80-phon contour = 1,000 Hz tone at 80 dB SPL.
Listeners adjusted other frequencies until they sounded equally loud, producing this family of curves.

Key takeaways for students:

At low levels (20–40 phons), bass and treble need much more level to be perceived as equally loud.
At higher levels (80–100 phons), the curves flatten — the ear is more balanced across the spectrum.
This explains why music sounds “thin” when quiet but “full” when louder.
It’s also why old stereos had a “loudness” button — to boost bass and treble at low volumes.

Equal-Loudness Contours in Action

Key Concept: Our ears don’t hear all frequencies equally well.
Example: Bass must be louder than midrange to be perceived equally loud.
Why It Matters: Mixing at low volume often makes bass and treble feel weak.

Interactive Demo: Equal Loudness Experiment