This revision of SAE J1849 was developed during the second phase of a planned three-phase process. Phase one was performed to further develop test procedures and minimum performance requirements for electronic siren systems with a single speaker and electromechanical sirens. Phase two was conducted to specify an additional acoustical test for siren systems with a large speaker or a large speaker array. The intent of phase three will be to specify tests and performance requirements for individual siren components so that an electronic siren system, created from components tested accordingly, will meet the acoustical performance requirements developed during phase one.
The primary goal of phase one was to further develop the measurement methods for siren performance specified in Title 13, Article 8 of the California Code of Regulations and previous versions of SAE J1849. As an outcome of phase one, the acoustical measurement methods are specified in more detail and reduce the uncertainty in the results. These methods produce data that are less dependent on the design of the sirens tested, and are more indicative of the sound levels produced across the entire frequency range of the measured signals. Phase one also produced a more comprehensive set of environmental tests than these other documents, acoustical performance re-testing subsequent to these environmental tests, and a specified procedure for the signal frequency measurement.
The required warm-up period for the SPL measurement was increased to 10 min in phase one. Measurement results obtained after a ten-minute warm-up period are much less sensitive to the magnetic circuit design of the siren loudspeaker under test (with respect to its thermal properties), since most loudspeakers have more closely approached thermal equilibrium after warming up for 10 min. A 10 min warm-up period also produces measurement results that better represent siren performance during periods of extended use on emergency vehicles. Since the SPL decreases with time as the loudspeaker heats, a longer warm-up period results in measured SPL values that are lower than those measured after only one minute.
Acoustical measurement instruments indicate SPL values obtained by averaging continuously over a given period specified by the exponential-time averaging constant, which is commonly referred to as the averaging time. For time-varying signals such as a siren signal, the averaging time will have a direct effect on the range of the fluctuations that occur during the measurement. In order to obtain results derived from the SPL produced across the entire frequency range of the measured signal, a long-term average SPL measurement is specified. This measurement replaced the measurement of the minimum, relatively instantaneous rms value of the SPL. The long-term averaging time was chosen to minimize the range of fluctuations observed when determining the average SPL produced across the entire frequency range of the siren signal measured.
Requirements for the SPL measured with the long-term time setting are lower than those measured with the fast time setting since the former measurement averages over the entire signal cycle, which includes more low level portions of the signal that occur over a wider frequency bandwidth. For measurements done with the fast time setting, the requirements for the SPL of faster cycling signals are lower than the requirements for slower cycling signals. This is due to the fact that a much wider bandwidth is sampled over the averaging time used with this setting for faster cycling signals than relatively slower signals. Therefore, with all other parameters equal, a faster cycling signal will produce lower measured maximum rms levels since more low-level portions are included in the sampled signal.
To reduce the uncertainty in the acoustical measurements, mechanical level recorders are not specified. These recorders are subject to overshoot, and have variable controls (e.g., writing speed) with nonstandardized settings that can affect the measurement results by changing the effective averaging time of the measurement. Measurements of the SPL produced by a siren off the device axis can be performed by continuously rotating the loudspeaker on a turntable while measuring the SPL at a fixed point. However, using a continuously rotating turntable makes it difficult to accurately determine the SPL produced by a siren loudspeaker as a function of angle since the SPL also varies with time in a cyclic manner. More accurate results are obtained with the turntable paused at each specified angle for a period at least as long as the siren cycle or the instrument averaging time, whichever is longer.
The goal of phase two was to develop a test that would determine whether the SPL measured for a given large speaker or speaker array at 3.00 meters can be considered to approximate a far field measurement result. Procedures for performing this test and reducing the acquired data are described in 5.11. Requirements for the test results are specified in 6.11.
The procedures for the vibration and corrosion tests specified in 5.2 and 5.3 respectively were revised to cite SAE J575 with no exceptions.
Electromagnetic Radiated and Conducted Emissions testing has been updated to reference CISPR 25 as the new radiated and conducted emissions test method since SAE 1113-41 is obsolete.