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 P 1, and another playing a louder version of the same sound with power P 2, but everything else (how far away, frequency) is kept the same. –The difference in decibels between the two is defined to be 10 log 10 (P 2 /P 1 ) 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/cm 2. The difference in sound pressure level between two sounds with p 1 and p 2 is therefore 20 log 10 (p 2 /p 1 ) dB The “acoustic amplitude” of sound is measured in reference to p 1 = p ref = dynes/cm 2. –The human ear is insensitive to sound pressure levels below p ref.

Audio Amplitude IntensityTypical Examples 0 dBThreshold of hearing 20 dBRustling of paper 25 dBRecording studio (ambient level) 40 dBResident (ambient level) 50 dBOffice (ambient level) dBTypical conversation 80 dBHeavy road traffic 90 dBHome audio listening level dBThreshold of pain 140 dBRock 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 sound0 Hz– 20 Hz –Audible sound20 Hz– 20 KHz –Ultrasound20 KHz – 1 GHz –Hypersound1 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