Eli5 – What is the difference between Subsonic sound and Infrasonic sound?

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Even googling it, it doesn’t make sense. I know it’s about sound that can’t be heard but….???

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Subsonic and supersonic refer to speed, like, an object could travel at less than the speed of sound in air (subsonic) or more than the speed of sound (supersonic).

Infrasonic and ultrasonic refer to a frequency, like the frequency is less than what humans can hear (less than 16 Hertz, too low a bass for us to hear), or higher than what humans can hear (more than 20,000 Hertz, higher pitch than what humans can hear).

Subsonic is anything too low pitch to be heard like rumbles. Infrasonic are notes too low to be heard by human hearing. Subsonic has a wider catch all for sounds, where infrasonic mainly goes to what humans can hear (but some animals may hear stuff we can’t).

Subsonic or infrasonic is sound at ultra low frequencies below the capabilities of human hearing.

Sound is a set of waves of air that moves back and forth. Low frequency waves sounds low pitched. High frequency waves sounds high pitched.

Infrasonic is lower than audible by humans. It’s a frequency.
Subsonic is slower than the speed is sound. It’s an actual velocity that can be measured in mph.

The human ear can only consciously register sounds from 20 vibrations a second (low pitch) and 20,000 vibrations per second (high pitch) anything lower is considered infrasonic and anything higher is ultrasonic. Just like for colors, if it’s outside the upper wavelength of light we can see it’s ultraviolet if it’s outside the lower wavelength it’s infrared.

Subsonic is something entirely different. Sound can’t be subsonic as the speed of sound through air is a set value (it changes with air density) that sound will always travel at. If something is subsonic it is travelling slower than the speed of sound through the air. When I walk that’s subsonic motion. Something travelling faster than the speed of sound is supersonic. Most bullets are supersonic.

Now where you may be confused is with speakers. Some speakers have a limit they call a subsonic filter. Basically every speaker has a range of frequencies it can produce. Many don’t bother going below 30 or 40 vibrations a second as the tone is so low it will get lost in the other music. However if the recording equipment is high enough quality on the original track, the speaker could try to play below that and it will end up messing with the other sounds it’s trying to produce. So a subsonic filter will cut out frequencies below a certain point say 50 vibrations a second.