Watts watt? Speakersover and under and some trouble shooting...

Mathematical lesson time again people! YAY!!!!!!!!!!! lol

It all comes down to man a few years ago...a man with the last name... Ohm...blah blah blah blah

In essence, you have whats called Ohm Law.

You can work out so many different things with ohms law...Voltage, current, power(watts) and resistance....here is a few formulas and I will try to give an example for each one.

But first a simple way to give you a rough indication of your possible power.

A car voltage system is 12V...not quite... 12.4V when car is not started and 14.4 with car running.
Now, lets say the fuse in the actual amp itself (not the one at the battery) is.... 30 amps.

Lets times 14.4 by 30 amps.... That gives us 432 Wrms. BUT that is the power(watts) being removed from the battery. As a general rule of thumb amps only run at 50% efficency. So that then only gives us 216Wrms. That then get divide between all the channels. This is also the rule of thumb for standard operating procedure which is usually 4ohms.

Now for the Real Mcoy.

As you just saw, Watts = Voltage x Amperes.
Watts also = Voltage squared / ohms. So, 100 volts(on your speaker cables) squared equals 1000 divide that by 4ohms(the standard impedance of a car speaker and you get 250 Wrms.
And also = Amperes squared x ohms. This equation as a general rule isn't used because car audio is usually high voltage and as such has low amperage.

Now to work out Voltage....
Volts = the square root of (watts x ohms) So, lets say we have 100 Wrms... 100 x 4 = 400. The square root of 400 = 20 So the voltage on the speaker cables would be 20Volts AC. (AC = Alternating current...Positive and Negative switch sides depending on the frequency of the signal)
Voltes = Watts / Amperes, 100wrms / 8amps = 12.5 Volts. So in other words a 100W spotlight is drawing approxiametly 8 amps, two spotlights would be 16amps.
Volts = Amperes x Ohms. Once again usually not used in car audio as amperage is quite low when it comes to the speakers.

Amperes = Voltage / Ohms. Not really used once again....
Amperes = Watts x Volts. 100watts x 12 Volts = 8amps
Amperes = the square root of (watts / ohms)

You can also the Ohms Law for measuring resistance but I wont go into that here....but before I finish this awwfully long thread...Lets take that amp Claiming 1000Watts and see what sort of number we come up with eh?

What numbers do we have? 1000 Watts and we know the speakers are 4ohm.

So lets work out how much current it's putting out.....Bear in mind the big current draw and wattage hits are acheived during bass notes. So only last a split second....

Amps = the square root of (watts / ohms )
Amps = the square root of (1000 / 4 )
Amps = the square root of 250
Amps = 15.8

So now we know three numbers. Amps is 15.8, Watts is 1000 and 4 ohms

We are missing Voltage...

Voltage = Amperes x Ohms
Voltage = 15.8 x 4
Voltage = 63.2 Volts!

So all in all, to develop 1000Wrms you need an amplifier that can produce
63.2 Volts @ 15.8A
Thats a massive amount of power....lets see how much power we will need from the battery.
Remember what I said about amps being only 50 efficent?

We have 1000W @ 14.4 Volts add into that equation the effiecency and you will need another 50% of the watts we already have...
So we need to draw 1500W @ 14.4 Volts
Amperes = Watts / Voltage
Amperes = 1500 / 14.4
Amperes = 104!!!!!!!!!!!!

14.4Volts @ 104Amps is a massive battery drain. Considering the standard alternator can only deliver approx 14.4Volts @ 60Amps

You do the maths...

I only hope I didn't boggle your mind. Any more questions feel free to ask away

A lesson in speakers and amplifiers.

In this post I will cover a couple of topics.
Over/Under Powering speakers.
Maxpower Vs RMS power ratings.

If anyone else can think of any other topics that need to be covered then please feel free to post, ideas, corrections (I maybe king but I’m not perfect) and questions.

The topic of over/under powering speakers is one full of myths. I will try to explain my side of things from what I consider a logical point of view.

In essence a speaker is a mechanical object, it has a cone that moves via the voice coil attached to the bottom of it ( a tight coil of wire) this voice coil travels between two magnets. Now magnets have polarity, North and South or positive and negative. The voice coil is forced to move by switching the polarity of the magnets, remember in school playing with magnets and one end would attract the magnet and the other end would push the magnet away? Exactly the same principle. You apply Alternating Current to the speaker through the speaker wires. Just in a precise and controlled manner. Now, the laws of physics state that where there is movement there is friction. Friction causes heat. So, onto the first part. Over powering a speaker.

Over powering a speaker is the act of forcing a speaker to handle more watts (power) then it is capable of handling. What happens when you feed a speaker too much power? You cause distortion. So, lets discuss distortion a little bit. Distortion is your music signal being “clipped” I’m sure you’ve all seen a sine wave before. A sine wave is a very simple music signal. One note and no harmonics. It goes up (speaker goes out) and then it goes down (speaker moves backwards) It is one fluid motion. If we now put more power into the speaker. That signal no longer becomes smooth. It will have flat spots. When there is a flat spot the speaker is being held in one position. This causes heat and heat can cause your voicecoil to melt, leaving you with a dead speaker.

Under powering a speaker. The common myth is that it is fine to underpower a speaker. WRONG! It can be just as dangerous to underpower a speaker as it is to overpower a speaker. For Example. Lets say we have a 100W speaker hooked upto a 20W amplifier. Sounds okay huh? Little bit quite though. Lets turn the volume up! So now we are driving the amplifier causing it to distort, this signal then gets fed to the speaker. Once again, those flat spots are causing the speaker to be held in position causing heat and potential voice coil melt down. Another meltdown. On another note, because the amplifier is distorting you also risk destroying the output transistors in the amplifier. Leaving you with dead amplifier and speaker. So, you say “well that’s fine, just don’t turn it up till it’s distorting.” If only it was that simple. The human ear is actually a horrible listening device. The “normal” human cannot hear distortion cannot detect distortion until it reaches 10% of the original music signal. It takes less then 1% to destroy the speaker and amplifier.

