Olaf van der Spek wrote:
HDMI out - 1
1. What does it output?
It is easy to damage speaker drivers with steady state pure tones, even at low power levels!
1. I presume the signal it is fed; it should be a pass-through. It was not tried.
2. Speaker drivers are similar to electric motors. There is a voice coil -- enameled wire wound around a tube which fits around a pole piece in the magnet structure. That tube is called a former, and it is attached to the cone, which has a structure called a spider attaching the cone and former/voice coil to the basket (frame), keeping it centered. The surround is the soft material around the rim of the cone which attaches it to the basket.
In normal use playing program material, the signal is usually not steady, it is quite dynamic, making the coil move back/forth vigorously sometimes, and very minutely sometimes, most often simultaneously. Typical power range might be from well under 1W to 50W+ with program material when the volume is set to what I'd call medium-high. The fact that the signal is not steady is the main reason why the coil usually doesn't get too hot, it cools off between high power periods. Usually the biggest risk of damage is from an unexpected sudden peak, which might force the voice coil farther than it is designed to go, which can permanently break the elasticity of the suspension (spider & surround), break the wires to the voice coil or deform the former. But these are very unusual results with modest power amps.
When fed a steady state tone, over time, the coil heats up much quicker because it never gets a chance to cool down with a lower/different signal. Do this long enough with enough power, and the coil can get hot enough to burn the enamel. Most modern speakers use a very tight gap for the voice coil, because this improves efficiency. When the enamel on the coil burns, it bubbles, then after it cools, it is no longer flush with the coil, it forms little lumps on the surface. Those burn artifacts now rub against the wall of the gap, and case a scratchy noise whenever the speaker is played, sometimes intermittently, sometimes with only certain kinds of signals. If the burn is severe, it can actually fuse voice coil to the gap wall or break the wire or short it.
All the above can happen while playing program material at very high volume, but usually only when the users are under the influence of alcohol or drugs -- the audible distortion when speakers or amps are overdriven is usually too annoying/upsetting when people are sober, and they will turn the volume down before real damage occurs.
The only way to repair a burnt coil is to remove the whole cone, and replace the voice coil/former -- which often cannot be done except at the factory, depending on exactly how the driver was manufactured.
That's probably more than you wanted to know... but you asked.