Loudspeakers & Power Ratings Part III: The Test Results Continued
After 90 minutes of testing without failure, I pump it up to 200 watts RMS, in the hope of putting this speaker to bed. Well, at this point, the smell was reminiscent of a garbage dump on a really hot day. The amp was running out of voltage, so either this bad boy gives up or I’d have to retest with the amp set to bridged mode. (6 minutes later...) OK!! We have TOAST!! At 1 hour and 36 minutes into the test, the pink noise speaker gave up with a puff of smoke. I dare not put my hand in there for fear of getting burnt by the hot frame. The heat from the speaker was intense enough that it can be felt from 6 inches above the driver. I removed the speaker and indeed, the frame was too hot to touch. After 10 minutes of cooling down, I noticed the speaker was so hot, it burned a hole through the polyester fibers lining the back of the test box! The white noise tested speaker by comparison was only slightly warm to the touch. This, in my mind was proof positive that the actual power delivered by 28.3 volts of white noise was considerably less real power to this speaker load than 28.3 volts of pink noise. (Photo Below...)
200 WATTS RMS OF PINK NOISE RESULTS
Upon dissection, it appeared the VC wire got so hot it expanded right off the bobbin (or former) and was sitting in the bottom of the gap. I had to remove it a turn at a time, and so it appeared like a plate of spaghetti. (photo above)
THIS IS WHAT A NORMAL UNBURNT VC LOOKS LIKE
Upon inspection of the deceased, it became apparent why it took so much power to burn this speaker. The VC wire was wound on an Aluminum bobbin, which in turn was directly coupled to an aluminum cone. This combination of parts allowed much heat to be drawn away from the VC, keeping it cooler than almost all other 1” VC's of this height and size would be. Kudos to the designers at Axiom for an excellent design job.
I remove the deceased, and replace it with subject # 3, the speaker which was going to get tested with a tone burst. Bear in mind, the tone burst was DESIGNED to kill the speaker. It was designed to do NOTHING else. The frequency is at the speakers free air resonance so this will maximize the excursion required for the given voltage, PLUS the impedance magnitude is so high at resonance, the current demand is only ¼ of what it is at 200 Hz. (See Impedance magnitude curve). The combination of high excursion and low current draw is a guarantee that this signal will break the speaker long before it is burnt.
1100 WATT PEAKS DESTROY THIS SPEAKER QUICKLY
The RMS level was set with the signal constant, and then the duty cycle was set for the signal to be on for 11 cycles and off for 99 cycles. That means the speaker was excited for 1/10th of the time so the RMS level drops by 10 db relative to the always on condition. This required more voltage than the amp was able to deliver when hooked up in the stereo mode. I therefore limited the peaks to 1100 watts for the tone burst part of the test. In under 60 seconds, I noticed the speaker (which sounded OK) looked warped. A closer examination revealed the speaker cone was torn near the surround/cone bond-line, and had partially separated from the surround. (See photo above)
This failure results from the enormous peak causing the speaker to move with sudden explosive force. It breaks at the point where the cone/surround subassembly goes from being thickest to thinnest, the point where the cone attaches to the inside edge of the surround flange. (See photo). Bear in mind, 1100 watts (peak from the 550 watt RMS tone burst) is almost 1.5 horsepower. Considering the duty cycle of this signal, the RMS value is only 55 watts!!
TONE BURST 11 CYCLES ON 99 CYCLES OFF
Oscillogram of same tone burst below, viewed over 11 cycles.
11 CYCLES OF 55 HZ
Conclusions
There are two ways to kill a loudspeaker; thermal or mechanical.
In this experiment we destroyed one of these small drivers
in each way. The most dangerous and
destructive signal used was one composed of a single low frequency tone burst
on for 11 cycles and off for 99. Despite
this signal containing only 55 watts of RMS power, the speaker failed as a
result of the peak voltage, correlating to a power (into nominal impedance) of
1100 watts peak. Peaks of very short
duration can serve to destroy a loudspeaker even though they are of such a
short duration, they are not able to heat up the speaker significantly. Frequency content and Peak to RMS ratio
(crest factor) are critical in determining the potential for loudspeaker
destruction of a test signal. The
loudspeakers impedance magnitude gives us clues as to the potential harm from a
signal based on the variation of impedance with frequency.
High frequencies are NOT DANGEROUS to the typical woofer, and the notion that clipped signals of a given RMS value are more dangerous to a woofer than unclipped signals is not necessarily correct. Clipped signals do put a higher percentage of amplifier power into the tweeter or high frequency device, and are therefore more dangerous to multi driver systems or high frequency units. Low frequency drivers (woofers) have very high impedances at high frequencies and are therefore not able to easily draw power from the amplifier at the highest frequencies. Without specifying the frequency content and crest factor of the test signal used, the POWER HANDLING NUMBER IS MEANINGLESS!
