Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[TJ (Tom) Cornish]

He had managed to fry the voice coil of a 400 watt woofer with what was probably 12w of highly distorted power.

There are two forms of the "under-powering" myth.
1. Amps that are too small blow tweeters due to harmonics appearing from the amp clipping.  That was the basis of my last post, and if I have time I will try to put some math behind my statements.
2. The shape of the waveform damages speakers.  This is usually brought up in the context of woofers.

You are referring to version 2.

This is equally a myth, and incorrect.   Distorted waveforms including square waves are a core component in synthesis, and are present any time there's an electric guitar or keyboard in the music.  If it were true that the shape of the waveform mattered, no loudspeaker would last 2 seconds. 

What causes woofer damage is the same thing that causes tweeter damage - either excursion damage from too much peak voltage, or thermal damage from too much sustained power.

In simple terms, there is no way a 400 watt speaker can be damaged by a 12 watt signal, however the real world is a little more complicated. 

Music theoretically (some genres aside - those popular in car audio, for example) has dynamic content, and therefore sometimes is loud, sometimes is quieter.  This is quantified by crest factor - the ratio between loudest parts of the music to the average.  Typical rock music has a crest factor of 12dB or more, some electronic music may have a crest factor of as little as 3dB, particularly when you look at the low-frequency band - long, sustained bass notes.

Drivers are rated in various ways, but generally are rated with a certain crest factor in mind.  This is crudely described with terms like "program", "peak", "continuous", "RMS", etc.  If you're playing rock, you probably can use the "program" rating.  If you're playing long, sustained bass notes, it's possible even the "continuous" rating may be too optimistic. A "400 watt" peak driver may have a continuous rating of only 50 watts or so, and even that assumes that the driver is moving, creating air movement to cool the coils.  The DC capacity of the "400 watt" driver may only be 10 or 20 watts.

Your observation of the toasted voice coil clearly indicates a driver that has been thermally damaged due to over-powering.  I'm unfamiliar with the rating conventions used in car audio, but this fact is self-evident.  Most likely the dash amp puts out a lot more power than the 12 watts you're claiming, and/or the woofer is rated optimistically.

I stand by my statement that a larger amp in the hands of someone who apparently has no awareness of damage - the smell of the hot coil/former, the horrible sound from the clipped signal, woofers bouncing off the magnet, etc. would only speed destruction.

I'm not saying that this guy or any of the other customers I've dealt with wouldn't have totally smoked their speakers with the recommended amount of power but I can say that I see and have seen many speakers come back as charred locked up blobs of nothing after being exposed to high levels of distortion over extended periods of time on amplifiers that were far below the rated power handling of the woofer.

[Jean-Pierre Coetzee]

There are two forms of the "under-powering" myth.
1. Amps that are too small blow tweeters due to harmonics appearing from the amp clipping.  That was the basis of my last post, and if I have time I will try to put some math behind my statements.
2. The shape of the waveform damages speakers.  This is usually brought up in the context of woofers.

You are referring to version 2.

This is equally a myth, and incorrect.   Distorted waveforms including square waves are a core component in synthesis, and are present any time there's an electric guitar or keyboard in the music.  If it were true that the shape of the waveform mattered, no loudspeaker would last 2 seconds. 

What causes woofer damage is the same thing that causes tweeter damage - either excursion damage from too much peak voltage, or thermal damage from too much sustained power.

In simple terms, there is no way a 400 watt speaker can be damaged by a 12 watt signal, however the real world is a little more complicated. 

Music theoretically (some genres aside - those popular in car audio, for example) has dynamic content, and therefore sometimes is loud, sometimes is quieter.  This is quantified by crest factor - the ratio between loudest parts of the music to the average.  Typical rock music has a crest factor of 12dB or more, some electronic music may have a crest factor of as little as 3dB, particularly when you look at the low-frequency band - long, sustained bass notes.

Drivers are rated in various ways, but generally are rated with a certain crest factor in mind.  This is crudely described with terms like "program", "peak", "continuous", "RMS", etc.  If you're playing rock, you probably can use the "program" rating.  If you're playing long, sustained bass notes, it's possible even the "continuous" rating may be too optimistic. A "400 watt" peak driver may have a continuous rating of only 50 watts or so, and even that assumes that the driver is moving, creating air movement to cool the coils.  The DC capacity of the "400 watt" driver may only be 10 or 20 watts.

