Sound

• Sound - vibration (wave) of the air. More specifically, it's a mechanical wave that propagates through matter.
• Acoustics - the scientific study of sound.
• Pitch - related to the frequency or the wavelength.
• Volume - related to the amplitude of the wave. Measured in decibels which are on a logarithmic scale.
• Hearing - the ear contains a membrane sensitive to the vibration of the air, called eardrum. The vibrations are transmitted through a liquid medium in the cochlea to specialized cells that convert them to nerve impulses.
• The hearing is limited in range both in frequency and in volume.

Sound in Multimedia

• Digital audio - sample sound. The audio clip is composed of sound samples for every nth of a second. Higher sampling rates mean higher sound quality.
• Digital representation of sound has several purposes:
• record, store, and reproduce sound more accurately and reliably than analog devices
• edit audio media directly in digital form
• create audio content using software
• integrate audio content in multimedia projects.

Sound Representation

• Sampling rate - number of samples per second. 3 usual rates: 44.1kHz (CD quality), 22.05 kHz, and 11.025 kHz.
• Bitdepth or sample size or resolution - the number of bits used to represent each sample. 8-bit samples can have 256 values while 16-bit samples 65,536 values.
• Quantization - since the audio signal is continuous, a discretization of the sample values is necessary. A value is rounded to the closest integer. This can result in a loss of precision of the representation.
• Higher sound quality results in larger files.

Space vs Quality

nBit Depth	Sample Rate	Bit Rate	File Size of one stereo minute	File size of a three minute song
n16	44,100	1.35 Mbit/sec	10.1 Mb	30.3 Mb
n16	48,000	1.46 Mbit/sec	11.0 Mb	33 Mb
n24	96,000	4.39 Mbit/sec	33.0 Mb	99 Mb
mp3 file	128 k/bit rate	0.13 Mbit/Sec	0.94 Mb	2.82 Mb

Audio File Formats

• Uncompressed format: PCM (Pulse-Code Modulation). Usually stored in .wav (wave format), or .aiff (audio interchange file format).
• Compressed: lossy or lossless compression.
• Lossless compression: .wav, .aifc (compressed aiff), .wma (windows media audio).
• Lossy compression: mp3, mp2, mpg (audio mpeg compression), some version of .wma.
• MIDI file format - Musical Instrument Digital Interface. Analog to vector graphics. Such a format contains an integer identifier for each instrument and a score.
• To create a MIDI file you need a sequencer program. To playback or convert a MIDI file to an audio file you need a sound synthesizer.

Software

• Playback: Windows Media Player, RealPlayer, iTunes, QuickTime, WinAmp, etc. Some of these can record music CDs from audio files.
• Recording and editing: Audacity, SoundMax, SoundForge, GoldWave, Adobe Audition, Steinberg WaveLab, Sonic Foundry, etc.
• MIDI creation: GarageBand, Sony Acid Pro, FL Studio, Steinberg Cubase, Pro Tools
• See http://emusician.com and http://www.audiomelody.com for more info info.

Sound Editing

• Multiple tracks: a project storing various sound sources separately.
• Trimming: removing the empty slices at the beginning and ending of a track.
• Splicing, assembly: removing the extra noises from the recording.
• Normalizing: bringing the whole project to a consistent level of volume.
• Resampling or downsampling: going from a higher to a lower resolution.
• Special effects: time stretching, reversing, reverb, etc.