สูตรที่แน่นอน คงไม่มีมั๊งครับ เพราะผู้ผลิต บางราย พยายามจะบอก watt แม้จะเป็น PMPO อยู่แล้ว ให้สูงขึ้นไปอีก เนื่องจากไม่มีมาตรฐานรองรับการวัดแบบนี้ .... คือ วัดกันตามใจชอบเลยอ่ะ....
From
http://www.fact-index.com/p/pm/pmpo.htmlPMPOPMPO stands for Peak Momentary Performance Output. It is a dubious measure of the power output of a loudspeaker of more interest to advertising copy-writers than to consumers. The PMPO is calculated by the vendor or tester of the speakers as the maximum power in watts under perfect conditions that are impossible to achieve in practical use. No sound system can sustain its PMPO for more than a few milliseconds without being destroyed.
A much more serious and reliable measure is watts rms. Rms stands for root mean square. The rms power is found by averaging the instantaneous power output over a long period of time.
To get an idea of the relationship between PMPO watts and watts rms, consider the following numbers advertised for some current loudspeakers. These models have been selected at random, and inclusion in or exclusion from this list is neither a recommendation nor a criticism.
* Teac PM-100 3D surround-sound speakers: 16W rms, 180W PMPO
* Kinyo "200W" PC speakers: 3W rms, 200W PMPO
* Philips Fun Power Plus MMS-102 PC speakers: 10W rms, 120W PMPO (however, the Philips data sheet mentions only the rms value; the PMPO value is claimed by retailers)
This list shows that PMPO figures are hugely exaggerated compared with the rms values used by professionals.
AC power, which includes audio, is rated in watts rms, which is equivalent in heating value to DC watts. This is the honest method. A given direct coupled amplifier (transformers are rare in non-distribution equipment) can obviously only swing its output from ground to VCC at best. This limits a simple car amplifier to something
a 12 volt swing peak to peak.
For example, an ideal (100% efficient) amplifier with a 12 volt peak-to-peak supply can drive a signal with an amplitude of 6V. Into an 8 ohm loudspeaker this would deliver
6V x 6V / 8ohms = 4.5 watts peak instantaneous.
If this signal is sinusoidal, its rms value is 6V x 0.707 = 4.242 V(rms). This voltage into a speaker load of 8 ohms gives a power of :
4.242V x 4.242V / 8R = 2.25 watts(rms)
With a 4 ohm speaker these figures double. You can see which the marketing person wants to use. You can also see why some car audio uses 2 ohm speaker loads.
With a differential (push-pull) output the swing is doubled so the power increases by a factor of four. Therefore, with a 14.4 V supply, which is the highest voltage usually available from a fresh car battery, the maximum possible output into a four ohm speaker is about 14.4 x 14.4 x 0.707 / 4R or about 46 watts rms.
"Music Power" is a somewhat arbitrary multiplier based on the fact that amplifiers cannot provide full power for long, but can produce high power in short bursts. This allows them to reproduce normal audio, which has a high peak-to-average ratio.
To get more power, high-end equipment uses a DC to DC converter to generate a higher supply voltage (at the expense of drawing more current from the battery).
The true power output of an amplifier can be estimated by examining the DC input. Linear amplifiers tend to be about 60% efficient at best. An amplifier labelled "500W PMPO" but fitted with a 5 amp fuse can therefore deliver an average power of 5A x 14.4V x 60%, or about 43 watts.
From
http://cbll.net/articles/pmpoWhat is PMPO? It's An advertising gimmick!PMPO is a measure of audio amplifier power output. It is deceiving because it considers an amplifier's power output under perfect conditions and only for a short instant.
PMPO
Have you ever wondered how that pair of speakers they are selling at the discount store for $10 can put out 120 watts of power? Well, depending on your definition of output power...they can...and it has fooled many people! I know people have told me that their home stereo unit has 2600 watts of power output...even though it is connected to a standard 120-volt power supply that is only allowed to draw 2000 watts before tripping the circuit breakers!
The answer to this issue lies in how they measure the output power of the device. Usually if you look at these huge numbers, you will see the letters "PMPO" underneath. PMPO stands for "Peak Music Power Output" or "Peak Momentary Power Output". Notice the word peak. The vendor of the product calculates PMPO based on the maximum power output of the device under perfect conditions and 100% efficiency. These conditions are impossible to obtain, and no device can sustain the PMPO power for any significant length of time without being ruined. The PMPO power relies on the fact that amplifiers can provide short bursts of very high power. Over time, these short bursts when averaged with the lower output powers, gives the real value.
The Real Power
Vendors use PMPO power to hide underpowered power supplies and amplifiers. When you see something with a PMPO measurement on it, turn it around and look at the specifications. Often, the input power from the AC mains is 5 or more times LESS than that of the PMPO power. You also may see a more accurate measurement, called RMS (Root Means Square) power and DIN power. These types of measurements average output over time, creating a much more reasonable reading. Don't be fooled by cheap products which appear to put out big power.
Another PMPO vs. RMS Package
So, how do you find the RMS Power from the PMPO Power?
I have been asked this, and there is no single answer or mathematical formula to convert these values. The peak output of the amplifier is going to differ based upon the amplifier's design, and the type of sound that is being amplified. Often each electronics manufacturer will create their own proprietary formula for figuring PMPO power.
Usually, the PMPO power is about five to ten times higher than the RMS Power.There is NO mathematical formula that will convert PMPO to RMS values or vice-versa.
from
http://www.drumnbass.be/forum/thread.php?threadid=2257Some general advice on reading the specs when buying an amp:
Watts (power)If you buy an amp don't take any sales waffle about the amount of watts it delivers, because that's the least important thing in an amp and it's a measure that can very easily be tampered with. Remember seeing tiny little PC speakers boasting to have 250W, or car stereos that have 4x100W. A load of bull Big Grin
You need to know the impedance in ohms and the THD (distortion percentage) before you can compare the Watt specs of two different amps. Also the specs have to be measured for the full 20Hz-20kHz bandwidth using pink noise.
