Jump to content
LegacyGT.com

Overpowered/Underpowered Sub Woofers


CCSubaru
 Share

Recommended Posts

roosien092, to be clear, you are not describing "underpowering" a sub, you're describing running DC voltage through the voice coil (distortion).

 

Running less power than a sub is rated for will not damage it. Overheating the voice coil due to short term DC voltages can.

 

correct. What i was trying to say is that a lot of people who use smaller amps are not happy with the sound output and therefore turn the gain up causing distortion and damage(cranking the gain). Obviously the power the sub recieves is dependent on the volume as well so saying that underpowering a sub damages it is like saying that turning the volume down damages it as well. However, it is better to underpower it than to overpower it, if you exceed the thermal limits of the sub then you are in trouble. That being said, the closer your are to the specs of the sub, the better it will sound (obviously)

Link to comment
Share on other sites

  • Replies 128
  • Created
  • Last Reply

Top Posters In This Topic

roosien092, to be clear, you are not describing "underpowering" a sub, you're describing running DC voltage through the voice coil (distortion).

 

Running less power than a sub is rated for will not damage it. Overheating the voice coil due to short term DC voltages can.

 

I'd love to hear the explanation on "short-term DC power"...

Link to comment
Share on other sites

biz77, I'm simplifying, but let's take a 60Hz sine wave. If you clip off the top and bottom 25%, take a look at the waveform. For a short period of real time, you're effectively providing DC voltage (around .21ms) to the voice coil. (.42ms total distorted time)

 

If you measure the voltage provided from when the clipping starts at t=0, between t=0 and t=.21ms, you're going to see the voltage rail, and this static voltage will appear to be DC.

 

This type of action will melt many voice coils. (Excluding seriously overbuilt drivers, but almost everything today isn't manufactured with that kind of margin). This can melt voice coils, as heat buildup during that time frame will compound, and since the woofer is now not moving for periods of it's oscillation, there is less airflow than design spec over the voice coil.

Edited by ean611
Link to comment
Share on other sites

biz77, I'm simplifying, but let's take a 60Hz sine wave. If you clip off the top and bottom 25%, take a look at the waveform. For a short period of real time, you're effectively providing DC voltage (around .21ms) to the voice coil. (.42ms total distorted time)

 

If you measure the voltage provided from when the clipping starts at t=0, between t=0 and t=.21ms, you're going to see the voltage rail, and this static voltage will appear to be DC.

 

This type of action will melt many voice coils. (Excluding seriously overbuilt drivers, but almost everything today isn't manufactured with that kind of margin). This can melt voice coils, as heat buildup during that time frame will compound, and since the woofer is now not moving for periods of it's oscillation, there is less airflow than design spec over the voice coil.

 

In a properly functioning amplifier you are NOT seeing DC. You may see some ripple or offset, but your not seeing straight DC. Look at the definition of DC and then look at the explanation you gave. You are talking on the order of milliseconds. This is unfortunately another one of those myths that is so engrained that it is tough to show people the error of thier ways. Unfortunately people take white papers etc out of context and come up with this stuff. This myth is along the same lines as "too little power is more harmful to speakers than too much power." and is a close cousin of "clipping and distortion destroys speakers."

 

 

Nonetheless, straight DC will not kill a speaker and I can prove it...

 

The ONLY thing that will kill a speaker is TOO MUCH POWER. If anyone trys to tell you that a speaker driver dies any other death, please just kick them in the balls, turn your back and walk away.

Edited by biz77
Link to comment
Share on other sites

mwiener2, I've got melted voice coils to show for it

 

Biz77, the ripple is often too small to matter in this case. An average sub woofer will have virtually no response above 200-300Hz.

 

Also, when you're running distorted power through a speaker, you're turning your theoretical sine wave into a square wave. (not fully, but the power calculation becomes more complex). This will output more power than the amp is rated for, assuming the transistor and power supply can support the voltage. The amp is rated at a power and THD level, for instance 50 w @ .1%THD. That does not mean that the amp cannot output more power at an unacceptable level of distortion (85w @ 20%THD). In a good number of cases, you're seeing this, and it will melt the voice coil.

Amps typically do not publish how much power they can truly output if you're not concerned about harmonic distortion.

