1. Poor matching between the speakers and the power amplifier
Inexperienced sound operators assume that an oversized output from the power amplifier is what damages the tweeter unit, but that is not the case. In professional settings, a loudspeaker can generally withstand a large signal surge of three times its rated power, and can momentarily withstand a peak surge of five times its rated power without any problem. So it is extremely rare for an oversized power amplifier to burn out a tweeter, other than through an accidental strong surge or prolonged microphone feedback.
As is well known, a speaker cabinet contains several drivers, and the power each driver handles is distributed differently according to the crossover frequency.
As for a speaker's rated power, professional speakers generally state maximum pink noise power, meaning the rated power refers to the analog signal power the speaker can handle with pink noise or a wide frequency band. Take a two-way speaker with a crossover frequency of 1.6 kHz and a rated power of 100 W: at rated power the woofer is allotted 78 W of output power, while the tweeter is allotted only 22 W. So the speaker can handle 100 W of pink noise power or ordinary program signal power fed to it; but when tested with a 100 W single-frequency signal, either the tweeter or the woofer may be damaged.
If a speaker is three-way, with the mid-to-high crossover at 4 kHz, then the tweeter handles only 5% of the nominal power. If that power is distributed improperly, the tweeter unit is easily damaged.
Under normal conditions, if the signal fed to the speaker doubles, the power at the tweeter increases by only 5 W. But if the power amplifier lacks power, so that the signal overloads and clips, the higher-order harmonic content soars. A 1 kHz sine signal that overloads and clips toward a square wave will, besides the 1 kHz sine wave, generate a large amount of odd-order harmonics, sine energy at 3 kHz, 5 kHz and so on, greatly increasing the proportion of high-frequency content in the signal and pushing the high-frequency spectral energy far beyond what the tweeter can handle. Even if the total signal power has not yet reached the speaker's rated power, the tweeter is already overloaded and damaged. This situation is more dangerous than a brief signal overload that does not clip. When the signal is undistorted, a brief overload of a 1 kHz signal deposits its power in the higher-powered woofer, which does not necessarily exceed the driver's short-term maximum power, so it generally does not cause a power-distribution error in the speaker that damages the driver unit. Therefore, under normal operating conditions, the rated output power of the power amplifier should be 2 to 3 times the rated power of the speaker, so as to ensure that the power amplifier does not cause distortion when the speaker is at its maximum power.
2. Improper use of the crossover
An improperly chosen crossover frequency at the input, or an unreasonable operating frequency range for the drivers, is another cause of tweeter damage. When using a crossover, the crossover frequency should be chosen sensibly and strictly according to the operating frequency range the manufacturer gives for the drivers. If the crossover frequency for a tweeter is set too low, the power burden it carries is too heavy and the tweeter unit burns out easily; the same is true of a midrange horn.
3. Improper adjustment of the equalizer
Adjusting the equalizer is also crucial. A frequency equalizer is there to compensate for the various defects of the room's sound field and for the unevenness of the speakers at each frequency, and it should be adjusted using a real spectrum analyzer or other instruments. After adjustment, the transmission frequency characteristic should be fairly flat within a certain range. Many sound operators without audio knowledge adjust it at random, and quite a few push the high- and low-frequency sections of the equalizer far too high, producing a "V" shape. If those frequencies are boosted more than 10 dB compared with the mid frequencies (the adjustment range of an equalizer is usually 12 dB), then the phase distortion introduced by the equalizer will seriously color the music, and at the same time it will very easily burn out the speaker's tweeter unit; this is also a main cause of blown speakers. Of course, an audio system should be designed according to actual conditions, taking comprehensive account of venue size, purpose and architectural acoustics, and the maximum continuous sound pressure level should be determined from the actual operating conditions, which then determines the maximum SPL value of the speaker.
4. Adjusting the volume
Quite a few users set the attenuator on the power amplifier to -6 dB or -10 dB, that is, the volume knob at 70%-80% or even the midpoint, and then rely on raising the mixer input to reach a suitable volume, believing that leaving the power amplifier some margin keeps the speaker safe; in fact this too is wrong. The attenuator knob on the power amplifier attenuates the input signal, so attenuating the amplifier input by 6 dB means that, to keep the same volume, the mixer or preamp must output 6 dB more, with double the voltage, and the upper dynamic margin of the input, commonly called headroom, is cut in half. If a sudden large signal then arrives, the mixer output overloads 6 dB sooner and produces a clipped waveform. Although the amplifier has not overloaded, its input is a clipped waveform with an excessive high-frequency component, so not only is the treble distorted, the tweeter unit may also burn out.
The analysis above makes one thing very clear to us: an important reason a speaker burns out its tweeter unit is that the power amplifier has too little power, not too much. The signal the power amplifier sends out is itself a clipped signal, which damages the speaker. So when configuring audio equipment you must build the right understanding and adopt the "big horse pulling a small cart" approach, to prevent the power amplifier from sending out a clipped signal that damages the high and mid frequency driver units. When designing an audio system, the design power of the amplifier and the speakers must be matched according to the principles above, and in practice the equipment at each stage must be applied sensibly, so that the equipment is protected and the audio system at the same time achieves the best possible result.
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