Microphones were originally developed alongside the telephone. Many microphone types (and telephone types) have been invented since then. Today, many people carry a cellphone, and therefore a microphone, with them at all times.
What kind of microphones are used in cellphones? Cellphones use tiny electret condenser or MEMS microphones. These mics require very little power to function, which is easily supplied by the cellphone battery. They also fit nicely within the circuitry of the typical cellphone.
In this article, we’ll discuss electret microphones and cell phones in greater detail as well as the potential for using external microphones with cell phones.
What Is An Electret Microphone?
An electret microphone is a type of condenser microphone that has a permanently-polarized capsule.
The capsule, which acts as a parallel-plate capacitor, is charged via a quasi-permanent electret material that is applied to either the front plate (diaphragm) or stationary backplate.
With a permanent charge across the plates, any change in capacitance in the electret condenser capsule creates an inversely proportional change in voltage across the plates.
V = Q/C
Voltage = Charge / Capacitance.
The capacitance of the capsule changes are the distance between the plates varies. Therefore, as the diaphragm moves back and forth, an AC voltage is created across the plates. This AC voltage is our mic signal.
C = εo(A/d)
Capacitance = Dielectric Constant * Area Of The Plates / Distance Between The Plates.
Electret material is cheap and electret capsules are simple and inexpensive to manufacture. The fact that they require no polarizing voltage means that their power needs are low.
It is actually the impedance converter (a simple JFET or junction-gate field-effect transistor) that requires power. These JFETs convert the high impedance mic signal from the electret capsule to a signal with a usable impedance for use in the rest of the system (in this case, the cellphone).
Cellphones nowadays are digital, and so it’s arguable that the analog-to-digital audio converters in these cellphones are also part of the microphone. These ADCs also require power to convert the analog signal to digital audio for proper use within the cellphone.
What Is A MEMS Microphone?
A MEMS (MicroElectrical-Mechanical System) microphone is a mic that is etched into a silicone wafer or “chip” using MEMS technology.
MEMS microphones feature pressure-sensitive diaphragms and are generally built with integrated preamplifiers/impedance converters. On top of that, they also include analog-to-digital converters etches into the same chip for use with digital cellphones.
The MEMS microphones used in cellphones today are based on the design of the electret condenser microphone, though they have recently surpassed traditional electret mics as the most common microphone in cellphones.
With highly automated processes, layers of different materials are deposited on top of a silicon wafer. These materials are then etched away in order to create a moveable membrane and a fixed backplate over a cavity in the base wafer. The materials in the moveable membrane and/or the fixed backplate are capable of holding a permanent charge.
The MEMS diaphragm is permanently charge against a backplate. As it moves, it causes a varying capacitance between the plates, just like the aforementioned electret condenser mic.
As the capacitance charges with a fixed charged in the MEMS microphone “capsule,” a coincidental AC voltage (mic signal) is produced across the plates.
This mic signal is then processed, within the same chip, by an impedance converter “amplifier” and an analog-to-digital audio converter. The MEMS microphone then outputs a strong digital mic signal for the cellphone to properly use.
MEMS microphone, like electret microphones, require very little power (supplied by the microphone battery) in order to function. The power is only required for the impedance conversion and the analog-to-digital signal conversion.
Why Are Cellphones Designed With MEMS And Electret Mics Over Other Mic Types?
The simple reason as to why cellphone utilize MEMS and electret microphones is cost-effectiveness.
Electret condenser capsules and MEMS microphones are incredibly inexpensive and relatively easy to create. This means that cellphone manufacturers are able to make more profit from their cellphones while also having a constant supply of microphones to keep up with their production demands.
The sound quality of these cheap microphones is not a huge factor in the overall cellphone quality. The truth is that phones and cellphone have never had great audio, and that’s perfectly acceptable. So long as communication is possible, the cellphone microphone is a success.
In other words, these inexpensive electret and MEMS mics are not all that great at capturing accurate audio, but it doesn’t really matter so long as they capture the important frequencies of human speech. Cellphones do not require top-of the line mics.
The MEMS and electret mics used in cellphones are also very tiny and can easily fit in the locations they are designed to fit within. As cellphones continue to get flatter, they require smaller and smaller microphones. Electret mics and especially MEMS microphones are suitable for this technological challenge.
So to recap, cellphones use electret condenser and/or MEMS microphones for the following reasons:
- Cost effectiveness.
- High-quality audio is not required.
- Size restraints.
Can Cellphones Use External Microphones?
So we’ve discussed the microphones that are designed within the bodies of modern cellphones. However, the original question was “what kind of microphones do cellphones use?”
So even though cellphones are built with electret and MEMS internal microphones, they can also “use” external microphones.
Cellphones can connect to wireless Bluetooth microphones (modern smartphones all have Bluetooth capabilities).
Smart phones (which make up a great number of the cell phones on the market today) can also accept signal from external microphones though their headphone jacks (TRRS) and through their charging ports.
With the proper connectors, adapters, power supplies, wireless systems, and/or analog-to-digital converters, it’s possible for any microphone to send audio to a smart phone.
For more information on using external microphones with cell phones, check out my article How To Connect An External Microphone To A Smartphone.
What smartphone has the best microphone? Though some smartphones may have better microphones than other, there is no standout phone with the “best microphone.” If we’re looking for a great smartphone mic, we’re better off looking for a plug-in mic. The best external mics for smartphones include:
- Rode smartLav+ (link to check the price on Amazon).
- IK Multimedia iRig Mic Studio (link to check the price on Amazon).
Where is the microphone in a smartphone? Smartphones typically have several microphones in their design. They are generally found at the ends of the smartphone (near the top and bottom) though they may be found elsewhere.