Preston Electronics, LLC
Lee's Summit, MO


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Speaker selection and amplifier power ratings
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Too often we receive speaker systems for warranty repair in which the woofer voice coils are frozen and charred. The voice coil that drives the cone has overheated and burnt. (Sometimes the speaker cone even catches fire!) Often these are brought to us in the belief that the repair will be covered by warranty. Unfortunately, in practically all of these cases, the failure is due to excessive power applied to the speaker, which is specifically excluded from coverage by manufacturer warranties.  (Please note: this article is written from the perspective of a live musical performance sound system.  Other applications such as studio monitoring, home theaters, etc, may have additional considerations, although the principles here are still valid.)

We have identified two primary causes for speaker failure:
1 - Continuous power output from the amplifier is greater than the speaker is designed to handle.  This usually burns up the voice coil in the woofers.
2 - Excessive distortion caused by trying to get more volume (power) from the amplifier than it is capable of delivering.  (Underpowered system.) This causes the amplifier to go into clipping, which produces excessive high frequency energy and burns up the tweeters or horns.

The following article describes the reasons for these issues and will hopefully enable you to properly match the ratings of the speakers and the amplifier in your next system and avoid such catastrophes.

Cause #1: Excessive power to speakers.  Usually, the reason for these occurrences is because the amplifier power rating was much greater than the speaker was designed to handle.  We see this situation frequently in speakers used in DJ applications, where the program material contains continuous heavy bass signals.  Even worse, often the amplifiers are overdriven into clipping due to the desire for even more bass.

Unfortunately, amplifier ratings and speaker power ratings as described by the respective manufacturers are not directly related to each other, and the consumer often chooses a speaker with a rating that appears to match the amplifier, when in fact the speaker is severely underrated. That is, an amplifier rated at 100 watts does not necessarily match a speaker rated at "100 watts".

In order to properly understand the rules for speaker selection, a certain amount of technical background is necessary. Stay with me....

First, a definition: POWER is the rate at which energy is converted from one form to another. The basic international unit of energy is called a JOULE and is defined as the amount of work energy performed by applying a certain amount of force (called a NEWTON) through a distance of one meter. The number of Joules of energy converted each second defines the amount of Power in WATTS. 1 Watt = 1 Joule/second; 5 watts = 5 Joules/second, etc.  For instance, if your exercycle had a generator connected to a lamp, the brightness of the lamp would depend on how hard and fast you pedaled.  The faster and harder you pedal, the more power you generate and the brighter will be the lamp.

The purpose of an amplifier and speaker system is to convert the raw electrical energy from the AC power line into acoustic (sound) energy that you can hear. During this process, energy is converted from matter (such as coal or gas) into heat, then mechanical energy, then (possibly through magnetic energy) into electricity. From your AC power outlet the electrical energy is controlled by the amplifier in order to build a weak electrical signal into a strong signal, which is fed to the speaker to create magnetic energy, which pushes the speaker cone back and forth (mechanical energy), which moves air molecules in order to convert some of that mechanical energy into sound energy. The more energy (per second) is converted into sound, the louder the sound will be. So volume depends on the amount of electrical POWER used to create the sound by the speaker.

There are a number of methods used to describe the power of an electrical signal. These methods are given names such as "instantaneous power", "average power", "RMS Power", "peak power", "Music power", "Program Power" and others.

In order to move the speaker cone in the complex manner required to accurately reproduce sounds, the power must be continually and rapidly varying. The power being used at any instant during the process is called "instantaneous power". However, since that is always changing, it is not useful for describing the capability of an amplifier or speaker.

"Peak power" is the maximum amount of instantaneous power present at the highest level during the signal. In a speaker, the peak power would occur at (approximately) the instant the cone reaches its most forward (or rearward) extended position. In an amplifier, the maximum peak power output to a speaker is limited by the amplifier power supply. If the level controls are increased beyond the point where the amplifier reaches the limits of the power supply, a severe form of distortion known as "clipping" occurs. As an analogy, think of tying a weight to a string and whirling it up and down in a circle in the room. If the string is short, the circle is small. This is like an amplifier delivering a low volume tone to a speaker. If you lengthen the string, the circle gets bigger. An amplifier would be delivering more power to the speaker, and the sound would be louder. If you lengthen the string too much, the weight will hit the ceiling and/or the floor, and the weight no longer moves in a circle. The size of the circle (and the amount of sound) is limited by the ceiling and the floor (the power supply in the amplifier). In an amplifier, the peak power rating is useful for describing the maximum instantaneous limit of its capability for pulse sounds such as drumbeats and bass notes.

