2. Sound Waves
• Sound is caused by vibrations in the air
– Guitar strings, vocal cords, speaker cones
• Air molecules are pushed together, causing
subtle air pressure changes
– Ear drums interpret the pressure changes as
sounds
• Analog sound waves represent the shifts in air
pressure
4. Amplitude
• Distance from the peak to the trough of the
wave
• The higher the amplitude, the louder the
sound
5. Frequency
• Number of wave cycles per second
• Measured in Hz
• The higher the frequency, the higher the pitch
6. Cycle/Phase
• Cycle
– Single sequence of pressure changes from zero to
high to low, and then back to zero
• Phase
– Position in the cycle
– Measured in degrees
• Wavelength
– Time it takes to complete
one cycle
7. Interaction of Sound Waves
• In Phase
– Waves that are perfectly in phase reinforce each
other
• Out of Phase
– Waves that are perfectly out of phase cancel each
other
8. Interaction of Sound Waves
• Waves that are out of phase in varying
amounts combine to form a more complex
wave
9. Recording Sound Waves
• Microphones convert pressure changes into
voltage changes
– High pressure becomes positive voltage
– Low pressure becomes negative voltage
• Voltage changes can be recorded as changes
in magnetic strength on tape for analog
recording
• Digital recording is done by sampling the
analog waveform
10. Sampling
• Each sample is a snapshot of the voltage value
of the analog wave
• The more often the sample is taken, the more
accurate the digitization of the sound will be
12. Bit Depth
• Size of each sample
• Higher the bit depth, the better audio quality
is possible
13. Audio File Formats
• Uncompressed
– WAV (AIFF on the Mac)
• Files can get very large
• 5 MB per minute for a mono file at 44,100 sampling rate
• Lossless compression
– File formats that save space by not recording data for
silence
– FLAC, WavPack
• Lossy compression
– Complex algorithms that attempt to reproduce sound
using less data
– MP3, Ogg Vorbis, WMA (includes DRM), ra, AAC (I-Tunes)
