Sound
Sound is produced by vibration, causing a disturbance in air pressure.
Example: When force is applied to a tuning fork, the arm of the fork vibrates in the direction of the strike. The air molecules around the fork become pressurized, and exhibit high potential energy. Because all states seek equilibrium, the tuning fork vibrates in the opposite direction of the strike, decompressing the air molecules previously pressurized. As they expend their energy and decompress they excite neighboring molecules to a state of dynamism. The process is repeated in a domino effect, leading to propagation of the sound wave.
The pattern of oscillating pressure is called a waveform.
Pitch is created by wavelength. The number of vibrations per second determines the wavelength (more vibrations = shorter wavelength = higher frequency = higher pitch). The note, middle C, vibrates at 256 times per second. D above middle C vibrates at 288 times per second. The human ear can detect between 16 vibrations per second and 20,000 vibrations per second. The size of the vibration determines amplitude (volume). The bigger the vibration the louder the sound.
Speech waveforms:
The letter "A" as a waveform.
The letter "E" as a waveform.
The letter "I" as a waveform.
The letter "O" as a waveform.
The letter "U" as a waveform.
Frequency
Amplitude
Doppler Effect
The Doppler Effect is the change in pitch one hears when, for example, an ambulance passes you on the roadside with its siren on. The change in pitch happens because when the soundwaves are coming toward you they are traveling at the speed of sound plus the speed of the vehicle, which compresses the waves and pushes the pitch higher. Going away from you the sound waves travel at the speed of sound minus the speed of the vehicle. (This effect happens with light waves also, known as the red shift).
The largest musical measurement of pitch is the octave. Taking the pitch one
octave higher doubles the frequency, while going one octave lower cuts it in
half. The octave is divided into 12 half steps that correspond to notes on a
pitched instrument such as a piano.
The internationally recognized standard, called concert pitch, is that the
A above middle C has a frequency of 440Hz.
The characteristic of an audio waveform is referred to as its timbre or tone
color.
The sine wave with a frequency corresponding to the actual note being played
is called the fundamental frequency. The frequencies of harmonics are all multiples
of the fundamental frequency.
Global changes in amplitude to harmonics over time also determine the sonic
signature. They are -
Acoustics is the study of sonic reflection, described as echo and reverberation.
Sonic qualities of a listening environment are perceived as ambiance. Acoustic
amplification depends on reflection. Examples are a band shell, a guitar body,
and speaker cabinets.
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Analog audio works with a wide range of voltage levels, but they break up into
two general catagories: signal levels, which are much lower, and program material,
where they are much higher.
Microphones emit very weak signals of about one millivolt. This is called mic
level. Line levels come in two categories: consumer and professional. Consumer
level is .316 volts, while professional is 1.23 volts, or significantly hotter.
To accomodate the different voltage levels the audio chain is divided into
stages. The primary division is between preamps and power amps. Audio information
is modified and mixed at the lower preamp levels. The result is sent to the
amps whose sole purpose is to increase the signal level of program material.
Meters monitor signal levels in volume units. 0 VU is the optimum signal level
a circuit can handle without distortion.
Optimal audio frequency response is 20Hz to 20kHz. The dynamic range of an
audio CD is 96dB, while audio cassette is 48dB.
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A dynamic microphone consists of a diaphragm attached to a coil of wire suspended
in a magnetic field. As sound waves (pressurized air) move the diaphragm in
and out, the wire moves. This causes fluctuations in the magnetic field, which
in turn induce an alternating current in the coil. This current is a direct
corollary of the original sound waves.
The current travels through the wire to a speaker. The speaker has its own
coil suspended in a magnetic field. The alternating current causes fluctuations
in the field causing another diaphragm to move in and out. These movements reproduce
the same air pressure that provided the original sound wave.
In this example various types of current have passed along the information:
air pressure to magnetic field to electric current and back again to sound wave.
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Analog versus Digital
Analog signals are continuous and analogous to the original physical phenomenon.
Digital signals are measurements taken at discrete time/space intervals.
The number of bits of resolution determine dynamic range. Each bit contributes
6dB of dynamic range. 8 bit audio, then, has a range roughly equal to cassettes
(48dB). 16 bit audio equals the range of audio CDs, and approximates the range
of the human ear (96dB).
Besides bit resolution there are two other factors that determine throughput and storage: sampling rate and the choice between stereo and mono. Mono 8 bit audio sampled at 22 KHz occupies about 1.25MB of disk space per minute. Stereo 16 bit audio sampled at 44KHz takes up about 10MB per minute.
The formula for determining the size of a mono digital recording (in bytes):
sampling rate * duration of recording in seconds * (bit resolution / 8)
- multiply the result by 2 for a stereo recording
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Audio Formats
Audio files can be either uncompressed or compressed. Most of the early audio formats were uncompressed which means that sound files are bigger and take longer to download. Common uncompressed audio formats include WAV, the native audio format for Windows, and AIFF (Audio Interchange File Format), the native format for Macs.
MP3 is short for MPEG-1 Audio Layer 3. The higher the audio layer, the more complex the encoding which leads to better sound at higher compression.
MP3s can be encoded at varying bit rates ( KB per second). The higher the
bit rate, the better the sound quality. But the trade-off is a larger file.
MP3s can be encoded at bit rates from 8 kbps to over 1000 kbps. The usual standard
is 128 kbps which provides CD like playback. At this rate, a minute of
music is about1 MB in size.
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MIDI (Musical Instrument Digital Interface)
Another common type of audio file is MIDI (Musical Instrument Digital Interface). Technically it's not a digital audio format because a MIDI file doesn't actually contain a sound recording. A file or sequence contains a set of instructions for how a sound should be played back by a computer's sound card or a MIDI instrument like a keyboard. Think of MIDI like digital sheet music that your computer can read and play.
A MIDI sequence has to be created on a MIDI device which means that MIDIs are instrumental only though some sequencing programs can incorporate voice and sound effects. Playback must also be through a MIDI device so the sound quality of a file is dependent on the quality of your sound card or synthesizer. The advantages of MIDI are small files and easy sound editing. Sequencing programs let you edit musical notes much like you edit words with a word processor.
The inclusion of microprocessors in instruments, like keyboards, paved the
way for MIDI, which is a serial communications protocol designed for electronic
instruments. MIDI does for audio what Postscript does for graphics.
MIDI describes the elements of a musical performance rather than casting them
into bit streams of digital audio. MIDI is device and resolution independent.
The fundamental set up in MIDI is a master unit and a slave unit. To this set
up is added a sequencer, a device that records, edits, and plays back MIDI data
in real time.
The advantages of MIDI are:
Malleable files.
Audio quality is maintained- there is no recording loss.
Small file size. A typical four minute MIDI song is about 50K.
MIDI connections
All MIDI devices employ 5 pin DIN connectors. While all MIDI devices share
the same jack, there are three types of connectors-
MIDI in: accepts signal from another device.
MIDI out: sends signal generated inside the device to other devices.
MIDI thru: Passes data through to other devices on the chain.
MIDI messages
There are many messages sent from the master to the slave-