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- presented by Prof Trevor Cox

Microphones

Introduction

This article will discuss microphone types and parameters (such as sensitivity, frequency response and directional response) with aim of helping you choose and use microphones appropriately for given applications.

A microphone is a transducer that converts acoustic energy to electrical energy.

There are five key types of microphone you may use

  • Moving coil
  • Ribbon
  • Condenser
  • Electret
  • Crystal

All employ different mechanisms to convert sound energy to electrical energy. Hence all have different advantages and disadvantages. You will hence need to choose the right type of microphone for the right type of application.

The microphone is not expected to deliver electrical power- it operates into HIGH ELECTRICAL IMPEDANCE such that there is (approximately) zero current - VOLTAGE is the output variable. Consequently, it is conventional to talk of the "Open Circuit" response

The microphone may be sensitive to any combination of acoustic variables, but the simplest case is a microphone that is responsive to pressure input.

Microphone Sensitivity

When choosing a microphone pressure sensitivity is an important parameter. A microphone’s sensitivity (pressure sensitivity) is defined as the voltage generated in response to a certain pressure input.

The common notation is:

M0 (Volts/Pascal)

For example: A microphone is exposed to 94dB SPL generates 50mV output. What is the open circuit low frequency pressure sensitivity?

An SPL 94dB corresponds to an RMS acoustic pressure of

=1Pa

This gives a microphone sensitivity of 50mV/Pa

Alternatively a microphone’s sensitivity can be expressed in logarithmic form. As with pressure and power, voltages can be expressed in decibels relative to a reference.

i.e. dB re 1 V/Pa

So converting to this scale…

dB re 1 V/Pa

Generally the greater the pressure sensitivity the more sensitive the microphone is to quieter sounds. Also the greater the signal will be produced relative to noise in cables, etc.

However, as we shall see a high sensitivity may not be the first consideration when choosing a microphone for a given application.

Frequency Response

The frequency response of a microphone is the characteristic graph obtained by recording the voltage output level in dB, while the microphone is exposed to a range (sweep) of pure sinusoidal tones of equal intensity. The frequency response is often given as a graph or stated as variation within a given range, e.g.

Frequency response 3dB from 20Hz - 20kHz

The frequency response gives important information about the tonal balance of the microphone under different acoustic conditions. For high quality instrument grade microphones a large flat range (20Hz to 20KHz) is required. However different microphones exist for different types of application. For example when recording vocals a presence boost around 2kHz to 5kHz improves diction and intelligibility. Many celebrated studio microphones are chosen for their own distinctive warmth and clarity in much the same way as musical instruments.

Moving Coil Microphone

diagram of moving coil mic

The moving coil microphone works on the principle of Electromagnetic Induction. As a copper wire coil moves in the magnetic field a voltage is generated as given by…

where V is resulting voltage from B is magnetic field, l is the length of the copper wire and u is the velocity at which it passes thought the field. An incoming sound causes pressure variations which cause a corresponding movement of the microphone’s diaphragm and hence the attached coil in a magnetic field. Moving coil microphones are cheap and robust making them good for the rigors of live performance and touring. They are especially suited for the close miking of Bass and Guitar speaker cabinets and Drum kits. They are also good for live vocals as their resonance peak of around 5kHz provides an inbuilt presence boost that improves speech/singing intelligibility

picture of mic

The Shure SM58 shown is a classic moving coil microphone being an industry standard for live vocal, cabinet and drum miking. They are rugged and don’t require phantom power (see later).

However the inertia of the coil reduces high frequency response. Hence they are NOT best suited to studio applications where quality and subtlety are important such as high quality vocal recording or acoustic instrument micking.

The above shows the frequency response of an SM58.

  • Frequency range 20 Hz...15 kHz
  • Sensitivity of SM58 … 2.8mV/Pa (-54.5 dBV/Pa)

Condenser Microphones

Condenser mic

A thin plastic diaphragm coated with an extremely thin vaporized metal (gold or aluminum) is stretched over a shallow cavity closed by a metal back plate. The resultant capacitor can be charged with 48 Volts DC phantom power. As the diaphragm moves in sympathy with sound pressure the distance with respect to the back plate varies, so does capacitance (ability to hold charges) causing current and hence a change in voltage (mechano/electrical transduction).The output impedance must be very high (100Mohms) to achieved useful signal which then must be amplified (pre-amp stage).

Size and shape of diaphragm doesn’t have to be dictated by suitable positioning along magnetic field, hence it can be a very light disk (12-25mm diameter).

The resultant sound quality is hence very good, capacitor mics being the standard for top quality music and audio recording.

Convenience is reduced by the need for phantom power and a susceptibility to humidity.

u87

The Neumann U87 is a classic condenser microphone be an industry standard for vocals and orchestra instruments. The microphone has a warm sound clear sound.

U87 Frequency Response (cardioid)

mic picture

  • Frequency range 20 Hz...20 kHz
  • Pressure Sensitivity 20/28/22 mV/Pa @ 1kHz for omni/polar/figure8 respectively
  • Maximum SPL for THD 0.5% 117 dB (cardioid)
  • Maximum SPL for THD 0.5% with pre attenuation 127 dB
  • http://www.neumann.com

Factors influencing open circuit sensitivity:

If the polarizing field Vpol too big then sparks can puncture the thin metal diaphragm.If the displacement response of the diaphragm is too big or then the microphone.