Your observation of the toasted voice coil clearly indicates a driver that has been thermally damaged due to over-powering.  I'm unfamiliar with the rating conventions used in car audio, but this fact is self-evident.  Most likely the dash amp puts out a lot more power than the 12 watts you're claiming, and/or the woofer is rated optimistically.

I stand by my statement that a larger amp in the hands of someone who apparently has no awareness of damage - the smell of the hot coil/former, the horrible sound from the clipped signal, woofers bouncing off the magnet, etc. would only speed destruction.

Was going to type something like this earlier but decided it would be entirely off topic and has been addressed about a 1000 times on this forums already...

For my 2c though, the two modes of failure (Mechanical and thermal) are correct, peak power generally relates to instantaneous maximum which would mean pretty much how much power you could pass into the voicecoil before it will drive itself right out of the airgap, program/continues as you explained relates to thermal failure and as you stated crest factors, can we make a topic with what you just said, have JR and Tom Danley proof it and edit and then sticky it?

[Chuck Simon]

Was going to type something like this earlier but decided it would be entirely off topic and has been addressed about a 1000 times on this forums already...

For my 2c though, the two modes of failure (Mechanical and thermal) are correct, peak power generally relates to instantaneous maximum which would mean pretty much how much power you could pass into the voicecoil before it will drive itself right out of the airgap, program/continues as you explained relates to thermal failure and as you stated crest factors, can we make a topic with what you just said, have JR and Tom Danley proof it and edit and then sticky it?

I don't think it is necessary for JR, Tom Danley or anyone else here to proof anything written by TJ.  He is talking facts not opinion, and he knows what he is talking about.

[Brian Jojade]

If too little power blew speakers, we'd have to run them WFO all of the time.  Thank goodness that's not how they really work.

Power ratings are one of those things that have numbers that are all over the board and can mean many different things.  Peak ratings are just a number thrown out there by marketing guys with small units to make a product seem more impressive than it is.  The continuous RMS rating is the number that you want to focus on.  That's what a speaker should be able to handle for a long period of time. However, this assumes that the signal is a general sine wave.  Sending a square wave to the speaker versus a sine wave results in the speaker having to dissipate double the power.  So, a speaker rated at 400 watts would only really be able to handle 200 watts of a square wave.

Most amplifiers have protection circuits in place to prevent the amp from delivering square waves.  However, if you drive an amplifier past clipping, and the protection isn't in place, the resulting output clips the signal into a quasi square wave.  Now, if the signal is a square wave, that's twice the power of a sine wave.  A 100 watt amp producing a sine wave could potentially produce 200 watts of a square wave, or even more, depending on how modestly the rating was stated.  Using this math, a 100 watt amp could potentially deliver enough power to burn up a 400 watt speaker.

I highly doubt that an amplifier rated at 12 watts RMS would be capable of burning up a driver rated at 400 watts RMS though.  Either the amp is rated extremely low, or the driver is rated extremely high. 

[John Rutirasiri]

If this myth/logical fallacy would never be repeated again, we would all be a lot better off.
I will challenge anyone to find an example where putting a larger amp on a speaker has prevented it from failing.

While it is true that extreme clipping increases HF content, that only causes speaker damage if the "too-small" amp is big enough to over-power the tweeter.  Saying this for emphasis - that is OVER-powering. It's not the shape of the waveform that wrecks stuff, it's very simply too much power either instantaneously or cumulatively.

The logical fallacy that the "under-powering" parrots miss is that if a user is stupid enough to drive a small amp into extreme clipping, what's going to stop them from driving a large amp into clipping and blowing up the speaker faster?

I'm not picking on you John; this is just a pet peeve of mine and has been thoroughly and repeatedly debunked.

Tom, I think it's nomenclature here, and I was only talking about high freq drivers.

