Information on the range of Police and Emergency Sirens

[brian]

Q: Where can I obtain information on the range of Police and Emergency Sirens?
A: Guide to Test Methods, Performance Requirements, and Installation Practices for Electronic Sirens Used on Law Enforcement Vehicles NIJ Guide 500–00 Randall Wagner, National Institute of Standards and Technology (No charge for the document)
1.INTRODUCTION: Sirens are devices that produce warning sounds. Siren sounds are intended to help alert the public that an emergency vehicle (e.g., police car, ambulance, fire truck) is nearby and responding to an emergency. These sounds should be recognized as the call for the right-of-way of the vehicle.
There are three types of emergency vehicle sirens: electronic, mechanical, and electromechanical. Mechanical and electromechanical sirens are powered by a mechanical connection to the vehicle drive train or an electric motor. These sirens produce sound by pumping air through a perforated disk revolving in front of a fixed structure with ports. As the disk turns, the perforations pulse the air flow through the ports. The successive pulses of air produce the familiar mechanical siren sound, which modulates in frequency and amplitude as the disk and air flow speed up, and slow
down. An electronic siren is a system of components that includes one or two electronic siren loudspeakers (seldom more than two) and an electronic siren amplifier powered by the vehicle electrical system. Most sirens sold for law enforcement vehicles are electronic. The size of most electronic siren loudspeakers permits them to fit in smaller spaces than mechanical and electromechanical sirens.
This guide is written for persons who select, install, or operate electronic sirens on law enforcement vehicles, or instruct those who do. It discusses the contents of several documents, listed later in the guide, that specify test methods, performance requirements and installation practices for electronic siren systems. The goal is to familiarize law enforcement personnel with these documents and the meaning of compliance with each one. Apart from this issue, the basic characteristics of electronic siren systems are discussed, as well as special features and installation options. The related issues of occupational hearing loss and exposure to siren noise are also addressed. All of the material presented in the first five sections of the guide is fairly basic in nature. Technical details are limited to section 6.
also see
SAE J1849 APR 2008, Emergency vehicle Sirens (The SAE J Standard costs ~$63 list, $50 for SAE members)
RATIONALE: 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 Title13, 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.