Like ice creams in a parlor, microphones too come in various flavors. Different types of microphones are designed for varied purposes. This Techspirited article explores the various categories and the uses of different microphones.
The world’s first microphone was independently invented by David Edward Hughes in England, and Emile Berliner and Thomas Edison in the US, during mid-1877. Hughes first used the term ‘microphone’ to describe his transmitter’s ability to transmit weak sounds electrically to Bell’s telephone receiver. Unlike fellow inventor Thomas Edison, Hughes decided not to have his invention patented, but instead gave it as a gift to the world.
Imagine it’s the ’80s. Your hair is a jungle, and there are tattoos crawling up your sinewy arms. The bandanna on your forehead shouts ‘punk’, even as the print on your sleeveless T-shirt advocates peace. Once you don your black leather jacket and squeeze into those undersized stone-washed denim jeans, you will be ready to Rock Them Like a Hurricane, right? Well maybe not just yet. There is still one key equipment missing from your arsenal which is preventing you from reaching your rock-star dreams―a good microphone.
Fast forward to the present day, and though fashion has evolved (for the better!), the importance of having a good microphone cannot be undermined. And this is true not just for live performances. As we will see, there are different types of microphones having a variety of uses. For choosing the right one for your application, you need to first understand the theory behind each kind. But before that, let’s find out what a microphone exactly is?
Microphones are transducers. They convert sound frequencies into a proportional and measurable electric element (voltage or current). This element can then be amplified (for speakers) and even transmitted over long distances (e.g. radio). Nowadays, with advanced electronics, it can even be processed to make the output (given to the speakers) better. So, now that we know what microphones are, let’s classify them into different types.
Basically, there are six parameters used to classify microphones:
- Diaphragm Size
- Microphone Directionality
- Microphone Transducer Type
- Microphone Frequency Response
- Wired and Wireless
- Miscellaneous Microphones
Microphone Transducer Type
The transducer is the sensor element inside a microphone. It is the very heart of the microphone, that converts sound energy into electrical energy.
The following are the main types of microphones differing on the basis of the transducers used in them.
Condenser microphones work on the principle of capacitance. A capacitor is a device that can store charge. It basically consists of two parallel plates separated by a dielectric (non-conductive element). The distance between the plates is directly proportional to the amount of charge (therefore the voltage) that can be stored on the plates. The condenser microphone employs this principle by having a diaphragm as one of the plates. As the sound waves hit the diaphragm, it moves, changing the distance between it and the other plate. This changes the capacitance, and thus, voltage across the plates vary proportionally as well.
Condenser microphones require an external power source, known as a phantom power source, so as to establish the voltage across the capacitor. The required voltage can be as high as 48v DC.
These are used in almost all audio applications, right from inexpensive home mics to hi-fi (high fidelity) recording ones. The condenser mic technology and working principle is also employed in many of its derivatives.
Dynamic microphones work on the principle of electric induction. It’s the same principle that governs the generation of electricity from dam water, but on a much smaller scale of course. According to this, if a conductive element is moved through an electric field, electric current is induced through it. In dynamic microphones, the conducting element is a copper wire, called the voice coil. It is placed within a magnetic field produced by a suitable magnet. The voice coil is connected to a diaphragm that moves in response to being hit by sound waves. This causes proportional electric current to flow through it.
Dynamic mics are less sensitive than condenser mics. However, this proves to be advantageous in some applications, where ambient noise is likely to be picked up by the very sensitive condenser mics.
They find use in home applications, cheap karaoke instruments, and in the music industry as musical instrument microphones.
These are actually a subtype of dynamic microphones. In them, a light and thin conductive ribbon is attached to a movable diaphragm and suspended between the two poles of a permanent magnet. As the sound waves move the diaphragm, the ribbon vibrates and voltage gets induced in it. This voltage is received via the contacts placed on the ends of the ribbon.
