Audio Filters: The working and classification of microphones – Part 3

In Part 1 and Part 2 of this series, we covered the physical properties of sound and acoustic waves. In this tutorial, we’re learning about the importance of converting sound into electrical signals. 

As sound waves present in vibration form, they first must be electrically converted to be processed by electronic circuits. As mechanical energy, sound must be precisely represented as an electrical waveform (analog) for any successful signal-processing operations. 

In the air medium, the device used for converting sound waves into electrical (audio) signals is likely familiar to you — it’s called a microphone. 

The term “microphone” was first coined in 1875. It’s a combination of “microscope,” in reference to magnifying an item, and “telephone,” in relation to transmitting sound. A microphone is a device that converts speech or sound into electrical signals. It’s used for several purposes, including radio, television, recording, live music, VOIP, and others. 

How microphones work
A microphone is a type of transducer, which is a device that converts one form of energy into another. More specifically, it converts mechanical energy (wave forms) into electrical energy.

Different microphones use different methods for converting the energy but they all use a diaphragm, which is a thin membrane that vibrates in the presence of sound waves. When the diaphragm vibrates, this typically affects another component — such as a coil in a dynamic microphone or a plate in a condenser microphone.

The coil, for example, is also bound to a magnet, which produces a magnetic field. Whenever the coil in a dynamic mic moves in the magnetic field, it creates an electrical current. This electrical signal is produced from the microphone, which is passed on to the amplifier (or a preamplifier).

Since microphones use an inductor (such as the coil) or some kind of capacitor, they’re not purely resistive. So, it’s worth noting that the resistance of an inductor and a capacitor will vary based on the sound frequency — and different microphones will demonstrate different impedances based on the frequency range.

In addition, a microphone’s frequency response depends on the technology of the microphone’s conversion. 

Now, let’s review how the different types of microphones work.

Classification of microphones
Microphones are typically categorized based on two features: the conversion technology used and their application.

The types of conversion technology include: 

1. Dynamic
2. Condenser
3. Ribbon
4. Crystal 

Dynamic microphones – A simple microphone that uses a coil and magnet to convert sounds into electrical signals, based on the principle of electromagnetic induction. A coil is attached to the diaphragm, which is surrounded by a permanent magnet. When the diaphragm vibrates, the attached coil also vibrates. An electrical signal that’s proportional to the vibration is, then, generated. 

• Pros: Cost-effective to design and durable 
• Cons: Unreliable frequency response (at least compared to a condenser or ribbon microphone)

The inner working of a dynamic microphone

Whenever the space between the plates changes, the Q charge is stored between them and this causes a current in a resistance that’s connected in series with the capacitor.

• Pros: Responds well to a dynamic range of frequencies
• Cons: Costly and requires a power supply to charge the plates

The inner working of a condenser microphone

Ribbon microphone – These microphones have a metallic ribbon, suspended between a permanent magnet. Whenever sound waves hit the ribbon, it vibrates. As the ribbon is in a magnetic field, any vibration induces a voltage.

The voltage produced is proportional to the velocity of the ribbon. The voltage is also the audio signal received from the microphone. 

• Pros: Adds boost to any bass
• Cons: Susceptible to external noises, such as a breeze or the wind

The inner working of a ribbon microphone

1. Omni-directional –  can pick up sound from all directions
2. Bi-directional – can pick the sound from front and back of the microphone, but not from its sides
3. Unidirectional – can only pick up sound from one direction, which is the front of the microphone

Microphones also display one of three distinct polar patterns. A polar pattern is a visual graph that indicates the sensitivity of a diaphragm at different angles.

By combining the directional property and the polar pattern that a microphone’s “pick-up” pattern is determined. The pick-up pattern refers to the sensitivity of the diaphragm and demonstrates how well a microphone will pick up sounds from different directions and angles.

In terms of its pick-up pattern, microphones can be categorized as follows… 

1. Omnidirectional – sensitive to all directions at all angles, equally. 

2. Bidirectional or figure-of-eight – sensitive to two, opposing directions, which are either the front and back, or the left and right sides of the microphone. This sensitivity is equal at all angles.

3. Cardioid – extremely sensitive at the front of the microphone, with decreasing sensitivity at the sides, and very little at the back. They provide isolation from unwanted ambient noises and are ideal for loud sounds.

Line levels
The output level of a microphone level is an extremely weak signal (only -60 dBm). However, the output strength can be increased by using a preamplifier (also known as a preamp), which sets the input signal to the line level.

The line level is the standard strength of an audio signal required to transmit the signal between audio sources to amplifiers, radios, TV, DVD players, etc. The typical line level voltage is 0.316 to 1.228 V.

Therefore, a preamp circuit is necessary in a microphone’s circuitry to convert its level to the line level. 

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Filed Under: Audio Electronics, Featured Contributions, Tutorials

 

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