Introduction - Microphones are one of the most basic kinds of transducers. By converting vibrations in the air into corresponding variations in electrical current. Because microphones are all around us and used in all kinds of settings I first passed them off as too simple and not worthy of examination, especially compared to all the high tech choices such as RFID, force sensors, blue tooth etc. However after some examination on the subject for research into a toy design I am working on I decided that there was alot of information on the general subject of microphones as a sensor.
Types of Microphones
Dynamic microphones contain a plastic membrane or diaphram. A metal coil inside is connected to the diaphram on one end and a magnet on the other When the diaphram moves in response to air vibrations the coil moves across the magnet creating a current throught induction. The resulting signal is then converted into sound by powering a small load(speaker). The signal can also be amplified.
CharacteristicsThe dynamic microphone has excellent frequency response and doesn't need to be powered internally. You see dynamic microphones used for public address systems in auditoriums and also in music performances. Their design makes them quite durable and forgiving.
The condenser microphone is constructed with a pair of metal plates that move closer or further apart in response to air vibrations. One rigid plate is connected to ground, the other moving plate is flexible and positively charged by an external voltage.
CharacteristicsThe Condenser microphone is good for crisp sound and can be used for high quality recordings.
An Electret Microphone is a variation on the Condenser type of microphone, but instead of charging the flexible plate with external voltage, the diaphram is permanently charged. The characteristics of an Electret microphone favor middle to high-frequency ranges which make them suited to voice communications but not so for music with heavy bass.
Other types of microphones:There are other types of microphones worth mentioning but that will not be looked at for this report. Carbon Microphones are the original invention and were used in Bell's first telephone and remained in phones right up through the late 20th Century. The carbon microphone works with a diaphram covering a quantity of carbon granules that as compressed by sound waves offer more or less resistance to an electric current running through it. Piezoelectric Microphones:The element under the diaphram in this microhone is a piezo. The voltage generated by this type of microphone is proportional to the force. These microphones are used for high force measurement such as explosive blasts and as you might expect are limited to situations where precision is not needed.
Characteristics of Microphones
Frequency Response-The ability of a microphone to convert accoustic frequencies into AC voltages.
Sensitivity- The ratio of electrical output (voltage) to the intensity of sound input. sensitivity is most often expressed in decibels.
Directivity- The ability of a microphone to respond to sound coming from different directions, these vary in degrees but generally are either Omnidirectional or responsive to sound from all directions or Directional- microphones that work better in response to one direction.
Impedance- This characteristic represents the microphones resistance to the flow of an AC signal. In general the microphone should have a low impedence to the input devise's high impedance. The ratio or rule of thumb is to have the load impedence to be 10 times the source (microphone) impedance. Matching a high impedance source to a low impedance load will result in significant signal loss.
Microphones and Op Amps-Like peas and carrots No study of Microphone as sensors would be complete without the other half of this equation, Op Amps. Basically the microphone outputs too small a signal to be useful. So what I needed to look at for this report was using an Op Amp to boost the signal to at least Line Level.
Op Amps are "differential amplifiers"... the trick with Op Amps is using a technique called negative feedback. Voltage is fed from the output to the inverting terminal. By using a resister the across those pins the amplifier can regulate it's output based on the resistence level.
Op Amp IC's I looked at for my amplifier circuits
One of the most popular Op Amps for audio is the LM386 by National Semiconductor. The majority of audio amplifier circuits that I came across used this IC.
Building Op Amps
1. Jameco super snooper ear kit.
As a kit this probably the smallest board out there, even slightly smaller than the Vellemen amplifier. The small size of this probably was beyond my soldering skills at this time as I couldn't get it working. I did make on stupid error. Both IC's were suppose to plug into a small chip holder that was to be soldered to the board. I soldered the IC's straight onto the board which probably fried the chips.
2. From a schematic and plans in "Electronic Sensors for the Evil Genius" pg.6 High power audio amplifier circuit
This circuit was by far the best performing so far. The parts list was pretty straight forward, again this circuit is one of many that uses the LM386. Because it was a breadboard I guess that my sloppy soldering didn't get a chance to mess it up. The amplificaton while not large is plenty for line level into another piece of hardware.
3. Electronics Goldmine "General Purpose Amplifier".
This kit was easy to solder. The instructions and the fact that there was so little to solder made this kit very enjoyable to put together. The amplifier itself was not working very well, it seemed very powerful but the kit was plagued with feedback noise. In the instructions there was a note that it might do this if a microphone was too close to the speaker. I did separate the speaker and mic with a longer mike cord but probably not enough.
Tips for success in building Op Amps
- Solder quickly and at as low a temperature as possible.
- Solder not sensitive components first like resistors and capacitors
- Always solder an IC mount to the board last and insert the IC with gentle pressure
- Remove IC for changes or fixes to soldering.
My goals for this study of microphones
1. Learn enough about microphones to select the right type for my Toy Workshop Final Project. http://mutantears.blogspot.com/
2. To construct an Op amp that can fit inside the toy and amplify the ambient sounds around the user.
3. Keep an eye on the cost by using inexpensive microphones to better aproximate real concerns in toy production.
4. Learn something about the directional characteristics of microphones and apply that to my toy design.
5. Keep my toy fun and inexpensive.