1.Audio Devices and Applications

2. Contents Microphone Sensitivity Nature of Response and Directional Characteristics Measurement Microphones Various Types of Microphones Various Types of Loudspeakers Characteristic Impedance of Loud Speakers Headphone Types The basics of Magnetic Recording Sound Cards, Sound Mixers PA Systems & Installations Digital Consoles 3. Microphone working 4. Microphone Sensitivity Sensitivity is an important characteristic of microphone It tells that how much electrical output a microphone has produced for a certain sound pressure level For a same sound pressure level input if two microphones are producing different electrical outputs then the microphone with higher electrical output is said to be more sensitive Sensitivity is defined as output in millivolts for the sound pressure of 1 pascal at 1 Khz 5. Measuring microphone sensitivity 6. Microphone Sensitivity Select a measuring point (about 5 to 6 ft) in front of the loudspeaker and place the SLM there Adjust the system until the SLM reads 94 dB (a band of pink noise from250 to 5000 Hz is excellent for this purpose) Now substitute the microphone to be tested for the SLM Take the microphone open circuit voltage reading on the micro-voltmeter 7. Nature of Response and Directional Characteristics A controlled directional response can be obtained by employing a sensing diaphragm, both faces of which are exposed to the sound field of interest Such diaphragms experience a driving force that depends on the spatial rate of change of pressure rather than on the pressure itself The diaphragm may be circular as in a capacitor or moving coil microphone or rectangular as in a ribbon microphone The principal axis of the microphone is directed perpendicular to the plane containing the diaphragm 8. Nature of Response and Directional Characteristics This axis makes an angle with the direction of the incident sound When has the value /2, both faces of the diaphragm experience identical pressures and the net driving force on the diaphragm is zero 9. Nature of Response and Directional Characteristics Now when is 0, the sound wave is incident normally on the diaphragm and the driving force on the left face of the diaphragm will be the pressure in the sound wave at the left faces location multiplied by the area of the left face The pressure difference can be calculated by taking the product of the space rate of change of acoustic pressure, known as the pressure gradient, with the effective acoustical distance separating the two sides of diaphragm 10. Nature of Response and Directional Characteristics 11. Frequency response of microphone Frequency response refers to the way a microphone responds to different frequencies It is a characteristic of all microphones that some frequencies are magnified and others are attenuated For example, a frequency response which favors high frequencies means that the resulting audio output will sound more trebly than the original sound For high quality instrument grade microphones a large flat range (20Hz to 20KHz) is required 12. Frequency response of microphone 13. Measurement Microphones Some microphones are intended for testing speakers, measuring noise levels and otherwise quantifying an acoustic experience These are calibrated transducers and are usually supplied with a calibration certificate that states absolute sensitivity against frequency Measurement microphones are dominantly air capacitor or electret capacitor microphones While ceramic piezoelectric units may still be encountered 14. Measurement Microphones The standard sizes in terms of capsule diameter are 1 inch, 1/2 inch, 1/4 inch, and 18 inch The larger units have higher sensitivity and lower noise floors The 1-inch unit is favored for making measurements in quiet environments at frequencies below about 8 kHz The inch unit is a general purpose one but has high frequency limitations Broad frequency band measurements usually require the 1/4 or 18 -inch variety 15. Measurement Microphones When the microphone capsules are smaller than 1/2 inch in diameter it is impossible to incorporate the necessary circuitry and connector in a uniform cylinder having a diameter equal to that of the capsule In such instances it is necessary to enclose the circuitry and connector in a larger cylinder that is joined to the capsule by a smoothly tapered section matching the larger diameter to the smaller diameter 16. Various Types of Microphones Various types of microphones are available in the market as listed below Moving coil(dynamic microphone) Ribbon Carbon microphone Condenser Electret Crystal(Piezoelectic microphone) 17. Various Types of Microphones Fiber optic microphone laser microphone Water microphone Microelectromechanical systems(MEMS microphone) 18. Moving coil(dynamic microphone) Dynamic microphones work via electromagnetic induction They are robust, relatively inexpensive and resistant to moisture With moving coil microphones a small movable induction coil is attached to the diaphragm When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying current in the coil through electro-magnetic induction 19. Moving coil(dynamic microphone) These are the advantages of this microphone type: - Relatively robust to mechanical stress - High SPL capability (useful when singing or playing loud instruments) - No supply voltage needed Due to the coil mass, moving coil microphones provide a limited reproduction spectrum and poor pulse behavior They are suitable for close miking, because non-linear distortions are rare with high sound pressure levels They are primarily used for live applications, sometimes also in the studio. 