Multimedia Sound

What is Sound? Sound, sound wave, acoustics Sound is a continuous wave that travels through a medium Sound wave: energy causes disturbance in a medium, made of pressure differences (measure pressure level at a location) Acoustics is the study of sound: generation, transmission, and reception of sound waves Example is striking a drum Head of drum vibrates => disturbs air molecules close to head Regions of molecules with pressure above and below equilibrium Sound transmitted by molecules bumping into each other

Measurement of Sound A sound source is transferring energy into a medium in the form of sound waves (acoustical energy) Sound volume related to pressure amplitude: - sound pressure level (SPL) SPL is measured in decibels based on ratios and logarithms because of the extremely wide range of sound pressure that is audible to humans (from one trillionth=10 12 of an acoustic watt to one acoustic watt). 0 dB SPL - no sound heard (hearing threshold) 35 dB SPL - quiet home 70 dB SPL - noisy street 110 dB SPL - thunder 120 dB SPL - discomfort (threshold of pain)

Sampling: x(n)=x(nT), where T is the sampling period (horizontal). The opposite transformation, x(n) -> x(t), is called interpolation. Quantization: Q() is a rounding function which maps the values of x(n) into N levels (vertical) Coding: Convert discrete values to binary digits Digital to Analog Conversion

Sampling Sample Rate: refers to the rate at which sound samples are recorded from the incoming analog audio source and it is recorded along X axis. Measured in Hz Higher the frequency better is the recorded signal. CD quality audio : 44.1 KHz Sampling Size: refers to the number of bits used to record the incoming signal it is recorded along Y axis. Mono & Stereo Channelising

Quantization & Clipping Quantization: The value of each sound sample during ADC process is round off to the nearest integer value. Clipping: During ADC process if the amplitude of the sample is greater than the interval available the value is clipped at the top and the bottom.

Nyquist’s Sampling Theorem Sampling Frequency is Very Important in order to accurately reproduce a digital version of an Analog Wave form Nyquist’s Theorem: “The Sampling frequency for a signal must be at least twice the highest frequency component in the signal.”

Digital Audio File Size Disk space required = Sample Size (Hz) X Sampling Rate X Channel multiplication Factor A 16 bit sound system recording signal at 44 KHz in stereo will take up: 16 * * 2 = bits or 172 KB per second

Audio Editing Terminology Trimming: the process of removing blank spaces. Splicing: the process of removing unwanted sound. Reassembling: Cutting, Copying, Pasting of sound clips. Volume Control: Increase & Decrease of either portion or whole of the recorded audio. Fade in fade out: smoothing of beginning & end of audio file. Resampling: Process of reducing the sound quality to reduce the size.