Microfono capacitivo

Transcript

1 Edoardo Milotti 22//27 Microfono capacitivo In questa semplice esperienza analizziamo il comportamento di un microfono capacitivo. Un microfono capacitivo è un condensatore in cui una delle armature è elastica, e il movimento dell armatura dovuto al suono modula la capacità. Si consideri dunque un circuito RC che viene alimentato da un generatore AC, e in cui la capacità può variare lentamente nel tempo: questo significa che il periodo di un oscillazione del generatore è molto minore del tempo caratteristico di oscillazione dell onda sonora: in questo modo possiamo utilizzare la soluzione standard del circuito RC. Sappiamo che la tensione ai capi del condensatore è ( + i! RC) +! 2 R 2 C 2 ˆV e "i arctan! RC dove la notazione utilizzata è la stessa contenuta nella nota sul circuito RC discussa a lezione. Supponiamo ora che la capacità possa variare, che il valore centrale della capacità sia C, e che le variazioni siano indicate da!c. Allora # ( ) 2 ˆV # +! 2 R 2 C + "C $! 2 R 2 C ' +! 2 R 2 2 C +! 2 R 2 2 % & C ( ) "C +! 2 R 2 C 2 + 2! 2 R 2 C "C Supponiamo ora di alimentare il generatore proprio con la frequenza di taglio che corrisponde alla capacità centrale! 3dB RC, e allora troviamo! "C 3/2 2 C

2 Domanda: come si dimostra che questa scelta massimizza la sensibilità del circuito alle variazioni esterne? Se la variazione viene indotta da una variazione di distanza tra le armature del condensatore, allora sappiamo che C!S d ; C e quindi!s d + "d # C % $ "d ( & ' ) * d! "C! # "d 3/2 3/2 2 C 2 d Domanda: supponendo di utilizzare il condensatore come microfono, come si fa ad massimizzare la sua sensibilità? Esempio: condensatore ad aria con armature quadrate di 5 mm di lato, e distanza tra le armature di.5 mm. Si ricordi che! " #2 F m -, allora C! " F, e dunque se si vuole che la frequenza di taglio stia a MHz, si deve avere! 3dB 2" ( MHz) RC e quindi R " 3.6 k# 2! ( MHz)C Il segnale in uscita è modulato in ampiezza, e lo si può demodulare come al solito con un diodo, come nello schema seguente:

3 Nello schema riportato sopra D è un diodo per radiofrequenza e C2 è un condensatore di disaccoppiamento con una capacità elevata Questo tipo di schema viene implementato commercialmente in alcuni microfoni di qualità, come i microfoni della serie MKH della Sennheiser (v. scheda allegata)

