Full Description (PDF)

HEADPHONE LIMITERS

Under Health and Safety regulations, employers are required to reduce the risk of damage to the hearing of employees from exposure to noise to the lowest level reasonably practicable. Noise is generally defined as unwanted sound and regulations concentrate on the control of noise. There are, however, sounds which are not unwanted but can be at such a level, sometimes unwittingly, as to present a risk of exceeding an individual’s daily personal noise exposure recommended level. Such a situation can arise from the use of headphones worn for long continuous spells whilst generating high sound levels. One BBC designed solution to this problem is the Headphone Limiter.

The small units are designed to protect headphone users from the effects of harmful sound levels. The units are powered from the incoming audio signal and are available to suit a wide range of different headphone impedance/sensitivity combinations. They may usually be set to limit the SPL in the range 85–110dBA.

The limiters are manufactured for Canford Audio under a BBC licence and are supplied fitted to a chosen pair of headphones and calibrated to a defined SPL. They are not available separately, other than to approved service companies who have appropriate calibration equipment approved by Canford. For repair purposes only, the hybrid circuits may be purchased separately.

Limiter Types

	Stereo	Mono
For high impedance headphone capsules:	EP5/24A	EP5/24B or EP5/29B
(approx 1k–2k ohm)
For low impedance headphone capsules:	EP5/25A	EP5/25B or EP5/29A
(approx 8 ohm – 600 ohm)
For low impedance, high-sensitivity
headphone capsules:	-	EP5/29C or EP5/29D
(approx 8 ohm–90 ohm; sensitivity >90dBA/mW)

Technical Specification:

Input:	Audio frequency signal balanced or unbalanced, maximum 35V rms continuous (worst case) for all types, except 17V rms for EP5/29C and EP5/29D.
Output:	As input, undistorted until onset of limiting. Insertion loss varies with calibration conditions
Sound limiting level:	Typically 85–110dBA, tolerance ±2dB. (Note: Not all levels are achievable with some headphone types).

Notes:

The above is a guide only and headphones need to be individually type assessed to see which Limiter is best suited. Assessment of a headphone not previously investigated and documented by Canford is undertaken subject to a fee. There is no charge where the headphone or headset has previously been assessed for the fitting of a limiter set to the limiting level specified. The limiter is fitted 200mm from the plug, unless otherwise specified. The fitting and calibration charge does not include changing or fitting a plug (other than that normally supplied under the stock code) on the headphone or headset; if this service is required, a quotation will be offered against a specific enquiry. Orders can be accepted over the telephone, but work cannot proceed until written confirmation of specification is received.

A range of headphones are available as standard products with limiters fitted inside the earcups, see ‘Level limited headphones and headsets’. Please contact Canford’s Tech Support team on 0191 418 1144 (email: techsupport@canford.co.uk) for further information on fitting limiters not covered in the standard product range.