There are many types of VLF receiver designs, but for the most part they fall into two categories. There are magnetic wave receivers, and electric wave receivers. Electromagnetic waves are a combination of electric and magnetic waves, but these 2 types of receivers are designed to pick up either the electric field or the magnetic field. The electric field receivers use antennas, and the magnetic field receivers use coils of various sizes. Also, there are receivers that pick up the VLF transmitter signals with the purpose of recording the signal strength, and there are receivers that pick up waves in the Ionosphere caused by lightning or solar activity.
The biggest problem with all of these receiver designs is that they pick up a lot of 60 HZ (or 50 HZ in Europe) power line hum.They also have problems with picking up anything else that generates RF noise, static electrical noise, motors starting up, etc.
The goal is to eliminate as much noise as possible in order to isolate the desired signals. For receivers that are designed to pick up waves caused by lightning or solar events, they are narrow-band receivers which are most often modified audio amplifiers designed to pick up moving wave or static fields. The noise can be eliminated by filters, coils, chokes, etc., but the thing that helps the most is to get away from cities and power lines.
Also, narrow band receivers are designed to pick up Sferics, Whistlers, and Tweeks. Sferics (short for “atmospherics”) sound like bacon cooking on the stove. These are caused by lightning somewhere far away, possibly on the other side of the Earth! Tweeks are also caused by lightning, and have a kind of bird chirp sound. Whistlers are not as common, and they occur because the lower frequencies travel more slowly than the higher frequencies. Remember we are not talking about acoustic (sound) waves. These are magnetic / electric waves which are converted to sound by a special type of receiver. Whistler waves travel around the Ionosphere and their frequencies are dispersed - a phenomena called dispersion. The higher frequencies hit the receiver antenna or coil first, and then gradually the lower and lower frequencies are received. The sounds can last 1 - 5 seconds, and sound like someone is whistling.