4 Microphones RF Condenser Microphones.47 Operating Principle of the RF Condenser Microphone RF condenser microphones constitute a unique, sophisticated class of condenser microphones. They fulfil the very highest demands made on sound quality and versatility, and are renowned for their high quality, highly accurate sound reproduction, ruggedness, all-weather operation, wide frequency response and db Studio condenser microphone MKH 4 extremely low inherent self-noise. Usually, a high capsule polarisation voltage would be necessary, however, RF condenser microphones use a comparatively low RF voltage, generated by a lownoise oscillator. It is modulated by the capacitance changes produced by the sound waves moving the capsule diaphragm. Following the demodulation, a low-noise audio frequency signal with very low source impedance is available, which can be used to directly drive ordinary bipolar transistors. As bipolar transistors produce less random noise than the field-effect transistors usually needed, electrical equalisation technology can be included to increase the microphone s dynamic range and linearise its frequency response. As a result, an excellent low frequency response can be achieved even with small capsules, which would normally only be possible using larger capsules. In addition, small capsules achieve a considerably better high frequency response than large capsules. The RF process results in a considerable reduction of the electrical impedance of the capsule and enables RF condenser microphones to be operated in all weather conditions. A further advantage of this process is that an ungrounded audio frequency signal is available although the capsule is grounded. This is why all RF condenser microphones possess a genuine fully floating, balanced output without having to use an audio frequency transformer. 4 Hz Hz 2 Hz 5 Hz khz 2 khz 5 khz khz 2 khz One-third octave band analysis of the MKH 4 inherent noise compared to a conventional low-noise studio condenser microphone % Other studio condenser microphones Correlation of frequency and intermodulation distortion MKH 4 5 k 2k 5k k 2kHz Sennheiser s Revolutionary Symmetrical Microphone Capsule The MKH 2 to 8 microphones have Sennheiser s unique and revolutionary symmetrical push-pull capsule. Besides the normal back plate, this capsule is fitted with an additional front plate, with the diaphragm being positioned between them. Both plates are acoustically transparent. Any impedance changes in the air gaps between the diaphragm and the respective plate are opposite and therefore cancel each other out, which leads to an unchanging acoustic impedance. This balanced design produces extremely low distortion figures and a higher capsule output that gives a much lower noise figure. The result is a microphone series unmatched in clarity and neutrality. Sennheiser s symmetrical push-pull transducer Power Supply Units All RF condenser microphones use 48 V phantom powering (P 48). The TU-3 version of the MKH 46 operates using 2 V AB (T) powering. All models are powered externally via the microphone cable. The MZA 4 P 48 U (or MZA 4 TU) battery unit is available for both types of powering. Connections All RF condenser microphones are fitted with a 3-pin XLR connector as standard. Standard polarity results in a positive voltage at pin 2 against pin 3, when pressure increases in front of the diaphragm. MZA 4 Publ. 6/2

5 Microphones RF Symmetrical Capsule Condenser Microphone The MKH 2 is usually used as a main microphone for orchestras and solo instruments, and picks up the natural acoustics of the room, though it does have a much wider range of applications as omni-directional microphones do not suffer from proximity effect. Features Exceptionally flat frequency response, switchable for near and diffuse field applications High-frequency response can be accentuated by means of a pressure build-up ring Slightly increasing directivity at high frequencies to adjust tonal balance Exceptionally low inherent self-noise Transformerless and fully floating balanced output Switchable pre-attenuation Black, anodised light metal body Delivery includes: MKH 2, MZS 4 shock mount, MZW 4 windshield, pressure build-up ring Effect of diffuse-field correction.5 MKH 2 Cat. No Recommended Accessories Battery power supply unit MZA 4 P 48 U Cat. No. 296 Table stand MZT 44 Cat. No. 799 MZT Cat. No. 883 Dual clip MZD 3, for MS stereo Cat. No. 299 Fish pole MZB 45 Cat. No. 972 Shock mount MZS 4 Cat. No. 37 Foam windshield MZW 4 Cat. No Popshield MZP 4 Cat. No. 332 Shock mount MZS 2- Cat. No. 369 Basket windshield MZW 2- Cat. No. 366 Hairy cover MZH 2- Cat. No. 36 Product Variants Cable pictured is an accessory not supplied with the microphone Technical Data Pick-up pattern omni-directional Frequency response 2 2, Hz Sensitivity (free field, no load) ( khz) 25 (8) mv/pa ± db Nominal impedance 5 Ω Min. terminating impedance kω Equivalent noise level A-weighted (DIN IEC 65) (8) db CCIR-weighted (CCIR 468-3) 2 (28) db Max. sound pressure level Power supply Supply current 34 (42) db at khz phantom 48 ± 4 V 2 ma Dimensions in mm Ø 25 x 53 Weight approx. g Values in parentheses with attenuator switched on ( db) Profile Omni-directional studio microphone with switchable diffuse-field correction, switchable pre-attenuation and pressure build-up ring, frequency response 2 Hz 2, Hz, sensitivity (free field, no load) 25 (8) mv/pa ± db at khz, nominal impedance 5 Ω, min. terminating impedance kω, equivalent noise level A-weighted (8) db, CCIR-weighted 2 (28) db, max. SPL 34 (42) db at khz, phantom powering 48 ± 4 V, supply current 2 ma, dimensions in mm Ø 25 x 53, weight approx. g. Values in parentheses with attenuator switched on ( db). Publ. 6/2