There exists a problem in how modern facilities manufacture pieces of metal, which I know was real in one example that I can remember. This example was one in which “Stainless-Steel Ice Cubes” were meant to be kept in a freezer, and later put into drinks to cool them off, without ever diluting the drinks, due to melting ice for example.
When I purchased a set of stainless-steel ice cubes years ago, what I noticed was that they had an invisible film of residue on them, from the manufacturing process. I noticed this film, because touching it would irritate my skin. So what I needed to do was, to put those ice cubes through my dishwasher, after which they no longer irritated my skin from being handled, and after which they were also safe to plunk into drinks.
The fact that they had such a residue actually implies, that the people in charge of the manufacturing process do not understand what the machines are doing. After all, if these stainless-steel ice cubes are meant to be put into drinks, then it’s actually a part of the design objective that they not have any toxic residue on them, or, that the customer be warned in a way that can’t be misunderstood, that he or she needs to put them through the dishwasher before using them.
And I suspect that this problem is more prevalent with the way modern countries produce stainless-steel, resulting in low greenhouse gas emissions, than it would have been using old-fashioned methods of working steel. After all, nobody seems to recall this problem from ‘the old days’ of steel manufacture.
But the exact same problem can take place, if other types of metal are being mass-produced. For example, if a 3.5mm headphone jack is being manufactured with copper surfaces, and if the process is skipped, which would normally be standard, to gold-plate those copper surfaces, then those surfaces could also end up with an invisible residue from the manufacturing process. If that happened, the next problem would be that the 3.5mm jack fits into its socket snugly, but that a proper connection does not form, so that sound output from the headphones may initially be muffled.
The reason why sound reproduction would be affected would be the fact that such a film would effectively place a resistor with unknown resistance in series with the headphones. And the problem with that would be, that headphones and loudspeakers do not have exactly the same resistance at every frequency. The resistance of such devices is actually referred to as their “impedance”, and gets lower for the highest and lowest audible frequencies. In the mid-range, those devices probably have considerably more than 8Ω, which their impedance probably comes down to, at the outermost frequencies. What this means is that during normal operation, headphone-drivers and speakers will draw more current at the highest and lowest audible frequencies, in order to result in a frequency-response that’s reasonably flat. Therefore, if the value of series resistance is any greater than 4Ω, the voltage actually appearing at the voice-coils will collapse at these frequencies, and sound output will become noticeably abnormal.
If somebody suspects that they have a headphone jack which is doing this, the only good advice would be to use a dry cloth, and to wipe any invisible residue off it.
My reader should be aware that I can’t be 100% sure, that the second case, of the headphone jack, was a real experience of mine. This may have happened to me, when I tested a new set of earbuds for the first time. But only hours later, that set of earbuds worked flawlessly, and continued to do so ever since. Of course, if there ever was any residue on its headphone jack, that could just have rubbed off outside my being aware of it, however silly that sounds.
In general, I’d say that I need to have observed such manufacturing deficiencies at least twice, before concluding that there’s a pattern to them.