20. Moving coil(dynamic microphone) 21. Ribbon microphone Ribbon microphones use a thin, usually corrugated metal ribbon suspended in a magnetic field The ribbon is electrically connected to the microphone's output, and its vibration within the magnetic field generates the electrical signal Basic ribbon microphones detect sound in a bi- directional (also called figure-eight) pattern because the ribbon, which is open to sound both front and back, responds to the pressure gradient rather than the sound pressure 22. Ribbon microphone 23. Ribbon microphone 24. Carbon microphone A carbon microphone uses a capsule or button containing carbon granules pressed between two metal plates A voltage is applied across the metal plates, causing a small current to flow through the carbon One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules to change 25. Carbon microphone The changes in resistance cause a corresponding change in the current flowing through the microphone, producing the electrical signal Carbon microphones were once commonly used in telephones; they have extremely low-quality sound reproduction and a very limited frequency response range, but are very robust devices Unlike other microphone types, the carbon microphone can also be used as a type of amplifier, using a small amount of sound energy to control a larger amount of electrical energy 26. Carbon microphone 27. Condenser microphone The condenser microphone, invented at Bell Labs in 1916 by E. C. Wente is also called a capacitor microphone or electrostatic microphone capacitors were historically called condensers Here, the diaphragm acts as one plate of a capacitor, and the vibrations produce changes in the distance between the plates There are two types, depending on the method of extracting the audio signal from the transducer: DC- biased and radio frequency (RF) or high frequency (HF) condenser microphones 28. Condenser microphone With a DC-biased microphone, the plates are biased with a fixed charge (Q) The voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation (C = QV), where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts RF condenser microphones use a comparatively low RF voltage, generated by a low-noise oscillator. 29. Condenser microphone Condenser microphones span the range from telephone transmitters through inexpensive karaoke microphones to high-fidelity recording microphones They generally produce a high-quality audio signal and are now the popular choice in laboratory and recording studio applications They require a power source, provided either via microphone inputs on equipment from a small battery Power is necessary for establishing the capacitor plate voltage, and is also needed to power the microphone electronics 30. Condenser microphone 31. Electret microphone An electret microphone is a type of capacitor microphone The externally applied charge under condenser microphones is replaced by a permanent charge in an electret material An electret is a ferroelectric material that has been permanently electrically charged or polarized Nearly all cell-phone, computer, headset microphones are electret types 32. Electret microphone 33. Crystal microphone A crystal microphone microphone uses the phenomenon of piezoelectricity the ability of some materials to produce a voltage when subjected to pressure to convert vibrations into an electrical signal The high impedance of the crystal microphone made it very susceptible to handling noise, both from the microphone itself and from the connecting cable Piezoelectric transducers are often used as contact microphones to amplify sound from acoustic musical instruments, to sense drum hits, for triggering electronic samples, and to record sound in challenging environments, such as underwater under high pressure 34. Crystal microphone 35. Fiber optic microphone A fiber optic microphone converts acoustic waves into electrical signals by sensing changes in light intensity During operation, light from a laser source travels through an optical fiber to illuminate the surface of a reflective diaphragm Sound vibrations of the diaphragm modulate the intensity of light reflecting off the diaphragm in a specific direction The modulated light is then transmitted over a second optical fiber to a photo detector, which transforms the intensity-modulated light into analog or digital audio for transmission or recording 36. Fiber optic microphone Fiber optic microphones do not react to or influence any electrical, magnetic, electrostatic or radioactive fields Fiber optic microphones are robust, resistant to environmental changes in heat and moisture, and can be produced for any directionality or impedance matching The distance between the microphone's light source and its photo detector may be up to several kilometers without need for any preamplifier or other electrical device, making fiber optic microphones suitable for industrial and surveillance acoustic monitoring 37. Fiber optic microphone 38. Laser microphone Laser microphones are often portrayed in movies as spy gadgets, because they can be used to pick up sound at a distance from the microphone equipment A laser beam is aimed at the surface of a window or other plane surface that is affected by sound The vibrations of this surface change the angle at which the beam is reflected, and the motion of the laser spot from the returning beam is detected and converted to an audio signal 39. Laser microphone 40. Water