Friday, December 27, 2013

FM Radio Station Microphone Part 15


Electret Microphone

A typical electret microphone preamp circuit uses an FET in a common source configuration. The two-terminal electret capsule contains a FET which must be externally powered by supply voltage V+. The resistor sets the gain and output impedance. The audio signal appears at the output, after a DC-blocking capacitor.

THE MICROPHONE is an important part of the radio station. In a simplified circuit, it must deliver reasonable sound quality and provide a usable frequency response and audio level.

An electret condenser microphone (ECM) consists of a very light diaphragm (moving plate) and back plate (stationary or static plate) and has a permanent charge implanted in an electret material to provide polarizing voltage.

The principle of operation is that sound waves impinging on the diaphragm cause the capacitance between it and the back plate to change synchronously, this in turn induces an AC voltage on the back plate.

1. Foil Electret Condenser Microphone (also called Middle or Classic) - Type of condenser microphone where the electret material is the diaphragm. (Sometimes referred to as "Front" type.)

2. Back Electret Condenser Microphone -Type of condenser microphone where the electret material is the back plate.

3. Front Electret Condenser Microphone -Type of condenser microphone where the electret material is the inside of the case of the microphone.

"It must be realized that all electret mics (indeed, all mics) have one limitation we cannot readily change, and that is maximum SPL (Sound Pressure Level). Because electret capsules have an integral amplifier, there will always be a level where they will distort.

A capsule having a 10k feed resistor and supplied from a 15V supply will output well over 1V RMS quite easily, simply by having it close enough to your mouth as you speak loudly.

Even professional microphones (including dynamic types) are quite capable of 0dBm in close proximity to a floor tom or a loud singer. As a result, close vocal work, drums and brass instruments (trumpet, sax, etc) are capable of extremely high SPL, and are not really suitable candidates for electret mics. It is possible to get good performance at up to 115dB SPL quite easily - possibly more. 

The sensitivity can be reduced, simply by reducing the value of the feed resistor. Again, there is a limit, as the internal FET amplifier can be driven into distortion regardless of what you do on the outside of the capsule. It is feasible to modify the capsule itself - but this is only possible with some models unless you are willing to make a few sacrifices (you can guarantee that you will ruin a couple in the process)." 

Frequency response
"Frequency response is the microphone's sensitivity performance in the frequency range of 0 to 20 kHz. Compared with the dynamic types, ECMs tend to have an extended response both at low and high frequencies, which is also smoother.In the case of dynamic types, the response limits are defined where the sensitivity has fallen by 3 dB relative to its value at 1 kHz. It is not stated in such terms for ECMs since they have a much wider frequency response." 

"To obtain the best results, avoid placing near reflective surfaces. If possible, mount in soft rubber to insulate from vibration. Treat ECMs as stat ic sensitive. Use specified heat sinking when soldering to ECMs. Use as little heat as possible for shortest time consistent with good joint. Avoid subjecting ECMs to high temperature and humidity that can degrade performance. The frequency response of ECMs may be too extended for communications application, in which case, it can be limited by suitable acoustic filtering in the intended housing or handset."

Microphone Setup
The legs on the electret microphone can extend through the cabinet. The mic is supported using a single strand color insulated copper wire. The mic electrical cable has a negative shield side to reduce interference. The microphone has a foam covering fabricated from a discarded headset. The wire support is bendable for best positioning.

Microphone Modifications
One improvement, since moving very close to the mic produces a much better signal and greater clarity, is a funnel that can direct the sound into the mic. This may be a great improvement and needs further testing. It may resemble a miniature old fashioned Thomas Edison "Gramophone." The cone can be fabricated from paper, cardboard or plastic from a bottle.

Microphone Measurement
The microphone resistance is measured at 1.37 ohms.

Microphone Impedance 

SPL - Sound Pressure Level

Microphone - Receiver that converts sound energy to electrical energy. Also described as a sound sensor.

Condenser - Another term used for a capacitor: two plates separated by a fixed distance capable of storing an electric charge.

Electret - An electrical property which describes the capability to retain electrical charges.

Pascal (Pa); bar (1Pa = 10 u bars) - Unit of pressure. For microphone applications, this unit refers to 
the pressure of sound on a microphone.


FM Radio Station Part 5 Index