No you didn't get it right. RMS voltage (or equivalent DC) is 70% of the peak value, not the peak to peak value. So 70% of 50V in this case, or 35V.
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Can't blow a speaker by under powering it.......
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No you didn't get it right. RMS voltage (or equivalent DC) is 70% of the peak value, not the peak to peak value. So 70% of 50V in this case, or 35V.
This point is lost on the clipped signal = stationary cone crew, and continues to be ignored every time it is pointed out they are wrong.
Still waitng for the Fourier Transform result lads.
In a nutshell:
Speakers are always damaged with more power than they can handle, whether that be in the form of mechanical damage (torn surrounds etc) or thermal damage (burnt voice coils etc).
There are a couple of modes where an amplifier rated for less power than a speaker can damage said speaker:
1) When an amp is driven into clipping, it will put out more than its rated power up to a theoretical maximum of 2x under extreme clipping (typically much less). If an amp can deliver more than about half the speaker's rated power before clipping, there is potential for damaging that speaker with it. Despite the amp being rated for less power than the speaker, the failure mode would still be overpowering. Not all that common, really.
2) The amount of power a speaker can handle before it reaches its mechanical limits at low frequencies is always less than the thermal rating which is the only power rating given. So a driver rated at, say 350W might reach its excursion limits at 100W at 80Hz. Clearly important for bass guitar applications. In situations where there is information at frequencies below the cabinet's tuning frequency, the power handling is massively derated because the cabinet cannot damp the movement of the cone below the tuning frequency. This causes mechanical damage like torn surrounds but can also result in burnt voice coils if the coil spends appreciable time outside the gap as Jerrold mentioned.
Once again the failure mode is actually overpowering though it kind of looks like underpowering.
I've been around this track a few times in my TB career so I'm not going to get into an involved argument about whether clipped waveforms have a DC component or not. However, I will say that any EE who believes a square wave symmetrical about 0V has a DC component should have his degree revoked. Hint: the DC component of any alternating signal is equal to the area under the curve.
Speakers are always damaged with more power than they can handle, whether that be in the form of mechanical damage (torn surrounds etc) or thermal damage (burnt voice coils etc).
There are a couple of modes where an amplifier rated for less power than a speaker can damage said speaker:
1) When an amp is driven into clipping, it will put out more than its rated power up to a theoretical maximum of 2x under extreme clipping (typically much less). If an amp can deliver more than about half the speaker's rated power before clipping, there is potential for damaging that speaker with it. Despite the amp being rated for less power than the speaker, the failure mode would still be overpowering. Not all that common, really.
2) The amount of power a speaker can handle before it reaches its mechanical limits at low frequencies is always less than the thermal rating which is the only power rating given. So a driver rated at, say 350W might reach its excursion limits at 100W at 80Hz. Clearly important for bass guitar applications. In situations where there is information at frequencies below the cabinet's tuning frequency, the power handling is massively derated because the cabinet cannot damp the movement of the cone below the tuning frequency. This causes mechanical damage like torn surrounds but can also result in burnt voice coils if the coil spends appreciable time outside the gap as Jerrold mentioned.
Once again the failure mode is actually overpowering though it kind of looks like underpowering.
I've been around this track a few times in my TB career so I'm not going to get into an involved argument about whether clipped waveforms have a DC component or not. However, I will say that any EE who believes a square wave symmetrical about 0V has a DC component should have his degree revoked. Hint: the DC component of any alternating signal is equal to the area under the curve.
A car battery can deliver a helluva lot more current than most other 12VDC sources and 8 Ohms is a pretty low resistance load, really. If a starter on a car draws between 150A and 350A, what's the load? It varies, based on resistance to rotation and temperature of the armature winding but is still not very far from 8 Ohms.
It's actually like a Zen koan: "Can you blow a speaker by underpowering it?" is almost as deceptive a thought experiment as "If a tree falls in the forest and no one is there to hear it, does it make a sound?"
Both queries force the asker (or the askee) to make a semantic interpretation before an answer can be given.
In the case of amps/speakers, the red herring is the word "underpowering"
Since conventional wisdom (as well as occasional empirical evidence) suggests that you can indeed blow a speaker by putting more than the rated power handling capacity through it (aka "overpowering"), that same line of semantic reasoning figures that if you can also blow a speaker by putting less than the rated power handling capacity through it, that must be called "underpowering" and this underpowering is what blew the speaker.
But that's wrong on both counts:
- The word "underpowering" has no useful meaning in terms of the amp>speaker relationship; it's only a useful word in terms of the sound system's sound pressure level output capacity relative to the room/audience size (presuming one also takes into account speaker sensetivity, dispersion, etc.)
