It refers to the ability of an amplifier to deliver a greater amount of power on brief musical peaks than the amount of power it is rated to deliver continuously.
That can be good in the sense that the peak power levels required by a lot of music can be vastly higher than the average levels that are required. Classical symphonic music is one of the most extreme examples of that.
However, high dynamic headroom can also be an indication that the amplifier's power supply and/or its thermal design are "weak," because it indicates that the amplifier cannot sustain its maximum power capability for very long.
Almarg...I don't agree with your description of the power supply as "weak". It may have been designed that way.
I like to think about this issue in terms of voltage. The amplifier delivers an electrical output which is the same shape as the input signal, but at higher voltage. When the output voltage is applied to a load (speaker) current will be drawn, and if the amp can't deliver enough current the voltage will fall. This limits amp power. Amp power may be limited by either the voltage it can output, or the current it can supply. It is better to be limited by current because music typically includes voltage peaks of such short duration that the power supply can provide the necessary current for a brief interval by drawing from its capacitance.
The "continuous rms power" by which audio amps are rated is really unsuitable for audio. The "headroom" measurement is important.
You also asked what the numbers mean. 3 db of headroom means that it has headroom that allows the amp to double its output for short periods of time.
I also disagree with Almarg about "signs of a weak power supply". The rest of the info is good.
I disagree with the weak power supply description also and that "headroom" is more important for tracking the musical signal properly. A good example is an amplifier I happen to own and I own it for the design we are discussing. The amp is rated at 60 watts per channel, but this is one of the most power "sounding" amps I have ever heard. A glance inside reveals a huge transformer with capacitors the size of the oil filter on your car. With something like 8 db of headroom, this amplifier seems impossible to run out of power. It played Vandersteens louder than any amp regardless of power rating including some well known 200 watt per channel amps. My neighbor said he liked feeling the bass on his face. Not that I make a habit of playing music this loud. It was a demonstration of what this system was capable of. This type of amplifier is also a good choice for speakers with a lower power rating. They will play louder without bottoming out on large scale dynamic peaks.
Note that I said that "high dynamic headroom CAN also be an indication that the amplifier's power supply and/or its thermal design are 'weak.'" I did NOT mean to imply that a high dynamic headroom figure NECESSARILY means that the amplifier is deficient in terms of its power supply design or its thermal design.
For instance, an el cheapo mid-fi receiver with a poorly regulated power supply, minimal storage capacitance, and marginally adequate heat sinking may very conceivably provide more dynamic headroom, relative to its continuous power rating, than a 200 pound $20K class A monster.
Keep in mind also that besides being a function of design, the dynamic headroom numbers are functions of specmanship. The more conservative the continuous power rating is, the greater the dynamic headroom number will be.
In addition to Al's comments, the other problem with "peak" power is there is no standard definition of how to measure it. One amp may be measured at the power level that can be sustained for 1 second while another amp may measured for 20 milliseconds.
That makes comparing the true performance of one amp to another based on that number unreliable.
Still another issue is that music does not follow any set rules as to what constitutes a peak. The peak power that may be helpful for a drum strike that has a peak duration that is a fraction of a second long is going to be useless for a sustained organ peddle note.
If you need a certain amount of power to reach a certain volume, the only way to make sure it is there is to have an amp that is capable of sustained output at the desired level.
This is without getting into the issue of what is actually on most recordings in terms of dynamic range. (Most people think the dynamic range in recordings is much greater that what is typically present.)
In plain English
...means an ability to reproduce a large interval between 'quiet' and 'loud'.
The sound system (mainly defined with amplifier and speakers for dynamic headroom) with large dynamic headroom does not compromise the sound quality with large increase or decrease of sound.
Eldartford wrote: "Almarg...I don't agree with your description of the power supply as "weak". It may have been designed that way."