microphone Early microphones did not produce intelligible speech, until Alexander Graham Bell made improvements including a variable resistance microphone/ transmitter Bell's liquid transmitter consisted of a metal cup filled with water with a small amount of sulfuric acid added A sound wave caused the diaphragm to move, forcing a needle to move up and down in the water The electrical resistance between the wire and the cup was then inversely proportional to the size of the water meniscus around the submerged needle 41. Water microphone The famous first phone conversation between Bell and Watson took place using a liquid microphone 42. MEMS microphone The MEMS (MicroElectrical-Mechanical System) microphone is also called a microphone chip or silicon microphone The pressure-sensitive diaphragm is etched directly into a silicon chip by MEMS techniques, and is usually accompanied with integrated preamplifier Most MEMS microphones are variants of the condenser microphone design Often MEMS microphones have built in analog-to-digital converter (ADC) circuits on the same CMOS chip making the chip a digital microphone and so more readily integrated with modern digital products 43. MEMS microphone 44. Loudspeaker working 45. Loudspeaker working The loudspeakers in radio, television or stereo system consists of a permanent magnet surrounding an electromagnet that is attached to the loudspeaker membrane or cone By varying the electric current through the wires around the electromagnet, the electromanget and the speaker cone can be made to move back and forth If the variation of the electric current is at the same frequencies of sound waves, the resulting vibration of the speaker cone will create sound waves, including that from voice and music 46. Various Types of Loudspeakers Crystal loudspeaker(Piezoelectric speakers) Dipole loudspeaker Electrostatic loudspeaker Dynamic loudspeaker Permanent magnet loudspeaker Woofers Midrange Tweeter 47. Crystal loudspeaker(Piezoelectric speakers) Piezoelectric speakers are frequently used as beepers in watches and other electronic devices Piezoelectric speakers are resistant to overloads that would normally destroy most high frequency drivers, and they can be used without a crossover due to their electrical properties There are also disadvantages: some amplifiers can oscillate when driving capacitive loads like most piezoelectric, which results in distortion or damage to the amplifier Frequency response is inferior to that of other technologies 48. Crystal loudspeaker(Piezoelectric speakers) Piezoelectric speakers can have extended high frequency output, and this is useful in some specialized circumstances; for instance, sonar applications in which piezoelectric variants are used as both output devices and input devices 49. Dipole loudspeaker A dipole speaker enclosure in its simplest form is constructed by mounting a loudspeaker driver on a flat panel The term dipole derives from the fact that the polar response consists of two lobes, with equal radiation forwards and backwards A dipole speaker works by creating air movement (as sound pressure waves) directly from the front and back surfaces of the driver 50. Dipole loudspeaker 51. Electrostatic loudspeaker An electrostatic loudspeaker is a loudspeaker design in which sound is generated by the force exerted on a membrane suspended in an electrostatic field The speakers use a thin flat diaphragm usually consisting of a plastic sheet coated with material such as graphite sandwiched between two electrically conductive grids, with a small air gap between the diaphragm and grids For low distortion operation, the diaphragm must operate with a constant charge on its surface, rather than with a constant voltage 52. Electrostatic loudspeaker By means of the conductive coating and an external high voltage supply the diaphragm is held at a DC potential of several kilovolts with respect to the grids The grids are driven by the audio signal; front and rear grid are driven in anti phase As a result a uniform electrostatic field proportional to the audio signal is produced between both grids. This causes a force to be exerted on the charged diaphragm, and its resulting movement drives the air on either side of it 53. Electrostatic loudspeaker 54. Dynamic loudspeaker These are the most common form of loudspeakers The voice coil in moving coil drivers is suspended in a magnetic field provided by the loudspeaker magnet structure As electric current flows through the voice coil (from an amplifier), the magnetic field created by the coil reacts against the magnet's fixed field and moves the voice coil (and so the cone) Alternating current will move the cone back and forth 55. Dynamic loudspeaker 56. Permanent magnet loudspeaker Tweeters produce the higher frequency spectrum sounds. High frequency sounds are in the range of 2000 Hz and up. Tweeters are very small and normally around 1 inch in diameter Midrange midrange drivers produce sounds in the midrange frequency. This is usually in the range of 200 500 Hz up to 2000 3000 Hz. Midrange speakers size range from 4 inches in diameter up to 8 inches in diameter Woofers produce the lower frequency sounds, but dont necessarily have to go to bottom of the spectrum. Their frequency range is typically from 500 Hz down to 80 Hz or below. Woofers speakers size range from 8 to 12 inches in diameter 57. Permanent magnet loudspeaker Subwoofers produce sounds in the lowest frequency range typically at 80 Hz and below. Ten inches and up are typical sizes for subwoofer speakers. Subwoofers are usually standalone speakers that get placed in a corner. The reason for their large size is the considerable amount of power required to produce low frequency sounds. Therefore, a large amplifier with its own power source is needed to give enough power to the subwoofer. 