IRSST - Institut de recherche Robert-Sauvé en santé et en sécurité du travail

Study of the Applicability of Parametric Speakers to the Development of New Backup Alarm Concepts


The use of backup alarms is mandatory on most heavy vehicles and remains a widely used means of alerting people working in their vicinity. Previous studies conducted by the IRSST have shown the limitations of the effectiveness of alarms used in the workplace to minimize risks to workers’ safety. Because of the loud and very distinctive sound they produce, workers and people living nearby often regard backup alarms as annoying. These nuisance and sound perception problems are amplified when workers work in semi-open or completely enclosed environments, such as warehouses, where sound bounces off walls, floors and ceilings, increasing the noise level and creating parasitic reflections that interfere with perception. Yet, there are no proven solutions to address all these problems.

The proposed study explores the use of parametric speaker technology for the design of a directional backup alarm. The technology has existed for many years for audio applications, and has recently become more accessible through miniaturization and lower costs. This type of loudspeaker produces a focused sound field that can be aimed toward and concentrated on specific directions or areas. Outside these areas, the sound produced is not or only slightly perceptible, which could be a solution to the problems mentioned previously. The work presented here is intended to be a proof of concept in which the case of forklifts is considered. The use of these vehicles is common on construction sites and widespread in enclosed and reverberant spaces (where trailers are loaded, warehouses).

In parallel with the study of the applicability of this new backup alarm technology, an in-depth comparison of the two types of commercial backup alarms currently available was carried out. Tonal alarms, with their characteristic “beep-beep,” are a significant source of environmental nuisance and have several limitations, such as large variations in sound levels depending on the distance. The broadband type of alarm, now available on the market, makes a “psht-psht” sound. A priori, this new type of alarm shows interesting characteristics in terms of directivity, homogeneity of the pressure field generated and reduction in environmental nuisance. However, few studies have compared the performances of these two types of alarms in the field, and no comparative laboratory study has been conducted to date.

In this study, two types of commercial alarms and two models of commercial parametric loudspeakers of different dimensions were first tested extensively in the laboratory (directivity and attenuation measurements according to distance). Three-dimensional measurements of the radiated sound pressure field were taken according to the type of alarm signal (tonal or broadband) and the environment (semi-anechoic or reverberant). The effects of ground conditions and mounting on a rigid surface were also investigated.

At the end of the project, a simplified prototype using a parametric speaker and two commercial alarms was tested on a forklift performing a backup manoeuvre in an industrial corridor (reflective environment).

The results show that the use of parametric speakers is a promising avenue for developing backup alarms, with notable advantages. One advantage is the very high degree of directivity, which is not achievable with currently available commercial alarms and which enables a sound signal’s diffusion to be limited to a specific area. Other advantages naturally follow from this high degree of directivity, such as limiting noise exposure for machine operators and workers nearby, and reducing environmental nuisance.

However, immediate application still faces a number of hurdles. The first hurdle is the low volume level compared to commercial alarms, which is due to the fact that the parametric speakers used were developed for audio applications and low voltage power signals. This problem could easily be overcome by developing a parametric speaker made specifically for backup alarms, with a five to ten volt power supply. A second hurdle is the possible appearance of a phantom sound source caused by reflection of the directional beam on a nearby surface. The study suggested a simple solution to this problem, by placing a perforated grille in front of the loudspeaker, which would diffuse the signal emitted and thus significantly reduce this adverse effect. The grille has the added advantage of providing mechanical protection, which is useful in an industrial context.

In the case of the two commercial alarms, the study also highlighted the generally superior performance of the broadband alarm compared to the tonal alarm. Indeed, all of the results show that, generally, compared to the tonal alarm, the broadband alarm is less sensitive to ground effects, effects related to the space, or effects related to sound wave diffraction. This conclusion applies both to laboratory measurements and to tests conducted under realistic conditions.