RMS v/s PMPO

Okay, so how many of you have been to a shop to buy that much awaited Music System but got confused at the last moment by the dazzling numbers on wattage (Yeah, curse that..).

Well, lets get versed with the acronyms  first.

RMS means Root Mean Square.

PMPO stands for Peak Momentary Power Output.

RMS, as the name suggests is the is the average power output of the speaker over a long period of time.

While, PMPO is the maximum power achieved for a speaker under perfect conditions.

RMS is derived from an equation which produces the most mathematically accurate measure of a speaker's power output. The RMS value is often seen listed as "wattsRMS" on product packaging.  An RMS power value is referred to as the real music power output of the speaker. It is a true indication of the output power of a given speaker across its range of pick up frequencies.

On the other hand, PMPO is the term, which is more often used as a marketing ploy. This is because the conditions under which the PMPO is achieved is next to impossible, plus the wattage that comes out at PMPO is only peak. You cannot drive your equipment at continuously that much power (who would really drive their home theatre at 3000W or 6000W).

And yes, there is no particular formula on how to convert PMPO to RMS or vice-versa.  PMPO is the ability to deliver a particularly large amount of power for a fraction of a second. This depends on the individual amp design & depends largely on the size of the filter caps.

Being cleared on this, next time you visit a store for music equipments, you are now more well informed about the product details. So don let that sales person get to you ;)

Speed of Sound & How our environment affects it.

Ever wondered in childhood why the thunder struck always after the lightning. Well we grew up to knew that sound travels slower than light ( Big deal “Light”,get over it).

Then came 9^th standard Physics & we all learned that sound travels with the speed of 342 m/sec. ( made us feel like a bit of Einstein, when we would tell our folks back home about speed of sound).

Now, for those of you who were happy with that fact, please stop here. No, seriously. Coz the next few para’s is gonna leave more confused than depressed.

Without much haggle, I’m gonna delve into the topic of how the speed of sound varies with humidity and Temperature.

Yes Sir, you read that correct. The speed that we thought was constant varies with the ever impressing factors of temperature and Humidity.

Now, many of you would have started checking on Google for the formula for the speed of sound & those of you already aware with the formula, know that the speed of Sound is defined by the formula:

Where  c: Speed of sound, P : Ambient Pressure & p: Gas Density

ᵧ as we know is the term dependent upon the freedom of the gaseous molecule. Since air is made up of diatomic molecules, the value of ᵧ is taken to be 1.41.

Thus our equation becomes  

So, how does temperature plays any role in determining the velocity of sound at all. Well, we use the equation of state of air of an ideal gas, which is PV= RT .

p is density ( mass per unit volume) in the equation, from which we derive the following :  

                              

Where R: Universal Gas constant, T: Absolute Temperature, M: Mean molecular weight of gas at sea level.

Since the values of R & M are constant, the velocity of sound maybe shown as temperature dependant as follows:

Where T: temperature in Kelvin , c (0): Reference speed of sound under defined conditions.

Thus, the speed of sound is seen to increase as the square root of absolute temperature.  Hence converting the same in centigrade we get,

where t: temperature in degree Celsius

Now, many will be asking them self that does'nt pressure any way affect the velocity. The answer to that is, that change in pressure indeed affects change in density of the air, but the inverse is not true. Change in density of air does not affects the change in pressure. Thus, there is no change in the velocity of sound due to any change in the density of air around it.