"RMS power" is practically the same as "Average power". Since an audio signal is constantly changing, mathematical methods were developed to accurately compare the voltage and current in an audio signal or AC power source to an equivalent DC level such as that produced by a battery. The method of comparison was based on the amount of heat each form of power would produce in a heating element such as a lamp bulb. So the values of voltage and current producing 10 watts of AC electricity would have the same average power as a battery supplying a steady 10 watts to a lamp, and would light it to exactly the same brightness. The Average Power (often called "RMS" power) is the most consistent method of comparing power levels between two devices. Most audio power amplifiers are rated for their Maximum Continuous Average ("RMS") power output capability of an essentially undistorted signal  to a specified load (speaker) impedance.  Under certain conditions, a relationship exists between the peak power and the "RMS" power rating of an amplifier. (The term "RMS" stands for "Root-Mean-Square" and describes the mathematical steps required to calculate the effective values of voltage or current which determine the average power of an AC sine wave, which is the graph of instantaneous signal  in a pure tone. Strictly speaking, "RMS" applies only to voltage and current, not power.  "RMS power" is technically meaningless, but has become commonly used to refer to Average Power because it is usually calculated using the RMS values of voltage and current, and also to distinguish it from other "types" of power.(*1))

"Music power" and "Program power" are rather nebulous terms that are often used in speaker ratings. They are explained by the manufacturers by saying that speakers are almost never used to produce pure tones (where average power is most easily measured) and that the power distribution in most music is erratic and of many frequencies.  A closely related industry standard method of rating speakers is called the IEC power rating.  The description usually given is that a complex waveform is used to evaluate the power capacity of the speaker. No exact formula or other mathematical relationship appears to be available, but from our research it appears that these ratings are approximately twice the equivalent average power. A detailed, rather technical explanation is given by Jon M. Risch of Peavey Electronics in this article.

Herein lies the problem. How can you match a speaker system to an amplifier when the amplifier is rated in "RMS" watts (average power) and the speaker is rated in Music Power or Program Power (which is definitely NOT average power)? Ultimately there must be some method of comparing apples to apples when it comes to the two.

Remember that average power was defined in terms of the "heating effect" produced by the AC power source. When audio power is applied to a speaker, much of the energy is converted to magnetism and then to sound. However, a significant portion of it gets converted to heat in the wire of the voice coil. When too much power is applied, the heat damages the insulation of the wires. They come loose and start to rub inside the speaker, or the insulation starts to burn. This causes a buzz or rattle from the speaker. If the wire gets even hotter, it fails like a burnt-out lamp bulb filament and the speaker quits entirely. Since it is heat that causes the speaker failure, you can see that it is the average power that is important in determining the failure limits of the speaker.

Now let's consider the amplifier power rating. If you examine a specification sheet of an amplifier, you will see that they are usually rated in Average ("RMS") power for an UNDISTORTED output. This is like swinging your weighted string so it almost touches the ceiling and the floor. But keep in mind that you can overdrive the amplifier into clipping. When this happens the power amplifier can produce up to TWICE AS MUCH power to the speakers as it is rated for. (Long technical explanation required - take my word for it.) So an amplifier rated at 100 watts clean can actually put out as much as 200 watts when heavily overdriven into severe distortion.

One other note about amplifier power ratings: The output power per channel usually depends on the IMPEDANCE of the speaker(s). For example, an amplifier might be rated at 100 watts per channel to an 8 ohm load and 190 watts to a 4 ohm load. Be sure you are comparing amp and speaker ratings for the same impedance value. We will assume you are only using one speaker per channel in this discussion.

Now, if you want to keep from smoking your speakers, you should consider the relationships between the speaker ratings and the maximum capability of the amplifier. The first thing to do is find out what form the speaker power rating is. If the speaker rating is clearly stated in RMS WATTS or AVERAGE POWER, you can compare it directly to the amplifier rating. If you are buying an amplifier rated at 100 watts/channel, a speaker rated at 100 watts RMS is usually adequate. However, if there is a possibility that it might be overdriven, (such as with lots of bass or heavy metal) you might want to buy a speaker rated at up to twice the power output rating of the amplifier to allow for the distorted output power.   (Distortion of bass notes is not as obvious as higher frequencies, and it's easy for enthusiastic DJs to overdrive the system without realizing it.)

