Summary. The loudness of broadband alarms is shown to be greater than that of tonal alarms of a similar A-weighted level. Listening tests, comparing different levels of broadband alarm noise in machine noise, showed that the assessment criteria adopted for tonal alarms are not applicable to broadband alarms, for which different criteria should be developed. It is not acceptable to ignore these differences.
1. Loudness
1.1 For some time it has been clear to those familiar with broadband alarms that, at an equal A-weighted level to tonal alarms, the broadband alarms are subjectively louder. The reason for this has now become clear, and loudness measurements have shown that the difference is about 5dBA in level or 5 phons in loudness.
1.2 Consider the equal loudness contours of Fig 1, which are from ISO 226:2003 and show the phon contours.
The area of interest to alarms is higher levels and higher frequencies, as in the box in Fig 1.
This area is shown in Fig 2, which also indicates typical regions of tonal and broadband alarms. The ear is most sensitive where the equal loudness contours dip down to lower decibel levels. Consequently, tonal alarms, which are typically from 1200Hz to 1500Hz, have their fundamental frequency in the region where hearing is least sensitive. On the other hand, broadband alarms spread into the 2kHz to 4kHz region where the ear is most sensitive. The equal loudness contours show that, for equal decibel levels, there is a loudness difference in excess of 5 phons between about 1500Hz and 3000Hz, with the higher frequencies in this range sounding louder than the lower frequencies at the same decibel level.
1.3 The main conclusion from this, that broadband alarms can operate at about 5dB lower in decibel level but have the same loudness as tonal alarms, was also shown by measurement.
The spectra of typical tonal and broadband alarms are shown in Fig 3. The tonal alarm has first spectrum peak at about 1450Hz with a level of 97dB. There is then little energy until a harmonic at 2900Hz, which is 20dB down on the fundamental. A third harmonic (4350Hz) is 10dB down and a fourth harmonic (5800Hz) is 25dB down on the fundamental. The broadband alarm has a fairly uniform spectrum over the range from below 1000Hz up to about 3000Hz and has energy at the sensitive frequency region of hearing.
Table 1 Loudness of Tonal and Broadband Alarms
Direct measurements of A-weighted sound pressure level and loudness were made from recordings of broad band and tonal alarms by feeding the signals directly, as an electrical input, to a CEL 593 sound level meter which contained a Loudness Module measuring Zwicker loudness. Input levels were adjusted to read approximately equal dBA and the loudness measured, leading to Table 1. The broadband alarm is 5 to 8 phon louder than the tonal alarms of similar dBA level.
1.4 Implications.
A number of the criteria for tonal alarms differ from those for broadband alarms; therefore Standards must reflect these differences. To ignore them is unacceptable.
