MIT 21M.380 Session 3: Microphones

21m.380 · Music and Technology Recording Techniques & Audio Production Microphones Session 3 1 Student presentation (pa1) • 2 Microphone specifications • Frequency range (Klepko 2004, p. 122) • Frequency response (Shure 2012a) • Self noise (Shure 2012b) or equivalent noise level (Williams 2010) • (Output) sensitivity (Dpa 2008–2013; Shure 2012b; Williams 2010) • Maximum sound pressure level (Dpa 2008–2013; Shure 2012b) • Dynamic range (Shure 2012b) • Polar pattern (directivity; discussed in detail later today) 3 Electric quantities Quantity Symbol Unit 𝑉 𝑃 V W Voltage Electric power 3.1 Nature Field quantity Energy quantity Table 1. Electric quantities Voltage 𝑉 and voltage level 𝐿𝑉 Common references 𝑉0 : 𝐿𝑉 = 20 ⋅ log10 ( • 1 V ≡ 0 dBV (electronics) • 0.7746 V ≡ 0 dBu (audio equipment) 3.2 Electric power 𝑃 and electric power level 𝐿𝑊 𝑉 ) 𝑉0 Equation 1. Voltage level 𝐿𝑉 𝐿𝑊 = 10 ⋅ log10 ( Common references 𝑃0 : • 1 W ≡ 0 dB (loudspeakers) 𝑃 ) 𝑃0 Equation 2. Electric power level 𝐿𝑊 • 1 mW ≡ 0 dBm (telephone equipment) 1 of 8 3.3 Doubling field quantities 1 Cf., Shure 2012c. • Doubling a field quantity (e.g., 𝑝, 𝑉) results in 6 dB increase • See slides for derivation 3.4 Doubling energy quantities 𝑝 (𝑡) → 𝑉 (𝑡) → 𝑝 (𝑡) • Doubling an energy quantity (e.g., 𝐼, 𝑃) results in 3 dB increase • See slides for derivation 4 Electroacoustic transducer principles1 Microphone type Power needed? Sound quality Robustness Dynamic microphones (electromagnetic induction) Moving coil no medium/good Ribbon no (very) good robust fragile Condenser microphones (capacitance) Regular condenser yes Electret condenser yes excellent (very) good fragile less fragile Piezo microphones (piezoelectric effect) Contact (pickup) mic no low robust • Job of a microphone: convert mechanical pressure waves to electrical signals (and vice versa for loudspeaker) • But how to do this? By exploiting one of several physical phenomena: – Electromagnetic induction – Capacitance – Piezoelectric effect 5 5.1 Dynamic microphones Electromagnetic induction • General: Current is induced in a (closed-loop) conductor that moves relative to a magnetic field. • Dynamic mics: diaphragm as/on conductor oscillates in magnetic field 5.2 Moving coil microphones • Diaphragm mounted on top of coil (conductor) • Available in moss: Shure Beta 58a, Shure sm57, Sennheiser md 421-ii, Sennheiser e 604, Audix d6, Blue Microphones encore 200, AudioTechnica atm250de (one of two capsules) 2 of 8 Figure 1. Electroacoustic transduction process Table 2. Comparison of different microphones by transducer principle 5.3 Ribbon microphones • Diaphragm is the conductor • Better high-frequency response can be achieved than with moving coil (less mass) • Ribbon microphones are very fragile (sensitive to wind!) • Available in moss: Royer r-101 6 Condenser microphones 6.1 Capacitance • General: An electric charge can be stored by a dual-plate capacitor • Condenser microphones: – Diaphragm forms one of two plates of a dual-plate capacitor – Movement of diaphragm changes distance between plates (and thus output voltage) 6.2 Large vs. small diaphragm condensers • Small-diaphragm condensers often cigar-shaped • Large-diaphragm condensers typically addressed from the side • But can’t always tell difference from outside (e.g., Neumann m50 vs. m49) • Small diaphragm condensers feature more neutral frequency response and polar pattern • Colored hf response of large-diaphragm mics can be desirable (vocals) • Available in moss: – Large-diaphragm condensers: Akg c 414 xl ii, Mojave Audio ma-200 – Small-diaphragm condensers: Audio-Technica at4041, Earthworks tc20, Audio-Technica atm250de (one of two capsules) 6.3 Tube condensers • Typically come with their own power supply • Operate best if warmed up before use • Often mounted upside down (to dissipate heat away from diaphragm) • Available in moss: Mojave Audio ma-200 3 of 8 6.4 Electret condensers • Pre-charged capacitor • Popular where small capsules are required (e.g., lavalier or in-ear mics) • Available in moss: Audio-Technica atm250de (one of two