Loudspeakers & Power Ratings Part III: The Test Results

[admin]

In parts one and two, the case was made for the idea that real power handling in loudspeakers was intimately tied to the nature of the signal, as well as the electromechanical parameters of the speaker under test.

In the final part of this three part article, Paul performs an experiment to determine if the underlying theory has merit, or if he is simply full of hot air. (Hot air is not good for either loudspeakers or authors). What he aims to prove is without specifying the frequency content and crest factor of the test signal used, the power handling number/rating in loudspeakers IS MEANINGLESS.

Discuss " Loudspeakers & Power Ratings Part III: The Test Results" here. Read the article.

 

[fmw]

Thanks for an excellent test and article.

 

[gene]

crickets crickets, honestly guys if this were a cable article or an article about bass extension of various online subwoofers, there would be 20-30 posts.

 

[cbraver]

I tried reading it but almost died. What does it say?

It needs some flavor. I added it for you...

 

[Haoleb]

Fortunately for the further sucess of the human race, Not all of the male species have the intellect of a doorknob, only to be stimulated by beer, football, and boobs.

I for one found the article very intersting, Especially considering I have 4 of those drivers in my speakers.

Now if you dont mind im going to get a beer, and put on some girls in bikini's in HD. I'll pass on the football.

 

[gene]

Ok next time I will throw in some pics of Sarah Palin

 

[fredk]

Too funny.

I need to find a good chunk of time when I am alert to read this one. Though I am technically inclined I know squat about electronics.

I must say I just about cried when I was what was once and Axiom woofer! I guess that means my ears are gonna blow long before my speakers.

Overall, this is a great series for someone like me who wants to learn more.

 

[Djizasse]

It can't quite understand everything. Had to digest the "crest factor" definition and read the article twice. Thanks for destructing stuff in the name of Science

 

[Rickster71]

Two questions:
Would a ported speaker be able to cool itself better, due to it's openings pumping air in and out?


What type of voice coil wire is better overall? An aluminum wire coated in copper, or a solid copper voice coil?
The aluminum wire would dissipate heat better, though an all copper wire is a better conductor.
What is better, or is it a trade off?

Thanks,
Rick

 

[Soundman]

Ok next time I will throw in some pics of Sarah Palin

I sense sarcasm. She's the best looking vice-presidental candidate I've seen.

 

[PENG]

I hope this article can make a dent into another popular misconception (I mean the other one being how important ACD ratings are) that more speakers are damaged by being underpowered than overpowered. I always find it hard to believe even some loudspeaker manufacturer’s sites are perpetuating such nonsense. The thing is people with low power amps will only tolerate audible distortions to a point before they reluctantly turned the volume down, hence before they blow the tweeters. On the other hand, people with high power amps (happened to me once) may get carried away and end up turning the volume up too high especially when they could not hear distortion during the quiet passage of a symphony.

Surely clipped signals are not good to speakers and will damage them, but there is no need to exaggerate it to the nth degree. I know for a fact you can use a 5WX2 (claimed) to power a pair of 20W (claimed) rated speakers all day long at levels you can hear major distortion and that boom box will not blow the speakers. On the other hand I guarantee you will blow those same speakers in no time with a 50W amp sending unclipped signals.

Now, it would be nice if Audioholics would do more tests using real music instead of test tones.

 

[fmw]

Now, it would be nice if Audioholics would do more tests using real music instead of test tones.

Wouldn't work with this test. You need a constant source so that your measurements will have meaning.

 

[Paul_Apollonio]

Answers for Rickster 71

Two questions:
Would a ported speaker be able to cool itself better, due to it's openings pumping air in and out?

Sorry if I sound like a politician, but in fact this could serve to help or to hurt. The major heat sink is the drivers metal (steel and aluminum) and having a box that is ported would allow a lower ambient temperature around the speakers major heatsink, its motor structure. With the cooler surroundings, the metal can absorb more heat from the voice coil, allowing for more power handling. On the other hand, a vented system will likely have less back pressure than a sealed box, and this can translate into less turbulence in the Voice coil gap. Less turbulence in the gap will provide less forced air cooling across the surface of the Voice coil, and that will hurt the power handling of the speaker. If I had to pick one effect over the other, I would say the vented box would give you a better overall long term power handling, at the expense of a smaller short term peak power handling compared to the sealed box.

What type of voice coil wire is better overall? An aluminum wire coated in copper, or a solid copper voice coil?