58. Permanent magnet loudspeaker 59. Characteristic Impedance of Loud Speakers The characteristic impedance is the ratio of the effective sound pressure to the particle velocity at that point in a free, plane, progressive sound wave It is equal to the product of the density of the medium times the speed of sound in the medium ( p0C ) It is analogous to the characteristic impedance of an infinitely long, dissipation-less, transmission line The unit is the Rayl, or Newtons/m3 60. Headphones types Moving Iron Moving Coil Electrodynamics Orthodynamic Electrostatic Electrets High Polymer 61. Moving Iron Early headphones relied on many turns of wire wound on to a magnetic yoke held close to a stiff disc made of a soft magnetic alloy A permanent magnet pulled the thin disc toward the yoke with a constant force and audio signals fed to the coil caused this force to vary in sympathy with the input They were very sensitive, needing hardly any power to drive them, and were very poor in sound quality due to the high mass and stiffness of the diaphragm 62. Moving Iron 63. Moving Coil Moving-coil headphones work in exactly the same way as moving-coil loudspeakers The major difference, of course, between moving-coil headphones and loudspeakers is that the former are much smaller, with lighter and more responsive diaphragms They can consequently sound much more open and detailed than loudspeakers using the moving-coil principle 64. Moving Coil 65. Electrodynamics Orthodynamic This type of headphone is the same family as the moving-coil type, except that the coil has been unwound and fixed to a thin, light, plastics diaphragm The ring shaped magnetic gap has been replaced by opposing bar magnets, which cause the magnetic field to be squashed parallel to the diaphragm The coil is now a thin conductor zig-zagging or spiraling its way across the surface of the diaphragm Unlike the moving coil type, this headphone has flat diaphragm 66. Electrodynamics Orthodynamic 67. Electrostatic The diaphragm is stretched under low mechanical tension between two perforated conductive plates to which the audio signals are fed via a step-up transformer The central diaphragm is kept charged to a very high voltage with respect to the outer plates using a special type of power supply and hence it contains a static charge and is light The diaphragm experiences electrostatic attraction toward both outer plates 68. Electrostatic The spacing between the plates and diaphragm, the voltage between them, and the tension on the diaphragm are all chosen carefully so that the film does not collapse on to either plate When an audio signal is fed to the transformer, it is stepped up at the secondary from a few volts to around a thousand volts This unbalances the forces on the diaphragm in sympathy with the audio signal, causing it to be attracted alternately to each plate and of course reproducing an analogue of the original sound 69. Electrostatic 70. Electrets Basically, the electret headphone is an electrostatic type but using a material that permanently retains electrostatic charge The electret has the advantages of the conventional electrostatic, but does not require an additional external power supply It is similarly restricted in maximum sound pressure level, although both types produce perfectly adequate sound pressure levels with conventional power amplifiers 71. Electrets 72. High Polymer High polymer is basically a generic name to cover piezoelectric plastics films such as polyvinylidene fluoride film High polymer films are very thin, some 8 to 300 m, and have very low mechanical stiffness, which makes them ideal for transducer diaphragms The basic film is made piezo electric by stretching it to up to four times its original length, depositing aluminum on each side for electrodes, and polarizing with a high DC electric field at 80100oC for about an hour 73. High Polymer When voltage is later applied across the film, it vibrates in a transverse direction, becoming alternately longer and shorter If the material is shaped into an arc, this lengthening and shortening are translated into a pulsating movement, which will generate sound waves in sympathy with the electrical input signal The high polymer headphone is also claimed to be much more sensitive than the electrostatic type and unaffected by humidity 74. High Polymer 75. The Basics of Magnetic Recording A sound recording is made onto magnetic tape by drawing the tape past a recording head at a constant speed The recording head (which is essentially an electromagnet) is energized by the recording amplifier of the tape recorder The electromagnet, which forms the head itself, has a small gap so that the magnetic flux created by the action of the current in the electromagnets coil is concentrated at this gap 76. The Basics of Magnetic Recording Because the tape moves and the energizing signal changes with time, a record of the flux at any given time is stored on the tape Replaying a magnetic tape involves dragging the tape back across a similar electromagnet called the playback head 77. The Basics of Magnetic Recording The changing flux detected at the minute gap in the playback head causes a current to flow in the heads coil This is applied to an amplifier to recover the