58.13.2 Earphones

Handsets normally have a high acoustic impedance and high gain output. Most headsets cannot achieve the same acoustic impedance and gain without amplification. With close coupled earphones, as used on over the head style and under the chin style headsets, this can be achieved easily either with or without an amplifier, according to the system specification with which they are interfacing. How­ever, to overcome the use of unhygienic eartips on single sided headsets, an acoustic low impedance earphone is used which has been specifically designed to work with an acoustic leak between the earphone diaphragm and the ear canal. The first headset to employ this principle is GN Netcom's Profile which has an adju­stable earphone to provide users with maximum comfort combined with optimum sound pressure output from the earphone.

58.14 The growing need for headsets

PABX and ACD system designers have always taken care to ac­count for the needs of the headset user. Now more and more key system and telephone instrument designers are also starting to recognise those needs particularly in view of the increasing telemar­keting activities in the UK.

In the US, every telephone instrument and system normally has a facility to operate with a headset. And since the UK tends to follow closely trend in the USA, we can expect that this will happen here. With screen based work booming, telephone system designers must ensure that all new equipment, including 2-wire sets, have a headset port as defined in BS 6301.

To facilitate optional handset/headset working a switch arrange­ment can be incorporated simply into many systems. The Dana-switch provided by GN Netcom for this purpose features in many key systems currently on the market.

There is also general feeling among leading headset designers that if OEMs worked with them at an early stage in the design of equipment, and there are headset companies perfectly willing and able to do this, then users would benefit considerably from in­creased versatility and a greater choice of headsets.

58.15 Headset approval process

In terms of approvals, much depends upon the telephone system that the headset is to be used with, and it is frequently the OEM who obtains the required approval, under DTI specification 85/013.

The approval process is a lot easier nowadays since the introduc­tion of the BABT simple attachment scheme. Previously, testing was carried out under the direct control of BABT and, in the event of failure, the modified apparatus had to repeat the entire test procedure, at considerable expense. Under the new arrangement, testing is carried out directly with one if the accredited test houses who then submit their report to BABT. Testing, and charging, is carried out until a failure occurs or the test is completed. In the event of a failure, the test house informs the designer who can then take the equipment away, rectify it and bring it back to continue testing at the point at which failure occurred.

The test is designed to ensure that:

1. The headset does not electrically damage the public telephone system.

2. That it meets electrical safety standards.

3. That no damage to the user can occur through acoustic shock.