What Is a Hidden Microphone?

Microphone, scientific name is microphone, translated from English microphone (microphone), also known as microphone, microphone. A microphone is an energy conversion device that converts a sound signal into an electrical signal. The classifications include moving coil, capacitive, electret and recently emerging silicon micro-microphones, in addition to liquid microphones and laser microphones. Most microphones are electret condenser microphones, which work by using a polymeric diaphragm with permanent charge isolation. ^[1]

Microphones can be divided into electric microphones and condenser microphones according to their transduction principles. Among them, the electric type can be subdivided into dynamic microphones and ribbon microphones. ^[2]

Most microphones are electrets

At the beginning of the 20th century, microphones developed from resistive conversion of acoustic and electric to inductive and capacitive conversion. A large number of new microphone technologies have gradually developed, including aluminum microphones and other microphones, as well as currently widely used condenser and electret microphones . The working principle of the coil microphone is that the diaphragm is vibrated by the human voice through the air, and then the magnetic coil winding on the diaphragm and the magnet surrounding the moving wheat head form a magnetic field cutting to form a weak wave current. The current is sent to the loudspeaker, and the fluctuating current is turned into sound in the reverse process. ^[5]

Microphone directivity

Directivity is also called the polar pattern of the microphone. It refers to the ability of the microphone to pick up sound from different directions. Generally divided into omnidirectional, heart-shaped, super-cardioid, 8-shaped. ^[6]

Omnidirectional (Omnidirectional) is also called non-directional, it has the same sensitivity to sound in all directions. Cardioid is a directional microphone with the strongest front-end sensitivity and the weakest back-end sensitivity. The Supercardioid pickup area is narrower than a cardioid microphone, but it also picks up sound at the back end. The figure 8 picks up sound from the front and back, but not from the side (90-degree angle). ^[6]

Microphone technical indicators

Sensitivity

Refers to the ratio of the open circuit voltage of a microphone to the sound pressure acting on its diaphragm. In fact, the microphone in the sound field will inevitably cause sound field scattering, so sensitivity has two definitions. One is the actual sound pressure acting on the diaphragm, called the sound pressure sensitivity, and the other is the sound pressure of the sound field without the microphone in the sound field, called the sound field sensitivity. The sound field sensitivity is divided into free field sensitivity and diffusion Field sensitivity. Usually the microphone for recording gives the sound pressure sensitivity, and the microphone for measurement gives the sound pressure or sound field sensitivity depending on the type of application. ^[6]

The unit of sensitivity is Volt / Pa (Volt / Pascal, V / Pa), which is usually expressed by the sensitivity level, and the reference sensitivity is 1V / Pa. ^[6]

Microphone frequency response

It means that when the microphone receives sound of different frequencies, the output signal will be amplified or attenuated with the change of frequency. The most ideal frequency response curve is a horizontal line, which represents that the output signal can directly and truly represent the characteristics of the original sound, but this ideal situation is not easy to achieve. Generally, the frequency response curve of a condenser microphone is flatter than that of a moving coil. The frequency response curves of common microphones are mostly high and low frequency attenuation, while mid and high frequency are slightly amplified; low frequency attenuation can reduce the interference of low frequency noise around the recording environment. ^[6]

In the frequency response graph, the horizontal axis is frequency and the unit is hertz. Most cases are expressed in logarithm; the vertical axis is sensitivity and the unit is decibel ^[6] . ^[6]

impedance

In the microphone specifications, the impedance value (in ohms) is listed. According to the Maximum Power Transfer Theorem, when the load impedance and the microphone impedance match, the load power will reach the maximum value. However, in most cases of impedance mismatch, the microphone can still be used, so this specification has not received much attention. Generally speaking, low impedances below 600 ohms; low impedances between 600 and 10,000 ohms; high impedances above 10,000 ohms. For example, the impedance of a microphone like Shure SM58 is 300 ohms. [6]

The 3-pin XLR connector can produce a balanced output signal, which can effectively eliminate external noise interference. The three pins will be marked with the numbers 1, 2, and 3; in the US regulations, 1 represents the ground wire, 2 represents the hot signal, and 3 represents the cold signal; in the European regulations, 1 represents the ground wire , 2 represents a cold signal, and 3 represents a hot signal. ^[6]

Equivalent noise

Used to describe the noise voltage of the microphone itself. ^[7]

Signal-to-noise ratio

It is measured by the logarithm of the ratio of the microphone output signal voltage to the microphone's intrinsic noise voltage. The S / N value of general high quality condenser microphone is 55 ~ 57dB. ^[8]

Directionality

Directivity describes the mode in which the sensitivity of the microphone changes with the spatial location of the sound source. All MEMS microphones from Analog Devices are omnidirectional microphones, that is, they are equally sensitive to sound from all directions, regardless of the orientation of the microphone. This picture looks the same whether the microphone's sound-receiving hole is in the xy, xz, or yz plane. ^[8]

Dynamic Range

The dynamic range of a microphone measures the difference between the maximum SPL and the minimum SPL of a microphone's linear response. It is different from SNR (in contrast, the dynamic range of an audio ADC or DAC is usually equivalent to SNR). ^[8]

Equivalent Input Noise (EIN)

The equivalent input noise (EIN) is the expression of the microphone's output noise level (SPL) as a theoretical external noise source applied to the microphone input. Inputs below the EIN level (SPL) are below the noise floor of the microphone and outside the dynamic range of the signal that the microphone can produce output. EIN can be derived from dynamic range or SNR parameters as shown below:

EIN = Maximum Acoustic Input-Dynamic Range

EIN = 94dB-SNR

Total harmonic distortion (THD)

Total Harmonic Distortion (THD) measures the distortion level of an output signal at a given pure single-tone input signal and is expressed as a percentage. This percentage is the ratio of the sum of the power of all harmonic frequencies above the fundamental frequency to the tone power of the fundamental frequency signal. ^[8]

Power Supply Rejection Ratio (PSRR)

The microphone's power supply rejection ratio (PSRR) measures its ability to suppress noise on the power supply pins so that it does not affect signal output. PSRR is measured by applying a 217 Hz, 100 mV peak-to-peak sine wave to the microphone's VDD pin. The PSRR measurement will show how much dB this input signal is attenuated from the microphone output. The reason why this parameter uses the 217 Hz frequency is because in GSM telephone applications, the 217 Hz switching frequency is usually a major source of noise in the power supply. ^[8]

Maximum acoustic input

Maximum acoustic input refers to the highest sound pressure level (SPL) that the microphone can withstand. SPL higher than this parameter will cause severe non-linear distortion of the output signal. The maximum acoustic input is specified by the peak SPL, not the root mean square value. ^[8]

Microphone connector

The 1 / 4-inch (6.3mm) connector and 3.5mm connector are divided into mono and stereo. The simple way to distinguish them is to see several black insulation rings on the connector. Stereo, an insulating ring represents mono.

1. Ground

2. Right channel in stereo; inverted signal in balanced mono; or as power input for mono

3. Left channel in stereo; positive phase signal in balanced mono; signal output terminal in unbalanced mono

4. Insulating ring ^[2]

Sound collection angle

Just as the focal length of the lens varies, the angle at which the microphone collects sound varies. The cardioid microphone can collect sound from multiple angles. The angle at which the supercardioid microphone collects sound is relatively small. The gun microphone collects sound at a narrower angle than the previous two. Unlike the lens, the critical point of the type of microphone is not precise. For single-person video recording, that is, shooting without a camera crew, the best microphone choice is a small gun microphone. ^[9]