  • operates at a lower bandwidth
  • has a disrupted field (polar effects)
  • experiences distortion

Practical Construction

The internal components of a instrumentation grade pressure microphone, such as this Bruel and Kjaer 4190

are shown in the exploded diagram....

Exploded diagram

The microphone has zero response at zero frequency due to:

i) pressure release vent (component 4)

ii) blocking capacitor

The DC blocking capacitor simply prevents the phantom power from enetering the head amplifier, allowing only audio signals to pass.

Circuit

Electret Microphones

These work as capacitor microphone except a permanent (electrostatic) charge applied to microphone during manufacture removing need for phantom power. However the diaphragm requires a larger mass to hold the electrostatic charge adversely effecting frequency response. Electrets tend to be cheap, compact and easy to mass-produce. Classic applications include built in microphones for portable cassettes and tie clip microphones.

However, recently some high quality examples have begun to appear such as the AKG C1000S. Called ‘Back Electrets’ they can have similar diaphragms to traditional condenser microphones by applying the electrostatic charge to the rigid back plate.

Electret

The AKG C1000S represents terrific value for money having a high quality response without the need for external phantom power of pre amplification (an internal nine volt battery is used for the preamp stage). They’re great for recording vocals and acoustic instruments, and are convenient enough for live applications though humidity can be a problem so they should be looked after. However, professional studios still prefer the more expensive large diaphragm phantom powered microphones like the U87 for vocals for their warmth and character.

AKG C1000S Frequency Response

Frequency response

  • Sensitivity 6 mV/Pa (-45 dBV)
  • MAXIMUM SPL FOR 0.5% THD: 137 dB
  • EQUIVALENT NOISE LEVEL 19 dB-A
  • http://www.akg-acoustics.com/

Ribbon Microphones

Ribbon mic

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 smother and higher frequency response. However the relatively low output requires a step up transformer.

Ribbon microphones are good for quality studio recording of acoustic instruments though can be delicate, for instance you wouldn’t want to put one in front of a bass cabinet.

R121

The ROYER R121 ribbon microphone gives a beautifully warm and smooth sound quality.

ROYER R121 Frequency Response

  • Frequency Response: 30-15,000 Hz +/- 3dB
  • Sensitivity: >-53 dBv Re. 1v/pa
  • Maximum SPL: >135dB

Crystal

These microphones utilises the ‘piezoelectric effect’. Piezo (Greek for Push) electric solids produce a voltage between surfaces when a mechanical stress is applied. Conversely they exhibit deformation when a voltage is applied. This is due to structure of their crystal lattice. Certain types of crystal such as Quartz and Rochelle Salt will have the charges of their respective molecules polarised by deformation. There is a cumulative effect through out the crystal creating a voltage.

Crystal mic

Here’s how the effect can be applied to a microphone.

Diagram

Sound waves couple their movement to a diaphragm which intern communicates the resulting vibration to an attached piezo electric crystal. Charges and hence voltages are proportional to the crystal’s bending. The frequency response of crystal microphones is often limited to a relatively narrow band restricting their application. Also older Rochelle Salt versions were sensitive to moisture, though modern choices of ceramic peizo electric crystals (barium titanate and lead zirconate) are more robust. Crystal microphones tend to be used for low quality audio applications such as telephone handsets since they don’t require phantom powering or amplification and are cheap to produce. However, there are some high quality versions, such as CADs HM50, which are ideal for specific applications.

Crystal mic

The HM 50’s is ‘A superb omnidirectional crystal microphone used by discriminating Harmonica musicians. The HM 50’s high output crystal element has been factory tuned to provide just the right presence or “bite” to create the ideal sound for blues and many other styles of play.’

Piezo electric transducers can be attached directly to the sound board of acoustic instruments such as guitars. The resulting vibration produces a signal large enough to be sent to guitar amplifiers. They are also are used in accelerometers for vibration analysis.

CAD HM50

Frequency response

  • Frequency Response: 30 Hz - 8 kHz
  • Open Circuit Voltage:, -49 dB (0 dB = 1 volt per microbar) @ max. gain 35 mV/Pascal

Directional Response

The directional response of microphones is an important consideration during sound recording and will hence be discussed in greater depth in a later lecture. For now the are a few important aspects to be aware of…

The directional response varies greatly will angle of incidence for high frequencies. Consider high and low frequency waves incident on a microphone diaphragm shown. At an steep angle of incidence there are proportionality less nodes and antinodes over the same surface area, hence a much reduced response. At low frequencies the effect is much less significant and the microphone can be though of as omni directional.

By combining an original sound wave or microphone signal with a phase inverted version of it’s self various important directional responses can be constructed. Notably cardioid, hyper-cardioid, and figure of eight patterns.

Directivities

microphone responses have an especially good rejection of sound from the rear so with correct positioning can be used to minimise spill from other instruments or potential sources of feedback such as on stage monitors. Next time you see a professionally micked drum kit notice how some of the microphones are positioned at angles to minimise overspill.

Hypercardioid microphone responses include a little more sound from the rear. The upshot of this is that reverberant sounds in larger concert halls can cancel so giving a good level of direct sound relative to the echoes and reverberations of the room. You may have seen vocal microphones at gigs set up with two monitors positioned in the dead spaces (130 & 270 degrees) of the hypercardioid pattern.

Directional microphones have an increase bass response with proximity (bass tip-up or proximity effect). This is due to the way sound propagates spherically in the near field as opposed to plane wave propagation in the far field. Hence as a perform moves closer to a microphone the sound can seem warmer and more intimate or muddier, conversely as a microphone is moved away from a source the sound gets thinner.