Section 1.1: Microphones

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Section 1.1: Microphones 1 2 3 4 5 6 • Microphone Design • Polar Response • Frequency Response • Sensitivity and Self Noise • SNR’s and SPL’s • Dynamic range, Transient response and Impedance GrassCA © 1 Section 1.1: Microphones Transducer: Any device or element which converts an input signal into an output signal of a different form. An electrical device which receives and transforms electrical energy into another form, such as magnetic material and winding used in acoustic transmitters and receivers. . Acoustic Mechanical Electrical Electrical Mechanical Acoustic GrassCA © 2 1.1: Microphone Design: Dynamic Dynamic microphones: versatile and ideal for general-purpose use. A simple design with few moving parts and resilient to rough handling. Suited to handling high SPL’s from certain musical instruments or amplified speaker cabs. No internal amplifier and do not require batteries or external power. The diaphragm is attached to the coil. The diaphragm vibrates in response to incoming sound waves, the coil moves backwards and forwards past the magnet. This creates a current in the coil which is channelled from the microphone via wires to an XLR pin and then to the mic pre-amp where signal boosting is required depending on the acoustic energy being received GrassCA © 3 1.1: Microphone Design: Dynamic Dynamics do not usually have the same flat frequency response as condensers. Instead they tend to have tailored frequency responses for particular applications. Neodynium magnets are more powerful than conventional magnetism. Neodynium microphones can be made smaller, with more linear frequency response and higher output level. GrassCA © 4 1.1: Microphone Design: Capacitor A capacitor to convert acoustical energy into electrical energy. An electronic component which stores energy in the form of an electrostatic field. A voltage is required across the capacitor for this to work. They require power from a battery or external source.(phantom power + 48volts) The resulting audio signal is stronger signal than that from a dynamic mic. Condensers also tend to be more sensitive and responsive than dynamics, making them well-suited to capturing subtle nuances or higher frequencies in a sound. They are not ideal for high SPL capture, as their sensitivity makes them prone to distort. Generally they have a flatter frequency response than dynamics and ribbons. GrassCA © 5 1.1: Microphone Design: Capacitor A capacitor has two plates with a voltage between them. In the condenser mic, one of these plates is made of very light material and acts as the diaphragm. The diaphragm vibrates from sound waves, changing the distance between the two plates and therefore changing the capacitance charge. Specifically, when the plates are closer together, capacitance increases and a charge current occurs. When the plates are further apart, capacitance decreases and a discharge current occurs. GrassCA © 6 1.1: Microphone Design: Capacitor(Phantom power) DIN 45596 DC electrical power through mic cables to operate/supply the active electronic circuitry. Commonly found as individual power source for each mic pre –amp or in banks of 8 in some digital consoles. The IEC standard gives 10 m Amps as the maximum allowed current per microphone. Normally 2 – 8 mA is required depending on Manufacturer. DI boxes require no more than 10mA and also have relevant Specs. GrassCA © 7 1.1: Microphone Design: Ribbon Ribbon mics are a type of 'velocity' or 'pressure gradient' microphone. Industry standard for recording and broadcast from about 1920 to 1950 and are one of the defining factors in the recordings from that period . Ribbons tend to emphasise the warm low-mids and gradually roll off at the top end. This gives them what at first listen can seem a dull sound when compared with capacitor microphones. Ribbon mics are coming back into favour due to some cheaper types being manufactured and the want for more retro sounding recordings by artists and producers. (recreated colouration) The frequency response of ribbons tends to be very flat in the lower midrange due to the lack of resonances within the ribbon elements themselves GrassCA © 8 1.1: Microphone Design: Ribbon Ribbon microphones employ electromagnetic induction to convert sound to voltage. A long thin strip of conductive foil moves within a magnetic field to generate a current hence voltage. The foil’s lower weight when compared to a moving coil gives it a smoother and higher frequency response. However the relatively low output requires a step up transformer. GrassCA © 9 1.2: Microphone design: Shotgun(Rifle) The shotgun is extremely sensitive along the main axis, but posseses pronounced extra lobes which vary drastically with frequency A shotgun mic takes directionality to