spun by an electric motor. Electronic sirens are high efficiency loudspeakers, with specialized amplifiers and tone generation. They usually imitate the sounds of mechanical sirens in order to be recognizable as sirens. To improve the efficiency of the siren, it uses a relatively low frequency, usually several hundred hertz. Lower frequency sound waves go around corners and through holes better. Sirens often use horns to aim the pressure waves. This uses the siren's energy more efficiently by aiming it. Exponential horns achieve similar efficiencies with less material. The frequency, i.e. the cycles per second of the sound of a mechanical siren is controlled by the speed of its rotor, and the number of openings. The wailing of a mechanical siren occurs as the rotor speeds and slows. Wailing usually identifies an attack or urgent emergency. The characteristic timbre or musical quality of a mechanical siren is caused because it is a triangle wave, when graphed as pressure over time. As the openings widen, the emitted pressure increases. As they close, it decreases. So, the characteristic frequency distribution of the sound has harmonics at odd (1, 3, 5...) multiples of the fundamental. The power of the harmonics roll off in an inverse square to their frequency. Distant sirens sound more "mellow" or "warmer" because their harsh high frequencies are absorbed by nearby objects. Two tone sirens are often designed to emit a minor third, musically considered a "sad" sound. To do this, they have two rotors with different numbers of openings. The upper tone is produced by a rotor with a count of openings divisible by six. The lower tone's rotor has a count of openings divisible by five. Unlike an organ, a mechanical siren's minor third is almost always physical, not tempered. To achieve tempered ratios in a mechanical siren, the rotors must either be geared, run by different motors, or have very large numbers of openings. Electronic sirens can easily produce a tempered minor third. A mechanical siren that can alternate between its tones uses solenoids to move rotary shutters that cut off the air supply to one rotor, then the other. This is often used to identify a fire warning. When testing, a frightening sound is not desirable. So, electronic sirens then usually emit musical tones: Westminster chimes is common. Mechanical sirens sometimes self-test by "growling", i.e. operating at low speeds. In music Sirens are also used as