Now I’m not saying that if you have a pair of 50W speakers that the amplifier MUST be 50W itself. That’s not always very practical. What I do suggest is a 10% Variance. I.E. 50 Watt speakers would ideally be suited to an amplifier somewhere in the 45-55Watt Range, 100Watt speakers would ideally be suited to 90-110Watt amplifiers. So on and so forth.

Max Power Vs. RMS power rating.
What is RMS? RMS stands for Root means square. Now I could go on all day about what it really means. But, in reality it’s not necessary. The simple version is, that RMS mean long term use. Or, longer the 1second. Max power is short term use, less then 1second. The technical term for Max Power is PMPO Peak Music Power Output.

Exceed your max power by more then one second and you are overpowering the speaker.

Unfortunatly, audio companies have always raved on and on about their high power speakers. I personally am yet to see a 4” two way handle 150Watts continuously. When purchasing speakers, always look on the box for RMS power or continous power handling. They are the real figures to pay attention to. If a brand of speakers doesn’t mention RMS power handling, put them back onto the shelf. They will most likely be so low that the company were too ashamed to mention it. Below is a rough guide to what size speakers handle what amount of power.

4” = approx 40 watts RMS
5 or 5.25” = approx 50 – 60 watts RMS
6 or 6.5” = approx 70 watts RMS
6x9’s = approx 90 – 110 watts RMS

Please remember that the above table is a rule of thumb only and is based upon aftermarket speakers only. The stock 6x9’s in your VT will NOT handle 110 watts being poked up their backside Wink Also a friend of mine who builds speakers, created a pair of 6x9’s that could be plugged into the wall. That’s 240 Volts x 10 amp = 2400 Watts at 50Hz!!! Oh yeah, they were kinda loud as well.

There is really no use for Max Power ratings. They are used primarily for advertising purposes. By both company and consumer. My last ride had was pulling 9,000Watts, but that was PMPO but RMS was 1170Watts. Big difference.

In general what I’m trying to say, is pay no attention to big power claims. Look for RMS capabilities. It’s the truth and not a wild claim.

The exact same can be said for the output of amplifiers.

Some trouble shooting tips, the list will be extended as more problems become apparent. To start this off I have included the most common faults I have come across and the solutions.

Q, I’ve installed a cd player but it won’t keep it’s memory, what is the problem?
A,You’ve accidentally reversed the constant and accessory power wire. Remove the head unit, swap these two wires over and everything should be fine. If further problems continue. You may have a faulty head unit and should get it looked at by a professional.

Q, I’ve installed a cd player and now the dash lights won’t work because the fuse is blown, if I replace the fuse it blows straight away.
A, The most likely fault here is that you’ve either left the Illumination wire uncovered behind the head unit and it is shorting out against the body of the car. OR, you have mistaken the illumination wire as your earth. The best Earth is a metal part of the car. IE Mounting screw for the radio. If you do need to use an earth wire from the car, test to see that it is always earth. The Illumination wire reads as an earth when use the multimeter but if you switch the head lights on the earth becomes a 12V power line. NEVER replace a fuse with a higher amperage fuse. Also, remember if a fuse blows then there is a fault, don’t just replace the fuse, check to see what the fault could be. NEVER use anything other then a fuse in place of a fuse. I’ve seen .22 bullet casings being used as fuses. That is not a good move as the .22 fuse probably has an amperage rating of approx 500amps! Your car will destroy itself before that blows!

Q, My sound is like a fly in a matchbox, where is my bass?!
A, It sounds like you’ve a speaker out of phase. This occurs when you accidentally swap the wires around on a speaker. As one speaker cone travels forward the other speaker travels backwards. This creates opposite sound waves that try to cancel each other out. Trace your speaker wires and find which ones are reversed and rectify the problem. If this problem is occurring with subwoofers then remember to check the wiring inside the speaker box – a commonly overlooked problem.

"Watts also = Voltage squared / ohms. So, 100 volts(on your speaker cables) squared equals 1000 divide that by 4ohms(the standard impedance of a car speaker and you get 250 Wrms."


Um.. 100 x 100 = 10000. Therefore 10000 / 4ohms = 2500Wrms

And where did u get 100V from anyway?

mrc25381 said:

"Watts also = Voltage squared / ohms. So, 100 volts(on your speaker cables) squared equals 1000 divide that by 4ohms(the standard impedance of a car speaker and you get 250 Wrms."


Um.. 100 x 100 = 10000. Therefore 10000 / 4ohms = 2500Wrms

And where did u get 100V from anyway?

Click to expand...

Quite right there.

100V?

It's an example.

Found anothery: Amperes = Watts x Volts. 100watts x 12 Volts = 8amps

Should be Amperes = Watts / Volts. 100watts / 12 Volts = 8amps

And on that note, it is a little confusing using 100W, because you started using 432Wrms on the paragraph before. I would've kept going using that example. And you've also gone back to using 12V when you said that cars run on 12.4V + 14.4V. I understand everything but had to read it over and over (even had the calculator out) before I got it because it's so confusing man.

A slight correction, the world wont stop or anything. RMS is only used for AC. To say that a dc supply can provide Wrms is meaningless.

But a pretty good description of a complicated subject.

Mike