When you drive an amp continuously into hard clip and the amp does not have any limiters, the average power (area under the curve) could be seriously more than a normal music signal, sine wave, or pink noise.  So actual wattage may be more than the AES pink noise with 6dB crest factor rating.  I suppose if you want to call this overpowering beyond the spec'ed rating, that would be correct.  I think most folks refer to it as driving an underpowered amp into clip, or underpowering a speaker. This has profound effect on tweeters and high freq drivers since they don't handle that much power to begin with.

The higher average power heats up the voice coil winding because of Ohmic resistance and to a small degree, the cone or diaphragm whose movement cools the voice coil cannot pump enough air given the increased the power going through the winding, or the ferrofluid can't dissipate heat fast enough.  Eventually the insulation coating on the tiny wires of the voice coil winding just flow and melt, causing shorted winding (or open if the wire just breaks).  The former and the collar can soften, leading to distortion of the coil and eventual destruction. 

The clipped waveform contains many inaudible high frequency harmonics simply not present in normal music, and at high amplitudes.  Can they kill tweeters and high freq drivers?  Probably more likely to kill the amp first (oscillation) but my guess is yes, high amplitude HF content can kill high freq drivers because it's still power.

FWIW, my nephew fried a couple of pretty robust JBL L100T speakers' titanium tweeters by underpowering them with a 50W/ch home receiver.  He could not get enough volume, so he just cranked it.  Now tell me how that happened with 3-way speakers rated for 200W RMS and robust 12" woofer?  I think the tweeters alone handled 40W continuous each.  Home stereo or not, it's the same principle.

I agree that overpowering is the usual case and underpowering should not damage speaker, but let's agree to disagree that you can also kill it by using an amp of insufficient power and drive it in continuous clip.

Best,
JR

[John Sabine]

Ok.. I think I see what TJ Cornish is saying. If you look at a speaker in pure technical terms you have a resistor (voice coil) that is designed to dissipate a certain amount of heat as measured in wattage. A 100 watt resistor will dissipate 100 watts worth of heat be it sine wave, square wave, sawtooth, DC, 100 watts is 100 watts. Coupled with the physical structure of the speaker the power handling of the voice coil is increased because of air being forced past the voice coil through the VC gap. In a perfect world in a perfect box with the perfect amp using numbers pulled purely from the sky a woofer with a 100 watt voice coil might handle 400 watts of power as long as it is allowed to move in the manner of which it was designed. However, if you halve the enclosure size for that woofer then the power handling will decrease because you are limiting the ability of the woofer to cool it's voice coil. You also lessen the power handling of a woofer (as a unit) by placing it in a too large enclosure because although you are giving it good amounts of cooling air you could allow the mechanical structure to exceed the limits for which it was designed. (Going out on a limb here). As far as distorted waveforms, I believe that if you driving a speaker at it's theoretical maximum power with a square wave then you will lower the electrical power handling of the speaker because it will not achieve full excursion and can't cool itself effectively, therefore a 400w RMS speaker might become a 300w RMS speaker being driven with 400W. This can also happen if you are driving a speaker designed to handle a certain wattage at a frequency the frequency range that the speaker was designed to operate in.
 The technicians among us are saying that a 100 watt resistor will handle 100 watts and will not burst into flames at lower wattages and speakers designed to handle 400 watts will handle 400 watts if operated as designed. The problems happen when we in the real world change a parameter in the equation such as box volume, frequency, etc. My customer who killed the 12" woofer with his radio was probably indeed getting far more than 12 watts out of his radio by turning the volume all the way up. This in itself isn't a problem except that his radio wasn't able to generate adequate output in the range where the woofer was designed to work but my customer was driving a full range signal into this 12" woofer so even though his radio couldn't produce enough power to actually move the woofer very well so that it could mechanically cool itself, it was able to generate enough power higher up in the frequency range to overheat the voice coil and kill the speaker. Too much power for the resistor (voice coil) to dissipate on its on without mechanical cooling. So indeed, too much power applied wrongly killed the speaker and kills many speakers I see (and I sell literally hundreds of speaker drivers a year and see a dozen or so come back smoked). The problem is that most of us don't live in an engineers world. People want speakers that won't fit certain places so they skimp on box volume or they don't feed the proper signal into the speaker or some actually kill them with distorted waveforms because it's actually PART OF THE MUSIC (especially rap, techno and dub step). To the engineer each of these customers over powered the structure in their speakers but to the unwashed masses (myself included)  dude bought 2 400 watt RMS speakers, installed them with an amplifier that was rated at 200w RMS, and blew up his SH**. This discussion is truly one of same planet, different worlds.

