ALTERNATING CURRENT (AC)

Transcription

1 ALL ABOUT NOISE ALTERNATING CURRENT (AC) Any type of electrical transmission where the current repeatedly changes direction, and the voltage varies between maxima and minima. Therefore, any electrical AUDIO signal may be called an AC signal. Whereas the voltage range of an audio signal is less than a few volts, that of electrical current is standardized in the volt or 220 volt range with a frequency of 60 or 50 Hz (see map). When electrical equipment is improperly grounded, a HUM is heard at one of those PITCHes, or a multiple thereof. Alternating current was introduced in the 1890's, and is now used almost universally to distribute electricity on a large scale. 1

2 Noise Generator When energy occurs at all frequencies, this is known as a distributed spectrum or noise. More specifically, when energy is distributed uniformly over frequency this is known as white noise. Pink noise, which sounds subjectively less tinny than white noise, exhibits a 3 db / octave reduction in high frequency energy. Below is a time domain graph of a noise signal. Notice the random nature of the signal, which is very different from a sine or square wave. Amplitude Amplitude v Time Graph of a Noise Signal Time (s) 2

3 NOISE Danger The red bar below shows how long it takes before a particular sound level becomes dangerous to the human ear. For example, a chain saw has a sound intensity of about 110 db. Without proper hearing protection, running a chain saw for only 2 minutes can become dangerous to the human ear! 3

4 NOISE DEFINED Truax.1 NOISE Acoustics / NoiseEtymologically the word can be traced back to Old French (noyse) and to 11th century Provençal (noysa, nosa, nausa), but its origin is uncertain. It has a variety of meanings and shadings of meaning, the most important of which are the following:1. Unwanted sound: The Oxford English Dictionary contains references to noise as unwanted SOUND dating back as far as

5 Fourier analysis Based on the theory that any complex signal may be reduced to a series of sine waves of varying amplitude, frequency and phase. also known as a fast Fourier transform. FFT Fourier analysis may be generally described as the representation of a sound's frequency components as a sum of pure sinusoidal waves (Truax, Handbook for Acoustic Ecology: 51). An analysis of a sound's frequency components is taken at a steady state to give an approximation of that sounds spectrum. As most natural sounds are spectrally dynamic, one single Fourier analysis could not possibly represent a sound in sine waves. By 'windowing', a Fast Fourier Transform takes several of these approximations and strings them together to better predict a sound's spectrum over time. 5

6 Subtractive Synthesis CREATIVE USES OF NOISE Subtractive synthesis is technique which creates musical timbres by filtering complex waveforms generated by oscillators. Subtractive synthesis is usually (but not exclusively) associated with analogue voltage controlled synthesizers such as the moog or the minimoog.it can produce very natural changes in a sound, owing to the intuitive way in which it works. Typically a lowpass filter is applied to the initial tone, because natural musical sounds generally include less of the harsh high frequencies found in square or sawtooth waves. Often the user has a choice of steeper and shallower filters. The cut-off frequency and resonance of the filter may be modulated by an envelope to create a more dynamic sound. 6

7 NOISE DEFINED Truax.2 2. Unmusical sound: The 19th century physicist Hermann von Helmholtz employed the term 'noise' to describe sound composed of non- PERIODIC vibrations (e.g. the rustling of leaves), by comparison with musical sounds, which consist of periodic vibrations. Noise is still used in this sense in expressions such as BROAD BAND NOISE, GAUSSIAN NOISE, NARROW BAND NOISE, RANDOM NOISE, RUSTLE NOISE or WHITE NOISE. 7

8 NOISE DEFINED Truax.3 &4 3. Any loud sound: In general usage today, noise often refers to particularly loud sounds. In this sense a noise abatement by-law prohibits certain loud sounds or establishes their permissible limits in DECIBELs. See: JET PAUSE, LOUDNESS, NOISE POLLUTION, SOUND INTRUSION, SOUND POLLUTION. 4. Disturbance in any COMMUNICATION system: In electronics and engineering, noise refers to any disturbances which do not represent part of the SIGNAL, such as static on a telephone or 'snow' on a television screen. See: BACKGROUND NOISE, SIGNAL-TO-NOISE RATIO. Compare: REDUNDANCY. 8

9 NOISE DEFINED Truax.5 The most satisfactory definition of noise for general use is still 'unwanted sound'. This makes noise a subjective term: one person's music may be another's noise. But it also provides the opportunity for a society to come to a general agreement as to which sounds constitute unwanted intrusions. It should be noted that each language preserves unique nuances of meaning for words representing noise. Thus in French one speaks of the bruit of a jet, but also the bruit of the birds or the bruit of the waves.. 9

10 NOISE DEFINED.1 10

11 NOISE DEFINED. (2) Subjectively speaking noise is unwanted sound. It is the sound that appeared in the mix that we didn't want to be there. It is often the result of distortion of the signal due to clipping or improper connections or the result of the operating noise of the electronic circuitry of the equipment being used. 11

12 NOISE DEFINED. (3) random activity of electrons - i.e. all audio systems have a noise floor or basic level of such activity 12

