A correspondent, having encountered such usages as 'Do you read me?' and 'I'm reading you loud and clear' in radio interaction, wonders what is meant by 'read' instead of 'hear'. It's an interesting example, as these are well-used expressions used in films and television where radiotelephony is a part of the plot, but they must seem odd to learners of English. For a start, the collocation of read and loud is unusual. And if it's radio, what is being read?
The interpretation is clear enough. The question asks about the quality of the signal being received. The response affirms that the signal is of good quality. The expressions are part of the jargon of two-way radio communication: read has been used along with copy ('Do you copy?'), receive, and other conventions, such as Roger (acknowledging receipt of the message). As radio is the medium, we might have expected a different form of words: 'Do you hear me?' and 'I'm hearing you loud and clear', where the collocations are normal. So what motivated read?
I've not been able to find a source which explicates the point. The usage has been around since 1930, according to the OED. (Receive in this sense is a little older; copy in this sense is not recorded in the OED.) My suspicion is that it was the ambiguity in hear which led to the search for alternatives. 'Do you hear me?' can actually have a negative interpretation, expressing attitudes ranging from mild insistence to aggressive rudeness - 'Are you listening to me?', 'I'm not going to say this again'... - the effect being reinforced by the absence of any facial expressions which might soften the force of the language.
But why read? Presumably it was motivated by the visual display on the receiving equipment, which would show a point of origin or frequency of the signal. This would often be the name of a place or a set of letters and numerals. But even if it were simply a waveform, then there was ample precedent for applying the verb in this way. Read had already been widely used in a range of senses other than that of interpreting conventional written language, as in to study, observe, or interpret something ('reading thoughts/my mind/ my heart/ the road...'). It seems to have been a natural development. As I say, I haven't found a source in the 1920s or 1930s which explicitly discusses this use of the verb, so if any reader is aware of one, do let me know.
Loud and clear was a collocation long before radio telephony. The first recorded use of it is in Lewis Carroll's Through the Looking-glass: 'I said it very loud and clear; I went and shouted in his ear.' That's how it stayed, resisting prescriptive criticism (that it should be loudly and clearly). And it remains the number one choice, beating the phonetic alphabet alternative (Lima and Charlie) and the fascinating 'I read you five by five'.
This was a response (also not in the OED) where the first number indicated the strength of a signal and the second indicated its readability. Each was rated on a scale from 1 to 5: signal strength went from 'loud' through 'good', 'weak', and 'very weak' to 'fading'; readability went from 'clear' through 'readable', 'unreadable', and 'distorted' to 'having interference'. So, if you read something '5 by 5' you could hear the signal well and also understand what was being said.
The origins of the expression are unclear, though it seems likely that it's another of those expressions based on parts of the body. One commentator suggests that it began with early pilots having to communicate with a controller, before taking off, by signalling with their hands, and showing with their fingers the quality of their radio reception. Plausible - but again, it would be nice to find a written source.