Chapter 6(Sections 6.4-6.6)

Sound

Sound

The speed of sound in a substance depends on: the mass of its constituent atoms, and the strength of the forces between the atoms.

The speed of sound is large when: the atoms have small mass — they’re easier

to move, and/or the forces between the atoms are larger — an

atom pushes harder on its neighbor.

Sound, cont’d

Typically we represent a sound wave as a transverse wave (even though it is not). A region of compression is drawn as a crest. A region of expansion is drawn as a trough.

Sound, cont’d

A waveform of a sound wave is a graph of the air-pressure fluctuations caused by the sound wave versus time. A pure tone is a

sound with a sinusoidal waveform.

A complex wave is a sound that is not pure.

Sound, cont’d

Pure tone Sinusoidal wave form

Sound, cont’d

Complex tone Spoken “ooo” sound

Sound, cont’d

Noise Air rushing over a microphone

Sound, cont’d

Noise is sound that has a random waveform. It does not have a definite wavelength or

period. Sound with frequencies below our audible

range is called infrasound. Below about 20 Hz.

Sound with frequencies above our audible range is called ultrasound. Above about 20,000 Hz.

Production of sound

Sound can be produced by: Causing a body to vibrate:

e.g., plucking a string. Varying an air flow:

e.g., buzzing your lips. Abrupt changes in an object’s temperature:

e.g., a lightning flash creates thunder. By creating a shock wave:

e.g., flying faster than the speed of sound.

Production of sound

A piano produces sound by: The player presses a key so that the hammer strikes

the wire. The wire vibrates

and transmits this vibration to the soundboard.

The soundboard then radiates the sound to the room.

Production of sound

A flute produces sound by: The player blows across the opening to create a

varying airflow. The airflow reaches

the end of the flute and radiates to the room.

Or the player opens a note-hole to release part of the wave.

The tube is then effectively shortened.

Propagation of sound

Imagine a person talking in the middle of an empty arena.

The amplitude of the sound wave decreases as one moves farther away from the person.

Propagation of sound, cont’d

In a room, we have to deal with the multiple reflections off the walls and other objects. It gets a lot more complicated to determine the

amplitude at an arbitrary place in the room. The process of repeated reflections of sound

in an enclosure is called reverberation.

Propagation of sound, cont’d

A hand clap in an open field is a simple pulse since there is no echo.

But in a gym, there are multiple echoes which tends to make the sound fade away gradually.

Propagation of sound, cont’d

This effect can be useful or detrimental. Speech might be easy to understand in a field

but can become muddled with too much reverberation.

Reverberation is useful in music to help blend sounds.

Perception of sound

We have to be careful when we discuss sound.

There are physical properties we can measure.

But our ears do not just measure these physical properties.

We have to deal with the perception of the sound.

Perception of sound, cont’d

Pitch is the perception of highness or lowness of a sound. The pitch depends primarily on the frequency

of the sound. It also depends on the duration.

A very short sound might sound like a click even if it has a definite frequency.

Perception of sound, cont’d

Loudness is the perception of whether a sound is easy to hear or painful to hear. It depends primarily on the amplitude of the

sound. It also depends on whether the sound is

played with other sounds (before, after, concurrently, etc).

It even depends on the frequency. Our ears are more sensitive to higher frequencies

and less sensitive to lower frequencies.

Perception of sound, cont’d

Our eardrums respond to sound pressure level. A louder sound creates a larger compression,

i.e., higher pressure, than a quiet sound. We typically call the sound pressure level just

the sound level. It is measured in decibels (dB).

0 dB corresponds to inaudible. Normal conversation is about 50 dB. ~120 dB starts causing pain.

Perception of sound, cont’d

The sound level of the quietest sound is called the threshold of hearing.

The sound level at which we start experiencing pain is called the threshold of pain.

The minimum increase in sound level that is noticeable is about 1 dB.

Perception of sound, cont’d

For a sound to be judged as “twice as loud,” the original sound must be increased by 10 dB. To make a sound “twice as loud,” you would

need ten equal sources. Two sounds with equal sound levels cause an

increase of 3 dB over a single such sound. A sound with 10 times the amplitude of

another is 20 dB higher

Perception of sound, cont’d

Tone quality is a measure by which two sounds of the same frequency and amplitude sound different. A sax sounds different from a trumpet playing

the same note because the two instruments have different tone qualities.

We typically refer to tone quality as timbre or tone color.

Perception of sound, cont’d

Any complex waveform is equivalent to a combination of two or more sinusoidal waveforms with definite amplitudes and specific frequencies.

Theses component waveforms are called harmonics.

The frequencies of the harmonics are whole-numbered multiples of the complex waveform’s frequency.

Sound, cont’d

The complex waveform (left) is a combination of three pure waveforms (right) The pure tones (harmonics) have frequencies 1, 2, and

3 times the frequency of the complex tone

Perception of sound, cont’d

The specific tone quality of a sound depends on: the number of harmonics that are present, and the relative amplitudes of these harmonics.

A spectrum analyzer displays a complex waveform in terms of the constituent harmonics.

END

Sound, cont’d

A waveform of a sound wave is a graph of the air-pressure fluctuations causes by the sound wave versus time. A pure tone is a sound

with a sinusoidal waveform.

A complex wave is a sound that is not pure.

Sound, cont’d

Various different waveforms of sounds

Perception of sound, cont’d

We can construct the waveform on the left by adding the three waveforms (harmonics) on the right.