Richard Woollons of Christchurch asks :-
When driving into a ground floor car park the AM radio stations go noisy and vanish but the FM radio stations can still be heard. Why is this?
Bob Bennett, a radiophysicist at the University of Canterbury, responded.
The different signal strength observed inside a modern ferro-concrete building has two main causes. First, the AM broadcasts are on frequencies about 1 megahertz and the FM stations on 100 MHz and so have different wavelengths. The relationship is frequency times wavelength equals velocity, (which for electromagnetic waves is near 300 million metres per second). Hence the AM wavelengths are about 300 metres and the FM ones about 3 metres.
Radio waves are “transverse” and the electric and magnetic fields that make up the wave are perpendicular to the direction of travel and to each other and this means that they need “elbow” room to travel in a confined space. For example, radio waves can travel inside a metal pipe but only if its diameter is greater than half the wavelength. A modern building is not all metal, just a network of steel reinforcing rods, but solid walls, floors and pillars restrict the waves similarly, also absorbing them. The lower frequency waves come in through the windows and doors, which restrict the signal entering, and once inside it is reduced the further it goes because the size is much less than the wavelength. The 100 MHz signals, with the internal size well over their 1½ metre half-wavelength, travel freely although there is some loss due to internal walls etc.
The type of modulation, AM or FM, also has an effect, although smaller. Amplitude modulation means that signal is made to vary in amplitude and the receiver converts these variations into an electrical signal that, through the loudspeaker, reproduces the sounds. As the signal weakens the output (and so the sound) falls.
Frequency modulation conveys the sound signals as variations in the frequency of the transmissions and decreases in the signal strength do not reduce the output sound level so noticeably until the signal becomes very small.