- Secondly, the thing that blew the speaker was in all liklihood amplifier clipping...which may have been caused because you were trying to exceed the sound system's maximum sound pressure level output capacity, but regardless, it wasn't because there was too little power going through the speaker.
You can damage a speaker by "underpowering" the PA system you're using it with in certain circumstances, but you can't damage a speaker by "underpowering" it.
See grasshopper?
Both queries force the asker (or the askee) to make a semantic interpretation before an answer can be given.
In the case of amps/speakers, the red herring is the word "underpowering"
Since conventional wisdom (as well as occasional empirical evidence) suggests that you can indeed blow a speaker by putting more than the rated power handling capacity through it (aka "overpowering"), that same line of semantic reasoning figures that if you can also blow a speaker by putting less than the rated power handling capacity through it, that must be called "underpowering" and this underpowering is what blew the speaker.
But that's wrong on both counts:
- The word "underpowering" has no useful meaning in terms of the amp>speaker relationship; it's only a useful word in terms of the sound system's sound pressure level output capacity relative to the room/audience size (presuming one also takes into account speaker sensetivity, dispersion, etc.)
- Secondly, the thing that blew the speaker was in all liklihood amplifier clipping...which may have been caused because you were trying to exceed the sound system's maximum sound pressure level output capacity, but regardless, it wasn't because there was too little power going through the speaker.
You can damage a speaker by "underpowering" the PA system you're using it with in certain circumstances, but you can't damage a speaker by "underpowering" it.
See grasshopper?
Ohm's law always applies. No matter how much current a 12V battery can supply, when connected to an 8 Ohm load it will supply 1.5A and 18W. Bob's example uses the more appropriate typical DC resistance of a voice coil of around 5-6 Ohms, which is why he said 25-28W.
150A from a 12V battery indicates a load of 0.08 Ohms. As far as I can recall car starting motors typically have resistances <1 Ohm.
More simply:
No speaker was ever blown with less power than it could handle.
How about if we look at what's happening to a speaker while receiving signal from an amplifier this way- the signal voltage varies in many ways- usually alternating about some zero point. It may not be a true zero point and if we watch a larger speaker while powered, especially when low frequencies are produced at high power levels, the cone can look like its not only moving with the audible signal, but also at extremely low frequency. This comes from my work with 12V audio systems and I saw this with sealed, vented and "infinite baffle" applications.
Any time the voice coil receives signal, current can be measured or calculated and if the current never stops or the cone's motion experiences resistance while current flows, the coil's temperature will increase. If the heat can't be dissipated, the coil will ultimately fail. When an instrument produces the signal, it's almost never a pure sine wave- it's very complex and can consist of the fundamental(s), a series of overtones and if more than one note is played, it will also show sum and difference frequencies. For this reason, we can't say that an amplifier or speaker receives signal within its design parameters. Add amplifier distortion and the range of harmonics increases. All of this causes the voice coil to become hotter and leads to its failure.
Inaccurate power handling specs/improper application of speakers aside, I think we need to look at the signal going to a speaker when the instrument's controls are at max, the amp's controls are jacked up and it's being driven to its limits, then consider that the speaker is almost never at rest and heat is almost always dissipated by the voice coil. If the coil's ability to lose heat isn't sufficient, buh bye!
Has anyone measured the temperature of a voice coil in a speaker with high power handling capability when driven by a low power amp that's being abused? I'd like to see that.
Any time the voice coil receives signal, current can be measured or calculated and if the current never stops or the cone's motion experiences resistance while current flows, the coil's temperature will increase. If the heat can't be dissipated, the coil will ultimately fail. When an instrument produces the signal, it's almost never a pure sine wave- it's very complex and can consist of the fundamental(s), a series of overtones and if more than one note is played, it will also show sum and difference frequencies. For this reason, we can't say that an amplifier or speaker receives signal within its design parameters. Add amplifier distortion and the range of harmonics increases. All of this causes the voice coil to become hotter and leads to its failure.
Inaccurate power handling specs/improper application of speakers aside, I think we need to look at the signal going to a speaker when the instrument's controls are at max, the amp's controls are jacked up and it's being driven to its limits, then consider that the speaker is almost never at rest and heat is almost always dissipated by the voice coil. If the coil's ability to lose heat isn't sufficient, buh bye!
Has anyone measured the temperature of a voice coil in a speaker with high power handling capability when driven by a low power amp that's being abused? I'd like to see that.