capsules) 6.5 Phantom (and other kinds of) power • Phantom power provided by external preamp through mic cable • Ideally +48 V, but most mics accept anything from 12 V to 52 V • Two reasons for phantom power: 1. Charge capacitor of a condenser microphone 2. Power internal preamp to amplify low-level output signal • Rule of 👍: Condensers need phantom, dynamic mics don’t. • Reality is more complex: – Tube condensers such as Mojave ma200 in moss come with own power supply (and hence do not need phantom from preamp) – Blue encore 200 in moss is an active dynamic mic (requires phantom) – Ribbon mics are dynamic, so generally do not require phantom (e.g., Royer r-101 in moss), but active ribbon mics do (e.g., Royer r-122) – Electret condenser come with pre-charged capacitor, but to amplify signal might still require 48 V (e.g., Audio-Technica atm250de in moss), or 3 V plug-in power from audio input or battery 7 7.1 Piezo microphones Piezoelectric effect • General: Certain materials (e.g., crytals) generate an electric charge when mechanical stress is applied to them. • Piezo mics: Mechanical stress conveyed through sound pressure wave 7.2 Contact microphones • Attached directly to a solid vibrating body • None available in moss 7.3 Hydrophones • For underwater recordings • None available in moss 4 of 8 (a) Omni (b) Wide cardioid + + − − (c) Open cardioid (d) Cardioid + + − − (e) Supercardioid (f) Hypercardioid (g) Figure-eight (h) Shotgun Directivity2 8 8.1 Figure 2. Microphone directivity patterns 2 Cf., Shure 2012d. How to read polar diagrams • Bird’s eye view • Extend (symmetrically) into 3d 8.2 Table 3. Notation of microphones with different directivity in a stage plan Stage plan notation • Distinguish omni-, uni-, and bi-directional microphones • Or use 8.3 for any mic where directivity is irrelevant Symbol Omnidirectional Unidirectional Bidirectional Comparison Property Omnidirectional Directional Gain to feedback ratio Feedback build-up Off-axis coloration Proximity effect Wind, handling, pop noises Distortion Channel separation Lower Slow Smooth and even No Less sensitive Lower Only in direct field Higher Fast Less smooth Yes More sensitive Higher Good 8.4 Directivity pattern Figure-eights • Pick up sound from front & rear, but much less from sides • Classic application: interviews • Available in moss: Royer r-101, akg c 414 xl ii (switchable) 5 of 8 Table 4. Characteristics of omnidirectional vs. directional microphones (Nymand 2005, p. 7) 8.5 Omnis • In theory pick up sound equally well from all directions • In practice become directional towards front for 𝑓 ≫ (why?) • Preferred by some engineers for superior tone quality • Available in moss: Earthworks tc20, akg c 414 xl ii (switchable) 8.6 Cardioids • Single preferred direction (less pronounced for 𝑓 ≪) • Popular for live applications, to avoid feedback loop with monitor loudspeaker • Available in moss: Audio-Technica at4041, akg c 414 xl ii (switchable), Shure sm57, Sennheiser md 421-ii, Sennheiser e 604, Audix d6,Blue Microphones encore 200, Audio-Technica atm250de (condenser capsule) 8.7 Wide & open cardioids • Cardioids that are somehwere between cardioid and omni • Wide cardioid closer to omni than open cardioid • But terminology varies, e.g., both might be referred to as subcardioids • Available in moss: akg c 414 xl ii (switchable) 8.8 Supercardioids & hypercardioids • Cardioids that are somewhere between cardioid and figure-eight • Hypercardioid closer to figure-eight than supercardioid • Available in moss: akg c 414 xl ii (switchable), Shure Beta 58a (super), Audio-Technica atm250de (dynamic capsule: hyper) 8.9 Pressure vs. pressure gradient • Simplest (idealized) microphone: a freely suspended diaphragm – What would its polar pattern be? Figure-eight! – Why? Diaphragm moves due to pressure difference front vs. rear – Such a difference across space is called a gradient (denoted with ∇) – But pressure gradient ∇𝑝 = 0 for sounds from the sides! – A pure pressure gradient transducer is bidirectional. • Question arises: How to even build an omni, then? – Answer: By putting the diaphragm into a closed capsule 6 of 8 – Sound can now only reach diaphragm from one side, not both – Hence diaphragm moves according to sound pressure 𝑝 in front of it – A pure pressure transducer is omnidirectional. • And how to build a cardioid? – Add slits on sides of capsule (any capsule is open to front, of course) – Acoustical labyrinths control phase of wavefront at rear of diaphragm – Resulting interference results in cardioid pickup pattern – A mixed pressure & pressure gradient transducer is unidirectional. 