Love recording music, speeches and anything that is worthy of documenting? Want to learn more about the technology behind this vital recording tool? The microphone have been used for decades, ever since people saw the need to record and amplify speeches and conversations. Hence, if you are an aspiring audio professional, you should be asking – How does a microphone work?
In today’s article, we will look deeper into the internal electrical workings of a microphone, and discover how it actually functions, in order to produce sound. Find out what makes the different types of microphones suitable for various recording situations in the audio world. Other, important aspects of microphones will also be covered. So don’t wait any longer, let’s learn more!
Also referred to as “mic” or “mike”, a microphone is essentially a device that is designed to convert sound waves into electrical signals. In the electrical engineering world, it is often known as an “acoustic to electric transducer” or “sensor”. The conversion of acoustic signals (sound waves) into electrical audio signals is facilitated by electromagnetic transducers.
Microphones are used in many applications such as in telephones, hearing aids, PA systems (used for lecture theatres and outdoor performances), film production, music production (both live and in the studio), broadcasting in radio and television studios, in computers for voice and speech recognition (or recording), and also for non-acoustic functions such as ultrasonic evaluation or knock sensors.
Most of the microphones that you see today, uses methods such as electromagnetic induction (dynamic microphones), piezoelectricity (piezoelectric microphones) and capacitance change (condenser microphones). All of them function to produce electrical audio signals from the variations in air pressure. Microphones are usually connected to pre-amplifiers, before its signal can be recorded.
Let us look at the various topics that we will cover in this article:
- Internal components
- Types of microphones
In order for speeches to be effectively heard by larger audiences, various methods have been used to amplify the spoken word. The earliest known device that can achieve this (dates to 600 BC), is a mask that has a uniquely designed mouth opening, which acoustically augments the voice in amphitheatres.
Then in 1665, Robert Hooke (an English physicist) was the first to experiment with mediums (other than air), and invented the “lovers’ telephone” (made up of a stretched wire, attached with a cup at each end). Johann Philipp Reis (a German inventor) then invented a sound transmitter, that consisted of a metallic strip attached to a vibrating membrane, which produces intermittent current.
This system was then improved on by Alexander Graham Bell, through his “liquid transmitter” design that is used in his telephone of 1876. In his improved version, the diaphragm was attached to a conductive rod in an acid solution. Shortly after, the “Carbon microphone” was invented, which was the first microphone that had the capability to enable a proper voice telephony.
The carbon microphone was developed by three independent engineers – David Edward Hughes from England, and Emile Berliner and Thomas Edison from America. In the mid-1877, Edison was awarded the first patent (for the carbon microphone) after a lengthy legal dispute. However, David Hughes had presented his working device to many witnesses several years before, thus, he was credited by most historians for its invention.
The carbon microphone’s technology became the basis for the development of today’s microphones and was vital in the development of telephony, broadcasting and the recording industries.
In a microphone, the sensitive transducer element (the component that converts one form of energy to another) is referred to as the “element” or “capsule”. Sound waves are first converted to mechanical motion through the use of a diaphragm, where the resulting motion of the diaphragm is then converted to an electrical signal. However, an exception to this method would be thermophone based microphones.
A practical, complete and “ready-to-use” microphone will also include a housing (typically a metal chassis), some means of transmitting the electrical audio signal from the element (transducer) to other electrical equipment, and often an electronic circuit to adapt the output of the capsule to the equipment being driven. Also take note that a wireless microphone is designed to include a radio transmitter.
Types of microphones
A microphone is identified by its transducer principle, such as condenser, dynamic, ribbon (amongst others) and also by its directional characteristics (cardioid, shotgun etc.). At times (although quite rare), other characteristics such as diaphragm size and the intended application of the microphone are also used to describe the microphone.
In this section, we will cover the types that are widely used or are very popular:
- Condenser microphone
- Dynamic microphone
- Ribbon microphone
- Piezoelectric microphone
Also known as a capacitor microphone (capacitors used to be called condensers in the past), its transducer consists of a diaphragm (acting as one plate of a capacitor) and a back-plate. The back-plate is electrically charged, either by battery or phantom power supply. It is important to take note that a “capacitor”, is an electrical component that stores electrical energy temporarily in an electrical field.
When sound waves excite the diaphragm, the resulting vibrations produce changes in the distance between the plates. As a result of this phenomenon, the electrical field that is created by the back-plate and the diaphragm (the capacitor) will also change. Thus, producing an electrical signal that corresponds to the acoustical sound waves. Microphones that uses this method are also known as DC-biased microphones.
In a DC-biased microphone, the plates are biased (supplied with a steady current or voltage) with a fixed charge (Q). The voltage that is maintained across the capacitor plates (in this case, the diaphragm and back-plate) changes in relation to vibrations in the air. This change is calculated by the formula (C = Q⁄V), where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts.
