Perception of Broadband alarms in machine noise Subjective measurements.
2.1 The noise from a number of stationary machines, with engines running fast, was recorded at a sand quarry (Sevenoaks – UK):
JCB 380 Excavator
Cat 725 Dumper
Volvo L150 E Wheel loader
Cat 962 Wheel Loader
Linde 2.5 Fork lift
2.2 Each machine’s noise at a constant level about 90dB(A) was mixed sequentially, in a digital editing package, with nine different levels of broadband sound. The first (inaudible or just-audible) level was increased in eight steps of 3dB to a very clearly audible level about 24dB higher. The combined machine and alarm sounds were presented via a loudspeaker to ten subjects facing it over 5m of hard ground.
The subjects were instructed to complete forms indicating when
The broadband alarm was not audible over the machine noise.
The broadband alarm was audible over the machine noise
The broadband alarm was both audible and judged to be at a sufficient level over the machine noise to be effective as an alarm signal.
This procedure was undertaken a total of three times for each of the five machines
2.3 . Three times, ten subjects recorded nine responses for each of five machines. Total number of responses therefore was 1350
2.4 Figs 4 to 8 show the subjects’ responses for the audibility of the alarm signals at different levels of alarm over the machine noise. The presentation levels to the subjects were typical of a location a few metres behind a machine.
Levels of the machine noise and alarm noise, as measured from signals on the CDRom which was used to present the sounds are in Table2.
| Alarm signals | Machines | ||
| Level number | LAeq dB | Type | LAeq dB |
| 1 | 59.4 | Cat 725 | 78.0 |
| 2 | 62.2 | JCB 380 | 82.9 |
| 3 | 64.6 | Cat 962 G | 86.7 |
| 4 | 68.1 | Volvo L 150 E | 85.5 |
| 5 | 70.8 | Linde 2.5 | 86.3 |
| 6 | 74.1 | ||
| 7 | 77.0 | ||
| 8 | 79.9 | ||
| 9 | 82.8 | ||
Table 2 Levels of Alarm and machine sounds Table 3 is derived from Figs 4 to 8 , showing the alarm level number , as in Table 2, when listening to alarms in over machine noise when The alarm was audible either to most subjects or to all subjects, where 30thirty is the maximum number of responses. (i.e. ten This number results from 10 subjects and three repetitions of the sounds).
Most subjects and all subjects considered the level of the alarm in over the machine noise to be suitable as an alerting alarm signal.
| Machine | Alarm level for Audibility over machine noise. Number of responses indicating most subjects or all subjects | Alarm level for good alarm function. Number of responses indicating most subjects or all subjects |
| Cat 725 | Level 4 29/30 - most Level 5 30/30 - all |
Level 6 21/30 - most Level 7 30/30 - all |
| JCB 380 | Level 5 29/30 - most Level 6 30/30 - all |
Level 8 29/30 - most Level 9 30/30 - all |
| Cat 962 G | Level 5 22/30 - most Level 6 30/30 - all |
Level 8 26/30 - most Level 9 30/30 - all |
| Volvo L150E | Level 4 29/30 - most Level 5 30/30 - all |
Level 7 29/30 - most Level 8 30/30 - all |
| Linde 2.5 | Level 5 25/30 - most Level 6 30/30 - all |
Level 7 24/30 - most Level 8 30/30 - all |
Table 3 Perception of alarms in over machine noise A score of 29/30 means that all ten subjects found the level satisfactory for two of the three presentations, whilst nine subjects found it satisfactory for the third presentation. As each level step corresponds to 3dB, the change in level between all people hearing the alarm in over the machine noise and all people considering the alarm to be of sufficient level to function as a good alarm, was 6 to 9dBA. A further comparison can be made between the machine level and the alarm level at which the subjects considered the alarm function to be effective, as in Table 4. This Table shows that this is generally when the A-weighted level of the alarm level is similar to or below the machine level. Where a range is given for alarm levels, this is for 29/30 and 30/30 responses.
| Machine | Machine level LAeq dB | Alarm level LAeq dB for effective action |
| Cat 725 | 78.0 | 77.0 |
| JCB 380 | 82.9 | 80 - 83 |
| Cat 962 G | 86.7 | 83 |
| Volvo L150E | 85.5 | 77-80 |
| Linde 2.5 | 86.3 | 80 |
Table 4 Comparison of Machine Levels and Alarm Levels
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