Physics
Directivity
Quick fact
A typical directional microphone can pick up sound from a specific direction at distances over 10 times greater than an omnidirectional microphone can achieve.
Why this is interesting
Have you ever wondered why a loudspeaker sounds clearer when you stand directly in front of it, but muffled from the side? That’s directivity—the hidden directional personality of every wave source.
Read the full explanation
Understanding Directivity
Imagine a bare light bulb: it shines light equally in all directions. That’s isotropic radiation. Now imagine a flashlight: it focuses light into a narrow beam. Directivity measures how much a real source resembles that flashlight. For sound waves, a large speaker cone acts like a flashlight at high frequencies, sending sound straight ahead, while at low frequencies it becomes more like a bare bulb. The shape of the emitted energy—the radiation pattern—tells you how directional the source is. This pattern is usually shown as a polar plot: the main lobe points in the strongest direction, and smaller side lobes indicate weaker, unintended directions.
A deeper explanation
Directivity arises from the size of the source relative to the wavelength. A source much smaller than the wavelength radiates almost equally in all directions (omnidirectional). As the source grows (e.g., a large speaker), different parts of its surface emit waves that interfere constructively in some directions and destructively in others. This interference creates the pattern. The directivity index quantifies how much the maximum intensity exceeds the average intensity over a sphere. In practice, directivity matters for noise control (avoiding unwanted side lobes), communication (focusing signal to a receiver), and imaging (ultrasound transducers). Understanding directivity helps engineers design everything from concert hall speakers to radar antennas.