What about straight aluminum wire without copper cladding? Aluminum is better at handling power than copper wire. No contest here. The cost of aluminum wire is higher, and the volume it occupies for a given height and diameter will be greater as it only has 62% conductivity of copper when compared on unit volume. This same effect however, means lower current density in the wire allowing the power absorbed to have a greater area over which to dissipate. ALUMINUM WORKS BETTER, Period.

The aluminum wire would dissipate heat better, though an all copper wire is a better conductor.

Copper has higher conductivity Per unit volume, NOT per unit mass.

What is better, or is it a trade off?

Thanks,
Rick

Very good questions Rickster. EVERYTHING we do as speaker engineers results in trade-offs. Anyone who tells you otherwise is ill or under informed.

 

[Paul_Apollonio]

It can't quite understand everything. Had to digest the "crest factor" definition and read the article twice. Thanks for destructing stuff in the name of Science

It is not difficult to understand, so perhaps I did not explain it well. Let me try again. Crest factor is the ratio of instantaneous Peak to long term RMS power in the signal.

Thats it.

Sincerely,

Paul Apollonio

 

[Paul_Apollonio]

I hope this article can make a dent into another popular misconception (I mean the other one being how important ACD ratings are) that more speakers are damaged by being underpowered than overpowered. I always find it hard to believe even some loudspeaker manufacturer’s sites are perpetuating such nonsense. The thing is people with low power amps will only tolerate audible distortions to a point before they reluctantly turned the volume down, hence before they blow the tweeters. On the other hand, people with high power amps (happened to me once) may get carried away and end up turning the volume up too high especially when they could not hear distortion during the quiet passage of a symphony.

Surely clipped signals are not good to speakers and will damage them, but there is no need to exaggerate it to the nth degree. I know for a fact you can use a 5WX2 (claimed) to power a pair of 20W (claimed) rated speakers all day long at levels you can hear major distortion and that boom box will not blow the speakers. On the other hand I guarantee you will blow those same speakers in no time with a 50W amp sending unclipped signals.

Now, it would be nice if Audioholics would do more tests using real music instead of test tones.

Actually the article speaks directly to this point. Perhaps it was not clear. Regardless, it is worth reiterating. Here goes. CLIPPED SIGNALS are dangerous to High Frequency devices, NOT woofers. It is because the percentage of power in the spectrum shifts from low to high frequencies that this is so. As the article clearly shows, high frequency energy is not absorbed as easily as low frequency energy by inductive woofers (anything with a conventional voice coil will be inductive). Therefore, the clipped signal is (with the RMS value held constant) LESS dangerous to the woofer, and more dangerous to a systems tweeter. If unclipped music has 10% of the power above 2khz, a clipped version of the same signal (test or music) will have considerably more. Therefore, the tweeter sees a much higher percentage of the signal when it is clipped than not. The woofer sees a lower percentage of the signal since it has shifted up. Also remember an amp that can deliver 500 watts RMS power (at low distortion) will put out 1000 watts (or close to it) when clipped (turning your sine wave into a square wave). Of course, the distortion (Which manifests itself as a series of harmonics) goes WAY UP. If this point is not clear, you might have to look at the impedance curves to see that the woofer has a higher impedance (resistance to current flow) at high frequencies than low frequencies. Since most of the signal power of music is in the range of 100 to 500 hz, when those musical signals are clipped, the harmonics which result go into the range of the high or mid frequency devices. Again, dangerous for the tweeter, not the woofer.

As for using music as a test signal, we would love to. This would make the tests far more entertaining. The problem is that music does not have a controlled crest factor or spectrum and is therefore not a reliable way to measure speakers. The best we can do is to use a repeatable signal with the same spectral distribution of energy and crest factor. The standards committees are lagging behind your intuition on this point with regard to adopting such a realistic standard.

I hope that helps to clarify. If not, let me know.

Sincerely,

Paul Apollonio

 

[Seth=L]

Paul, you may be the perfect person to ask this question.

I have an M&K MX-100 subwoofer in storage at the moment (because it is broken). I have had it fixed twice. I had it for several months (after I initially bought it) before I had a single problem with it. It sounded great, had awesome low frequency extention, as well as high output. One day I played the movie Resident Evil: Apocolypse and during one of the scenes there was some inaudible low frequencies present. I was watching this movie at near reference level in a small to medium room. My other relevant equippment was a pair of PSB Centuri 600i floor standing speakers and a Carver AV-505 multichannel amplifier.