[Sander Rooijens]

to the unwashed masses (myself included)  dude bought 2 400 watt RMS speakers, installed them with an amplifier that was rated at 200w RMS, and blew up his SH**. This discussion is truly one of same planet, different worlds.

And the reasons have nothing to do with "underpowering" but with idiots abusing gear.
It has already been said but worth it to repeat:

I stand by my statement that a larger amp in the hands of someone who apparently has no awareness of damage - the smell of the hot coil/former, the horrible sound from the clipped signal, woofers bouncing off the magnet, etc. would only speed destruction.

TJ is 100% right. (text is from TJ's earlier post, bold sections added by me)

S.R.

[Tim McCulloch]

Tom, I think it's nomenclature here, and I was only talking about high freq drivers.

When you drive an amp continuously into hard clip and the amp does not have any limiters, the average power (area under the curve) could be seriously more than a normal music signal, sine wave, or pink noise.  So actual wattage may be more than the AES pink noise with 6dB crest factor rating.  I suppose if you want to call this overpowering beyond the spec'ed rating, that would be correct.  I think most folks refer to it as driving an underpowered amp into clip, or underpowering a speaker. This has profound effect on tweeters and high freq drivers since they don't handle that much power to begin with.

The higher average power heats up the voice coil winding because of Ohmic resistance and to a small degree, the cone or diaphragm whose movement cools the voice coil cannot pump enough air given the increased the power going through the winding, or the ferrofluid can't dissipate heat fast enough.  Eventually the insulation coating on the tiny wires of the voice coil winding just flow and melt, causing shorted winding (or open if the wire just breaks).  The former and the collar can soften, leading to distortion of the coil and eventual destruction. 

The clipped waveform contains many inaudible high frequency harmonics simply not present in normal music, and at high amplitudes.  Can they kill tweeters and high freq drivers?  Probably more likely to kill the amp first (oscillation) but my guess is yes, high amplitude HF content can kill high freq drivers because it's still power.

FWIW, my nephew fried a couple of pretty robust JBL L100T speakers' titanium tweeters by underpowering them with a 50W/ch home receiver.  He could not get enough volume, so he just cranked it.  Now tell me how that happened with 3-way speakers rated for 200W RMS and robust 12" woofer?  I think the tweeters alone handled 40W continuous each.  Home stereo or not, it's the same principle.

I agree that overpowering is the usual case and underpowering should not damage speaker, but let's agree to disagree that you can also kill it by using an amp of insufficient power and drive it in continuous clip.

Best,
JR

It wasn't the harmonics, it was the continuous application of too much voltage, a rise in voice coil DC resistance from heat and the application of yet more voltage, resulting in yet more heat.

A bigger amp *might* have gotten the speakers to the user's desired level - briefly - until the power compression set in, then the result would be the same if the user just turned up more to compensate.

[Brad Weber]

In my 30 + years in the consumer electronics industry I find your statement that under powering speakers will not blow them to be inaccurate. I not only work with pro audio but sell and install home theater and car audio speakers. Many times I have had customers buy (against my advice) the biggest woofers available for their car yet skimp on a power amplifier and either buy an amp that is too small or buy one of the wonder amps on the market that offers 10,000 watts for $99.00 and come back with very nice woofers with very cooked voice coils. I've seen this scenario several times a year repeated over my many years in the industry. I once had a customer purchase the most expensive woofer I had in stock (A Punch Pro Series 12" subwoofer that I believe had a rating of 400 watts RMS). We tried to sell him an amplifier but he only wanted the speaker. He came back a week later with the speaker blown. The voice coil was completely locked up. I asked to see how he had hooked it up and what he was running the sub with. He took me to his car where I discovered that he had been driving the sub with one channel of his high powered in dash radio. I asked him to turn his radio up to the volume he usually listened at and he ran the volume up to maximum. He had managed to fry the voice coil of a 400 watt woofer with what was probably 12w of highly distorted power.
 I'm not saying that this guy or any of the other customers I've dealt with wouldn't have totally smoked their speakers with the recommended amount of power but I can say that I see and have seen many speakers come back as charred locked up blobs of nothing after being exposed to high levels of distortion over extended periods of time on amplifiers that were far below the rated power handling of the woofer.