But it's still a matter of heat. A starter gets hot because current is flowing for too long or its rotation meets resistance but that .08 Ohms is only at rest, before it has been used. A voice coil receiving DC will fail because it can't move further than its mechanical limit and the current remains, even though its resistance may increase and the heat can't be dissipated. A voice coil that receives constant signal that causes its temperature to increase beyond it's thermal dissipation capability by a wide enough margin will fail eventually.
A 12VDC power supply that can't deliver constant voltage to a load will have little effect WRT the voice coil's heat because it may fail before the voice coil (wall wart vs car battery).
I think we should test the theory that a low power amplifier can't damage speakers that are rated for more power. We have anecdotal evidence of this happening but we need an answer to the question of "Is it a matter of the speaker power ratings not being accurate, or is the low power equipment actually causing the damage that some say is impossible?".
Right, but most high power-handling woofers can easily handle the ~25W dissipated in the coil by connecting them to car batteries. The suspension of any woofer worth its salt will not allow a 12VDC signal to exceed Xmax/Xmech and as such won't even derate the power handling of the driver by putting a portion of the coil outside the gap.
I've actually tested this. National Semi chip amp on a board I designed. 40W max module but power supply voltage limited to 22W at 8Ohms vs. 100W Bryston 2B module. I could get about 35W out of the chip amp under severe clipping. I put a fan on the heatsink, drove it with 100Hz and cranked the input level until the resultant output waveform stopped changing. Connected it to an 8 Ohm, 100W SEAS woofer in a sealed box in the quasi-anechoic room we tested stuff in, shut the door and let it run for an hour. The driver survived. The Fluke infrared thermocouple indicated no significant difference in magnet assembly temperature between that setup and the 100W 2B running at 35W after the same amount of time.
No one is saying that it's impossible to damage a speaker with an amp rated for less power, though that possibility decreases to near nil as the amp gets less powerful relative to the speaker's power handling. I think I summed the issues up fairly well in post 123.
Yeah, I'm not talking DC. I'm talking that the speaker cone follows the voltage applied to it. Like the meter.
For the dynamic microphone, if you have the back of the diaphragm was sealed, and you pressurized the room, it would create a voltage as the diaphragm moved the coil. When the pressure and diaphragm are at equilibrium it stops, and the voltage output stops. But the air is still applying pressure.
For a speaker, the cone moves and pressurizes (or depressurizes the air, and when it stops, the voltage is still pushing. at equilibrium with the spider/spring.
The cone follows the voltage, and it's not always producing sound. Voltage applied that is not making sound is dissipated as heat.
For those quoting for Fourier, at any point in time, there is a single voltage. The speaker cone follows the voltage.
This person did a big analysis on tweeters blowing from clipping
http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/audio/clipping/page1.html
It's not a paper, it's not peer reviewed, but it does follow science and is unbiased.
http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/audio/clipping/page1.html
It's not a paper, it's not peer reviewed, but it does follow science and is unbiased.
bongomania
Commercial User
When you say the cone stops, be sure to add the part "for a tiny fraction of a second at a time, under a thousandth of a second in most cases"... Otherwise you are essentially telling a lie by using an idealized abstraction to ignore reality.
Mr. Foxen
Commercial User
Where does this idea that the heat changes if the coil isn't moving come from? Most energy fed to a speaker is wasted as heat whether its making sound from that energy or not.
Bob Lee (QSC)
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No, it would not. We're talking AC signals, not DC.
It is true that if you apply a DC voltage across the voice coil of a loudspeaker driver, it will accelerate the coil and cone with a force and eventually the spring force of the surround and spider will increase until it is equal in magnitude to the coil's magnetic force (but opposite in direction), and the motion stops.
At frequencies above resonance, the voltage applied across the driver is not positional, but accelerative.
Bob Lee (QSC)
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The current delivered into a load by an applied DC voltage depends not on the current capacity of the voltage source, but on the resistance of the load. Georg Ohm published this exact relationship in 1827, and it has become known as Ohm's Law. http://en.wikipedia.org/wiki/Ohm's_law
If a starter draws between 150 and 350A from a 12V source, then its resistance is anywhere between 34 and 80 mΩ, a couple orders of magnitude different from 8Ω.
Bob Lee (QSC)
In case you missed it, I work for QSC Audio!
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- Former Technical Communications Developer, QSC Audio
The force does but the position doesn't. Apply an AC signal of a constant voltage but varying frequency across a loudspeaker, and you'll see that the displacement varies inversely with the square of the frequency, all else being equal.
That's because although the accelerative force on the loudspeaker is the same at all those frequencies, the force vector will of course reverse direction more frequently at higher frequencies.
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