its mechanical limits at lower frequencies. Tweeters can also be damaged by amplifier clipping (amplifier circuits produce large amounts of energy at high frequencies in such cases) or by music or sine wave input at high frequencies. Each of these situations might pass more energy to a tweeter than it can survive without damage. In some jurisdictions, power handling has a legal meaning allowing comparisons between loudspeakers under consideration. Elsewhere, the variety of meanings for power handling capacity can be quite confusing. Impedance – typically 4 Ω (ohms), 8 Ω, etc. Baffle or enclosure type (enclosed systems only) – Sealed, bass reflex, etc. Number of drivers (complete speaker systems only) – two-way, three-way, etc. Class of loudspeaker: Class 1: maximum SPL 110-119 dB, the type of loudspeaker used for reproducing a person speaking in a small space or for background music; mainly used as fill speakers for Class 2 or Class 3 speakers; typically small 4" or 5" woofers and dome tweeters Class 2: maximum SPL 120-129 dB, the type of medium power-capable loudspeaker used for reinforcement in small to medium spaces or as fill speakers for Class 3 or Class 4 speakers; typically 5" to 8" woofers and dome tweeters Class 3: maximum SPL 130-139 dB, high power-capable loudspeakers used in main systems in small to medium spaces; also used as fill speakers for class 4 speakers; typically 6.5" to 12" woofers and 2" or 3" compression drivers for high frequencies Class 4: maximum SPL 140 dB and higher, very high power-capable loudspeakers used as mains in medium to large spaces (or for fill speakers for these medium to large spaces); 10" to 15" woofers and 3" compression drivers and optionally: Crossover frequency(ies) (multi-driver systems only) – The nominal frequency boundaries of the division between drivers. Frequency response – The measured, or specified, output over a specified range of frequencies for a constant input level varied across those frequencies. It sometimes includes a variance limit, such as within "± 2.5 dB." Thiele/Small parameters (individual drivers only) – these include the driver's Fs (resonance frequency), Qts (a driver's Q; more or less, its damping factor at resonant frequency), Vas (the equivalent air compliance volume of the driver), etc. Sensitivity – The sound pressure level produced by a loudspeaker in a non-reverberant environment, often specified in dB and measured at 1 meter with an input of 1 watt (2.83 rms volts