Microphones

Headset technical considerations

The ergonomics and mechanical design of modern headsets are of paramount importance. Different styles and configurations of head­sets, perhaps with a modular concept, are essential. Limited choice of headsets places suppliers and communication managers in an unnecessarily inadequate position.

The acoustic and electrical performance of the headset clearly is also fundamental to its design. In particular, the microphone and earphone transducers must be matched correctly, both to interface with the system being used and to suit user requirements. Ideally, the microphone and earphone transducers in the headset need to operate to the same electrical and acoustic performance as the handset. This is not always easy to achieve mainly due to size and weight considerations, and often sound pressure levels need to be compensated for by automatic gain control and adjustable volume control.

Most headsets use either standard electret or electromagnetic micro­phones. Both types can be boom mounted in miniature form, but the electret type is generally most popular. Acoustic tubes are used on some headsets but these generally provide poorer transmission quality than boom mounted microphones and are subject to consid­erable crosstalk problems.

Noise cancelling versions of both electret and electromagnetic microphones are also available. However the operation of noise cancelling microphones can often degrade transmission perfor­mance if users are not trained properly. There is a tendency to presume that a noise cancelling microphone will automatically provide a superior transmission performance. This is not the case. To explain this it is necessary to know how a noise cancelling microphone works.

The definition of a noise cancelling microphone is a microphone which functions according to the pressure gradient principle. It has two sound ports which give direct access to the front and the back of the microphone diaphragm. The sound ports are symmetrical around the diaphragm.

The noise cancelling microphone reacts to the difference of sound pressure on the diaphragm. It is necessary to distinguish between the sound field originating from a close sound source (called 'near field') and the sound field originating from a distant sound source ('far field'). Noise is a typical distant sound source. The microphone reacts differently to the two types of sound source. The near field frequency response closely resembles a 'normal' pressure micro­phone characteristic, while the far field frequency response shows a distinct proportional dependence on the frequency.

The directional characteristic of noise cancelling microphones is such that greater attenuation of microphone signals takes place when the sound activates the microphone simultaneously on each side of the diaphragm. In a polar form the characteristic would be shaped as a figure of eight. This means there are two noise reducing effects present in a noise cancelling microphone, partly the effect of direction and partly a strong dampening of low frequencies from the far field sound source.

The near field measurement less the far field measurement is termed noise cancellation. A good noise cancelling microphone has a noise cancellation of not less than 1 ldB.

Experience with use of noise cancelling microphones is that there is a very minimal real requirement for this type of microphone except in specialised uses such as military, heavy industrial, air traffic control etc. The main objection from other users being the distance dependence of the microphone i.e. the need for careful positioning of the microphone in close proximity to the lips of the user.

Дата добавления: 2015-03-31