The Carver and the subwoofer shut down during this scene. The Carver came back on, the M&K did not. If I remember correctly the mains fuse on the subwoofer blew, and I know for certain the two internal fuses blew as well. When this occured the woofers (there are two) slammed against the magnet structure making a loud clap sound (I assume this might have been the bleed off from the capacitors). I am unsure of what caused it do that. I took the subwoofer's amp plate off, I observed the power transformer was hot enough to burn my hand.

I took the subwoofer to a local repair shop and had it tested. The shop claimed the IC output device was damaged and in need of replacement, apparently the power transformer tested good. No other parts, other than the fuses, where replaced. When they repaired it the where unsure of what fuse size to use for the internal fuses (the mains fuse is clearly stated what it should be). I don't recal if the fuses where too low, or too high of a rating. I got the subwoofer home and tried it out. It worked for a short while (probably less than 30 minutes) and stopped working again.

The repair shop has a warranty, which they honored. They replaced the IC again, and put the correct fuses in it this time. I am unsure if they retested the entire subwoofer. It's conceivable that another part could be damaged and in need of replacement due to the neglagence to find the correct fuse value the first time because it blew again after a few short days, and I don't believe I was being very hard on the subwoofer. M&K made very good subwoofers designed for high output, and low bass. I would expect a $1000 subwoofer to perform as such.

Both woofers seem to be fine, there is no rubbing sound when you press on the cones and they both work when connected to an amplifier.

Have I just been horribly unlucky and am I just getting all these low frequencies imposing taxing loads on the subwoofer's amplifier, or is there something massively incorrect with the subwoofer's amplifier? I am aware these factors aren't likely enough to figure it out, but what's the likelyhood of it being one way or the other.

Thank you in advance for any advise or insight,

Seth

 

[PENG]

I hope that helps to clarify. If not, let me know.

Sincerely,

Paul Apollonio

Actually I did understand your article as I understand Fourier analysis quite well. In fact that was why I made the point that people tend to exaggerate how speakers are more likely damaged by being underpowered. As you explained so clearly, it is the tweeter that gets damaged due to the high frequency contents of the clipped signal. When a solid state amp clipped, it makes the speakers sound terrible. So I would bet that normal people would turn the volume down long before the tweeters melt down, because normal people are sensitive to the distorted mid/high frequency signals. In doing so they will save their tweeters every time!

Regarding my suggestion of testing with real music, again I think you and fmw might have misunderstood my point. I was just trying to say that while you can damage the tweeter with tones easily, if you use real music, a normal person would not like the distorted mid/high frequency signal and would turn the volume down right away before the tweeter gets too hot. In my house even if the music sounds loud but not distorted, someone will still grab the remote and turn it down if I am not quick enough to do it myself.

In fact, I would like to argue the opposite that a high power amp has a greater chance to kill a tweeter because a normal person can withstand much louder level of SPL if the music is loud but is free of distortion. That's how I fried a tweeter as I mentioned in my last post. In fact at the time I had no excuse as I wasn't even listening to classical music that has quite passages. I simply got it too loud (no audible distortion) for a little too long, when I first added an amp to the front channels.

Anyway, thanks for clarifying your point. As I said I understood it the first time, but you did an excellent job explaining it the second time. On the contrary I completely failed to make myself clear in my post clear.

 

[PENG]

Wouldn't work with this test. You need a constant source so that your measurements will have meaning.

I agree, I was being sarcastic when I made that statement. Basically I am not a big believer of the claim that more speakers are damaged by underpowered amps. I am sure it happens, but probably not that often. I damaged mine once by overpowering it, period. I can't imagine who can withstand distorted mid high frequencies at home in their private listening environment. People do turn boom box up way too loud in semi public or public places, but those things can blast all day long fully distorted with no problem. I guess they don't have real tweeters to worry about.

 

[Pyrrho]

crickets crickets, honestly guys if this were a cable article or an article about bass extension of various online subwoofers, there would be 20-30 posts.

I think this article is something that more people should read, so I am bringing the thread back up into the realm of the living. Thank you (and especially Paul, who wrote the article) for your excellent work on this!

 

[GranteedEV]

This thread got me thinking that speakers need phase-change cooling.
As in, like the stuff in refrigerators. (I miss the old chip-con prometeia btw)

anyways..yeah...that article was amusing... but would be funner with WIDEOS!!!!! ESPECIALLY of that tone burst....