the extreme by mounting the diaphragm halfway down a tube The frequency response of this mic is so bad it is usually electronically restricted to the voice range, where it is used to record dialogue for film & video GrassCA © 10 1.2: Mic Specification: Polar response(Pick-up pattern) Omni-directional • In practice a mic cannot achieve the theoretical equal pick up from 360 °. Used for ambience sometimes Cardioid (directional) • Most common pattern used in dynamics for close mic’ing . Drums, cabs, vocals, brass and acoustic guitars. Figure of eight (directional) • Single diaphragm , dual sided. Offers rejection from 90 and 270 degree off-axis sources GrassCA © 11 1.2: Mic Specification: Polar response Supercardioid Hypercardioid The directional response of a microphone refers to it’s sensitivity (output level) at various angles of incidence with respect to on-axis energy from the front-side of the mic. GrassCA © 12 1.2: Mic Specification: Polar response Bi-directional Shotgun Directional response displayed on a chart showing the mics sensitivity with respect to direction (and frequency) known as a polar plot or polar pattern. GrassCA © 13 1.2: Mic Specification: Cardioid vs Omni GrassCA © 14 1.2: Mic Specification: Polar response(Shotgun) Rifle mic: Sounds arriving of – axis are effectively cancelled due to phase distortion. Cardioid capsule used. Low frequency Low mid frequency Wanted range Wild life recording, sports capture, dialogue and broadcasting applications. LINK : Parabolic Mics GrassCA © 15 ” 1.2: Mic Specification: Frequency/Polar response Polar response is a function of frequency for directional mics, conversely the frequency response will be a function of polar angle. This is known as “off-axis colouration”. A result of this for cardioid mics is that sounds arriving from say 450 will be reproduced with HF loss(attenuation) whilst those from the rear of the mic will have relatively amplified LF LINK: Frequency Response GrassCA © 16 1.2: Mic Specification: Frequency response(Dynamic) SM 57 Instrument mic - Sharper roll-off 6 dB/octave - Prescence @ 5, 6 + 7 KHz - Attenuation @ 9.5 – 17.5 kHz SM 58 Vocal mic with pop grill - Vocal prescence @ 2, 3 - 4KHz - Smooth LPF roll-off 3 dB/oct - Attenuation @ 9- 15 kHz GrassCA © 17 1.2: Mic Specification: Frequency response(Capacitor) Audio Technica 4040 - Flat response 300hz to 3kHz - Prescence @ 6.5, and 11 KHz - Attenuation @ 9.5 – 17.5 kHz -Vocals , ac. Guitars - Versatile Audio Technica AE 3000 - Prescence @ 6 and 8KHz - Smooth LPF roll-off 3 dB/oct - Dip of 2 dB @ 450 Hz - Guitar cabs, toms and snare, - Timpani and overheads - High SPL’s GrassCA © 18 1.2: Mic Specification: Frequency response(Ribbon) It can be seen from frequency response graph that this mic will provide low- mid warmth and a slight lack of high frequency pickup. Could be good for guitar cabs or drum overheads. (colouration or sonic palette) GrassCA © 19 1.2: Mic Specification: Sensitivity An indication of electrical output obtained for a given Sound Pressure Level(SPL). The standard is either 74 or 94 dB. One level is 10 times greater(20dB). 74 db( = 1uB microbar), 94 dB (= 10uB or 1 Pascal) Sensitivity is usually expressed in V/Pa at 1 kHz. It is the output voltage measured when a sound wave is detected by the microphone in a specified load condition A capacitor mic may have a figure of -60 dBV Pa -1 meaning that its output level is 60 dB below 1 volt for a 94 dB SPL input - 62dB(0dB=1 V/µ bar) = - 42dB(0dB= 1 V/Pa). GrassCA © 20 1.2: Specification testing: Weightings Equal Loudness Curves represent the ears non-linearity to perception: less sensitive to LF and HF than in the upper midrange. This variation is dependent upon the sound intensity (SPL). The Fletcher-Munson curve (commonly known) shows the variation, and it is clear that any loss of sensitivity is highly dependent upon the actual SPL. GrassCA © 21 1.2: Specification testing: A - Weightings A-weighting is generally applied only to measurements of noise, it is essentially a tailored bandpass filter, having a defined roll-off above and below the centre frequency. The reference point is at 1kHz, where the gain is 0dB. The filter response is supposed to be the inverse of one of the curves of the equal loudness graph (Fletcher Munsen Curve/Equal Loudness contours/ Auditory perception) Frequency Response of the A-Weighting Filter GrassCA © 22 1.2: Mic Specification: Sensitivity Capacitor (Most sensetive): 5-18 mV Pa - ¹ : An SPL of 94 dB will give 5-15mVolts of electrical output - Neumann TL M 103: 23mV/Pa - Try and do your own research Ribbon (Least Sensetive): 1 – 2 mV Pa - ¹ will yield 15-20 decibels less than a capacitor microphone - Try and do your own research - Try and do your own research Dynamic: 1.5 – 3mV Pa - ¹ will give up to 3 m Volts of electrical output with 94 