The Relationshipof
Voltage, Loudness, Power and Decibels What a boring title! On the contrary, I think you will find the information contained herein to be both enlightening (maybe even surprising) and certainly valuable. This paper will help you to better understand how the major component parts of your stereo system interact. Inparticular, we will explore the elusive relationship between speaker output(volume) and amplifier output (power). Loudness, to a great degree, is subjective. What is loud to one person is not to another. Thus we need a measure that allows us quantify what we hear. The unit of measure most common is thedecibel
(dB). Most audiophiles are familiar with the term, but just what does it actually mean? Without getting too technical,the dB is a unit of measure for sound intensity or level. It is a logarithmicscale developed to express wide ranging quantities on a simple scale. Becauseit is logarithmic, it can be a little bit confusing. As an example, a doubling of volume is not equal to twice the number of dB. The chart below will help you to better understand the dB and how it relates to what you hear, and what is happening throughout your system as volume levels change. But first, we need tolook at the other end of the equation. Power,
in this discussion, refers to watts. Power defined is the rate at which energy is converted or dissipated, as in the caseof an amplifier driving a loudspeaker. It is important in our discussion indefining the general relationship. A simple chart will help to understand the relationshipof the terms we have introduced: Looking at the chart, and starting at the left, we cansee than an increase of 3dB results in a
voltage increase 1.4 times the original, adoubling of power, and yields only a subjective increase in loudness only 1.23times the original. To get a doubling of loudness, it is important to note that an increase of 10dB is necessary. And to reproduce that volume through ourloudspeakers, note that we require ten times more power from the amplifier! The implications of this comparison in a typical hi-fisystem is significant. Let us assume that you are listening to your stereo at acomfortable volume of 8OdB. We will further assume that this level requires 25peak watts from your amplifier. To just barely increase the volume, we mightincrease loudness by 3dB. As we can see from the chart, this will require adoubling of power from the amplifier, which will now be churning out 50 peakwatts to produce a loudness of 83dB. If we want to double the apparent volumefrom our original level we will need to create 9OdB of sound pressure,requiring 250 peak watts. It is obvious from the example above that adequateamplifier power is necessary to provide an accurate portrayal of music. This is especially true when attempting to reproduce realistic levels, or when drivinginefficient speakers. However, even small efficient speakers may requirecopious amounts of power to cleanly reproduce transient peaks. In the finalanalysis, it is difficult to have too much power. While a small amplifier ofonly a few watts output can produce surprising average loudness, the dynamicpeaks will usually suffer unless the sensitivity of the speaker is very high. Many individuals I have spoken with are concerned withhaving "too much" power. The perception is that an amplifier with anoutput greater than that recommended by the speaker manufacturer would be likelyto damage the speaker. This is not necessarily so. Interestingly, an underpowered amplifier is more likely to cause speaker damage! During my training atthe JBL factory for transducer servicing, I saw more speakers(tweeters/midranges) damaged from use with under powered amplifiers than onesthat succumbed to being overdriven. Sound strange? Not really, once youunderstand what is happening. When an amplifier is over-driven, it "clips"the wave-form. What was a clean sine wave becomes a distorted, almost square,wave. A square wave is extremely difficult for a speaker to reproduce, as itrequires virtually instantaneous starting and stopping of the diaphragm. Atsufficient power levels, the tweeter will simply die trying to reproduce thiswave-form. A given tweeter rated to handle 50 watts of clean undistortedsine-wave power, will be capable of handling only a fraction of that amount insquare-wave input. As you can see, clean, undistorted power is the key. A 25watt amplifier, constantly driven to clipping, is more dangerous than a 250watt amplifier that is never taxed. Of course, let reason prevail. I am notsaying that speakers can handle endless input, they cannot. However, extrapower does not mean that speaker damage is bound to occur. If common sense isused, any size amplifier can be employed. Many factors influence amplifier choice. Some of the moreimportant considerations are: speaker type (efficiency or sensitivity), roomsize, the type of music you listen to and, most importantly, how loud you liketo listen and even how far one sits from the speakers. In an extreme example,one might own an inefficient loudspeaker, have a very large room, and like tolisten to pipe organ music at realistic levels. This individual is going torequire a tremendous amount of power to satisfy his/her needs. The nextlistener, with the same speaker, may have a much smaller room, and preferschamber music at background levels. Here, power requirements might be only onetenth to one-fourth those required in the previous illustration. Requirements do vary widely. It is wise to take allfactors into account before making a decision regarding the amount of powernecessary for your situation.
[ 本帖最后由 kldguitar 于 2012-6-19 10:20 编辑 ] |