I'm not sure what you disagree with. Nobody designs power supplies for class AB amps to be able to sustain sinewave at full power for unlimited time - otherwise it would require heatsinks and transformer of the size of class A amp. This would not make any sense since average music power is only few percent of max power. Power supplies being much smaller are already compromised (weak). Al is just saying that within given power supply size (wattage) you could design for higher output voltage (getting better headroom) sacrificing output current and making it weaker for average power demand.
What I do know is that I had once a Luxman R107 receiver. Maybe 55 watts a channel and (back in the day, with my Cerwin vega D9 speakers) the system sounded extradorinarilly good (for the price). I had to investigate further. What I found out was the Luxman had 4db of dynamic headroom and my prior rig only 2dbs (same watts and, in those days, THD (remember THD?). Many of my engineer friends agreed that was the reason for the Luxmans great sound. Me? I just know it sounded better.
Kijanki...Agreed that "Unlimited time" would be more than "Continuous" in the definition of power. I don't know how the spec is exactly defined but I think that "continuous" could be 5 seconds or so. This would be long enough to reflect any power supply sag, but not long enough to get into heat sink issues.
Since the average power while playing music is a handfull of watts, but the peak may be hundreds (I have made measurements) I do believe that ability to follow peak voltage is very important. Power supply design is influenced by the need to advertize high "continuous rms power".
Using my MG1.6 speakers playing LOUD I measured average rms voltage in the 5 to 10 volt range but I still saw brief peak voltage corresponding to almost 600 watts. I was using 600 watt amps so I was able to see this voltage.
In the end I decided that while 600 watts for a MG1.6 is not ridiculous, 350 would probably be OK (so I bought CT D200 monoblocks).
Eldartford - I believe standard calls for 5 min rms power. http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=129b71f1c02566f96bb00521fdf0bd03&rgn=d
I remember that amp I used once at New Year's Eve party quit after about 10 min - thermal protection started switching output OFF and ON constantly.
My current amp Rowland 102 is rated 2x200W but Icepower modules inside are rated for 40W continuous and 55W FTC rated power. 150W that my amp outputs with my 6ohm speakers is only 33% louder than FTC rated 55W.
I remember listening to one amp that was very dynamic and had a lot of headroom but on heavy orchestral piece sound just sagged - I suspect that higher momentary power was achieved by higher voltage and power supply caps but when momentary power demand extended to few seconds whole headroom collapsed (small transformer?).
Luxman mentioned by Cerrot was top quality amp imported from Japan until they attempted to sell mass market product thru Costco and lost "image". Competition with cheap brands was not successful while Audio Dealers stopped carrying it. I would expect very good performance and honest ratings from Luxman.
I would say 101db. sensitive speakers with 1000 wpc.
would qualify as "Dynamic Headroom" as the speakers
only need about 7-10 watts to produce tremendous output.
ALL the rest, is "Headroom" for those transients in both
movies, and music.
Take a 100wpc amplifier that doubles all the way to .5ohms;
That is Dynamic Headroom, at its very Best.
Or another 100wpc amplifier, that produces 140wpc.@4ohms,
and maybe 150wpc. @2ohms very briefly.
The second amp. does NOT have ANY Dynamic Headroom.
This is why, not ALL 100watt amplifiers are the same; Nor
ANY others for that matter.
"Parts is parts"
Junk is, well...worthless.
Wonder why Loudspeaker Makers, went from "low impedance"
speakers in the 80's and 90's, but are now making much
MORE accurate speakers, that are not 86db. @4ohms; But
95-97 db. @8ohms!
Because they want MORE output, without needing an
"arc-welder", type amplifier.
There is NO such thing as "too MUCH Power!"
One does not typically listen to music with the volume
set to the "Maximum?"
Only enough to fill the room evenly with sound.
That is why I implement multiple amplifiers, so that
"Headroom" is NEVER an issue.
I Love MUSIC!
