Speech ScienceOct 7, 2009

Outline

Acoustics

Pure Tone

Simple harmonic motion

Particle movement in Sound

Pressure wave movement in Sound

Interference patterns

Complex tones

Frequency and Pitch

Intensity and Loudness

Wavelength

Resonance

Acoustics

What is acoustics

Study of sound

Speech is a continuously changing stream of sound

Clear understanding of nature of sound is necessary to understand the process of speech production and perception.

Acoustics

Sound

An audible disturbance in a medium caused by a vibrating source Has no physical

substance Invisible Most sounds are

complex

A visible pattern of sound waves. A Basic Guide to the Science of Acoustics by David C. Knight, Franklin Watts, Inc. New York (1960). p. 80.

Pure Tone

Pure tone

Simplest of all sound waves

Consists of only one frequency

Pure tones result from a vibration that repeats itself in exactly the same way (periodic) at a constant number of cycles per second (frequency).

Pure tone has a single frequency

A tuning fork produces a pure tone

http://www.youtube.com/watch?v=C5LS6scAL3E

Pure Tone

Tuning fork vibrates in simple harmonic motion (SMH)

Elasticity

The restoring force that causes a deformed structure to resume its original shape

Inertia The tendency for motion to

continue

Inertia and velocity occur simultaneously.

Pure Tone

Swing analogy: velocity gradation in SMH

Pure Tone

Velocity: The speed of an object.

Frequency: Number of completed

cycles per second (expressed as Hertz)

Period: The time taken for one

complete cycle of movement.

Dampening: Decrease in the

amplitude of displacement over time.

Quiz:

If the frequency were 20 Hz, then what is the period?

Relationship between frequency and period

f (in Hz) = 1/T T (in seconds) = 1/f

Pure Tone

Quiz

Which graph shows how the kinetic energy of an oscillating object varies with time in simple harmonic motion? The period of oscillation is T.

Pure Tone

Particle movement in sound: In pure tone, individual

air particles move in SHM in response to the movement of the pure tone vibrator in SHM.

When 540 Hz tuning fork is sounded, every air molecule in the room soon moves in place.

Each air particle completes 540 cycles per second.

Pure Tone

Movements of vibrating body could be displayed as an amplitude-by-time graph

It is known as waveform

Pure Tone

Both waves have same amplitude but differ in frequency.

Upper one is a low frequency wave, reduced number of cycles per second

Lower one is a high frequency wave, increased number of cycles per second.

Lower one has short period compared to upper one.

Pure Tone

Waveforms of different amplitudes

Pure Tone

Quiz

Among top two waves, which one has higher amplitude?

Among lower two waves, which one has lower frequency?

Pure Tone

Pressure wave movement in sound

The molecules of air vibrating in SHM disturb adjacent molecules, and therefore the disturbance is transmitted away from the source.

The disturbance takes the form of a pressure wave radiating outwards.

Pure tones are periodic, the pressure wave is repeated, followed by evenly spaced pressure waves.

http://www.youtube.com/watch?v=C5LS6scAL3E

Pure Tone

Pressure wave movement in sound

Movement of air particle is in the same direction as wave movement, this type of wave is called longitudinal wave. Sound waves are longitudinal.

Pure Tone

Pressure wave movement in sound

Transverse waves: particles move perpendicular to the direction of the wave

E.g., dipping a finger into water

Pure Tone

Pressure wave movement in sound

Energy released by the source of waves results in compression (area of higher pressure) and rarefaction (area of lower pressure) of solids, liquids or gases.

The alternating areas of compression and rarefaction constitute a pressure wave that moves away from tuning fork in all directions. This can be represented as sine wave.

Pure Tone

Pressure wave movement in sound

A waveform display how amplitude varies with time.

It can represent both particle motion as well as the pressure variation in the medium.

Waveform is a abstract representation of the particle’s displacement from rest during a certain time or variations in air pressure generated by the vibrating source and the air molecules.

Sound

Essential constituents of sound

Three prerequisites A source of energy A vibrating source A medium of transmission

Sound

Interference patterns Particles in air can be responsive to many

signals at once.

Signals of the same frequency can interfere with one another when signals are reflected from a barrier.

Sound

When two signals have a common frequency, then the resulting summed waveform depends on the phrase relationship between the signals

Every cycle has 3600, so that half of a cycle would be 1800, one fourth of a cycle would be 900, and three fourths, 2700.