information left on the tape Thus in a tape recording, sound signals are recorded as a magnetic pattern along the length of the tape This pattern is created due to the coating made of ferric iron oxide or chromium dioxide on the tape which possesses magnetic properties 78. B-H Curve The relation between the magnetizing field (H) and the resultant induction (B in an iron sample(assumed, initially, to be in a completely demagnetized condition) may be plotted as shown in Figure 79. Bias for magnetic tapes If a sound recording and reproduction system is to perform without adding distortion, a high degree of linearity is required From the B-H curve, it is apparent that the only linear region over which this relationship holds is between B1 and B2 For other regions, to get a distortion free output biasing is used In principle, a steady magnetic force may be applied, in conjunction with the varying force dependent on the audio signal, thereby biasing the audio signal portion of the overall magnetic effect into the initial linear region of the BH loop 80. Bias for magnetic tapes In other technique a system of ultrasonic AC bias is employed, which mixes the audio signal with a high- frequency signal current This bias signal, as it is known, does not get recorded because the wavelength of the signal is so small that the magnetic domains resulting from it neutralize themselves 81. Sound Card A sound card is an internal computer expansion card that facilitates the input and output of audio signals to and from a computer under control of computer programs Typical uses of sound cards include providing the audio component for multimedia applications such as music composition, editing video or audio, presentation, education and entertainment (games) and video projection 82. Sound Card Typical blocks of a sound card are as below ADC DAC PCI(Peripheral Controller Interface) I/O ports DSP(Digital Signal Processor) Memory 83. Sound Mixers A sound mixer is a device which takes two or more audio signals, mixes them together and provides one or more output signals As well as combining signals, mixers allow you to adjust levels, enhance sound with equalization and effects, create monitor feeds, record various mixes, etc 84. Sound Mixers Some of the most common uses for sound mixers include: Music studios and live performances: Combining different instruments into a stereo master mix and additional monitoring mixes. Television studios: Combining sound from microphones, tape machines and other sources. Field shoots: Combining multiple microphones into 2 or 4 channels for easier recording. 85. Sound Mixers Input channel controls Input Gain / Attenuation: The level of the signal as it enters the channel. In most cases this will be a pot (potentiometer) knob which adjusts the level Phantom Power: Turns phantom power on or off for the channel Equalization: Most mixers have at least two EQ controls (high and low frequencies). Auxiliary Channels: Auxiliary channels are a way to send a "copy" of the channel signal somewhere else to provide separate monitor feeds or to add effects Pan & Assignment: Each channel can be panned left or right on the master mix. Advanced mixers also allow the channel to be "assigned" in various ways 86. Sound Mixers Solo / Mute / PFL: These switches control how the channel is monitored. They do not affect the actual output of the channel Channel On / Off: Turns the entire channel on or off. Slider: The level of the channel signal as it leaves the channel and heads to the next stage (subgroup or master mix). 87. PA Systems & Installations A public address system (PA system) is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source 88. Digital Consoles Digital Console is an electronic device for combining, routing, and changing the dynamics of digital audio samples Digital mixing consoles are typically used in recording studios, public address systems, sound reinforcement systems, broadcasting, television, and film post- production 89. Digital Consoles Advantages: There is no added noise, distortion, or other signal degradation while the signal is in the digital domain, between the output of the analog to digital converter (ADC) and the input to the digital to analog converter (DAC) Aux sends can be mixed on the main faders rather than on a row of potentiometers Signal routing is often much more flexible than with an analog-based console 90. Digital Consoles Advantages: The setup of the console can be saved and loaded at will. This is particularly useful in live events where a setup for each band can be largely prepared in advance, saved, and then loaded as needed There are typically many on-board effects and virtual signal processors available, eliminating the need for additional hardware modules, and the associated cost, size, weight, cabling, signal quality issues, etc 91. Digital Consoles Disadvantages: There is an analog to digital conversion, then processing of the signal, then again digital to analog conversion, which degrades the sound quality. This is subject to debate, since the quality degradation is not always noticeable The number of faders is often less than the number of input channels. The extra input channels are not accessible until a bank of faders is switched to control them 92. Digital Consoles Disadvantages: Digital conversion and processing adds latency, or delay, into the signal The act of making adjustments is often slower for compact digital mixers which require the user to page through one or more layers of commands before reaching the desired control