Test of performance of horns by several manufacturers revealed that the predominant frequency was sometimes higher than the standard specified maximum of 2800 Hz, while all horns contained a rich spectrum of harmonic frequencies, starting below 500 Hz and reaching beyond 2800 Hz. To better describe such frequency content, definitions of excitation and fundamental frequencies, low and high sound frequency zones are added.
Added requirements for low and high frequency content will ensure that the horn sound is not concentrated in a narrow frequency band and will increase audibility of the horn signal in presence of masking sounds. Low frequency zone limit of 700 Hz will ensure at least five harmonics in the specified band ending at 3500 Hz.
The upper limit for predominant frequency increase from 2800 to 3500 Hz can only be supported when low frequency sound components are added to the requirements.
Low frequency requirement helps distinguish the horn sound from the warning sirens and back up alarms.
Rest time between successive sound measurements is added to avoid horn overheating.
Distance between horn and microphone is specified at 2.4 m to avoid near field measurements at low frequencies.
Moved predominant frequency definition from Requirements to Definitions section.
Replaced magnetic tape recorder with sound recorder to remove an obsolete term.
Clarified confusing g and displacement values in vibration test.