Kijanki...So it seems that the FTC power spec IS supposed to cover thermal considerations. That might be very reasonable for pro amps which play highly compressed and peak limited music at maximum volume. I play music on my system very loudly (but not compressed music) and my digital amps get barely warm. My comments above are aimed at home audio amps.
Eldartford - I don't listen very loud, perhaps 50% of max what makes already 1/10 of the power and because music does not contain peaks alone but most likely less than 50% volume most of the time it is another 1/10 of power. Music has also gaps - it is not continuous sinewave making it even less. I would assume that average power delivered to speakers is less than 1% of the max rms (around 1.5W).
Output stage of my amp is supplied from regulated 47V making 370W peak at 6 ohm. It's peak and not rms so overall headroom is not very big but it sounds big. It might be because of immediacy of response but also because of my moderate listening levels.
Take a 100wpc amplifier that doubles all the way to .5ohms; That is Dynamic Headroom, at its very Best.
Or another 100wpc amplifier, that produces 140wpc.@4ohms, and maybe 150wpc. @2ohms very briefly. The second amp. does NOT have ANY Dynamic Headroom.
While I agree with the general sense of your post, this statement reflects an incorrect definition of "dynamic headroom."
Re-stating what was said earlier in the thread, in slightly different terms, "dynamic headroom" refers to the ratio, usually expressed in db, of an amplifier's short-term maximum power capability to its rated continuous (long-term) maximum power capability.
An amplifier that can double down to 0.5 ohms is designed to be able to supply very large amounts of current. In many cases that kind of amplifier will have very little if any dynamic headroom based on the proper definition, because its maximum output power will typically be limited by the voltage range that its output can encompass.
At the other extreme, quoting Kijanki's accurate restatement of my earlier posts, "within given power supply size (wattage) you could design for higher output voltage (getting better headroom) sacrificing output current and making it weaker for average power demand."
Since the dynamic headroom of an amplifier is based on the proportion of short-term maximum power to long-term maximum power, it will be improved if the design (or the specmanship!) does one of the following:
1)Increases the short-term maximum power, without significantly affecting the long-term maximum power. This basically means increasing the voltage range that the output can swing without clipping.
2)Decreases the long-term maximum power without significantly affecting the short-term maximum power. This is why I mentioned that amplifiers with "weak" power supplies and/or marginal heat dissipation provisions can often have good dynamic headroom numbers.
3)Obviously, some combination of (1) and (2).
Mlsstl & Kijanki -- thank you for your excellent posts.
Correct me if I'm wrong. In my case dynamic / headroom and such doesn't matter.
If I use 30x2RMS, my ICE are 500x2@4, why would I care if they had 3db or 0.1db headroom? 30x2 is pretty loud.
Magfan -- No correction needed. You don't need any dynamic headroom provided that brief musical peaks do not approach the 500 watts. Keep in mind, though, that 500 watts is only about 12db louder than 30 watts.
Al - I like percents or ratios instead of dB. In case of 500W vs. 30W ratio of perceived loudness is:
R = (500/30)^(1/3.5)= 2.23 where "^" means "to power"
Before the post turned into a congressional assembly, I think Al's initial post is a legitimate answer.
Cyclonicman - Thank you for you valuable summary.
Thanks for all the comments. Feel free to battle it out. Interesting conversation
Kijanki...The 47 volts you cite is DC. Allowing for a 3 volt drop through the output transistors this suggests that the amp can swing +/- 44 volts. But that 44 volts is the peak of the signal. For a sine wave signal this would allow 228 watts RMS.
Almarg...We agree. My point is that an amp which is current-limited for high voltage output is a logical design for signals that have huge peak-to-average ratios (like music). Having current to spare is no help if you can't swing the voltage. We use amps with huge power specs because that assures the voltage swing. Having that swing continuously is not really necessary.
Edartford - It is class D amp with full H-bridged Mosfets. There is practically no voltage drop on them (possibly a volt total).
As for amps being design for "huge peak-to-average" - that's true but this average might vary. Some amps will handle Jazz nicely but give up at heavy orchestral piece (much higher average).
