Of our environment, we only perceive a fraction of a fraction. We only perceive colors at wavelengths from 380-750 nanometer also expressed as ~400–790THz. Our brains translate these frequencies to what we call red, orange, yellow, green, blue, indigo and violet: the colors of the rainbow. Most people see them. When someone says that "my speakers are yellow", we all can imagine a yellow speaker; approximately that is. It is harder to get an exact fix on the right hue of their yellow. Go to a Home Depot or other DIY shop. Try to find the exact paint tint to touch up something that's yellow. That's no surprise. After all, the spectrum of what we call yellow spans a colossal bandwidth of 18THz, give or take. What happens below 400THz and above 790THz is completely invisible to us unless we use special equipment.

Our hearing is equally challenged. It is limited to a theoretical range of 20Hz to 20kHz. This boils down to an upper limit of 15kHz when we reach adulthood; unless of course we screwed it up a lot sooner with excessively loud music. Next to frequency/pitch, our hearing is susceptible also to loudness. If our hearing is optimal, we only begin hearing things as loud as 16.5dB. The more years we put on our clock, the higher that threshold moves. Sorry, what did you say? On top of all this, our hearing is nonlinear. It is more sensitive to sounds between 2'000 and 7'000Hz. Of course our hearing is part of our genetic survival system. To stay alive, breed and propagate our species, our ancestors had to notice the approach of a sabre-tooth tiger in the high grass before he got too close to pounce. Perhaps the swish of the grass occupied a frequency range that trained us to become most sensitive there? Just like our vision, our perception of what goes on below 20Hz and above 20kHz is zero without aids; that is, if we refer to our perception via the ear canal. Our whole body forms an additional sensor for frequencies.

We feel frequencies in the infra red range below the visual red. This covers the bandwidth of 300GHz to 400THz. When exposed to infra red light, we feel it as warmth or heat. Now our entire skin area becomes a sensor. We could even mention micro waves like in a micro-wave oven that vibrate at 2.45GHz but we only experience their effects when it is too late. For hearing below 20Hz, our body acts as a reversed subwoofer. We do not consciously hear these frequencies but we still react to them mentally and physically. Infrasound or frequencies below 20Hz can cause anxiety and nausea in many people. Military morons have even constructed weapons based on this effect. On the other end of the hearing spectrum lives so-called ultrasound. That comes with its own controversy relative to its 'audibility' or having any effect on us.