A microphone is a transducer, converting sound wave energy to electrical energy. The sound wave is a longitudinal pressure wave (compressions and rarefactions of the air molecules) that causes a thin diaphragm in the microphone to vibrate. As we’ve seen in the two previous posts on dynamic microphones and ribbon microphones, a metal wire coil or metallic ribbon moves in the presence of a magnetic field, generating an electromotive force (EMF) voltage. There is a third type of microphone, the condenser microphone, that creates an output signal voltage by another physical process – the vibration of a metallic diaphragm that is part of an electrostatic capacitor in an electric circuit. Condenser microphones are very popular in the music studio owing to their clear, bright sound over an extended frequency range. They also have high sensitivity and accurate transient response. Condenser mics contain “active” circuits, requiring a voltage source to bias the electric capacitor and output amplifier circuit. Supplying this DC bias voltage through the microphone XLR cable is usually done by applying “phantom power” from the mixer console or audio interface.
The microphone (mic) shown in the photo at the top of this post is a Neumann TLM103 large-diaphragm condenser mic – a mainstay of most studios around the world. The Neumann TLM103 mic possesses a clear voicing with a wide presence boost for frequencies above 5 kHz. This mic is very well suited to bring out vocals and solo instruments in the mix.
The “inner workings” of a condenser mic are shown in the figure below.
Sound pressure waves strike a thin, metallic diaphragm that comprises the front conductor of a two-conductor electric capacitor. The difference in pressure between front and back (pressure gradient) causes the metallic diaphragm to move inward and outward from its rest position. The very thin, lightweight diaphragm can react swiftly and accurately to the incoming soundwave pressure differential. As the front conductor moves, the voltage across the capacitor changes, creating a time-varying output signal voltage that “images” the pressure variations of the input sound wave.
Let’s look more closely at the output voltage generated by the varying capacitance of the biased capacitor in the circuit diagram above.
The time-varying output voltage is taken from across the very large resistor R in the circuit. Consequently, there is a very high output impedance for the signal source. This is not a desirable situation for connecting to the pre-amp of a mixer console or audio interface, as we saw in an early post discussing the voltage divider effect in audio cable circuits. Therefore, we need to place an impedance converter (buffer amp) immediately after the RC circuit to drop the impedance of the signal source. Most modern condenser mics use a field-effect transistor (FET) amplifier to convert a high-impedance source to a low-impedance one. And there is usually some voltage gain of the signal provided by the amplifier, increasing the output signal level. The field-effect transistor circuit requires voltage biasing (48 VDC) which is usually supplied by phantom power coming from the XLR cable attached to the mic.
The important characteristics of a microphone include:
1. Polar response pattern
2. Frequency response
3. Output signal level
4. Output resistance (impedance)
Condenser mics typically have a cardioid polar pattern, as shown here for the Neumann TLM103 mic:
Some condenser mics have dual diaphragms, i.e., two back-to-back capacitors that share a common fixed inner plate. These dual-diaphragm condenser mics exhibit multiple polar response patterns by electrically combining the signals from the two capacitors in different ways. The ability to switch between a cardioid, bi-directional, or omni-directional polar pattern makes these mics very versatile in the recording studio.
The frequency response curve for the Neumann TLM103 condenser microphone is shown below.
The wide presence boost from 5 kHz to 15 kHz gives this microphone its bright, clear character that is well suited for bringing out vocals and solo instruments in the mix. The very flat response from 5 kHz down to 60 Hz ensures a very immediate, uncolored sound that is true to the original. The bass roll-off below 60 Hz reduces unwanted low-end rumble and noise. In addition to using this mic for vocals and solo instruments, it is commonly employed as an ambient mic in stereo recordings of classical orchestral music. The topic of stereo microphone techniques will be the subject of the next post.
Here are some notes on the output signal level and impedance of a condenser mic. Both signal level and output impedance are determined by the integrated buffer amplifier in the mic capsule. The spec sheet for the Neumann TLM103 condenser mic lists an output voltage sensitivity of 23 mV per Pascal of pressure at 1 kHz into a 1-kOhm load, and an output impedance of 50 Ohms. This 23 mV/Pa voltage sensitivity is quite a bit larger than the 1-3 mV/Pa sensitivities typically found in dynamic microphones. Likewise, the 50-Ohm output impedance is substantially lower than the 150-300 Ohm impedances commonly found in dynamic microphones.
Lastly, the Neumann TLM103 condenser mic has a very low self-noise level (7 dB-A) and a very high maximum sound pressure level (138 dB SPL) . This means that the TLM103 has an extremely large dynamic range of ~131 dB, making it capable of capturing the softest sound or loudest sound without adding noise or distortion – just another feature making this microphone a mainstay in most music recording studios.
In the next post, we’ll take a look at the topic of stereo microphone techniques.
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