The impedance is the 'resistance' of the speakers that are used. How lower this is, how higher the amount of watts the amp can deliver (because there is less resistance, duh). A typical speaker has an impedance of 8 ohms, but if you wire two speakers together in parallel they will have an impedance of 4 ohms (remember your physics lessons Big Grin ).
One and the same amp can eg deliver 300W into 8 ohm (0.05% THD, 20Hz-20kHz) and 500W into 4 ohm (0.05% THD, 20Hz-20kHz). Usually the amps are rated in 4 ohms because this looks more impressive on the front panel. It might seem
a great idea to hook up two speakers on every output of the amp to get increased power and it works perfectly, but you get some drawbacks
increased heat dissipation and slightly increased distortion.
Note: specs of 2 ohms impedance are often supplied but can be ignored, running an amp in 2 ohms will make it overheat and will increase the THD to unacceptable levels. It's even illegal in some countries such as the USA to run amps in 2 ohms because of fire risks Big Grin
The THD (Third Harmonic Distortion) is a measure for the amount of distortion an amp generates for a certain amount of power. Remember those supposed 250W PC speakers? They will undoubtly be able to deliver 250W but then at a distortion level of 50% and only for a very short time before they blow up Big Grin
Any good amp can deliver much more power than it is rated at, but when you increase the power, you will also increase the THD. So it is necessary to know the amount of THD that is generated if you want to have a reliable idea of the power the amp can deliver. The manufacturers measure this by playing test tones through the amp and increasing the power until they reach a certain THD level. This way they can find out that a certain amp can eg deliver 500W at 0.05% THD.
Typical THD figures are 0.05% and 0.02%. Any figures above 0.05% should be taken with a big grain of salt.
The 20Hz-20kHz indication is important too. It indicates the frequency range for which the specs are valid. Cheap amps typically quote only one frequency (eg 1kHz) at which the amp performs best. So if you get a spec of "200W @ 4 ohms, 0.05% THD - 1kHz" you know the amp generates 0.05% distortion at 1kHz, but how much distortion in other frequencies? Maybe the bass or high end is awfully distorted?
So look out for the frequency range indication. BTW very good amps quote this for much larger ranges, such as 5Hz - 200kHz, this is ofcourse a better measurement Wink
A sample correct Watt spec looks
this: 750W in 8 ohms with 0.05% THD (20Hz-20kHz)
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If you see PMPO in the specs then the advice is simple: don't buy it! Tongue
PMPO means Peak Momentary Performance Output, so it's the maximum power that the amp can deliver for a brief period. I mentioned this in my post for the so called 250W PC speakers, these would be rated in PMPO.It's clear that this is a very unwanted spec, you want to know how much power the amp can deliver continuously, not for a couple of milliseconds Big Grin Manufacturers that are serious about their amps will never mention the PMPO rating, so if you see this, don't buy it Big Grin
Another warning: sometimes the specs don't even say it when they used PMPO, but you can recognise this by looking at the THD. A PMPO rating will have a very large THD, way above 0.05%! So if the THD is not stated or has a high value it is probably measured in PMPO
.
Why watts are not important
The human hearing works in a logarithmic way, while speaker output works in a linear manner.
The difference in perceived loudness between two signals is measured in decibels (dB). An increase of 10dB means that it sounds twice as loud. So a 110dBspl sound is twice as loud as a 100dBspl sound. If an amp has to deliver twice the perceived loudness, it has to generate 10x the amount of power! So for every doubling in loudness you need and amp that is 10 times as heavy. This means that a 1000W amp is only 4 times as loud as a 10W amp!! Shocked
The formula for this is:
loudness difference = 10 * log ( power1 / power2 )
So if you want to know the difference between a 1000W amp and a 250W amp you calculate:
10 * log (1000 / 250) = 6.02dB which is not very much Big Grin
A couple more examples:
1000W vs 750W: 1.2 dB louder (no perceivable difference for the human ear!)
2000W vs 1000W: 3dB louder (barely perceivable)
So if you would chose a 2000W amp over a 1000W amp just because it seems to give you 1000W more you're completely on the wrong track!
For home studio use a 100W amp is more than sufficient, it will give you around 110dBspl @ 1 m using average speakers which is more than loud enough, it will make your walls shake Big Grin
Just take my advice, if the sales guys start ranting on about the watts just ignore him and bluntly ask for the damping factor and frequency response graphs. You will instantly earn his respect, get a better deal, and go home with a better amp
http://www.drumnbass.be/forum/thread.php?threadid=2257
อย่างไรก็ตาม มีการคำนวน PMPO กะ RMS โดยใช้ dynamic headroom มาเป็นตัวคูณ ....
RMS and peak power (PMPO) are related through the dynamic headroom specification given in dB. An amplifier with 3dB of dymanic headroom will have a peak power of double its RMS or average power.
So an amplifier rated at 100 watts RMS with a dynamic headroom of 3dB will have a peak power output of 200 watts.
Note that the dynamic headroom figure is given in decibels (dB), which use a logarithmic scale. Every 3dB increase results a doubling in power. So if an amplifier had a dynamic headroom of say 6 dB and an RMS power rating of 100 watts then its peak power output would be 400 watts (double 100 to 200 then double 200 to 400).
Note that
most good quality amplifiers have dynamic headrooms ranging from 1.5 to 3dB. Very high numbers typically mean the amplifier uses a poor power supply only able to provide short bursts of power.