8.10 Mathematical description • Equation describes mic directivity as weighted combination of – Pressure component 0 ≤ 𝐴𝑝 ≤ 1 – Pressure gradient component 0 ≤ 𝐴∇𝑝 ≤ 1 • Values for 𝐴𝑝 & 𝐴∇𝑝 depend on polar pattern (cf., table 5) • Above equation gives a polar function for angle 𝜙 • Assume 𝜙 = 0 for front direction (i.e., top of polar diagram) 8.11 𝐴 (𝜙) = 𝐴𝑝 + 𝐴∇𝑝 ⋅ cos 𝜙 Equation 3. Mathematical description of microphone directivity Table 5. Pressure and pressure gradient components for different microphone directivity patterns Polar pattern Proximity effect • Directional mics (i.e., those with a pressure gradient component) exhibit a boost in bass frequencies for close, on-axis sound sources. • Effect becomes weaker as source moves away from mic or off-axis • Effect does not occur at all for pure pressure microphones (omnis)! • Deliberately exploited by singers, salesmen, etc. References & further reading Dpa Microphones (2008–2013). How to read microphone specifications. url: http : / / www . dpamicrophones . com / mic - university / how - to - read microphone-specifications (visited on 01/19/2017). Eargle, John (2004a). “Care and maintenance of microphones.” In: The Microphone Book. 2nd ed. Focal Press. Chap. 20, pp. 334–7. isbn: 9780240519616. mit library: 002015037. — (2004b). “Classic microphones: The author’s view.” In: The Microphone Book. 2nd ed. Focal Press. Chap. 21, pp. 338–365. isbn: 978-0240519616. mit library: 002015037. Huber, David Miles (1988a). “Microphone accessoires.” In: Microphone Manual: Design and Application. 1st ed. Indianapolis, Indiana: Howard W. Sams & Company. Chap. 5, pp. 69–83. 7 of 8 Omni Wide cardioid Open cardioid Cardioid Supercardioid Hypercardioid Figure-eight 𝐴𝑝 ↕ ↕ 1 / 2/3 1/2 1/3 1/4 0 3 4 𝐴∇𝑝 0 / 1/3 1/2 2/3 3/4 1 1 4 Huber, David Miles (1988b). “Popular professonial microphones.” In: Microphone Manual: Design and Application. 1st ed. Indianapolis, Indiana: Howard W. Sams & Company. Chap. D, pp. 257–78. Klepko, John (2004). “Understanding microphones.” In: Audio Anecdotes: Tools, Tips, and Techniques for Digital Audio. Ed. by Ken Greenebaum and Ronen Barzel. Vol. I. Natick, ma: A K Peters, pp. 115–28. mit library: 001253727. Available at: Mit Learning Modules Materials. Nymand, Mikkel (2005). Directional Vs. Omnidirectional microphones. With focus on PA/Live applications. url: http://www.dpamicrophones.dk/ sitecore / shell / Controls / Rich % 20Text % 20Editor / ~ / media / PDF / MicUni/directional_vs_omni.pdf (visited on 08/27/2014). Rumsey, Francis and Tim McCormick (2009). “Microphones.” In: Sound and Recording. An Introduction. 6th ed. Focal Press. Chap. 3, pp. 47–78. mit library: 002147704. Shure (2012a). Understanding Mic Specifications. Part 2 – Frequency Response. url: https://youtu.be/7TH82dx7Qas (visited on 11/05/2013). — (2012b). Understanding Mic Specifications. Part 4 – Electrical Output. url: https://youtu.be/UYMaRhRt8Ts (visited on 11/05/2013). — (2012c). Understanding Mic Specifications. Part I – Operating Principle. url: https://youtu.be/Mxp3eCCQyas (visited on 11/05/2013). — (2012d). Understanding Mic Specifications. Part 3 – Polar Pattern. url: https://youtu.be/rYWnSuAxato (visited on 11/05/2013). Williams, Michael (2004). “More on microphones: a short tutorial.” In: Microphone Arrays for Stereo and Multichannel Sound Recording. Vol. I. Editrice Il Rostro. Chap. 4, pp. 79–166. — (2010). The interpretation of the microphone data sheets. url: http : / / microphone - data . com / /media / filestore / articles / MW % 20mic % 20specs-10.pdf (visited on 08/27/2014). 8 of 8