Edited by ean611
Link to comment
Share on other sites

Example for above:

http://legacygt.com/forums/images/editor/separator.gif

Amp has voltage rails at +42V and -42V (This is Class A, rather than class AB)

 

Amp could be rated at 400w RMS at 4Ohms. (leaving small margin over rating). Peak power of amp while distorted would be significantly higher, approaching a theoretical limit 882w at 100% duty cycle. If one were to add more margin, bringing the voltage bias significantly higher for the same rating, let's say +60V/-60V, the RMS power rating could be unchanged, but the max distorted output of the amp has increased significantly.

 

Bottom line, power output can increase beyond rating, and will only stop at electrical limits.

Link to comment
Share on other sites

Ean611, you are running off onto so many tangents here.

 

Let's address them one-by-one.

 

roosien092, to be clear, you are not describing "underpowering" a sub, you're describing running DC voltage through the voice coil (distortion).

 

Running less power than a sub is rated for will not damage it. Overheating the voice coil due to short term DC voltages can.

 

From "Truths and myths about clipping" by QSC, who builds professional sound reinforcement amplifiers and related equipment found in stadiums, theaters, at concerts etc:

 

http://media.qscaudio.com/forum/viewtopic.php?f=29&t=2736

 

"Clipping is bad for loudspeakers because it is DC.

Myth. A clipped signal is not DC. Even if the clipping is so severe that the waveform shape approaches that of a square wave, it’s still AC.

 

Even so, DC by itself is not necessarily dangerous for loudspeakers. In fact, a good way to check polarity of a woofer driver is to connect a small battery across it and see which way the cone moves for a given DC polarity; the amount of power dissipated is minuscule. However, a significant DC offset on an amplifier output is undesirable for several reasons: because it will dissipate power in the voice coil without producing any acoustical output; because it may cause the voice coil to travel out of the magnet gap, which would reduce the cooling; and because it shifts the driver’s at-rest position and thus makes the available excursion asymmetrical, reducing the overall safe excursion limits of the driver."

 

Example for above:

http://legacygt.com/forums/images/editor/separator.gif

Amp has voltage rails at +42V and -42V (This is Class A, rather than class AB)

 

Amp could be rated at 400w RMS at 4Ohms. (leaving small margin over rating). Peak power of amp while distorted would be significantly higher, approaching a theoretical limit 882w at 100% duty cycle. If one were to add more margin, bringing the voltage bias significantly higher for the same rating, let's say +60V/-60V, the RMS power rating could be unchanged, but the max distorted output of the amp has increased significantly.

 

Bottom line, power output can increase beyond rating, and will only stop at electrical limits.

 

Your point is moot. Your theoretical 882 watt amplifer could have been touted as a 100watt amplifer, or a 400 watt amplifier, or as an 800 watt amplifier depending on the rating method and how unscrupulous the manufacturer is. Your theoretical amplifier is capable of producing 882 watts, whether that is into an unclipped sine wave or if it is driven into complete distortion (square wave.) Here is the difference: Driven into a sine wave there is 3db of crest factor, which means the average of the musical signal is 3db down from the peak. Driven into a square wave there is 0db of crest factor. The amplifier did not magically become able to deliver twice as much power.

 

No one seems to be better at explaining this than our own msmith, so no reason for me to try any different twist on it:

 

"The only thing that thermally damages speakers is power... more specifically: average power over time.

 

I'll explain...

 

If you take a given amplifier, let's say 100 watts and operate it just below clipping with music material, the "Crest Factor" of the amplifier's output is equivalent to the "Crest Factor" of the program material.

 

"Crest Factor" is the difference between the average level of the signal and its peak level. For example, a pure sine wave has a "crest factor" of 3dB, meaning that it's peak level is 3dB higher than its average level. We all know that 3dB represents a power factor of 2, so another way to look at it is that the peak power of the signal is twice that of its average level. So, if we play a sine wave on our 100 watt amplifier, just below its clipping level, the average power (over time) the speaker is needing to dissipate is 50 watts.

 

A true square wave, by comparison, has a crest factor of 0db, so it has equal average and peak power. Our 100 watt amplifier, playing a square wave, unclipped, into our speaker requires that the speaker dissipates 100 watts of power (twice the heat as a sine wave).

 

Music has a significantly higher crest factor than sine waves or square waves. A highly dynamic recording (Sheffield Lab, Chesky, etc.) typically has a crest factor of 20dB or more, meaning that its average power is 100 times lower than its peak power. So, if we play our 100 watt amplifier just below clipping with the typical audiophile recording our speaker is only needing to dissipate 1 watt of average power over time.