If the speakers are rated in Program power or Music power, or if it is not clearly stated, you should also consider the additional rule of thumb that their average power rating is probably about half of the Program power rating. If you are buying an amp rated at 100 watts clean (200 watts maximum), your speakers should have an RMS Average Power rating of 100 to 200 watts, so the Program power rating of the speakers should be 200 to 400 watts each.  

In practice, for an application where the program material contains high average power levels, (especially lots of bass)  the speaker Program Power rating should be MORE than two times the amplifier RMS power rating. For maximum protection, select speakers with Program Power ratings of four times the amplifier power rating.   Keep in mind that these rules are based on  approximations and some assumptions, so no guarantee can be made that you still won't blow the speakers, but using these rules will greatly lessen the risk.

The speaker in the photo above was rated at 240 watts IEC for a duration of 8 hours, or 200 watts IEC for a duration of 100 hours.  This appears to translate to approximately 100-120 watts RMS. (The manufacturer doesn't give an RMS rating for this speaker.)  The user was driving it with an amplifier rated at 500 Watts RMS per channel (with both channels driven).  This amp is capable of producing over 1000 watts per channel when heavily distorted.  The system was being used in a DJ application.  More than likely the DJ was cranking it as loud as he could. (Click the picture for a large image.)

Case #2: Excessive distortion.  As mentioned previously, excessive distortion burns up tweeters.  This happens when the amplifier does not have enough power to produce the desired volume and the user has set the volume controls too high. The amplifier goes into clipping (described above),  the sound becomes harsh and distorted, and produces lots of high-frequency harmonic energy.  Also as described above, the amplifier can actually put out up to twice as much distorted power as it can clean power.  A lot of this extra distortion power is contained in those high-frequency harmonics, which are directed to the horns or tweeters in the speaker. The result is a burnt tweeter voice coil. 

There is much conflicting information available on the web and from supposed 'experts' that is based on the idea that distortion must be avoided at all costs.  Some even go so far as to say the amplifier power rating should be two times the speaker's IEC power rating(*2).   The rationale is that speakers can normally handle brief pulses well in excess of their average power rating.  While this is true, such applications are typically found in home theater and studio monitoring, where the average power level of the program material is normally well below the speaker rating. It is NOT a safe practice for systems where high continuous average volume is required, such as dance clubs, arenas, theaters and outdoor venues. If you go back and examine the ratings for the burnt speaker and the amp that was being used, you will see that the user thought he was following the (erroneous)  recommendations.

To avoid damage due to excessive distortion, be sure your system's amplifier power rating is great enough for the application to ensure that you have enough volume without overdriving it into distortion.  Select your speaker power ratings to match the amplifier using the guidelines above, adjusting for the type of program material.  If you are amplifying speech, speaker Program Power rating  = amplifier RMS power is probably adequate.  For recorded music or live acoustic-style performances,  speaker Program Power rating  = 2x amplifier RMS power would be appropriate.  For live rock or ethnic music, or heavy-bass DJ situations, you may wish to increase the safety margin beyond this. (*3)

A properly designed amplifier/speaker system for sound reinforcement will have enough amplifier power that clipping distortion of the amplifier will never occur at any desired volume level, AND speakers that can handle continuous power near the rated output of the amplifier. Using an amplifier with twice the power rating of the speaker (or more) is inviting a trip to the repair shop.

 

More information about speakers and impedance can be found here.

(*1) A full explanation of power is well beyond the scope of this article, and would only serve to confuse the reader.  Plenty of information is available in textbooks and on the web. If you are really interested, be prepared to learn about resistance, capacitance, inductance, trigonometry, vectors, and calculus.

(*2) http://www.sweetwater.com/shop/live-sound/power-amplifiers/buying-guide.php   (I disagree with this recommendation.)

(*3) JBL has an article on Speaker Power Requirements which includes three different descriptions of program content and their recommendations.  My recommendations are more conservative because of the number of burnt speakers we see.

Copyright 2007 - 2011 Neil R. Preston, Preston Electronics LLC. All rights reserved.   Comments invited - info1@prestonelectronics.com


 

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