Keeping workers safe with acoustic warnings


Do your systems of work use acoustic warnings (sirens, sounders, alarms, etcetera)? If you are using acoustic warning signals, how certain are you that they will have the desired effect, including keeping your people safe from harm?



Written by

Timothy Ward

Acoustic warning signals are in common use in the working environment. This might be alarms that warn of approaching vehicles or machinery, machine start-up alarms, or signals to warn that an event or condition (desired or otherwise) is about to happen or has happened. Whatever the case, acoustic warning signals should always have a fundamental characteristic: of being audible to the intended recipient.

The consequences of a warning that is not heard, or not appropriately responded to, can vary, however, the worst cases can be easily envisaged, for example, major or fatal injury from the coming together of people and machinery/vehicles, or catastrophic loss of machine assets.

In a recent case in which a Finch expert was involved, a worker had suffered catastrophic injuries when a machine was started while he was in a danger zone. He was working to clear a part of the machine at which blockages were common.

Two acoustic sounders were present on the machine, one at the control panel and one at the electrical control box, which sounded at the ‘pre-start’ stage. Our expert’s investigation showed that neither was clearly audible at the injured worker’s location.

Whilst multiple factors were involved in the event, it was clear that the audibility of warning signals played a part.

Legal duties

The employer has the responsibility to ensure that acoustic warning signals are generally suitable, which includes being easily recognisable. This applies to warning signals installed directly, and also to any warning devices intrinsic to machinery and equipment supplied to workers. Legal duties are therefore found under the Health and Safety (Safety Signs and Signals) Regulations 1996, and the Provision and Use of Work Equipment Regulations 1998 (PUWER). Under PUWER for example, the employer should ensure that where appropriate an audible, visible or other suitable warning is given whenever work equipment is about to start (Regulation 17), and that any warnings or warning devices are unambiguous, easily perceived and easily understood (Regulation 24). Given this, arguably, any ‘PUWER assessment’ that does not assess the audibility of acoustic warnings, is deficient.

There are complementary duties on the ‘supply side’ for work equipment. Suppliers of machinery must ensure where necessary that machinery gives acoustic and/or visual warning signals – covered by Essential Health and Safety Requirement (EHSR) 1.2.2 of the Supply of Machinery (Safety) Regulations 2008. Mobile machinery with a ride-on driver must have an acoustic warning device (EHSR 3.6.1). Warning devices (acoustic or light signals) must be unambiguous and easily perceived (EHSR Machinery manufacturers should be guided by the standards BS EN 981:1996+A1:2008, and BS EN ISO 7731:2008, in designing acoustic warning signals. A CE mark (or equivalent) on a machine indicates that the manufacturer has checked that it complies with all relevant EHSR.

Selecting and designing appropriate acoustic signals

What makes an acoustic warning audible, and be able to convey the appropriate information such as urgency or need for action? It is just a case of, the louder the better? Not necessarily: a startle or fright reaction should be avoided, as this can delay the person in taking the appropriate action. Advice in BS EN ISO 7731:2008 is that the signal should be designed to be audible in comparison to the background or ambient noise against which it is competing, and so should be distinctive in frequency content. It should not have a very rapid onset or change in sound level. Two dominant components are recommended, with time-varying signals preferred, for example pulsing on/off or varying frequencies. Needless to say, acoustic warnings should have these qualities in those areas in which they are intended to be heard.

What if the people who need to hear the warning are wearing hearing protection? Acoustic warning signal design can also take this into account. Distinctiveness remains the key, as both the warning signal and the background noise will be reduced. Employers should however check both distinctiveness and overall sound level with hearing protection.

The Health and Safety (Safety Signs and Signals) Regulations 1996 state that acoustic signals must be ‘considerably higher than the level of ambient noise’, and ‘easily recognizable’. Guidance in HSE publication L64 to the phrase ‘considerably higher’ states “acoustic signals should be set at a level which is considerably higher in terms of frequency than the ambient noise, for example 10 dB above the level of ambient noise at that frequency.”. This, in the author’s view is ambiguous, and simplistic, and BS EN ISO 7731:2008 gives much more sophisticated and helpful advice on making sure that acoustic warnings are audible. It may be that HSE agrees on this point, since its published operational guidance[1] (internal advice to inspectors) recommends that BS EN ISO 7731:2008 is used in assessing acoustic warning signals in the workplace, for example when investigating accidents and incidents.

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