 

Modern commercial recordings typically exhibit crest factors of around 10dB, meaning that the average power is 10 times lower than the peak power. So, our 100 watt amp just below clipping would deliver an average power over time of 10 watts that the speaker has to dissipate.

 

Okay, so what happens when we clip the amplifier (which we all do at times). When the amplifier enters into clipping, the peak power no longer increases, but here's the KEY... THE AVERAGE POWER CONTINUES TO INCREASE. We can often tolerate a fair amount of clipping... as much as 10 dB or more above clipping with a reasonably dynamic recording... a bit less with a compressed commercial recording.

 

So, if we turn the volume up 10dB higher than the clipping level with our Sheffield Lab recording, we have now reduced the crest factor of the signal reaching the speakers by 10dB... so instead of needing to dissipate 1 watt average, we are asking the speaker to dissipate 10 watts average, and we're probably ok.

 

If we turn up the volume 6dB past clipping on a compressed commercial recording (or bass music recording), we have taken the crest factor of the signal from a starting point of 10dB to only 4dB, asking the speaker to dissipate an average power of 40 watts instead of 10 watts... that's FOUR TIMES the average power, which generates four times the heat.

 

SO, in most cases, the reason clipping can damage a speaker really has nothing to do with anything other than an increase in average power over time. It's really not the shape of the wave or distortion... it's simply more power over time."

Link to comment
Share on other sites

SO, in most cases, the reason clipping can damage a speaker really has nothing to do with anything other than an increase in average power over time. It's really not the shape of the wave or distortion... it's simply more power over time.

 

Biz77, looking over how I went through that, I think we're saying the same thing. The point of bringing up the voltage rail was exactly that point, which is simply pointing out how a small "underpowered" amp can still damage a speaker. The reason is still overheating/overpowering the voice coil, but the point is that you can blow up a 75w speaker with a "50w" amp.

Link to comment
Share on other sites

Biz77, looking over how I went through that, I think we're saying the same thing. The point of bringing up the voltage rail was exactly that point, which is simply pointing out how a small "underpowered" amp can still damage a speaker. The reason is still overheating/overpowering the voice coil, but the point is that you can blow up a 75w speaker with a "50w" amp.

 

 

You made it sound like that theoretical amplifier would deliver a maximum of 400 watts while unclipped, but once you start clipping the amp it becomes an 800+ watt amp, which isn't how it works. That amp is always capable of delivering that 800 watts. How the power is applied over time is what matters.

Link to comment
Share on other sites

You're all stuck on this whole electrical side of the speaker.

 

If you have a sub rated at 100w and you turn your 35w amp 'up to 11' and it distorts really badly, you can actually physically damage the cone, the surround, the voice coil, etc. Paper rips, wires break, the speaker smashes through the grill.

(Updated 8/22/17)

2005 Outback FMT

Running on Electrons

Link to comment
Share on other sites

You're all stuck on this whole electrical side of the speaker.

 

If you have a sub rated at 100w and you turn your 35w amp 'up to 11' and it distorts really badly, you can actually physically damage the cone, the surround, the voice coil, etc. Paper rips, wires break, the speaker smashes through the grill.

 

Distortion doesn't kill speakers. Too much power does. What you are describing is mechanical failure from over powering.

Link to comment
Share on other sites

Distortion can cause mechanical failure. What we hear as distortion is the speaker hitting the end of it's travel.

 

 

The amp has enough peak power to move the speaker one direction, but then doesn't have enough power to stop it moving and reverse direction. The momentum of the speaker carries it to the end of it's travel and can cause mechanical failure.

(Updated 8/22/17)

2005 Outback FMT

Running on Electrons

Link to comment
Share on other sites

Distortion can cause mechanical failure. What we hear as distortion is the speaker hitting the end of it's travel.

 

 

The amp has enough peak power to move the speaker one direction, but then doesn't have enough power to stop it moving and reverse direction. The momentum of the speaker carries it to the end of it's travel and can cause mechanical failure.

 

Utter BS.

 

THE ONLY THING THAT DESTROYS A SPEAKER IS TOO MUCH POWER!!!!

 

You are describing a speaker that has surpassed its mechanical limits from (drum roll)... too much power.

 

In a sense all speakers introduce distortion when reporducing sound as they are far from perfect in converting the eletrical energy from an amplifier into mechanical energy, but what you describe above regarding momentum, blah, blah BS, blah is completely false.

