I have two sets of speakers. One set is rated at 60W RMS and the other is 75 RMS (4 ohm each) I know the speakers in parallel will ohm at two ohm per channel, but what will the speakers RMS rating be?

 

I could be wrong but...

RMS rsting of the speakers never changes. The output of the amp changes depending on the load and the amp's output.

400w 2 channel amp, output at 2 or 4 ohms is 2x200
running this amp to (4x) 4 ohm speakers in parallel will result in a 2 ohm load. Amp is rated at 2x200 @ 2ohms so output is 200w to each channel, 2 speakers on each channel = 100w to speaker

Now some amps the 2 ohm load is different from the 4 ohm load so you'll need to check that.

 

RMS is the optimum wattage the speaker should be run at. Peak wattage is akin to the red line in RPM for an engine. If the speakers have an RMS of 75 and peak of 150 (Usually peak is twice RMS) then 100 watts ought to be fine

Also, be sure that the amp or head unit is 2 Ohm stable. If you run something at 2 ohms but it was not designed to do so it will over heat and do one or more of several things:

1. Put out distorted signal (clipping), which can blow a speaker faster than running more power to it than it was designed to handle. Back when I installed car stereo as a side job, the theory was it only take 1 watt of distorted signal to blow any speaker.

2. It will over heat and shut itself off temporarily or reduce its output (depending how smart it is)

3. It will over heat and actually be permanently damaged. I saw a flame come out of an amp because it was overloaded and run too long.

 

Ohms drop in parallel so the RMS of the speakers must change somehow. Does the RMS value of the two speakers add up?

 

The RMS remains constant. The total ohm loads drops which only means the amp is putting out to a lower resistance load which usually, but not always, means more wattage output form the amp. The amount of wattage the speakers can handle will never change, regardless of what you do to them. Their limit is completely static.

 

Nope.

Ohms "drop" is the load the AMP sees. Has zero change on what the speaker specs are.

 

^ bingo. Great explanation.

 

I remember when not all amps were 2 ohm stable. Yeah, that was a long long time ago

 

^^^^^^^
Yeah that!!

 

Think of speakers as buckets. Their capacity never expands. There is a safe fill level (RMS) for every day use and carrying, then there is a max fill (Max wattage) but you prolly don't want to do much with the bucket like that. Even if you increase the size of the hose (Amp or head unit) from a straw to a garden hose to a fire hose, those buckets will always will always have the same limits.