13 NOISE Two kinds of noise Two kinds of noise are intentionally generated and used for a variety of purposes such as sound sources for electroacoustic composition and sound reinforcement setup and calibration. These are white noise and pink noise. 13

14 NOISE: White & Pink Noise Asound or signal consisting of all audible frequencies with equal intensity. The term is used analogously to the term 'white light' in optics which denotes the simultaneous presence of colours of all frequencies. BANDs of FILTERed white noise are sometimes referred to as coloured noise, but the analogy to colour has not been applied systematically to sound. The term pink noise refers to a kind of NOISE where each OCTAVE band has the same intensity, and therefore does not sound as bright as white noise since the intensity of the SPECTRUM does not increase with higher frequencies. 14

15 NOISE: White & Pink Noise This is a frequency domain graph of both white and pink noise signals. Notice that the white noise signal has a flat frequency response whilst the pink noise signal has a high frequency drop-off of 3 db per octave (doubling of frequency). Level (db) Frequency (Hz) Level v Frequency Graph of Pink and White Noise Signals White Pink Due to the distribution of energy over all frequencies, noise is not particularly musical as it does not have a discernible pitch. In musical synthesis, noise is useful in creating percussive sounds and adding breath noise to instrument sounds. 15

16 NOISE: White Noise.1 Random noise or audio rate voltage which has equal energy per Hertz in the 20-20,000 Hertz range. Rain, a shower, a waterfall, etc. are natural examples which resemble white noise. WHite noise tends to have a distinct high frequency colour to humans as half the energy is in the highest octave which we can perceive: 10,000-20,000 Hertz. In white noise, half of its energy will be in the 10,000-20,000 Hz band while pink noise will have roughly 1/10th of its energy there. 16

17 NOISE: White Noise.2 This is the same noise heard in electronic devices and is the result of random movements of electrons in the components of those devices as a result of heat. One of the characteristics of white noise is the perception that it is louder in the high frequency regions. This is because white noise has equal amounts of energy per hertz. This is a result of the fact that with each octave increase in frequency there is a corresponding doubling of frequency. This also results in a doubling in the range of frequencies available between octaves. Consider the range of frequencies in the octave described by 100 to 200 hertz compared to that of the octave described by 5000 to hertz. Clearly the second octave has a much greater range of frequencies. Consequently white noise, because it has equal energy at all frequencies, exhibits a 3 db per octave 17 rise in level over its range of frequencies

18 NOISE: Pink Noise.1 A type of noise with equal energy in each octave vs white noise which has its energy distributed throughout the frequency range of audio. Pink noise more closely approximates the frequency/energy distribution of conventional music. pink noise 18

19 NOISE: Pink Noise.2 Pink noise is the result of a white noise signal being rolled off by a 3 db per octave low pass filter with a start point at a sub-audio frequency. This results is a noise which has equal energy per octave. Because pink noise is more balanced in its levels over a wide frequency range it is useful in testing and calibrating audio equipment which must perform equally well over the range of audible frequencies 19

20 NOISE: for simple calibration.1 For home theater use, aside from increased accuracy, there is an important benefit of using full range pink noise to calibrate your speakers. That is the fact that with this signal, TIMBRE changes are VERY audible from speaker to speaker. By ear, you can readily hear any changes in tone caused by room influences or non-identical speaker systems as the signal is switched from speaker to speaker. The pink noise should sound identical in every speaker (except for the sub). You can make tweeter level adjustment, or speaker position adjustments by ear and get very uniform results, and better imaging as a result.as a calibration signal source, you can take advantage of the greater accuracy of full bandwidth pink noise. 20

21 NOISE: for simple calibration.2 on your SPL meter, set the "weighting" to "A", rather than the more common "C". This "A" weighting makes the meter more sensitive to the mid range frequencies and less so to the bass and treble, but nowhere near as extreme as the bandwidth limited test tone built into receivers, pre/pros, and on test DVDs. After you get all the main and surround speakers to the same level, swtich to "C" weighting and note any differences: this will be mainly due to the greater sensitivity in the bass region of the meter. If the needle is still relatively easy to read, you might want to do any final touch-ups in the level of any speakers that give largely different readings from the "A" weighting reading.. Some by-ear level setting will probably need to be done in conjunction with the readings from the meter. 21

22 NOISE: step by step calibration Things required : db Meter, Pink Noise Source First you must choose what the actual reference level is going to be. This is usually anywhere from about 79dB to 85dB SPL (Sound Pressure Level). Now turn on or generate in-band pink noise 22

23 Distortion Distortion is usually thought of as an undesirable change in the waveform as it passes through electronic equipment or any kind of transducer, although, like noise, this is a highly subjective and context dependent judgement. Some sound synthesis techniques such as non-linear waveshaping use distortion to achieve their ends. The familiar sound of the rock guitar is largely dependent on distortion techniques. Any technique which changes the signal can be considered a form of distortion, so even a simple equalizer may be thought to distort the signal. 23

24 Distortion: Types Clipping Frequency Harmonic Phase Transient 24

25 Distortion: Types: Clipping 25

26 Distortion: Types: Frequency 26

27 Distortion: Types: Harmonic 27

28 Distortion: Types: Phase 28

29 Distortion: Types; Transient 29