 

 

Perhaps you should have looked into some of the links I poseted. From QSC "Truths and myths about clipping":

 

Clipping is bad for loudspeakers because it really heats up the voice coils.

Myth. Dynamic loudspeakers are notoriously inefficient; they turn well over 90% of the audio power put into them into waste heat instead of acoustical energy. A clipped signal does produce heat in the voice coil, but so does an unclipped signal, too.

 

A variation of this myth is that extra heating occurs because the voice coil and cone stop moving during the clipped portion of the audio waveform. This also is untrue; even if the instantaneous signal voltage stays the same for some short period of time, like a millisecond or so, the cone stays in motion because at frequencies above the loudspeaker’s resonance the voltage is an accelerative force.

 

Clipping at any power level is capable of damaging your loudspeakers.

Myth. Some may tell you that a clipped signal from even a very low-power amp will blow out a high-power loudspeaker driver, but that’s untrue. Picture a 50-watt amp driving a 500-watt (continuous) loudspeaker driver. Even if the amp is driven into very severe clipping, it will still put out less than 100 watts, and therefore will not be a threat to the loudspeaker (it probably will not sound good, though).

 

Try that with an amp rated at, say, 400 watts or higher, though, and the results may be very different. This is because an amp, when it clips, can usually put out much more power than it is rated for. Prolonged, sustained clipping may cause that amp to put more than 500 watts into the loudspeaker for some significant time and cause a thermal failure (melting) in the voice coil. On the other hand, in a system where the amplifier and loudspeaker are well matched (i.e., amplifier power is roughly equal to the loudspeaker's program power rating), very brief and occasional clipped peaks are generally harmless.

Edited by biz77
Link to comment
Share on other sites

You made it sound like that theoretical amplifier would deliver a maximum of 400 watts while unclipped, but once you start clipping the amp it becomes an 800+ watt amp, which isn't how it works. That amp is always capable of delivering that 800 watts. How the power is applied over time is what matters.

 

I didn't say it becomes an 800w amp, as it is not an 800w amp in effective music power, or any realistic rating. However, understanding the max theoretical power output (square wave to the voltage rails) will produce significantly more power than the amp is rated for, but not in a way that will sound anything like the music you're trying to reproduce.

 

I think we're debating semantics here, as we've all destroyed woofers at one point or another, typically proceeded by speaker distortion.

Link to comment
Share on other sites

biz77, I agree.

 

Oddly enough, re-reading most of what is here, we're both pretty accurate, but coming at things at a different angle.

 

Btw, .21ms is an eternity in my world. 6.3 million cycles might as well be DC :p

Link to comment
Share on other sites

  • 5 months later...
Rage33 et al. You cannot "Underpower" a sub woofer and damage it that way, period. (he likely had a crappy sub and a crappy amp, see following)

 

What an amp does is amplify a semi-sinusoidal wave from a low voltage to a higher voltage. The way the amp works is that there are two limits to the amp. One is the voltage that the MOSFET operates at, which once you try to exceed it, you get distortion, which can sometimes but not usually damage a sub, but this is not "underpowering". The second limit is the power supply to the MOSFET. If the FET is biased at a certian voltage, but once the current draw is demanded to support the voltage, the voltage may drop on the rails, also causing distortion. This is how they can claim power greater than the amp can really produce, becauce theoretically, a FET biased at a certian voltage can produce power at a level proportional to the voltage. (Power = v^2/r = i^2*r).

 

For example, if the FET in the amp FET bias has 40v peak to peak, in ideal conditions it can produce 400w at 4ohms. However, most amps are between 50 and 65% efficent (class AB), and for subs, somtimes around 80% efficent (class D) This means that if you look at power consumption, you have to consume a minimum of 500w of power to produce 400w for the sub. (more typical would be 650-800w, or in a car, 50-60A current @ 13.6V). What this means is that if the fuses are not set with some margin over these values, the power supply cannot consume that much power, and the rating is bullsh*t. (Also, typically one of the rails will fail first in a Class AB setup, as if both rails are same distance from the signal, that's Class A, but class A isn't typically used in cars as it's ~25% efficient, but for the sake of this explination, that's just a detail.)

 

There are two types of power in a FET, static power and dynamic power. Static power is typically small, and is used to bias the FET, and the dynamic power is when the FET is changing state. This means it is possible to consume a small amount of static power, rate an amp at a high wattage, because it is biased with very high voltage rails, but not have a power supply to support it, therefore causing the design th "fail" under load, and not produce the power desired by the end user.

