Actually, in the case of Focal themselves, the best bet is to pull up their data sheet and take a look for yourself!
1) Go to the Focal link here:
http://www.focal.com/en/access/465-165-ac.html2) If you scroll down towards the bottom of the page, you'll see a "Documentation" section. Click on "Access 165 AC Technical sheet"
3) Look at the graph labelled "Frequency Response/Impedance". You want to focus on the black line, not the red one. Along the vertical axis is output/volume, and along the horizontal access is frequency. The theoretically perfect speaker should output all frequencies at the exact same volume level - in other words, the graph should be as close as possible to a straight line. The black solid line shows you how the speaker performs on axis - i.e. pointing directly at your ear. The black dotted line shows you how the speaker performs 30 degrees off axis (i.e. angled 30 degrees away from your ear). In a car your speakers are never going to be pointing directly at your ear (except maybe tweeters, but still uncommon) - typically they will be somewhere between 30 deg - 45 deg off axis. Because of this, when you look at a car audio graph you generally always want to look at the off axis measurement.
4) Notice that the off axis measurement (the dotted line) is almost dead flat from about 100hz to about 15,000hz? That's a great indication that this speaker has a very nicely balanced frequency response, which is not very common for a speaker at this price point. Frequency response isn't by any means the ONLY thing that determines how good a speaker sounds, but it is a big one.
5) Look at the parameters and specifications on the right. Notice the "SPL" figure of 87.19 dB/w/m. This is a measure of the speaker's efficiency - i.e. how efficiently can it convert input power into sound energy. Every speaker will lose some power in the form of heat - some speakers will waste more power then others. If a speaker with higher efficiency/sensitivity will go louder then one with lower efficiency/sensitivity when given the same input power. In this case the 87.19dB 1w/1m figure on this speaker tells us that if we were to feed this speaker 1w of power, it will produce 87.19dB of output in the form of sound energy. Another speaker with a sensitivity of 85dB, if given 1w of power, would only output 85dB of sound energy. Sensitivity doesn't directly impact on how good a speaker sounds, however it does tell you how loud the speaker will play for each W of input power. As a general rule a sensitivity of 83-84.9 is pretty poor, 85dB - 86.9dB is solid, 87dB -89dB is excellent. Sensitivity levels below 83dB and above 89dB 1w/1m are pretty rare. The sensitivity of this speaker is 87.19 which is on the good side, so it will be able to run quite easily off minimal amplifier power (or even off a head unit). As a general rule, you need to double your input power to gain 3dB of output, so if speaker A has 3dB higher sensitivity then speaker B, it means that speaker B will need twice as much power to reach the same output level.
6) Notice the "Sd" value - this refers to the surface area of the cone (a.k.a. the cone area). Also notice the "xmax" value in the specifications - this tells you the maximum linear excursion (i.e. how far the cone can move inwards and outwards, before hitting it's limits). A speaker is a bit like a car engine, in that every speaker effectively has a bore (cone area) and stroke (excursion). The displacement of the speaker can be calculated by multiplying the cone area by the excusion, and this displacement figure effectively tells you how much air the speaker can move. This will effective determine the mechanical power handling of the speaker and is critical to a speaker's ability to reproduce low frequencies at high volume levels. This speaker as an Sd of 136.85 cm^2 and an xmax of 2.5mm (0.25cm) so you could say it's displacement is roughly 34.2cm^3,
7) Notice the "Fs" value. This is the resonant frequency of the speaker - as a general rule, the lower the "FS" value, the lower the speaker is going to be able to play if running in free air. In fact the "FS" pretty much completely determines how low the speaker is naturally capable of playing, before other factors (like enclosure, etc) come in to play.
We'll leave the technical stuff there for the time being, as I don't want too intimidating with it, but using the information above we can now look at the technical sheet of the 570 AC (link:
http://www.focal.com/en/access/464-570-ac.html) and see how the two compare:
1. Frequency response165AC: Off axis response is mostly flat from 125hz to 15khz
570 AC: Off axis response is mostly flat from 140hz to 5khz
Result:
165AC wins by a mile. Not only does it play a little bit lower (125hz vs 140hz) it also plays a LOT higher (15khz vs 5khz) which means you are going to get a lot more detail with things like guitar strings, triangles, cymbals female vocals, and anything else that occurs high up in the frequency range. The 165AC is going to have MUCH better sound quality then the 570AC.
4. Resonance Frequency165AC: 73.17 Hz
570 AC: 78.9 Hz
Result:
The 165AC wins here, which means that in theory it should be able to play lower frequencies better then the 570AC can. This matches up with what we saw in the frequency response graph, as the 165AC played flat down to 125hz while the 570AC only played flat down to 145hz. This is a pretty important number, as it plays an important role in determining how good your midbass performance will be. The 165AC should do a better job of producing deep male vocals, bass guitars and drums.
3. SPL Sensitivity / Efficiency165AC: 87.19dB @ 1w/1m
570 AC: 88.65dB @ 1w/1m
Result:
570AC wins by a significant margin - if all you care about is maximum volume, then the 570AC will go noticeably louder. Not night and day louder, but noticeably. It has rougnly 1.5dB higher sensitiivty which means that the 165AC off 75w would go just as loud as the 570AC off 50w. would go off 75W. In the grand scheme of things this difference in output is worth mentioning, but if sound quality is your main goal then the frequency response difference is going to be far, far more important.
4. Displacement165AC: Sd = 153.93cm^2, xmax = 0.25cm, displacement = 38.48
570 AC: Sd = 136.85cm^2, xmax = 0.25cm, displacement = 34.21
Result:
The 570AC wins, but by a very small margin - only about 11% greater displacement which, in the grand scheme of things, is pretty much irrelevant. Not worth considering in your decision.
ConclusionIf you don't care at all about sound quality, and the only thing you care about is getting as loud as you possible can, then the 570AC is the way to go - it will go noticably louder. If you care even the slightest amount about sound quality, then the 165AC is the no brainer here. The difference in sound quality will be HUGE - pretty much night and day.