Broadcast radio and recorded sound work through a chain of physical and legal steps. A microphone turns air movement into a small electrical signal, a studio sends that signal down balanced cables to a transmitter or a recorder, and the listener receives it through a system that has been measured, regulated and licensed. The same chain explains why a voice can sound close on one station and thin on another, and why two songs at the same volume on your radio can still be treated differently under copyright.
If you want the full technical and regulatory background, this radio and sound notebook sets out the studio, transmission and operating rules in long form. What follows is a learner's summary of the main parts.
What does microphone distance actually change?
Microphone distance is the first thing that shapes a recorded or broadcast voice. Put a cardioid microphone 10 centimetres from a mouth and you get a strong bass lift, known as proximity effect, plus a real risk of plosives on p and b sounds. Move it to 30 centimetres and the bass lift almost disappears, the breath noise drops, and the room starts to arrive in the signal. Move it to a metre and you are recording the room as much as the singer.
In a studio, the working distance for speech is usually between 15 and 30 centimetres, with a pop filter in between. For a group around one microphone, the distance is set by the need to keep everyone at the same level, so the quietest singer stands closest. The rule is not a number but a relationship: every time you double the distance, the direct sound drops by about 6 dB while the reflected sound stays roughly the same. That is why a close microphone sounds dry and a distant one sounds like a hall.
Distance also decides how much of the room you can fix later. A close take can be equalised and compressed without pulling up air conditioning rumble. A distant take has already committed to the room, and no plug-in will remove a bad reflection cleanly.
How does studio wiring keep hum and feedback out?
Studio wiring is about keeping the wanted signal stronger than the unwanted one. A moving coil microphone puts out a few millivolts, so any mains hum induced in the cable is a serious problem. The standard answer is a balanced line: two signal conductors twisted together and a separate shield, with the signal carried as the difference between the two conductors. Any hum that hits both conductors equally is cancelled at the receiving end. An unbalanced cable, such as a guitar lead, has one signal conductor and a shield, and it picks up hum over any length worth mentioning.
Anti feedback wiring is mostly about gain structure and polarity. Keep the microphone behind the loudspeaker, not in front of it. Use the quietest gain stage first, so the preamplifier does the work and the power amplifier does not amplify noise. If two microphones hear the same source at different distances, comb filtering appears; move one or change its polarity. In a small studio, a star earth, where every piece of equipment earths to one point, removes the earth loops that cause low hum.
Cable quality matters less than cable routing. Keep audio cables away from mains cables, cross them at right angles when you must, and do not run a microphone line beside a lighting dimmer. A cheap balanced cable routed well beats an expensive one routed badly.
How does FM stereo multiplex carry two channels?
FM stereo multiplex is a way of sending left and right through one carrier without doubling the bandwidth. The transmitter creates a sum signal, left plus right, called the M signal, and a difference signal, left minus right, called the S signal. The M signal occupies the centre of the audio band, from about 30 Hz to 15 kHz. The S signal is used to amplitude modulate a 38 kHz subcarrier, and that double sideband sits above the M signal. A 19 kHz pilot tone is added so the receiver knows the subcarrier is there and can lock to it.
A mono receiver ignores everything above 15 kHz and plays the M signal, which is why a stereo broadcast is still listenable on an old mono set. A stereo receiver adds and subtracts M and S to recover left and right. If the S signal is weak or noisy, the receiver blends towards mono, which is why stereo hiss often increases as a signal gets weaker.
The system is elegant but not free. The 38 kHz subcarrier and its sidebands take up spectrum, so FM stereo is more sensitive to multipath than mono. That is one reason a station may sound clean in a car park and rough a kilometre away.
Why does AM radio travel further at night?
AM radio travels further at night because the ionosphere changes after sunset. During the day, the sun heats the lower ionosphere, the D layer, which absorbs medium wave signals. At night that layer weakens and partly disappears, so signals that would have been absorbed instead reflect off the higher F layer and return to earth far from the transmitter. This is skywave propagation, and it is why a station on 1,000 kHz that covers one city by day can be heard several hundred kilometres away after dark.
The effect is not uniform. It depends on frequency, ground conductivity, transmitter power and the antenna height and class. In the United States, AM stations are licensed by class, and many are required to reduce power or change their antenna pattern at night to protect other stations on the same frequency. That is why a station you hear clearly at noon can fade or disappear at midnight, and why distant stations appear and vanish as the ionosphere moves.
Night time AM is also where the Emergency Alert System matters. A station may be interrupted by a national or local alert, and the rules for when and how that happens are part of the licence, not an optional extra.
What are the loudness and copyright rules behind the output?
Loudness is measured in LUFS, loudness units relative to full scale. The measurement uses a filter that mimics human hearing and a gate that ignores silence, so a quiet passage does not drag the average down. Broadcasters and streaming platforms set target levels, often around minus 23 LUFS for broadcast in Europe and minus 14 LUFS for some streaming services. The point is not to make everything as loud as possible but to make everything equally loud, so the listener does not reach for the volume control between items.
A limiter can stop peaks, but it cannot create loudness without distortion. If you push a mix too hard, the LUFS reading rises while the quality falls. The practical rule is to mix for the target, not for the meter.
Copyright is the other rule behind the output. A recorded song has two copyrights: one in the musical work, owned by the songwriter or publisher, and one in the sound recording, owned by the performer or label. Playing a record on air needs permission for both, which is why stations pay royalties through collection societies. The rules differ between countries and between interactive and non interactive services, and they apply whether the listener hears the song on FM, AM or a stream.
For a learner, the useful habit is to treat every output as having three layers: the physical chain from microphone to antenna, the measured level in LUFS, and the legal permission for the material. Get those three right and the rest is craft.