Sampling & Synthesis – Glossary

Frequency– Frequency is the number of times a wavelength occurs in a single second, and with music we measure frequency in Hertz or Kilohertz, also known as cycles per second. The faster the sound vibrates the higher the frequency, this suggests the sound would be at a higher pitch. Humans are limited to a certain range of frequencies, starting at 20Hz to 20Khz. Some examples of low frequency sounds could be, thunder, a bass drum, or a male voice and examples of high frequency sounds could be a birds chirp, a glass breaking or a female voice. http://www.youtube.com/watch?v=VBV5a0NmlOQ – Example of high frequency. http://www.youtube.com/watch?v=SS9d13zAN1k – An example of a low frequency sweep starting at 1Hz to 35Hz

 

Wavelength– Wavelength is the distance between any point on a wave and the equivalent point on the next phase. It defines the actual length of the wave. The shorter the wavelength, the higher the frequency will be which means the pitch will be higher than a longer wavelength. Each vertical line, in a waveform, from left to right represents a pitch, or frequency, in Hertz.

Amplitude– The amplitude of sound is a measure of its change over a period of time. Peak to peak amplitude is the measurement of the change between the highest and lowest value. Amplitude is often understood as the volume of a sound, but it specifically refers to the level of impact on the air pressure caused by a sound wave. The theory is that when an AM signal is transmitted, there are two waves. One is called the carrier wave which remains at a constant amplitude and frequency. At the transmission stage of the sound waves process, the other wave, or non carrier signal, varies the wave according to the sound being transmitted. So the degree to which the signal modulates in relation to the carrier wave determines the sound that comes out of the receiver. This means that the modulated signal causes the speaker to move at different rates, and these variances in movement cause different sounds to project from the speaker.

Sound is perceived by the movement of air, so when a sound wave is generated by vibration, the amplitude is the parameter used to measure its influence on the air. A louder sound will have a higher amplitude than a low sound, however this does not mean amplitude is directly related to volume. Factors such as environment and interference can make a high amplitude sound less audible than a lower amplitude sound.

 

Fundamental– The lowest resonating frequency of something is called the fundamental frequency. The fundamental frequency is also called the first harmonic of the instrument or sound wave, it is the the frequency with which the waveform completes one ‘cycle’. Each natural frequency of an object or instrument produces, has its own characteristic vibrational mode or standing wave pattern. These patterns are created only within the object or instrument at specific frequencies of vibration. These frequencies are known as harmonic frequencies, or simply harmonics. At any frequency other than a harmonic frequency, the resulting disturbance or distortion of the medium is irregular and non repeating which relates to non integers. For musical instruments and other objects that vibrate in a regular and periodic fashion, the harmonic frequencies are related to each other by simple whole number ratios or Integers. This is the reason why instruments sound pleasant.

Harmonics– One of the most noticeable differences between two sounds is the difference in pitch, it is the frequency of the sound that mostly determines its pitch. When we hear a single note we are concious of its pitch but we are unconscious of many other things that are going on, because a musical note consists of many different unnoticeable frequencies known as harmonics. These harmonics all merge to form an impression of one pitch or fundamental, the rest blend in to reinforce and colour the main note. Pythagoras notices that when a vibrating string on a guitar was stopped by hand halfway along the length, the pitch went up an octave. If he stopped at a quarter length, it went another octave higher, at an eighth, another octave higher. If stopped at one third the length of the string, the pitch was an octave plus a fifth higher than the fundamental pitch. With frequency and harmonics, every time a number is doubled it goes up an octave. So when a string is plucked on a guitar, all of its harmonics will be vibrating simultaneously. At any one frozen moment the shape of the string will exhibit a very complex wiggly shape which results in all of these simultaneous vibrations, the result is one pitch with a particular tone colour. A plucked G string on a violin will not sound the same as a plucked G string on a viola or guitar because they have different tone colours and the harmonic series will determine them. This is because the various harmonics are not all the same strength.

Integer– This is whole number frequency of harmonics. The ear is sensitive to ratios of pitches rather than distinguishing musical intervals. The intervals which are perceived to be most consonant are composed of small integer ratios of frequency. Example:

The octave, fifth, and fourth are the intervals which have been considered to be consonant throughout history by essentially all cultures, so they form a logical base for the building up of musical scales. A typical strategy for using these universally consonant intervals is the circle of fifths. The term musical integer refers to a step up or down in pitch which is specified by the ratio of frequencies involved. For instance, an octave is a music interval defined by the ratio 2:1 regardless of its starting frequency. From 100Hz to 200Hz is an octave, as is the interval from 2000Hz to 4000Hz.  The intervals which are generally the most consonant to the human ear are intervals represented by smaller integer ratios. Intervals represented by exact integer ratios are said to be just intervals, and the temperament which keeps all intervals at exact whole number ratios is Just temperament.

Examples of just musical intervals: 2:1 octave
3:2 fifth
4:3 fourth
5:4 major third
6:5 minor third

Integer Multiple– Integral multiples of fundamental frequency are called overtones where as the fundamental frequency is itself, a harmonic. Harmonics and overtones are both integral multiples of a fundamental frequency. An integer Multiple is the overtone above the fundamental frequency. Its frequencies being added onto the fundamental frequency, hence why they are called overtones. Therefore we can say that the fundamental frequency is the first harmonic or integer multiple by one. The double frequency or integer multiple by two is the second harmonic and the first overtone. higher harmonics and overtones will follow the same pattern in numbering therefore a formula could be written as (n)=(n+1)

Subtractive Synthesis– Subtractive synthesis is often referred to as analogue synthesis because this is how the synthesisers were used to make patches. The process of subtractive synthesis is very simple as it goes: Oscillator – Filter – Amplifier. This means the sound is generated by the oscillator and frequencies are then subtracted with a filter and finally the signal goes through the amp envelope. Subtractive synthesis means taking away elements of a wave to create a new wave or sound and then control the loudness over time.

 This picture shows the frequencies being blocked out with the frequencies being allowed through the signal chain. This is the fundamental idea on subtractive synthesis.

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