FWIW, my nephew fried a couple of pretty robust JBL L100T speakers' titanium tweeters by underpowering them with a 50W/ch home receiver.  He could not get enough volume, so he just cranked it.  Now tell me how that happened with 3-way speakers rated for 200W RMS and robust 12" woofer?  I think the tweeters alone handled 40W continuous each.  Home stereo or not, it's the same principle.

Neither example given seems to relate to "underpowering" the speakers and they instead both appear address driving the system beyond its capabilities.  You could take the same amps and some similar speakers with the same power ratings and if the speakers provided more output (i.e. had a higher sensitivity) then everything might have been just fine.  In such cases it is the systems, not the speakers, that are underpowered.

[Edgar Hernandez]

Ok.. I think I see what TJ Cornish is saying. If you look at a speaker in pure technical terms you have a resistor (voice coil) that is designed to dissipate a certain amount of heat as measured in wattage. A 100 watt resistor will dissipate 100 watts worth of heat be it sine wave, square wave, sawtooth, DC, 100 watts is 100 watts. Coupled with the physical structure of the speaker the power handling of the voice coil is increased because of air being forced past the voice coil through the VC gap. In a perfect world in a perfect box with the perfect amp using numbers pulled purely from the sky a woofer with a 100 watt voice coil might handle 400 watts of power as long as it is allowed to move in the manner of which it was designed. However, if you halve the enclosure size for that woofer then the power handling will decrease because you are limiting the ability of the woofer to cool it's voice coil. You also lessen the power handling of a woofer (as a unit) by placing it in a too large enclosure because although you are giving it good amounts of cooling air you could allow the mechanical structure to exceed the limits for which it was designed. (Going out on a limb here). As far as distorted waveforms, I believe that if you driving a speaker at it's theoretical maximum power with a square wave then you will lower the electrical power handling of the speaker because it will not achieve full excursion and can't cool itself effectively, therefore a 400w RMS speaker might become a 300w RMS speaker being driven with 400W. This can also happen if you are driving a speaker designed to handle a certain wattage at a frequency the frequency range that the speaker was designed to operate in.
 The technicians among us are saying that a 100 watt resistor will handle 100 watts and will not burst into flames at lower wattages and speakers designed to handle 400 watts will handle 400 watts if operated as designed. The problems happen when we in the real world change a parameter in the equation such as box volume, frequency, etc. My customer who killed the 12" woofer with his radio was probably indeed getting far more than 12 watts out of his radio by turning the volume all the way up. This in itself isn't a problem except that his radio wasn't able to generate adequate output in the range where the woofer was designed to work but my customer was driving a full range signal into this 12" woofer so even though his radio couldn't produce enough power to actually move the woofer very well so that it could mechanically cool itself, it was able to generate enough power higher up in the frequency range to overheat the voice coil and kill the speaker. Too much power for the resistor (voice coil) to dissipate on its on without mechanical cooling. So indeed, too much power applied wrongly killed the speaker and kills many speakers I see (and I sell literally hundreds of speaker drivers a year and see a dozen or so come back smoked). The problem is that most of us don't live in an engineers world. People want speakers that won't fit certain places so they skimp on box volume or they don't feed the proper signal into the speaker or some actually kill them with distorted waveforms because it's actually PART OF THE MUSIC (especially rap, techno and dub step). To the engineer each of these customers over powered the structure in their speakers but to the unwashed masses (myself included)  dude bought 2 400 watt RMS speakers, installed them with an amplifier that was rated at 200w RMS, and blew up his SH**. This discussion is truly one of same planet, different worlds.

Since it doesn’t make any sense I have to conclude that your customer did not told you the truth, let imagine that this customer came to your shop with a burn woofer and a 2000 watts power amp, your answer to the customer would be; you over powered the Sub, right?, so why take my real amp if I can request a replacement of a defective sub?

[ProSoundWeb Community]