what is dynamic headroom?

what is dynamic headroom and do i need to worry about it? i am purchasing a used B&K st-3140 amp and after looking at the specs, it has the lowest dynamic headroom of any B&K amp at 1.0dB. how does this affect sound? same thing with dampening factor? the B&K's is 100. what does this mean?
The dynamic headroom shows the ability of an amp to handle large power output (loud music) that exceeds normal power requirements for short duration of time without distortion. The link below gives good explanation and example:
The dumping factor shows how output signal is proportionally divided between impedances of the speaker and output impedances of the amp - 100 is very good - it means 1% or less of the signal is lost on the output impedance of the amp.
I hope this helps.
The dynamic headroom bit is very well explained in the link provided by Zoya so I will not repeat it.

Re. Damping factor: Damping factor is defined as the ratio of loud speaker impedance to amplifier output impedance. Nominally & simplistically, 8 Ohms is used for the speaker. So, if your B&K has df=100, it means that the B&K output impedance = 8/100 = 0.08 = 80 milli Ohms. This is pretty good.
B-U-T.......d.f. varies with frequency!!! So, in your B&K specs does it say d.f.=100 @ what freq??? Usually lesser amps have poorer d.f. as the freq. rises 'cuz they were too cheap to design a robust power supply!! This of course keeps cost of the amp down. Really hi-end amps have high d.f. across the audio band but then they are very heavy (power supply) & they cost much, much more.
What d.f. really does is that it controls or DAMPS the woofer driver in the speaker. By the amp presenting a very low impedance to the woofer driver, the amp provides a near short-circuit path to ground for the woofer back EMF. This allows the woofer driver to stop its pistonic motion after the bass note(s) go away in the music signal. We perceive this as tight, articulate & fast bass. When the d.f. is low (like in many tube amps) we feel that the bass is wooly, slow & boomy.
So, 100 would be a very good # if it is across the audio band ( tho I doubt it for B&K!).
Dynamic hr is really just the ability of an amp to put out more power than its rating would suggest. For example, an amp rated at 100w per channel that can put out 200w for short durations ( a few milliseconds) has 3db of headroom. There are various reasons for amps to have different hr values. Form instance, some amps are rated very conservatively and this will account for high hr values. Other amps have well regulated power supplies which means they will probably have low values of hr. On the other hand, some amps' power supplies are loosely regulated and will appear to have more hr.

Power is logrythmic in nature and one or two db may not mean much (and may not even be audible on music).

I would be more concerned with an amps' ability to be able to deliver more power into low impedance loads.

Damping factor is just a ratio of the amps' output impedance to the speakers nominal impedance. For example, an amp with an output impedance of .1 ohm and a speaker with an impedance of 8 ohms has a damping factor of 80 (8 divided by .1). Damping factor relates to an amps' ability to control a speaker's cone movement. Higher is better, but anything above 80 or so is questionable whether it improves anything.
Actually Bombaywalla, there is such a thing as having too high a damping factor as well as too low - one is overdamped, the other underdamped. The ideal damping factor would be what is called the "critical damping factor", but allowing for a deviation of about half the value around the critical point usually won't cause much difference. I think that for most speaker systems, this "critical" point probably occurs between a DF of 10 to 100 or so (if it's a lot higher, then the speaker was an underdamped design to begin with), so most amps, even most tube amps (excepting low- or no-feedback single-ended designs), should possess a sufficiently high damping factor to control the woofer well on any speaker having a reasonable bass impedance minimum. I have always found that the overdamped bass sound that results with certain SS amps having DF's in the hundreds or thousands is less pleasant on the ears than the more moderate damping that results with amps having DF's in the <100 range. Of course, there is another benefit to an amp's having a relatively low output impedance, namely the reduction of amplitude response deviations caused by the speaker's impedance curve varying with frequency. But since amps with very low output impedances (and thus very high damping factors) typically achieve those specs through the heavy application of global negative feedback in the circuit, I feel there is still often a sonic price to be paid in terms of transient and harmonic purity when engineering for lowest output impedance as opposed to allowing for a bit of response modification, dependent on the speaker used, with a moderate negative feedback design. (Remember, all of this is a separate issue from the question of adequate current supply to meet low-impedance high-volume bass demand.) And like you say, FWIW, IMHO, YMMV, etc...

Fishcall, it is good to bear in mind that between a higher-powered and a lower-powered amp, the more powerful amp may spec out with less dynamic headroom beyond its rated RMS power limits, but still actually be capable of deliving more power on dynamic peaks precisely because those limits are much higher to begin with. On the other hand, you could look at an amp featuring a small headroom margin as being designed with a skimpy power supply for its rated output. It can all be kind of arbitrary anyhow, since manufacturers can manipulate rated power and distortion figures to be conservative or liberal, and besides those numbers aren't arrived at by playing an actual music signal into actual loudspeakers, and may not take into account the fashion in which a given amp approaches clipping. But in any case, it's most important to make sure you get an amp having adequate power reserves to more than comfortably drive your particular speakers, in your size of room, playing your types of music at your preferred volumes, and then you won't have to worry about rated headroom specs.
Don't worry about the dynamic head room spec - a lot of ss amps have a value less than 1.0, including the big ticket ones. This value can be relatively meaningless if the duration of the extra power is not specified.

