The directional nature of sound does not change abruptly within the audible range. However, there are certain characteristics of sound that can vary with frequency and affect its directional properties.
In general, higher frequency sounds tend to be more directional than lower frequency sounds. This is because the wavelength of a sound wave is inversely proportional to its frequency. Higher frequency sounds have shorter wavelengths, which makes them more likely to exhibit directional behavior and be influenced by obstacles and reflections.
At lower frequencies, sound waves can diffract more easily around obstacles and spread out in a more omni-directional manner. This is why low-frequency sounds, such as the rumble of thunder or the bass in music, can often be heard from different directions and seem to fill a space more uniformly.
Regarding your question about the junction falling within our hearing range, it is important to note that the range of human hearing is generally considered to be between 20 Hz and 20,000 Hz. Within this range, there is a transition zone where the directivity characteristics of sound may change, but it is not a distinct cutoff.
The reason why the transition zone falls within our hearing range is due to a combination of factors. Our auditory system is most sensitive to sounds in the range of human speech and environmental sounds, which tend to fall within the mid-frequency range. Additionally, the physical properties of sound waves, such as diffraction and interference, play a role in how they propagate and interact with the environment. These factors contribute to the complex nature of sound and its directional behavior, which can vary across different frequency ranges.
It's also worth mentioning that technology and engineering applications can influence the directional control of sound to some extent. Devices like directional microphones and speakers can be designed to manipulate sound directionality for specific purposes, but they generally operate within the audible frequency range and cannot selectively aim sounds below our hearing range.
In summary, while there is no sharp change in the directional nature of sound within the audible range, there are frequency-dependent characteristics that can affect its directionality. The range of human hearing encompasses the frequencies to which our auditory system is most sensitive and where sound behaves in a way that aligns with our perception and environmental interactions.