dB SPL at the input - Neumann KA 184 :15mV /Pa - Shure SM 57 : - 56.0 dBV/Pa* (1.6 mV) / -41 dBV/Pa GrassCA © 23 1.2: Mic Specification: Sensitivity More amplification is needed to bring ribbon mics and dynamics up to line level than is the case with capacitors: Example: 1) Speech may yield 0.15 mV from a ribbon mic, so to amplify this to 775 mV requires a gain of 5160 or 74 dB 2) A capacitor mic of 1mV uB-1 sensitivity requires 775 times the amplification or 57dB of gain Any noise (electrical interference) will have a better chance of being unheard with a more sensetive mic. This illustrates that high quality mixers and microphone cables are required to get the best out of low-output mics. GrassCA © 24 1.2: Mic Specification: Equivalent self-noise Microphone noise figures are A-weighted self- noise. A typical value of a quality capacitor mic is around 18dBA. This means that its output voltage noise is equivalent to the mic being placed in a soundfield with a loudness level of 18dBA A mic with a self-noise in the region of 25dBA is very poor. If it was used to capture dialogue say from 2 metres, the hiss(noise) would be apparent in the recording. The noise in a capacitor mic comes from the head amplifier. Ribbons and moving coil dynamics are passive ,not noiseless. A 200 ohm passive resistance at 20° C generates a noise output between 20hz and 20kHz of 0.26uVolts. This is due to thermal excitation of the coil and windings. GrassCA © 25 1.2: Mic Specification: Equivalent self-noise . Average self-noise ratings depend on capsule size, which is the primary factor. 1/2" mics are generally 12dBA - 16dBA self-noise 1" mics, 6dBA - 10dBA. Very small diaphragm mics (1/4" / 6mm or less) will be more like 20dBA - 24dBATube mics may have higher ratings; very old designs tend to be higher as well. Sens = 2mV at 94 dBSPL, noise = 0.00026mV » 2 ÷ 0.00026 = 7600 dB 20 log 7600= 77dB;; 94-77 = 17dBA The best capacitor mics have figures of 6 dBA SPL GrassCA © 26 1.2: Mic Specification: Signal to noise ratio Example: Signal/noise ratio (A-weighted): 76 dB Max SPL = The point where the mic distorts, or clips the waveform. [More=better] Self noise = The amount of noise the mic creates all on it's own. (i.e. hiss). [Less=better] Dynamic range = The range between self noise and Max SPL. [More=better] Signal to Noise ratio = The range between self noise and a reference signal. [More=better] GrassCA © 27 1.2: Mic Specification: Maximum SPL This is a specification which indicates the amount of SPL a mic can handle before a specified level of distortion sets in For example the rode nt2a has a maximum SPL of 147db@1%THD Obviously if a mic can withstand high SPLs it would be more suitable for drum Mic’ing for example. Some mics have a switch which attenuates the signal from the capsule before it enters the internal pre amp. This switch is known as a pad and allows a useful extension to a mic’s maximum SPL GrassCA © 28 1.2: Mic Specification: Dynamic range Manufacturer s choose the amount of distortion to specify the SPL at, as long as the THD appears in the printed literature. Normally, microphone manufacturers all over the world specify the equivalent sound pressure level at 0.5% THD Audio equipment dynamic range is calculated as the difference between the total noise floor (measured in dBA and the equivalent sound pressure level (measured in dB), where a certain amount of total harmonic distortion appears (the maximum SPL) GrassCA © 29 1.2: Mic Specification: Transient Response The transient response of a mic (which has no accepted measure) is a measure of how quickly a mic’s diaphragm will react when it is hit by an acoustic wavefront. Example: the weight of a dynamic mics coil + diaphragm make for a large mass compared to the small power of a sound wave. This makes dynamic mics slow to react. Condenser mics on the other hand , having gold sprayed Mylar diaphragms9 extremely light0, offering very little mechanical resistance to an acoustic wave and can track the wavefront very accurately. Condenser Microphone Dynamic Microphone GrassCA © 30 1.2: Mic Specification: Impedance(Z) Impedance : The amount of opposition a device has to an AC current (such as an audio signal). Technically speaking, it is the combined effect of capacitance, inductance, and resistance on a signal. Low Impedance (less than 600Ω) Medium Impedance (600Ω - 10,000Ω) High Impedance (greater than 10,000Ω) Hi-Z or Low-Z. Impedance is measured in ohms, shown with the Greek Omega symbol Ω. A microphone with the specification 600Ω has an impedance of 600 ohms which should hopefully match a mixer pre-amp. Mixer pre-amps also have an ohms rating. Be aware that what one system calls "low impedance" may not be the same as your low impedance microphone . The ohms value must be known exactly. GrassCA © 31 The End GrassCA © 32