For some time I have suspected that frequencies above 20 kHz are still sensed by our ears and brain even though our conscious awareness does not "hear" it. A subconscious filtering process if you like, much like how our vision works as the brain only passes on relevant information to our conscious mind.
I can understand some people who have added super tweeters to their speakers and swear black and blue that even though they know they can't hear the sound from them - that they still get a greater sense of enjoyment listening to music with the super tweeters in place and miss them when they are removed again.
Here is some interesting research done involving brain scans while adding or removing frequencies above 20 kHz and suggest precisely what I had suspected...
Here is some interesting research done involving brain scans while adding or removing frequencies above 20 kHz and suggest precisely what I had suspected.
You already got good answers, and I can save you some searching and reading. The quote below from my Audio Expert book explains the problem with the Oohashi tests.
There was also a study by Tsutomu Oohashi that's often cited by audiophiles as proof that we can hear or otherwise perceive ultrasonic content. The problem with this study is they used one loudspeaker to play many high-frequency components at once, so IM distortion in the tweeters created difference frequencies within the audible range. When the Oohashi experiment was repeated by Shogo Kiryu and Kaoru Ashihara using six separate speakers, none of the test subjects were able to distinguish the ultrasonic content. This is from their summary:
When the stimulus was divided into six bands of frequencies and presented through six loudspeakers in order to reduce intermodulation distortions, no subject could detect any ultrasounds. It was concluded that addition of ultrasounds might affect sound impression by means of some nonlinear interaction that might occur in the loudspeakers.
I've also seen claims proving the audibility of ultrasonic content where a 15 KHz sine wave is played, then switched to a square wave. Proponents believe that the quality change heard proves the audibility of ultrasonic frequencies. But this doesn't take into account that loudspeakers and power amplifiers can be nonlinear at those high frequencies, thereby affecting the audible spectrum. Further, most hardware generators used to create test tones output a fixed peak level. When the peak (not average) levels are the same, a square wave has 2 dB more energy at the fundamental frequency than a sine wave. So, of course, the waves could sound different.
Finally, it's worth mentioning that few microphones, and even fewer loudspeakers, can handle frequencies much higher 20 KHz. Aside from tiny-diaphragm condenser microphones meant for acoustic testing, the response of most microphones and speakers is down several dB by 20 KHz if not lower.