SWE 423: Multimedia Systems

Chapter 3: Audio Technology (1)

Audio• Audio is a wave resulting from air pressure

disturbance that reaches our eardrum generating the sound we hear.– Humans can hear frequencies in the range 20-20,000

Hz.

• ‘Acoustics’ is the branch of physics that studies sound

Characteristics of Audio

• Audio has normal wave properties– Reflection– Refraction– Diffraction

• A sound wave has several different properties:– Amplitude (loudness/intensity)– Frequency (pitch)– Envelope (waveform)

Audio Amplitude

• Audio amplitude is often expressed in decibels (dB)

• Sound pressure levels (loudness or volume) are measured in a logarithmic scale (deciBel, dB) used to describe a ratio– Suppose we have two loudspeakers, the first playing a

sound with power P1, and another playing a louder version of the same sound with power P2, but everything else (how far away, frequency) is kept the same.

– The difference in decibels between the two is defined to be

10 log10 (P2/P1) dB

Audio Amplitude• In microphones, audio is captured as analog signals

(continuous amplitude and time) that respond proportionally to the sound pressure, p.

• The power in a sound wave, all else equal, goes as the square of the pressure.– Expressed in dynes/cm2.

• The difference in sound pressure level between two sounds with p1 and p2 is therefore 20 log10 (p2/p1) dB

• The “acoustic amplitude” of sound is measured in reference to p1 = pref = 0.0002 dynes/cm2.– The human ear is insensitive to sound pressure levels below

pref.

Audio AmplitudeIntensity Typical Examples

0 dB Threshold of hearing

20 dB Rustling of paper

25 dB Recording studio (ambient level)

40 dB Resident (ambient level)

50 dB Office (ambient level)

60 - 70 dB Typical conversation

80 dB Heavy road traffic

90 dB Home audio listening level

120 - 130 dB Threshold of pain

140 dB Rock singer screaming into microphone

Audio Frequency• Audio frequency is the number of high-to-low pressure

cycles that occurs per second.– In music, frequency is referred to as pitch.

• Different living organisms have different abilities to hear high frequency sounds– Dogs: up to 50KHz

– Cats: up to 60 KHz

– Bats: up to 120 KHz

– Dolphins: up to 160KHz

– Humans: • Called the audible band.

• The exact audible band differs from one to another and deteriorates with age.

Audio Frequency

• The frequency range of sounds can be divided into– Infra sound 0 Hz – 20 Hz– Audible sound 20 Hz – 20 KHz– Ultrasound 20 KHz – 1 GHz– Hypersound 1 GHz – 10 GHz

• Sound waves propagate at a speed of around 344 m/s in humid air at room temperature (20 C)– Hence, audio wave lengths typically vary from 17 m

(corresponding to 20Hz) to 1.7 cm (corresponding to 20KHz).

• Sound can be divided into periodic (e.g. whistling wind, bird songs, sound from music) and nonperiodic (e.g. speech, sneezes and rushing water).

Audio Frequency

• Most sounds are combinations of different frequencies and wave shapes. Hence, the spectrum of a typical audio signal contains one or more fundamental frequency, their harmonics, and possibly a few cross-modulation products.– Fundamental frequency– Harmonics

• The harmonics and their amplitude determine the tone quality or timbre.

Audio Envelope

• When sound is generated, it does not last forever. The rise and fall of the intensity of the sound is known as the envelope.

• A typical envelope consists of four sections: attack, decay, sustain and release.

Audio Envelope

• Attack: The intensity of a note increases from silence to a high level

• Decay: The intensity decreases to a middle level.• Sustain: The middle level is sustained for a short period of

time• Release: The intensity drops from the sustain level to zero.

Audio Envelope

• Different instruments have different envelope shapes– Violin notes have slower attacks but a longer

sustain period.– Guitar notes have quick attacks and a slower

release