Originally posted by: Peter
Sub-hearing frequencies on earphones? Dream on. Earphones can't even reproduce low frequencies /within/ our hearing range - you need WAY bigger membranes for that.
Originally posted by: BrownTown
One might also point out that the mp3 codec uses perceptual coding to achieve data compression, this means that it throws away any signals that are not able to be heard.
Originally posted by: Rubycon
Originally posted by: BrownTown
One might also point out that the mp3 codec uses perceptual coding to achieve data compression, this means that it throws away any signals that are not able to be heard.
High frequencies are affected mostly since they take up much more space. The lowest bitrate can still accurately reveal low frequencies. In some products (namely video recording gear) that have a severe roll off of low frequency the first thing that gets blamed is the compression methodology. This is simply incorrect for the reason just mentioned.
If one were to encode full spectrum pink noise (20-20k) they would see the output affected especially past 15kHz. How this looks depends on the codec and the bitrate chosen.
Originally posted by: BrownTown
The reason that the higher frequencies use more space can be seen for to reasons. First off if you are doing just a PCM representation of the wave like a ".wav" file then the bit rate is equal to twice the highest frequency you wish to represent. In this case the highest frequency is ~20kHz and the sampling frequency used is 44.1kHz, however in most sorts of sounds you might be recording the amount of information in the higher frequencies is very low, not to mention your ear can't hear that high of frequencies very well. So you could cut the sampling frequency in half and still get most of the information (for example all the voice information and most instruments although the sounds won't be as "rich" for loss of the high frequency harmonics). With the perceptual coding files like ".mp3" they compare the information from an input signal to a graph of humans ability to perceive sounds, they essential take the flourier transform of the signal for small windows and then create a "masking curve" based on human perception and the way that sounds of similar frequencies can mask each other. The energy at the different frequencies is then compared to the masking curve and anything below the curve is thrown out completely, anything above the curve is coded at a bitrate proportional to its intensity over the masking curve. What Rubycon is pointing out is because there is usually very little information at the high frequencies and because they are harder to hear to begin with you usually end up throwing them out all together. And like discussed before the bandwidth of this "high frequency" rage is very large, so you can GREATLY reduce the size of an audio file when they are removed (and lose no quality). Something like the mp3 can produce a sound file 1/10the the size of a 44100Hz 16bit PCM file that sounds just as good to most people.
Originally posted by: bobsmith1492
So, interesting discussion but what the heck was the OP talking about?
This effort will use directed ultrasound technology to enable the capability to significantly reduce sound emissions from large scale tactical military hardware. Theory predicts that nonlinear effects of high-power acoustic radiation on the atmosphere can cause acoustic energy to dissipate rather than radiate. This theory has been confirmed in some limited experiments; this program will apply it to reducing acoustic emissions of U.S. equipment. Reduction in noise levels by at least 30dB would enable U.S. forces to effectively operate considerably closer to enemy forces without being detected aurally
Originally posted by: silverpig
Originally posted by: bobsmith1492
So, interesting discussion but what the heck was the OP talking about?
It sounds like he wants an mp3 that plays sounds that you can't hear that will cancel out noise that you can hear. That doesn't exist though.
Originally posted by: Peter
Sub-hearing frequencies on earphones? Dream on. Earphones can't even reproduce low frequencies /within/ our hearing range - you need WAY bigger membranes for that.
Originally posted by: SonicIce
Ok what if you had a nice pair of open headphones, a good microphone, and a processor. The mic would pick up sounds coming to your ears from your environment, and be processed to be equal and opposite to the environment sound so you would effectively hear nothing? If the ear peices were pointing the opposite direction maybe?
Originally posted by: bobsmith1492
Originally posted by: SonicIce
Ok what if you had a nice pair of open headphones, a good microphone, and a processor. The mic would pick up sounds coming to your ears from your environment, and be processed to be equal and opposite to the environment sound so you would effectively hear nothing? If the ear peices were pointing the opposite direction maybe?
That's the concept behind noise-canceling headphones except the idea is to put the microphone exactly where your ear is and then generate an output signal using negative feedback to so that the amplitude of signals picked up by the microphone at your ear is zero.