A
Directional sound means that the sound is concentrated in an angle that, beyond that angle, the volume of the sound decreases rapidly. If it exceeds this angle, the sound is completely out of the way, which is related to ambient noise levels. If the use of the environment itself is more noisy, polysound treasure more than a certain angle after the volume drop below the level of environmental noise, it can not be heard, but if in a very quiet place itself, the environmental noise is originally very low, it is possible that the volume is lower, or higher than the environmental noise level, then, you can still hear a little sound.