As far as damping factor is concerned, a ratio of speaker impedance to total output impedance greater than 10 will adequately control most cone drivers. The speaker cable impedance has to be added to the amp output impedance for the total impedance. An amp with a low DF and long, capacitive speaker cables is the only reason to look at the DF if it's less than 50 or so, IMO.

Alone, neither one of these specs will tell you very much about how the amp will sound in your system and room.

Good info in your post borne out of experience I note & point well taken. Thanks for sharing the same.

BTW, I wrote some basics on D.F. without meaning to write how much is high enough (as Fishcall posed a basic question); rather, pointed out that d.f. is freq. dependent & that if the B&K can hold df=100 across the band, it would be doing very nicely. Why 100? 'cuz Fishcall read that off his amp's spec sheet so I stuck with that #. Is it enough for his room, his music, his volume level? Don't know!

How much df is enough is very debatable & many view points exist. The Gamut website has a paper saying that df=25 is more than enough. This # falls in your 10-100 range but it is rather subjective whether df=25 will suffice. What I find when the df is on the lower side that such amps cannot be used in any system that one pleases i.e. the amp & speaker have to be carefully matched. I consider this to be a negative aspect as the user's favourite speaker might be precluded from the list. Others might not feel this way at all. However, I agree with your view point that too much of df can be detrimental. It seems easier to pick out the lower limit (where bass is wooly) & the upper limit (where the bass is dry) but I'm not sure that 10-100 is that range as I've not done any tests myself to quantify this range. I agree that each one of us needs to choose the level of DF & its impact on sound in their resp. system.
Aleph 5 50 nominal is the damping factor
Aleph 3 100 nominal is the damping factor
SF Power 2 100 damping factor
ML 33 greater than 800 @ 20 hz, this is a very good figure to look at as this is typically where you want the greatest control of the drivers, in the low bass region. The Aleph products because they don't over control drivers are considered more tube like. Tubes also don't control drivers as well as SS.
This didn't answer your question about DH but should give a reference on Damping Factor.
wow! thanks for all the great responses. i love this site! the B&K's dampening factor is 100 at 50Hz. that's all the info that the specs give, but it looks like i do not need to worry with the specs and see how she sounds.....
Gs: "As far as damping factor is concerned, a ratio of speaker impedance to total output impedance greater than 10 will adequately control most cone drivers."

Does this mean that an amp with an output impedance of .1 ohm will be able to control a speaker with an impedance of 1 ohm ? That is a 10:1 ratio. : )

Other than that, the higher the output impedance of the amp and / or the lower the impedance of the speaker, the more the damping factor is reduced. The lower the ratio of damping factor, the higher the potential for the speaker to load down the output of the amp. This can result in higher distortion levels, frequency response abberations, poorer transient response, etc... Given typical SS amplifier designs, an 8 ohm load would be more desirable than a 4 ohm load in terms of "control" even though power transfer is reduced. As you go lower in impedance on the speaker, the amp is not only working harder to pass more current and dissipate more heat, it could become less linear depending on the design. When you start combining low impedance with low sensitivity AND high levels of reactance in a speaker, you really hurt yourself in terms of limiting the amount of amplifiers that you have available to choose from. Unfortunately, some very good speakers combine all of those attributes, making the need for "tank-like" amps a necessity for use with those speakers. Sean
Here are three decent online articles on damping.
Probably in the order they should be read if you are not familiar with the topic.

Murphy: http://www.trueaudio.com/post_013.htm
Pierce: http://www.diyspeakers.net/Articles/Richard%20Pierce%20DAMPING%20FACTOR.pdf
Tomcik: http://www.otlamp.com/articles/tomcik/index.html
Clueless: The first two articles are interesting to me because they seem to imply that differences in damping factor shouldn't matter much in audible terms, which could be seen as somewhat contradicting the notion of critical damping as explained in the third article. Of course my own listening experience with various amps possessing different damping factors can't confirm or deny any conclusion decisively, since many other factors are always in play (such as the impact of negative feedback on amplifier sound) without being held constant. But thinking about this stuff always puts me in mind of resistive loading of MC cartridges, which I see as an analogous situation, and in that case I *have* heard audible differences corresponding to system resistance changes with everything else held constant, and those results would seem to support the notion of there being underdamped, critically damped, and overdamped conditions possible.