The answer depends on what you mean by "cannot be heard." There are two main interpretations:
1. Frequencies Beyond Human Hearing
Humans can typically hear sounds between 20 Hz and 20,000 Hz. This range is called audible sound. Frequencies below 20 Hz are called infrasound, and those above 20,000 Hz are called ultrasound.
- Infrasound: While we can't hear infrasound, we can feel it as vibrations. It's also used in various applications, like earthquake prediction and weather forecasting.
- Ultrasound: Ultrasound is used in medical imaging, sonar, and other technologies. Some animals, like bats and dolphins, can hear ultrasound.
Therefore, frequencies below 20 Hz (infrasound) and above 20,000 Hz (ultrasound) cannot be heard by humans.
2. Frequencies That Are Too Quiet
Even within the audible range, sounds can be too quiet to hear. The threshold of hearing is the minimum sound intensity a person can detect. This varies depending on the frequency and individual factors.
- Example: A sound at 1000 Hz may be audible at a low intensity, but the same sound at 100 Hz would need to be much louder to be heard.
Therefore, any frequency can be too quiet to hear, even within the audible range.
In conclusion, the answer to the question "What frequency cannot be heard?" depends on how you define "cannot be heard." Humans cannot hear frequencies below 20 Hz or above 20,000 Hz. Additionally, any frequency can be too quiet to hear if it falls below the threshold of hearing.
