I need a little education on basic car stereo systems. Normal listening volume, no metal rattling base, ear splitting loudness, etc. Just a radio (Retro Sound Hermosa 24 watts RMS/45 W peak X 4 channels), no amp, no sub, etc. For a given amp output from a radio, does a lower impedance speaker (say 3 ohm in lieu of 4 ohm) mean the speaker will perform better, be louder at the same volume setting, or ? Does a lower RMS amp range of a speaker mean it will sound better at lower amp inputs? What is a good frequency response number? Sensitivity? What speaker spec is the best to compare for good sound quality? Looking at 6 x 9 speakers.
In an ideal world if you were to have 100 watts into an 8 ohm load then it should double into a 4 ohm load and then double again into a 2 ohm load and so on but no amplifier is 100% efficient.
Normally too when a load decreases it makes the amplifier run hotter and also will have it pulling more power.
These are just general statements as there are a lot of variables out there but if an amplifier is rated to run into a 4 ohm load, run it that way or with a higher load (8 ohms, 16 ohms, and so on). Some amplifiers can be run at a lower load than what is specified by the manufacturer and in those cases, you are kinda rolling the dice as to how things will last or what might or might not break.
A speaker has what they call a nominal impedance. Not to get too technical but a speaker at some frequencies may measure 4 ohms, at another frequency measure 3 ohms, and then at even another frequency measure 20 ohms or more but basically it's rated at 4 ohms and if you wire it to a radio rated to handle a 4 ohm speaker, all should be well.
Another specification of a speaker is sensitivity. Basically the higher the number, the louder it will be. A 4 ohm speaker with 92dB effeciency will be louder than another 4 ohm speaker with 89dB effeciency. Again, this is basic and what I would suggest is not to look super hard into the specs although things need to be compabible, but listen with your ears and buy what sounds best to you.
Another aspect on the system is making sure things are connected right and the speakers are mounted properly. I had a car in the other day that the customer said it sounds bad and he turned it on and right away I could tell some speakers were not wired properly. We faded the radio to the rear speakers and jacked up the bass and there was some bass coming out of both but when we went to just the left or the right speaker, the bass became more pronounced and louder. This told me one speaker was wired up backwards.
On rear deck speakers, or any speaker for that matter, they need to be mounted properly so that they perform properly. On the edge of the 6x9 speaker there is a flange and this flange must seal against the opening it's mounted to. On my 68 Nova I bought a replacement perforated rear deck cover and mounted the speakers to the underside of the metal package shelf. Due to the speaker I had to make a spacer since the tweeter protruded above the flange but basically the edge or perimeter of the speakers flange and the spacer sealed up against everything so that now the sound eminating off of the backside of the speaker cannot come into contact with the sound eminating off of the frontside of the speaker so it can create the most bass it can. In other words if the speaker is not mounted properly and it's just laying loose on the rear seat it will have good highs but the lower you go into the midrange and even lower into the bass sounds, it won't sound as well as a speaker mounted properly. Think of mounting a 6x9 like how you would put a motor together. You don't run a 6x9 laying loose, nor do you run a piston motor without rings.