It is good to take note that for a parallel-plate capacitor, the “capacitance” (the ability of a body to store an electrical charge) of the plates is actually inversely proportional to the distance between them. Also keep in mind that the “element” or “capsule” in a microphone, is basically the assembly of fixed and movable plates.
Electret condenser microphone
Unlike its DC-biased condenser counterparts (with capacitors that are externally charged), the electret condenser microphone uses a different method that employs an electret material, which have been permanently charge. To put it simply, an “electret” is a ferroelectric material that has been polarized (permanently electrically charged).
The term “electret” is derived from the words “electrostatic” and “magnet”. The way a ferroelectric material is “polarized”, is by having all of the static charges in the material itself to be aligned. This is similar to the way a magnet is created, by having the magnetic domains in a piece of iron to be aligned.
The electret microphone is used in many situations, from high-quality recordings and “lavalier” use, to built-in microphones in telephones and hand-held recorders. Though they were once deemed as lower quality condensers, the best ones in the market can now compete with other condensers in terms of sound quality. They are also capable of producing an ultra-flat response, inherent in measurement microphones.
In general, electret condenser microphones do not require polarizing voltage, but most of them are usually designed to include an integrated preamplifier, that needs power (often mistakenly called polarizing power or bias). In sound reinforcement and studio recording applications, the built-in preamplifier is typically phantom powered.
Also referred to as magneto-dynamic microphones, dynamic microphones function by using the method of electromagnetic induction. They are known to be very durable, much more affordable (as compared to condenser microphones) and also highly resistant to moisture. This, together with their ability to be able to handle high SPL before feedback, makes them suitable for live sound.
Dynamic microphones consists of a small movable induction coil (placed within the magnetic field of a permanent magnet) that is attached to the diaphragm. When sound waves come into contact with the diaphragm, it causes the diaphragm to vibrate. This in turn, causes the attached coil to also move within the magnetic field, producing a varying current in the coil via electromagnetic induction.
In most cases, a single dynamic membrane will not be able to effectively respond to all audio frequencies. Hence, some microphones incorporate multiple layers of membranes that are optimized for various sections of the frequency spectrum and then combine the resulting signals. The final combination process is complex and designs that can achieve this, tend to be rare and costly.
It is important to take note that there are designs out there, specifically optimized for capturing parts of the audio frequency spectrum. A prominent example is the “AKG D 112”, that is engineered for responding to bass frequencies rather than treble. In the audio engineering field, it is common practice to have several types of microphones used at the same time, in order to get a quality recording.
Here’s a great video illustrating how microphones function!
Ribbon microphones consists of a metal ribbon (typically corrugated), suspended within a magnetic field. When sound waves hit the metal ribbon, it starts vibrating within the magnetic field. This vibration then generates the electrical signal, which is transmitted electronically to the microphone’s output. Both ribbon and moving coil microphones produce sound through magnetic induction.
The most basic design of ribbon microphones allow the capturing of sound in a bi-directional (also known as figure-eight) pattern due to the metal ribbon being open on both sides. Furthermore, the ribbon has a low mass and thus responds to the air velocity rather than the sound pressure.
In some older ribbon microphone designs, the ribbon had to be suspended very loosely, in order to obtain a decent low-frequency response. However, this causes the microphones to be very fragile. Modern ribbon materials that are currently used, such as the new nano-materials, eliminates those problems and also enhances the effective dynamic range of ribbon microphones at low frequencies.
Ribbon microphones generally don’t need phantom power (it may damage some older ribbon mics). However, some new modern designs include a pre-amplifier, which is driven by phantom power. Also, take note that the circuits of a modern passive ribbon microphone (types that do not have a pre-amplifier), are engineered to resist potential damage to the ribbon and transformer by phantom power.
Otherwise known as “crystal microphone”, the piezoelectric microphone employs the phenomenon of “piezoelectricity” (the ability of some materials to produce a voltage when subjected to pressure) in order to convert mechanical energy (vibrations) into an electrical audio signal. An example of such material is “potassium sodium tartrate”, which is a piezoelectric crystal that functions as a transducer.
You will find piezoelectric transducers being widely used as contact microphones, that functions to amplify sound from acoustic musical instruments (such as an acoustic guitar), to capture drum hits, for the triggering of electronic samples, and for sound recording in challenging circumstances (such as in underwater under high pressure).
When you take a closer look at the “saddle-mounted” pickups on acoustic guitars, these are basically piezoelectric devices that comes into contact with the strings that pass over the saddle. This microphone design is very different from the usual “magnetic coil” pickups (commonly installed on most electric guitars), which uses magnetic induction, instead of mechanical coupling, to pick up vibration.
We have finally reached the end of the article. With all of this information, I hope that you people are now more confident in using the various types of microphones for different recording applications!
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