Reading a few 'choosing speaker with amp' guides, they generally say:
pick a speaker with rated/nominal input RMS power close to the amps continuous RMS output,
peak maximum power ain't as important,
high sensitivity speakers for low RMS amp,
low sensitivity for high RMS amps.
Looking at the headunit manuals, my old JVC said 50w max and 19w RMS, meanwhile my Sony only states 55w max and left out the RMS (their user manuals are pretty poorly written & vague too imo)
So with no info about what the Sony units RMS is and didn't wanna fit a low power speaker if the unit was able to pump a full 55w to fry the coils, I decided to investigate more into what the amp actually puts out to get a better overview.
Following a guide on how to test amps, I played a pure sine wave at various frequencies & volume on the headunit using a tone generator app on my phone via bluetooth, with the sound settings maxed out and measured the AC voltage coming out of one speaker channel.
recorded the voltage at every Volume level at various frequencies for both the JVC & Sony unit.
Initially I used a 12v 12.5amp power supply to run the headunit and discovered the amp can only output upto 12v max, same as what it was powered by.
Since the max output is dependant on the input supply I ran the test in the car with the engine running.
I also measured the input voltage at every volume and various settings.
The JVC draws quite abit of juice and the input reads a consistant 13.4v
Sony began at 13.7v and drops to 13.64v when the volume is above 33/50 but it was surprising to find the headunits LCD display brightness had a big effect in drawing soo much power.
With the display colour set to it's darkest setting the input was 13.7v but when turned to full bright it dropped to 13.3v, which will affect the amps max output so I turned it very dim for the tests.
Both units have a non-linear S-curve output vs volume.
The JVC has a wierd curve rising exponentially from 8-21, with the low freq being amplified more than higher freq.
Volume 21-31 oddly flatlines and then it slowly rises from 31-50 till it outputs a maximum of 14.5v (input supply is 13.64v during max volume)
The Sony in comparison has a much smoother S-curve, amplifying low freq more than high freq equally, and reaches a max output of 14.3v with the input at 13.64v
Using the equation Audio power = V(squared) / R
the JVC pushed out max 14.5v = 14.5 x 14.5 / 4ohm = 52.5watts (close to it's stated 50w max power)
It's RMS rating of 19w is approx 8.7v which according to the graph is around Vol 19, that's just above the volume I normally always listen at.
The Sony pushed out max 14.3v = 143.3 x 14.3 / 4ohm = 51watts (little bit off from the stated 55w)
So with the 'max output' figure meaning the absolute maximum juice the amp could constantly pump out with a pure sine wave, I could use the given JVC figure of 19w RMS to roughly estimate Sonys RMS as 21w
Later on I found on Sonys web site for their other headunit models that they stated 55w per channel and 20w RMS, so the estimate was close enough. wish they wrote it in the manual too
After days of researching every speaker make & model till my head is pounding (not a pun
) I narrowed down my selection to this coaxial Kenwood KFC-E1765
It handles 30w continuous RMS power
300w max peak power
good freq range of 30-22000hz
highly sensitive at 92dB/w
fits within the front door
and good price at £34 a pair
See how it goes when it arrives next week