Originally Posted by popalock
Quick question for you guys.
I don't know the exact
definition if "dynamic capabiliy," ...
Dynamic Capability would be:
- Whether the speaker can provide the "dynamics" of the program. This is the changes in volume in music or a soundtrack.
- Analogous to the video world, where if you have a lowest light level of some "blackish," how many times brighter can the display get? 100? 500? 5000?
- And even given that ratio, are we happy if it doesn't get "bright" (loud) enough for the application? Film might be 12-16 foot-lamberts. But other applications like sports might require more.
- Similarly, while we could technically say that any weak speaker that can provide the 20dB swings from average to peak levels has that 20dB "dynamic capability" (contrast level), I think it's understood that it's meant "has dynamic capability that's more difficult to obtain: at higher absolute output levels in SPL." So, while any speaker can provide 20dB swings at some volume level, only some speakers can do so where the average level is 80dB and the peaks are at 100dB at the listening position.
- This should include the "quality" of the dynamics: does the distortion rise to noticeable/objectionable levels on those content peaks?
- This can be achieved by either higher efficiency or higher power handling.
It's like subwoofers
Probably more intuitive than video is subwoofers. We can argue all day about which sub was more musical and had those owner/marketing/home_theater_mag attributes of "pounding," "dynamic," "palpable," "room-filling," "thunderous," etc. But those are all subjective, relative terms; they have their place, but they convey limited meaning to me.
The sub that a reviewer loved in her room might only achieve 105dB max at 12 foot distance. If she's listening at -10dBFS to film soundtracks, that peak output might be enough, as long as:
- distortion didn't rise to objectionable levels
- all the desired frequencies are at that level of 105dB (recognizing that LF is exponentially difficult to attain as you go down, so there has to be a user goal of 10Hz, 20Hz, 30Hz, etc.)
If the above aren't met, I'd suggest that sub was not dynamically-capable enough for her application.
Someone else might need to listen at -20dBFS and can get by with less sub. Someone else will want -0dBFS or even +5dBFS, and thus they need 115dB or 120dB at their seats.
The latter require a more dynamically capable sub(s).
Can a small-diameter driver provide the required bass? Maybe, but there are tradeoffs in output, extension, box size, distortion, amp requirement, etc.
Can a low-efficency driver provide the required bass? Maybe, but there are tradeoffs.
As Bill explained, if the speaker has been pushed at a high average level and then is asked to provide a big peak, it'll have more trouble than if the driver was cooler. A hot voice coil has a different resistance than a cold one.
Just like everything else: engines, people, trampolines: speakers perform better when not operating at their absolute limits.I can lift some maximum weight only a few times before I'm spent. If you need a hundred heavy rocks to be moved by hand, you'd better choose a guy who can lift much heavier rocks easily. Don't choose me, who can barely move one of those rocks once.
Thermal Compression - average levels
While a speaker manufacturer may claim that the model can handle 100w RMS, there is in fact distortion at some fraction of that number. 10w may yield .5dB compression/distortion. 50w may yield 2dB compression/distortion on your average levels if there's a long passage of demanding music, for instance. You have to decide if that's acceptable, or would you rather the speaker were loafing because it can handle 300w RMS, so the application workload requires a smaller percentage effort. Or find a higher efficency speaker.
Magnetic / Physical Compression - peak levels
And then, regardless of what the continuous average levels are doing to the speaker, you have peak limits in even best case scenarios of cold voice coils. The magnetic field will behave differently depending on the excursion. There are magnetic flux eddy currents. The speaker's max power handling may be "don't feed more than X watts in a .5 second peak or else ABC type distortion will rise beyond what we deem an acceptable 10%," but more likely it's "don't feed more than X watts in some peak or else the speakerer will break." I believe the experts say that some fraction of X peak watts will still yield objectionable distortion.
Edited by Eyleron - 11/30/12 at 11:10am