Carbon microphone

The carbon microphone is a microphone, the electro- acoustic transducing principle is based on that effect sound generated by pressure fluctuations of the contact resistance changes by graphite parts between its terminals. Carbon microphones are rarely used.

Charcoal meal microphone

For conversion serves the pressure-dependent contact resistance between carbon particles ( carbon granules, carbon semolina ), which are compressed by a sound- receiving sheet membrane.

Construction

A sound- permeable housing is closed at the top with a metallic membrane. The housing is filled with charcoal meal (made from anthracite ). Under the coal semolina is the counter electrode.

Function

Between the diaphragm and the counter electrode, an electrical DC voltage to be applied across a load resistor (consumer ). Sound waves are transmitted across the membrane to the coal meal. The principle is based on a quasi " loose connection " between the carbon granules. The microscopic changes in position of the particles cause a modulation of the flowing direct current. In this case, however, the typical pressure-dependent graphite contact resistance plays a role.

The load resistor can be direct the earpiece ( electromagnetic transducer ) for phones - a gain is only needed with long distances.

Different application requirements can cater to a certain extent by different grain sizes.

Use

Charcoal meal microphones were used in large numbers in phones. It is believed that the development of telephony was greatly accelerated by the invention of the carbon microphone. Speech intelligibility was so good enough.

In the sound or music recordings carbon microphones are due to the low reproduction quality is not used.

In the 1960s and 1970s, the coal microphones in telecommunications by the dynamic microphones acoustically better, but were particularly replaced piezo microphones, in turn, the electret condenser microphone gave way in the 1980s, which has integrated the necessary amplifier electronics. The operating voltage is as usual obtained from the connecting line. This carbon microphones can be used in phones usually 1:1 replaced by electronic versions.

In the professional sound engineer, the carbon microphone has been displaced in the 1920 's and 1930's from the condenser microphone. Today in sophisticated audio technology dynamic microphones and condenser microphones are used.

Carbon microphones were also used as a throat microphone and direct modulation of tube- equipped stations.

Designs

Discrete graphite parts were used instead of coal grit, which are moved by the sound of each other - serve on pins in the model left from 1889 hanging by springs relative to a wood diaphragm moving graphite rolls.

Converter of carbon microphone left (rear view ): on graphite pencils hanging graphite rollers are moved by springs relative to a wood diaphragm

Drawing from a patent

The typical, previously used in telephones to closed design

Properties

Positive qualities:

• Has self- reinforcing properties, directly to the earpiece be connected in series and does not require additional reinforcement
• Low production costs ( mass product)

Negative characteristics:

• Severe noise by oxidation processes at the tips of the carbon grit
• Distortion often more than 5 percent
• Poorly reproducible transfer characteristics, position and vibration- dependent
• Elaborately maintenance or limited life
• Strong moisture- dependent parameters