Ch 11: Hearing Pt 1 Physical Aspects of Sound

Perceptual definition of sound

The experience we have when we hear

Physical definition

Pressure changes in the air or other medium

A sound stimulus occurs when

The movements or vibrations of an object cause pressure changes in air (or water or any other elastic medium that can transmit vibrations)

Process through which loudspeakers produce sound

Air molecules are pushed together as the diaphragm moves out, increase in pressure (condensation)

Diaphragm also moves in, pulling the air molecules apart, decrease in pressure (rarefaction)

The cycle of this process creates

Alternating high and low pressure regions that travel through the air

Process is repeated hundreds or thousands of times per second thus the speaker creates a pattern of alternating high and low pressure regions in the air (as neighbouring air molecules affect each other) called a

Sound wave

Note that while air pressure changes (sound wave move outward from the speaker) …

The air molecules at each location move back and forth but stay in about the same place

Pattern of increases and decreased in pressure is transmitted that eventually reaches ear

Pattern moving, air molecules staying in same location

Pure tone

A simple kind of sound wave which occurs when changes in air pressure occur in a pattern described by a mathematical function called a sine wave

Sine wave (sinusoid)

A smooth, repetitive oscillation that follows a mathematical function

Shape is a smooth wave- like curve that oscillates above and below zero in a perfectly regular pattern

Frequency

Number of cycles per second that the pressure changes repeat

Amplitude

Size of the pressure change

Sound frequency

Measured in Hertz (Hz)

1 Hz = 1 cycle per second

Higher frequency sounds are generally perceived as

Higher pitched sounds

Pitch vs frequency

Pitch → psychological perception

Frequency → a measured physical property of the sound wave

Human frequency range

20 - 20 000 Hz

Dog frequency range

40 - 60 000 Hz

Cat frequency range

45 - 64 000Hz

Bat frequency range

20 000 - 120 000 Hz

Dolphin frequency range

150 - 150 000 Hz

Elephant

1 - 20 000 Hz

Key points about frequency ranges

Humans typically hear 20 - 20000 Hz

Many animals can hear much higher or lower frequencies than humans

Allows them to detect sounds used for communication, navigating or hunting

Examples of how different animals use frequencies

Bats use ultrasonic sounds (20 - 129 KHz) for echolocation

Dolphins can hear up to 150 kHz and use high frequency clicks for navigating and communication

Elephants can detect very low frequencies (infrasound) which may help with long distance communication

What do we use to convert the large scale of sound pressures (amplitudes) that we experience in our environment into a more manageable scale

Decibel (dB), unit of sound

Decibels

Used to shrink large ranges of pressure by using logarithms

Logarithms

Convert numbers into exponents

The logarithm of a number is the exponent to which the base much be raised to produce that number (log 1000 =3)

Micropascal (uPa)

An extremely small unit of pressure used to measure tiny changes in air pressure produced by sound waves (one millionth)

Sound waves create tiny fluctuations in air pressure as they travel

What value is used as the reference pressure for calculating sound

20 uPa = 0 dB SPL

Normal hearing can detect a 1000 Hz tone with a sound pressure of approx 20 uPa

0 dB SPL does not mean there is no sound, it represents a very faint sound near the standard reference threshold of human hearing

Equation for transforming sound pressure level into decibels

dB = 20 x log (p/ po)

po = 20

What does using common logarithms allow us to do

We can convert the extremely large range of amplitudes we encounter in the environment to a range of 7 log units (0, 20, 40, 60, 100, 120, 140)

Pure tones are important because they are the

Fundamental building blocks of sound

However, they are rare in environment

Sounds in the environment have waveforms that are

Much more complex than the pure tone’s sign wave pattern of pressure changes

Periodic tone

Means the waveform of the complex tone repeats

The repetition rate is called the … of the tone

Fundamental frequency

Complex tones

Made up of several pure tone’s components added together

What is each component of a complex tone called

A harmonic of the tone

The repetition rate (fundamental frequency) of the tone is also referred to as the

First harmonic

First harmonic

A pure tone with a frequency equal to the fundamental frequency

Higher harmonics

Pure tones with frequencies that are whole number multiples (eg 2, 3, 4) of the fundamental frequency

Frequency spectra

Provide a way of indicating a complex tone’s fundamental frequency and harmonics that add up to the tone’s complex waveform

What would happen if a harmonic was removed

Changes the tone’s waveform but not the rate of repetition

Why does the fundamental frequency stay the same when a harmonic is removed

The spacing between harmonics equals the repetition rate so even when the fundamental is removed, the spacing remains (still information in the waveform indicating the frequency of the fundamental)