Sound

The sound interference is acute in concert halls.

If the halls are designed poorly, then it may reverberates excessively

This will cause sound to persist for long period of time

Complex Tones

A tone having more than a single frequency component.

Complex periodic sound waves are those in which the pattern of vibration repeats itself exactly over time.

Complex aperiodic sound wave is one in which the vibration is random and displays no repeatable pattern.

Harmonics Periodic complex vibrations

produce signals in which the component frequencies are integral multiples of the lowest frequency of pattern repetition, or fundamental frequency.

The frequency of each harmonic is whole number multiple of the f0 or first harmonic.

E.g., fo/ first harmonic = 100 Hz; Second = 200 Hz Third = 300 Hz Fourth = 400 Hz Fifth = 500 Hz

http://www.zainea.com/perc1.gif

http://www.sfu.ca/sonic-studio/handbook/Graphics/Law_of_Superposition.gif

Spectrum Amplitude of each

harmonic can be displayed using a amplitude spectrum

Spectrum displays each harmonic as a function of amplitude and frequency.

Aperiodic complex tone

Complex aperiodic tones consists of more than one frequency, but the frequencies are not harmonically related

Summary

Periodic Aperiodic

Simple One component frequency- A pure tone

Complex Two or more component frequencies that are harmonically related: a fundamental frequency plus harmonics-A complex tone

Two or more component frequencies not harmonically related: no fundamental frequency; no harmonicsNoise

Complex Periodic Tones

Sine waves

Square waves

Triangle waves

Sawtooth waves

Quiz

Periodic or aperiodic?

Quiz

Periodic or aperiodic? Periodic or aperiodic?

Fourier Analysis

Complex waves composed of sinusoid waves of frequency, amplitude, and temporal relations.

Fourier analysis allows us to analyze sounds as a number of harmonics.

Based on the discovery that any periodic waveform could be represented as a summation of sinusoidal vibrations.

Frequency and Pitch

Frequency and Pitch

Frequency is the physical attribute of a vibrating source. Measured in Hertz (Hz) or cycles per second

(cps). Example: Vocal folds vibrate between 80 and

500 Hz.

Pitch is a sensation or psychological event.

As frequency increases so does pitch, but not in a linear or one-to-one manner.

Frequency and Pitch The units for pitch are

mels.

How do listeners judge the pitch of complex tones?

For complex periodic tones, listeners judge the pitch corresponding to the fundamental frequency of the harmonic series.

Intensity and Loudness

Loudness and Intensity

Decibel: Relative intensity of two sounds. Unit of measure Logarithmic scale

Loudness and Intensity

Intensity or sound pressure: Physical property of the acoustic signal that

correspond to the amplitude of vibration. Measured using sound level meter in decibels

(dB) which is a logarithmic system.

Loudness: Psychological sensation; perceptual. Increases with intensity, but not a linear or one-

to-one relationship. A phon is a unit of equal loudness.

Intensity and Loudness

Equal loudness levels at different frequencies

Heavy line at the bottom is the absolute threshold of audibility

The lighter lines are the phon curves of equal loudness (tones of 1 k Hz)

For quiet sounds, the amount of intensity needed to cause the perception of equal loudness is large between extreme and middle frequencies.

Wavelength

Wavelength is the distance from any point in one cycle to the corresponding point in the next cycle.

It’s represented by the Greek letter lambda (λ).

It depends on two factors:

λ = c/f Frequency Velocity of sound wave

propagation in the medium

Wavelength

The higher the frequency the shorter the wavelength.

λ = c/f c = velocity of the

sound (344 meters per seond)

f = frequency

High frequency sounds are more directional than low frequency sounds.

Resonance

Resonance

Resonance: Vibratory response to an applied force. Everything that vibrates has a natural or resonant

frequency Swing analogy

Natural resonant frequency: It is a frequency at which a system oscillates with

greatest amplitude when driven by a vibrating source.

Resonant frequency of a vibrating source depends on its physical characteristics

Resonance

The human vocal tract is a tube open at one end.

Can calculate the natural resonant frequency using the formula: f = c/λ; where f = frequency, c = the constant

34,400 cm, and λ = wavelength.

The other resonant frequencies will be odd multiples of the natural resonant frequency.

Conclusion

Pure tone is the simplest sound and has only one frequency and it is periodic.

In pure tone, movement of vibrating body is in simple harmonic motion.

Particle movement and pressure wave movement in sound.

Sound interferences