The induced voltage is proportional to the change in velocity of the sound waves hitting the diaphragm. Hence, these are also called velocity microphones.
Though ribbon microphones are old technology, they are seemingly making a comeback, as more and more audio enthusiasts are demanding the return of the vintage sound. Used on countless golden-age recordings, these mics have a unique sound. They are not as harsh as the condenser, and not as blunt as the dynamic mics either. Perhaps, we will see more of their kind in the future.
Microphone Frequency Response
The audible sound frequency ranges from 20Hz to 20,000Hz. A microphone can be designed to be responsive to only a part or whole of this range. As such, there are the following two different types based on their frequency response.
Microphones with a Flat Frequency Response
These mics have equal response to all the frequencies of the audible frequency range. The name is derived from the fact that the graphical plot of frequencies vs. sensitivity of such microphones is a flat line.
Microphones with a Tailored Frequency Response
Mics can be constructed to be more sensitive to a certain band of frequencies, while attenuating (rejecting) others. For instance, a microphone constructed to be more sensitive to frequencies in the range of 2,000Hz to 8,000Hz is best suited for live vocals. A graph of such a microphone would show a peak in the selected range.
Like teenage kids these days, some microphones only hear what they want to hear! Microphone directionality refers to the fact that different microphones hear from different directions. The following are the varying types.
These microphones are capable of picking up sounds equally from all directions. They have the advantage of not requiring to be positioned, but are more susceptible to ambient noises.
Omnidirectional mics are used when all sounds in a room need to be recorded. They are used especially in on-site TV interviews. They are also well-suited for use by inexperienced individuals, who are less likely to hold the directional mics correctly.
These have maximum sensitivity on their frontal side and the least on their rear side, making them ideal for large stages.
They are used in studio recordings, live performances, and for making public addresses.
Super Cardioid Microphones
These have an even narrower pickup range on the front than cardioid microphones, and are thus resistant to all surrounding ambient noises. However, they have a small amount of sensitivity to their rear, making them vulnerable to rear noises, especially feedback from speakers.
These are used in high quality studio recordings.
Bidirectional (figure of 8) Microphones
Microphones which have nearly equal sensitivity on their front and rear sides, while being resistant to the noises to their sides, are called bidirectional microphones. A polar pattern graph of these microphones would result in a pattern of the digit 8.
These are typically used when a person is interviewing another using a single microphone.
Wired and Wireless Microphones
Microphones pick up sound which is then converted to an electric quantity. To transmit this quantity, some form of communication link needs to be established. The following are the two types differing according to their communication links.
If there is a wire hanging from the lower end of your microphone, it is a wired microphone. Traditionally, a copper wire is used to transmit the electric quantity produced by the mic to the rest of the audio processing hardware. In case of optical microphones, fiber optic cables are used instead in place of copper wires.
Usually, most microphones are wired. They are used where mobility is not an issue, such as live singing performances, speeches, etc.
These use some form of wireless means to transmit the sound. Generally, RF (Radio Frequency) based transmission is employed, but nowadays, more advanced digital technologies such as blue-tooth are also used.
These are used when a lot of mobility is desired. For example, a dancing and singing combination performance.
The size of a microphone’s diaphragm changes its audio output, and therefore, its applications.
Large Diaphragm Size
A large diaphragm is softer, and therefore, easier to move, making these mics more sensitive. The microphones fitted with them also don’t require a lot of amplification. They are mostly used in studio recordings to record just about any kind of sound, including vocals and instruments as well.
Small Diaphragm Size
A smaller diaphragm is also stiffer. This allows it to handle higher sound pressures without clipping or tearing. These are used especially in microphones designed to record sharper, harsher sounds, like drumbeats or guitar strums.
Medium Diaphragm Size
This are a relatively newer concept, with the size falling between the large and small categories. These are believed to possess the best of both. They are used in specialized studio recordings.
The following are a few miscellaneous types of microphones.
