EVERYDAY FORMULAS

  

These are formulas that I use EVERYDAY  

You might ask “Why would I need these formulas?  What for?  Who cares!”

One of the reasons I use these formulas and why I teach this is for a REALLY fundamental reason.  Without some very basic information, I cannot diagnose a problem or help a retailer/installer/consumer figure out what is the issue/problem with their system.  I am NOT physically there to measure and figure out what is REALLY going on.  

I need information.  YOU need information!  

Without some basic rudimentary scientific information Nothing can be accomplished…NOTHING!!!!

For example, a classic story I tell all the time is a consumer comes into this car audio shop and tells the salesman/installer that his system “Don’t HIT!!”  Of course my response and what I teach in my seminars is to respond this way – “HOW HARD DO IT HIT???”  as I do not recognize that scientific measurement (It don’t hit) “measurement methodology?  90% of ALL salesman would reply “Oh my god I am so sorry lets look at what you have and get you more power”  This would seem reasonable and a logical response to a customer (or a friend) that tells you “It don’t HIT!”.  Add MORE power!! That makes TOTAL sense.  But here is the “rub”.  I really don’t know  what “It Don’t Hit”means , it isn’t very scientific.   My guess is it probably means that his system (amplifier?) isn’t making enough power OR that his woofer isn’t playing loud enough for him (SPL wise)  Just a guess 🙂 

Makes sense, but not really. 

(For more detailed information, or to continue the “It Don’t Hit Story” go to this article)

So here are the rudimentary simple formulas that you should use everyday

(if you do car audio for a living anyway..or if you just want to REALLY know what is going on?)

Remember the FORMULA’S!!!

AC Volts Squared Divided by the Voltage

(this is part of that thing called Ohms Law- it AIN’T a suggestion!)

AC volts measured at the amplifiers subwoofer output
(Any DVM (Digital Volt Meter) set to AC volts can do this!!!!)

Using a simple (this is a Radio Shack DVM shown below) volt meter you can measure
ANY systems power output!!

DMM

RadioShack Model 22-813 All of $35.99

Let’s test this “formula” out.  Go out to your customers car (or your car, or your display for that matter) and connect the DMM (DVM, O’Scope, whatever) to the subwoofer output of the car audio amplifier.  Set the meter to AC volts (since ALL audio in the signal path  is AC volts).

As shown below

AC volts Test

Have someone crank the system for all its worth with some stupid BASS music
(I know you have some!!)

Read the peak AC volts out.  Let’s say the biggest number you saw was 49 volts

So…we take the formula and “work it”:

49 X 49 divided by 2 ohms
(because the customer has 2 – 4 ohm 12 inch woofers in a sealed box- which equals 2 ohms)

Which equals 2401 watts divided by 2 (ohms)  which equals 1200 watts

If this is a 1000 watt 1 ohm amp.  It ROCKS!

If it is a 1500 watt 1 ohm amps…IT’S  PRETTY GOOD

If it is a 2000 watt 2 ohm amp …then it SUCKS!!

REMEMBER – The meter is STUPID!!…..  It is….. what it is!!!!

Now we need to ask the question ……IF this particular amplifier is making this much power
(1200 watts cause you measured it, it wasn’t a guess!!)……..
then HOW MUCH CURRENT IS THIS AMPLIFIER DRAWING??
Hmmm???   Well lets calculate that

 HOW TO CALCULATE CURRENT DRAW OF ANY CAR AUDIO SYSTEM (FULL POWER)

Current draw is a simple formula based on Class A/B amplifiers “efficiency” – which these amps are typically 50% efficient.    Before anyone starts with “How did you get that?  I’ve seen Class A/B amplifiers at over 60% and Blah blah blah”.      So here is my comeback.  I’ve TESTED IN-CAR over 5,000 amplifiers REAL-WORLD.  NOT hypothetical, not just whats written on a test spec.  So trust me on this , this is very accurate.  So…. for every 100 watts of energy coming in – Volts/Amps that equals watts……. you get 50 watts out .   Which is 50%   But that is Class A/B and Class D amplifiers are about 30% more efficient.  Or 0.7 times the number you get when calculating current draw (the formula) .

Let’s take a 1000 watt Class D amplifier and figure out the typical current draw

Current draw is a simple formula based on Class A/B amplifiers “efficiency” – typically 50%

Take the total power of the system X 2 and then divide by the DC volts
(measured, if possible, under full load -meaning CRANKED

THE FORMULA IS:

TOTAL POWER X 2 DIVIDED BY THE  DC VOLTAGE IN

So lets take that imaginary 1,000 watts X 2 divided by DC volts (12 ) = 166 amperes  (assuming a Class A/B amplifier) If it is a  Class D sub amplifier (which 95% are) then take that
number  X 0.7  then the current draw will be about 116 amperes (full power)

So this 1,000 watt amplifier we’ve been talking about  – OR ANY 1,000 watt Class D subwoofer amplifier will “pull” approximately 116 amps of current (FULL POWER)

Makes NO difference WHO’s 1,000 watt amplifier it is.  Mine , theirs, don’t care!!

Fortunately this is FULL power current draw and NOT typical of playing “Dynamic” music.

Meaning Fleetwood Mac, The Doors, Thomas Dolby, Sade, Michel Buble, Chris Rea, etc.

If your customer is playing Lil Wayne, and/or  “Bass that Ate the earth” type music , then that is a different story.  Now you got issues – because of the type of music played, NOT the system

SO whats the REAL DEAL with Current draw???

So now that you know all of this someone you know (or even you will say) B@#lSh#$t Larry!!

Below is a link to the tool page if your interested in knowing what tools I use:

TOOLS YOU GOT TO HAVE FOR  CAR AUDIO!!!!

SUMMARY OF THE FORMULAS

POWER FORMULA

AC VOLTS SQUARED DIVIDED BY THE IMPEDANCE

CURRENT FORMULA

TOTAL POWER X 2 DIVIDED BY THE  DC VOLTAGE IN

TEST TOOLS and where to buy them!