This subject has even generated a fight since the days of orkut, fact that I am right.
Previously only speakers with two layers in the coil were made. They were made for 3,2, 4, 6, 8 or 16, 25 ohm, and so on ... they had only two layers of winding in the coil and that was it.
Modernly, they are made with 4 layers of wire in the coil. Perhaps for greater gain. They are heavy coils and more inductive as far as I can believe ... even so, the so-called "impedance" with which many hit the foot and stubbornly claim to be the only relevant quantity that is compatible. ..
What happens: When using these loudspeakers in old amplifiers situations occurs what Ill describe:
A- In asymmetric source amplifiers with capacitor in the output, usually STKs, the sound becomes a spectacle, much gain, a lot of bass, but with a frightening, extremely violent heating of the amplifier to look like iron in the heatsink ...
B- already in asymmetric source that do not use capacitor in series with the speaker the result is even "worse". You can not open the volume a little more because it breaks! Instead of music we have cráác cráác tráác.
What is up? In the conventional amplifier not right with the inductance of the speaker? Does the damping factor of the amplifier become useless to the supposed voltage generated by the return of the cone? Does the amplifier end up interpreting the speaker as a short? How are modern amplifiers made for these speakers? What is the difference between them and what could be done, (if there is solution) to make things compatible? I already tried a series capacitor and they "helped" a little. When I test a loudspeaker for a client, usually a car, 4 ohms, I put a large 2R2 wire resistor in series with it, otherwise I cannot hear it.
Modern speakers use a totally different construction technology. Much more powerful magnets are used and the shape of the coil concentrates the lines of the magnetic field better in order to obtain a greater concentration of the lines of force.
This causes them to be able to reproduce a much wider range of frequencies reaching the bass even with smaller cones. The result of this is the displacement of the resonant frequency that is reflected when using old amplifiers.
The old amplifiers have bootstraps which maintain the impedance of the circuit depending on the characteristics of the old speakers where the resonance and the impedance is usually given at 1 kHz. For these speakers everything changes, thats why there is a strange behavior overloading circuits or distortions occurring.
An important possibility would be to analyze the amplifier circuit to be used, analyze the response curve of the modern loudspeaker and eventually change the boostrap (capacitor and resistor at the output) to the new characteristics.
See that, not matching the characteristics the energy that should be converted into sound does not go to the speaker and there we have the heating verified by the reader.