Why Speakers Fail/
Too Much Power/Too Little Power

Why Speakers Fail

Despite the best of engineering, the selection of the best drivers and the most careful production and quality control techniques, loudspeakers can still fail.  The purpose of this article is to identify some of the causes of such problems so that they can be avoided and your loudspeaker system can continue to deliver the quality sonic performance that was intended.
     Fewer than 1% of high-end speakers in the field ever fail or cause trouble for their owners.  There are two reasons for this excellent record.  Quality loudspeakers have always used relatively "high-order" crossovers.  This improves the crossover's control over the drivers' performance within each operating range, turning each driver off quickly outside of its band.  This control keeps a tweeter from attempting to reproduce midrange frequencies, and a midrange from trying to handle lower frequencies (tasks which neither driver is designed to perform).  Gentle or "low order" crossovers, a current audio fad, place extreme demands on the power handling capabilities of drivers, and sharply increase distortion, a sign of protest from the drivers.  Consequently, these two deficiencies sharply increase the prospect of the driver eventually degrading or failing.  Most high quality speakers protect against these problems even further by always choosing crossover frequencies very carefully, and using only the highest quality drivers.

Speaker Misuse #1: Too Much Power

     The first reason for speaker failure is the most obvious: playing any speaker at excessive levels can damage it.  If any distortion is audible, the volume is too loud and likely to cause damage.  But in some cases distortion may not be noticeable before damage occurs.  For example, home loudspeakers can easily reproduce the real dynamic range of live acoustic music; but no high quality speakers can reproduce the brutal levels of a rock concert.  Low fidelity PA speakers should be used if this is your goal.

     Caution: prolonged exposure to high sound pressure levels can permanently damage your hearing.  Moreover, your hearing " adapts" to high volumes, allowing you to incorrectly feel that potentially damaging volumes are "normal."  Protect your hearing as well as your investment in high quality loudspeakers by avoiding excessive playback levels.

     The average audiophile mistakenly believes that the position of the volume control provides some indication of the percentage of output power the amplifier is delivering at the time.  This is not true; in fact, most systems are clipping with the volume control set around "one o'clock."  Preamp/receiver designers probably set these levels so that less sophisticated buyers will be impressed by their product's power (I've only got the control set half-way and it plays this loud.")  Additionally, and not surprising, many audiophiles believe that the "power meters" on their amplifiers or receivers can be used to avoid clipping.  Unfortunately this is not so.  The meters used on amps are only decorative and cannot show actual power levels since they can only be calibrated to one impedance.  Actual speaker impedance varies with frequency, it might be 20 ohm at 40 HZ and 4 ohm at 150 HZ, and yet be rated at 8 ohm!  A very small number of power amplifiers have clipping indicators that actually do what they say.  If your amplifier has this capacity, it will be made clear in the instruction manual.  To guard against clipping, ask your dealer for guidance, or keep your volume control below "twelve o'clock" on the dial.

     The effects of too much power are either burned voice coils, deformed voice coil formers, loose voice coil windings, or lead-in wire fatigue.  The first three effects are the result of excessive heat.  More than 95% of the amplifier's power is converted to heat in the voice coils.  A burned voice coil results when the wire has actually melted.  Most voice coils are wound on aluminum bobbins to improve heat dissipation and cooling.  Unfortunately, when aluminum is heated to the point of deforming, it does not return to exactly its original shape.  The result can be a "rubbing driver" where the former touches the "pole piece" during its operation.  (The pole piece is a cylindrical piece of metal which, together with the magnet of the speaker, forms the magnetic gap in which the voice coil moves.)  While most high-end drivers use high temperature adhesives, excessive heat can still melt the adhesive enough to free turns of the coil.  This results in a rattling sound as these loose turns hit the pole piece.  Stop by our store, we'll show you these parts.

     We are all familiar with the concept of metal fatigue.  This occurs when a piece of metal is bent back and forth so far and so many times that it eventually breaks.  This can also occur in drivers which are overpowered.  Since woofers and midranges use "tinsel leads" or special heavy-duty wires attached to the voice coil, they don't fail this way.  But tweeters use a very thin voice coil wire to attach the voice coil to the terminals.  Excessive power can break those leads.  This is one of the causes for the "after-the-party syndrome," where the speaker was working at high volume levels, and then quit the next day at low levels.  The actual damage was done at the high levels, but then the wire finally broke at levels that would not normally cause any damage.

The second cause of speaker failure may be surprising: too little power can destroy drivers, especially tweeters.  To see why, we can look at a musical waveform below left:

     Notice that it looks like a combination of various sine waves.  If we take the same music and reproduce it at a level greater than the amplifier is capable of (clipping), we get something like the one on the right:  Now the signal  is looking more like a group of square waves rather than sine waves.  Square waves contain large amounts of high frequency information, and can destroy tweeters.  In this situation, clipping in the midrange creates large amounts of high frequency energy which damages the tweeter.  This is one of the reasons we look for high maximum amplifier power ratings: they will assure that musical peaks are not clipped and so turned into potentially damaging square waves.