1. With the same number of turns and volume, Air Core Coil's "inductance" value is much lower than that of solid inductors; air core inductors are more suitable for high frequency applications, while solid inductance has a serious frequency tendency; It is said that the hollow inductive reactance allows most of the audio or audio to pass, and the solid inductive reactance is selective, allowing only a part to pass.
2. The reason why these two kinds of inductive reactance are used is because of the need of audio and/or audio; in the frequency divider, the hollow inductive reactance is generally used in the preamplifier, which theoretically allows audio to pass through 20~20KHz; After the frequency, only the audio signals after "divided" such as 20~1KHz, 1K~10KHz, 10~20Hz, etc. are allowed to pass, so as to achieve the purpose of frequency division output.