Sound

Ranges of Hearing

Frequency / Wavelength / Period / Speed

AUD202Audio and Acoustics Theory

What You Need to Learn Today

What sound is and how it travels

The frequency range (Hz) and dynamic range (dB) of human hearing

How to calculate frequency, wavelength, period and speed of sound

Acoustics: the physics of sound

Psychoacoustics: the human perception of sound

Sound is Vibrations

When an object vibrates it causes nearby molecules to vibrate causing a chain reaction

Sound is Vibrations

Sound travels as a series of compressions and rarefactions through a medium

Sound is a Mechanical Wave

Sound travels through air as a longitudinal wave

However it can also travel as a transverse wave (such as on a guitar string or a wave in water)

One complete cycle is one compression and one rarefaction

1 cycle per second = 1cps = 1 Hertz = 1Hz

Frequency range of hearing

The human ear is capable of hearing frequencies between 20 and 20,000 cycles per second

(20Hz to 20kHz)

Frequency range of hearing

20Hz

40Hz

80Hz

100Hz

150Hz

200Hz

10kHz

15kHz

16kHz

17kHz

18kHz

19kHz

Dynamic Range of Hearing

The typically accepted amplitude range of human hearing is 120dBSPL

Sound amplitude is referenced to the Pascal (Pa) (the unit for pressure)

0dBSPL = 0.00002 Pascals (20

µPa)120dBSPL = 20 Pascals

Dynamic Range of Hearing

60 decibels of amplitude-60dBFS to 0dBFS

Sine Wave

The sine wave is a pure tone with no harmonics. It is a

‘simple waveform’

1kHz sine wave

Other Simple Waveforms

Sine

Square

Triangle

Sawtooth

HIGHER FREQUENCYshorter wavelength

LOWER FREQUENCYlonger wavelength

Characteristics of Waveforms

Frequency(number of cycles per second)

Wavelength(length of one cycle in metres)

Amplitude(strength or power of the wave)

Period(time for one cycle in seconds)

Units of Measurement

= velocity of sound (in metres per second)

f

v = wavelength (in metres)

= frequency in Hertz (Hertz is cycles per second)

= period of one cycle (in seconds)

Velocity of Sound Formula

The velocity of sound through air at 20 degrees Celsius is 344m/s

Wavelength (m) = speed of sound / frequency (Hz)

What is the wavelength of 20Hz?

= 344 / 20

= 17.2 metres

Frequency (Hz) = speed of sound / wavelength (m)

vf

f = 344 / 0.0172

f = 20,000Hz

What frequency has a wavelength of 1.72cm?

Tf 1frequency in Hz = 1 / period (in seconds)

f = 1/.002

f = 500Hzms = millisecds.

There are 1000ms in 1 second

What frequency has a period of 2 milliseconds?

period in seconds = 1 / frequency (Hz)

fT 1

What period has a frequency of 200Hz? Answer in milliseconds.

T = 1/200T = 0.005 secondsT = 5 milliseconds

What is the speed of sound at 1,263,719°C?

v = 331 + (0.6 x 1,263,719)

v = 758,562.8 m/s

The Sound Barrier

The Sound Barrier

Velocity of Sound in Various Mediums

Air > 344 m/sHydrogen > 1284 m/sWater > 1482 m/sHuman Brain > 1540 m/sGold > 2000 m/sSteel > 5200 m/sDiamond > 12000 m/s

Speed of Sound Through Air

Calculate the speed of sound at:

0°C5°C

10°C20°C30°C

v = 331.4 + (0.6xC)

Answers

0°C = 331.4 m/s5°C = 334.4 m/s

10°C = 337.4 m/s20°C = 343.4 m/s30°C = 349.4 m/s

What are the frequencies of the following wavelengths?

3.4m5m

50cm80cm

3.4m = 101 Hz5m = 68.8 Hz

50cm = 688 Hz80cm = 430 Hz

Answers

Calculate the Wavelength of:

140Hz in air (at 344m/s)

400Hz in steel (at 5000m/s)

500Hz for an electrical signal in wire (at 300,000km/s)

Range of Hearing

The ‘accepted’ range of hearing of a young undamaged ear is 20Hz to 20kHz

The typical amplitude range is 120dBSPL

The Decibel

The Bel is named after Alexander Graham Bell 1847 –1922.

The decibel (dB) is a tenth of a Bel

In audio, amplitude is measured in decibels

Loudness is Subjective

Amplitude is Objective

LoudnessLoudness is a subjective term describing the strength of our perception of sound.

Loudness is measured in Phons

Threshold of Pain 120dB

Threshold of hearing is

To Recap

Sound is audible pressure variations which travel as a series of compressions and rarefaction through a medium.

Sound is a mechanical wave which is transmitted as longitudinal and transverse waves.

Audio Engineering Society

www.aes.org

Username: jmcacademyPassword: student1

Papers related to NIHL on AES website

Hearing Loss from Noise and Music

Sound Pressure Levels in Symphony Orchestras and Hearing

How the Ear Works and Why Loud Sounds Cause Hearing Loss

Preventing Hearing Loss

Investigation of the Loud Music Exposure Hearing Loss

Moodle Key to enrol in Trimester 1 classes

amplitude

Next Week

Wave InteractionBeat FrequenciesPhase ConceptsComb Filtering

WaveformsNoise