Both CF and FM components each contain four harmonics (H1-4). Depending on how far away the bat is from an object, the bat can change the energy in each harmonic (Suga, 1984). .
A bat will change its biosonar pulses while hunting, in order to increase the precision of the process of locating the moving target. Typically a bat will emit long lower frequency calls, and as it gets closer and closer to the insect it will increase both the number and the frequency of the calls. This rapid vocalization allows the bat to track the twists and turns of the flying insect with great precision. For example, if the object is far away, the bats amplify the lower harmonics. However, if the object is detected as being closer, the bats will enhance the higher harmonics (Suga, 1990). Higher harmonics allow for more detailed perception of objects, especially at shorter ranges. The most predominant harmonic is H2 with CF2 at 61kHz and FM2 of 61kHz-49kHz. It is always a major part of the emitted pulse regardless of the range of the object. The other harmonics (H1, H3, and H4) are each several decibels weaker than H2 (Suga, 1984). .
A mustached bat in midair must rely on the Doppler shifts of its echoes to detect the relative velocity of objects in its surroundings. The Doppler shift is a phenomenon that occurs with all energy waves (e.g. light waves, or in this case, sound waves). If a stationary bat were to emit a biosonar pulse towards a stationary object, there would be no Doppler shift at all. However, if the bat and the object were moving towards each other, the waves emitted by the bat would become compressed after hitting the object, resulting in an echo returning to the bat with a higher frequency than the originally emitted pulse. In contrast, if the bat and the object being detected were moving further apart in distance, the waves would expand, resulting in the echo being perceived by the bat as having a lower frequency.