Also note that headroom is not necessarily "how much punch is available above maximum rated power". No one (virtually) listens at maximum rated power anyway (well perhaps a few have done so, but speaking realistically it is not an acceptable practice).
Anyway: your *available* headroom is the difference between the actual power level at which you're listening to, compared to the maximum available peak.
20dB, or more, is considered optimal headroom.
10dB is 10 times the power; 20dB is 100 times the power. So if you're listening at 2 watts then you need 200 watts available if you want that 20dB of headroom.
My rig is setup so that it virtually never clips. With high effeciency speakers (105dB SPL @1 watt 1 meter) if I'm listening at 2 watts per channel it's already quite uncomfortably loud for most people. So you need a decent sized amp and high efficiency speakers to stay 100% of out clip.
Class of operation has everything to do with headroom. A class A amplifier has by definition 0 db of headroom.
I have always looked at the headroom spec as a way to make cheaper amps look better due to the term itself. IOW 'headroom' seems to imply that an amp that *has* headroom is better than one that does not, but IMO/IME the opposite is true. Theoretically and often in the real world, class A is as good as it gets.
"A class A amplifier has by definition 0 db of headroom."
I'm not so sure of that. Class A is often designed with bias current in order of 150% o max output current to guarantee that transistors won't become nonlinear - hence some headroom might be possible.
In a Class A amp the output devices do not go into cutoff within the amp's linear region. Sure, you have the amp biased heavily, but that does not affect output voltage.
Many people are surprised to find out that the bias point does not affect the output power (that is a function of the power supply- class AB amps usually have a higher Vcc or B+ than class A amps do so you don't roast the output devices with too much dissipation). Bias affect the *distortion* of the amplifier- class A allows a push-pull amplifier to cancel harmonics at any point in its operating region. In the case of a single-ended circuit, it puts the operation at the most linear portion of the output device's curve.
Atmasphere, Your statement is simply not true. The class of operation has nothing to do with headroom. Headroom is the amplifiers ability to perform beyond it's rated power for a short amount of time, like during a dynamic peak in music. Class A operation eliminates switching distortion found in class AB amplifiers. Alternatives are A AB or AB1 which are high biased AB and run in Class A to a higher output rating than AB.
Also, your atatement of "headroom specs are a way to make cheaper amps look better" is also false. A much larger power supply is required to add headroom to an amplifier and the power supply is where the money is. The larger the storage capacitors the larger the tranformer has to be to keep the capacitors charged. This is where headroom comes from.
All of Krell's pure class A amplifiers have 3db of headroom. Classe Audio class A DR3B and DR3VHC have 8db of headroom. Bedini class A amplifiers have 3db of headroom. Just to mention a few.
Hmmm...I think someone who designs some of the best amps in the world knows what he is talking about.
Cerrot, I agree completely!
A much larger power supply is required to add headroom to an amplifier and the power supply is where the money is.
Although as I indicated earlier, headroom can also be "added" by subtraction. Either by underspecifying the continuous power capability of the amplifier, or by designing it such that its continuous power capability is reduced relative to its short term power capability. Doing the latter would likely reduce the cost of the amplifier, rather than increasing it.
Real bench measurements will find these out pretty quickly. It can work against a manufacturer, too, if they rate an amp at 40x2 rms and it will put out say.....80x2 rms. As has been noted, this will also artificially inflate the dynamic range spec. Most people looking for an amp in the 80 watt range will skip this amp unless they manage to run across the real, measured power.
For example, all Bryston amps come with real bench test results. The advertised spec is guaranteed minimum. All Bryston exceed there minimum ratings.
The headroom under unspecified dynamic conditions is icing on the cake. I don't know how much Bryson amps exceed there RMS rating, but it probably is only a fraction of a db.
Different philosophies by different designers I guess. One designer claims to have something and the other designer claims it doesn't exist.