exceed 90dB(A). Research has shown that sirens mounted behind the engine grille or under the wheel arches produces less unwanted noise inside the passenger cabin and to the side and rear of the vehicle while maintaining noise levels to give adequate warnings. The inclusion of broadband sound to sirens has the ability to increase localisation of sirens, as in a directional siren, as a spread of frequencies makes use of the three ways the brain detects a direction of a sound: Interaural level difference, interaural time difference and head-related transfer function. The worst installations are those where the siren sound is emitted above and slightly behind the vehicle occupants such as cases where a light-bar mounted speaker is used on a sedan or pickup. Vehicles with concealed sirens also tend to have high noise levels inside. In some cases, concealed or poor installations produce noise levels which can permanently damage vehicle occupants' hearing. Electric-motor-driven mechanical sirens may draw 50 to 200 amperes at 12 volts (DC) when spinning up to operating speed. Appropriate wiring and transient protection for engine control computers is a necessary part of an installation. Wiring should be similar in size to the wiring to the vehicle engine starter motor. Mechanical vehicle mounted devices usually have an electric brake, a solenoid that presses a friction pad against the siren rotor. When an emergency vehicle arrives on-scene or is cancelled en route, the operator can rapidly stop the siren. Multi-speaker electronic sirens often are alleged to have dead spots at certain angles to the vehicle's direction of travel. These are caused by phase differences. The sound coming from the speaker array can phase cancel in some situations. This phase cancellation occurs at single frequencies, based upon the spacing of the speakers. These phase differences also account for increases, based upon the frequency and the speaker spacing. However, sirens are designed to sweep the frequency of their sound output, typically, no less than one octave. This sweeping minimizes the effects of phase cancellation. The result is that the average sound output from a dual speaker siren system is 3 dB greater than a single speaker system. See also Blues and twos Chrysler air raid siren Civil defense siren Foghorn Klaxon Thunderbolt siren Warning system Notes and references External links Various examples Whistle Museum - Pictures of Siren whistles The Siren Archive - pictures of fire sirens