 

Where I was going with all of this is that like I said earlier, if you try to demand too much from the amp, the waveform will run into the voltage rails, and you will have a very horrible distortion, which can damage a sub, but is unlikely to, as you're at a low power, and should not be able to heat up the voice coil enough to melt it.

 

On the other hand, "overpowering" a sub would mean you're trying to push the cone further than the voice coil can allow, and this would mean that at the extremities, the voice coil is not moving, and may heat up and melt. Also, you would be at a higher voltage than the vendor spec'd the sub for, and can damage it. The solution here is don't turn it up too loud, and you'll be fine. My sub channel can push ~600w at 2 Ohms, and my pair of subs are rated at 250w each. However, unless I push the maximum power (read, VERY high volume), it will not be an issue. Keep in mind, a 10w sine wave @ 630Hz will be almost deafening on most speakers. (BA Z6/SPZ60s not withstanding, they're very inefficient).

 

Another note, the reason for overpowering is to get a bigger power supply and higher voltage bias rails. This means that when you run the amp under normal (read: not rock concert deafening or louder), you're running the amp at less of a percentage of it's maximum, which is generally desireable. Given the same amp vendor and model line (say JL Audio 250/1 and 500/1), the higher power amp will, in theory, be better on the same sub at the same volume, as it's operating at less of the amp's rated maximum. (It is often difficult to compare wattage numbers across different amp lines).

 

 

Just wanted to add, an example. There is a Kenwood Excelon amp in my room (belongs to a friend). It claims on the shell that it produces 1200w. (5 channel amp, KAC-X6500). This is impossible, as the math demonstrates. There are two fuses on it, 30A each. This means that the amp will draw a max of 60A (most likely a bit less as you don't want to tempt fate by getting too close to blow a fuse). This means at 13.6V, the Amp consumes 816w before the fuses will blow. In all likelyhood, the amp will consume around 600w at a typical max, as the fuses are there in case the power supply fails so it doesn't cause a fire. With 600w consumed, it has around 300-350w in real music power to distribute to all 5 channels. Sufficent to say, the amp, under no circumstance, can generate 1200w in music power.

 

For comparison, I have a Rockford Power1000 25 to life limited edition amp, 5 Channel. Fuse is 250A!!. This means it will consume 3400watts before the fuse blows, and again, it is likely to have margin. If we assume 2800w consumed under maximum nominal load, that means that at 50% efficiency (a conservative number, but safe to make assumptions with), the 1400W it's rated for is a very honest and expected number. Now, like the OP was asking, that amp I have can produce more power than most of the speakers in my car can handle, but I have to exceed the power limit of the speaker before I have a problem. Meaning, if I don't make myself deaf, I have no problem, and consume significantly less power.

 

If there are any questions on amp power, or anything on this topic, I'll update with information as I can.

 

...and learn to get your gains correctly, end thread.

 

ps. You ever haunt the12volt forums Biz?

Edited by ibhknl
Link to comment
Share on other sites

Distortion can cause mechanical failure. What we hear as distortion is the speaker hitting the end of it's travel.

 

 

The amp has enough peak power to move the speaker one direction, but then doesn't have enough power to stop it moving and reverse direction. The momentum of the speaker carries it to the end of it's travel and can cause mechanical failure.

 

 

Just my two cents...I completly agree.

I used to work at a sound audio shop in my area. they said the number one thing that people dont understand is that more times then none, underpowering a speaker can damage it worse then over powering it. When you under power the speaker you get so much distortion that it can ruin the coils in the speaker over time.

 

Your amp is trying to take a low whisper and turn it into a scream. Another example... trying to take a 1M pixel picture and blow it up the size of your wall. This is wear the distortion starts filling in the spaces.

 

As far as the power rating on amps. I used to sell them a few years ago, at that time they didnt have any base line for testing results. Some companies like sony would put 16V's to an amp and see how much power it would get in half a second. Then they would use that as their rating. Company's like alpine would under rate their equipment so that their amps would sound better then all the others in that product level. I heard of rumors that they were trying to get restrictions on how they rated amps but I dont know if it happend or not.

 

if you dont want to get into the nitty gritty, try to get the same amp and sub manufacterure because they are tuned together.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share




×
×
  • Create New...

Important Information

Terms of Use