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8 - MEASUREMENT AND THE OPERATIONAL CHARACTERISTICS

Although there are usually many informative characteristics published on the specification sheet of a microphone, the principle measurements that we use to estimate the operational characteristics of microphones, are the on- and off-axis frequency response of a microphone, and its directivity pattern at various frequencies throughout the audible frequency range.

8.1 - FREQUENCY RESPONSE

8.1.2 - The Electrodynamic Ribbon Microphone

The fundamental resonant frequency of a ribbon microphone is in the very low frequency range, well below 50Hz. Again if the diaphragm is free to vibrate almost without constraint, it would show a typical resonance curve centred around its fundamental resonance in the bass frequency range, as shown in Figure 29 (curve A).


Figure 29 - Typical Amplitude/Frequency Response at Resonance
for a Electrodynamic Ribbon Microphone
without any Constraints (curve A)
with Natural Damping (curve B)
with Additional Damping (curve C)

The mass of the ribbon with respect to the surrounding air means that there is already considerable natural damping (Figure 28 – curve B), but some additional damping is necessary to produce a good flat frequency response (Figure 28 – curve C). This additional damping is achieved by reducing the distance between the ribbon and the magnetic pole-pieces, thereby increasing the resistance due to the viscous air flow between the ribbon and the pole-pieces of the permanent magnet. Unfortunately reducing the distance between the magnet pole-pieces and the ribbon will also increase the risk of the ribbon actually touching the pole-pieces with disastrous results!

There are other factors such as the pressure-gradient acoustic coupling function that also limit the low frequency response of a ribbon microphone as described in section 4.2 (Pressure Gradient Acoustic Coupling) The final choice of damping coefficient will be a function of the required output sensitivity and the frequency range to be covered, whilst maintaining the best signal to noise ratio possible – unfortunately all these characteristics are interdependent.

The magnetic field around the ribbon is inversely proportional to the distance between the poles of the permanent magnet – in the two typical ribbon microphones shown in Figures 16 and 17 (section 5.2 - The Electromagnetic Ribbon Transducer), we can see that the distance is determined by the width of the ribbon. Two conflicting design constraints must be considered:

• Minimum distance between the pole-pieces for a strong magnetic field and maximum output voltage
• A wide ribbon for maximum surface and minimum electrical resistance, again to achieve maximum output voltage (a long ribbon introduces other problems).

Somewhere in between is the optimum compromise! Perhaps due to this design nightmare only a few remarkable ribbon microphones have been produced over the last seventy years. Figure 30 shows four well known examples:

• The RCA PB 90 & 77 DX (icons from the forties),
• The Coles 4038 (developed by the BBC in the fifties)
• The Beyer M160 (a cinema industry standard in Europe for over 30 years)
• 
  
  Figure 30 - Four Well Known Ribbon Microphones
  from left to right, RCA PB 90 & 77 DX, Coles 4038 and Beyer M160
  © photos courtesy of Klaus Blasquiz, Coles Engineering and Beyer
  Once the critical damping factor has been found, the frequency response curve is again typical of this type of microphone. We again can see a limited frequency band, low frequency roll-off below the fundamental resonant frequency, high frequency roll-off above about 8kHz to 10kHz, but otherwise the ribbon microphone has a remarkably linear frequency response over the whole of the working range from about 100Hz to 8kHz.
  
  The Beyer M160 (frequency response curve shown in Figure 31) is designed with a parallel double ribbon assembly (mounted one behind the other), together with a cavity resonator (at about 90Hz) in the body of the microphone to improve bass frequency response.
  
  
  Figure 31 - Frequency Response of a Ribbon Microphone
  (Beyer M160 No11794)
  © frequency response curve published by courtesy of Beyer
  In practice the impedance of the ribbon can be very low (about 0.2 ohms) and needs a transformer (about 1/20) to increase source impedance and signal voltage.