Known as a ‘transmitter’ back then, the carbon microphone was the first ever microphone. Though Thomas Edison was awarded the first patent, historians opine that David Edward Hughes had demonstrated his ‘transmitter’ to several witnesses a few years earlier.
Hughes employed a novel way to convert audible sound waves into proportional electricity. He packed granular particles of carbon inside his device, sandwiching them between a thicker plate on one side and a very thin plate on the other. He then passed electricity through them, measuring the output. When a person spoke into the carbon microphone, his/her sound waves would hit the thinner plate, causing a pressure change, compressing the carbon particles inside. This caused a change in the resistance, resulting in the electric current passing through them to vary proportionally.
Carbon microphones are capable of producing sufficiently high levels of audio signals from very low DC voltages. As such, they find use in long-distance communication over telephone lines, where the length of the cable itself results in a drop of voltage. They are used in mines and chemical factories, where higher line voltages may result in sparking. Carbon microphones are rugged and can withstand high voltage transients and electromagnetic effects. Thus, they are even used in backup communication lines by the military in case of wars of natural disasters.
Crystal microphones work on the principle of Piezo electricity. It refers to the property of certain materials to generate a potential difference in response to changes in the pressure applied to them. The generated voltage is directly proportional to the applied pressure. Piezo-electric microphones employ this principle to convert sound pressure into electric signals.
These microphones require careful handling, as they are susceptible to noise from their cables and from the microphone itself.
They find use in acoustic instruments such as drums to detect beats, on acoustic guitars in contact with strings, etc. They are also used for deep underwater recordings.
The liquid microphone was invented by Alexander Graham Bell. It was based on Ohm’s law, which states that current varies inversely to the resistance. It comprised a container filled with water diluted with sulfuric acid. A needle attached to a diaphragm was dipped in this mixture. Sound waves would cause the diaphragm to move, which in turn would cause the needle to move up and down in the liquid laced with acid. This varied the resistance of the water, and the current increased or decreased accordingly.
Though obsolete now, the liquid microphone was used in the famous first phone call between Alexander Graham Bell and Thomas A. Watson.
Unlike the other microphone types, fiber-optic microphones do not employ resistive, capacitive or inductive means. Instead, they use intensity-modulated light to convert sound to electricity. A laser source illuminates the surface of a reflective diaphragm. When sound causes the diaphragm to vibrate, the light intensity modulates in proportion to it. This modulated light beam then travels through an optic-fiber cable to a photo detector, which converts the light to proportional electricity.
Fiber-optic microphones have a high dynamic range and large frequency response. They are resistant to electromagnetic interference, and can be used in long-distance transmission, without needing any amplification.
They find use especially in industrial and medical (MRI) environment where electromagnetic interference is high. They can also be used for long-distance surveillance.
Having resemblance to the gizmos seen in sci-fi and spy movies, laser microphones are an amazing piece of technology. When a person within a room talks, sound waves cause micro-vibrations on many surfaces in and around the room; for example, the window pane. If a steady laser beam is focused on such a surface, the micro-vibrations can cause a small shift in the angle of the reflected beam. This shift can be detected and measured by a photo-detector, and converted to electricity, and eventually into sound.
Laser microphones are employed in espionage and military applications.
Microelectromechnical Microphones (MEMS)
MEMS microphones are extremely small microphones designed to fit on a silicon chip. They are based on the same working principle as condenser microphones. They usually also have an analog-to-digital converter module installed on the same chip. This converts the analog input into digital values, which are used by the modern electronic devices of today.
They find use in most modern-day electronic gadgets, such as cell phones, tablets, laptops, automotive industry, etc.
So there you have it, a complete list of the important types of microphones and their typical uses. Microphones are like people applying for a job. Most of them are capable, but only a few suit a particular profile the best. However, there can be only one who gets the job. If your selection procedure is right, your company will gain an asset and grow. If you get it wrong, then well, you know how it goes!