Wayne Turner, of Dunedin, asks :-

Why can they not get the volume even for TV programmes and commercials?

Phil Bones, an electrical engineer at the University of Canterbury, responded.

It seems like the TV advertising industry often try to capture attention using shouting promoters, just like the old fairground barkers who had to shout louder than the background sounds.

Another way for general TV adverts is for the advert makers to keep the average power the same as the programme but to use a narrower frequency range so that the amplitude can be increased.

The human ear is most sensitive at about 1200 Hertz (i.e. cycles per second). Young children can hear up to about 20,000Hz but this drops off with age, due to damage (general wear and tear) in the hearing receptors in the ear. Older people can hear up to about 10kHz, less if extra damage has occurred due to rifle shooting, rock concerts, playing music directly into the ears with earplugs, industrial noise at work, etc. But to understand a telephone conversation one only needs to use frequencies which range by about 1kHz. So there is no loss of understanding if the range of frequencies of the amplifiers is reduced to a few kHz and the amplitude increased in proportion. There is probably a rule of thumb the industry uses.

TVNZ does not increase the volume during adverts. As they state, "Unfortunately, the compression methods vary according to the production house and it isn't possible to predict in advance what the differences may be and take appropriate corrective action. In addition, it is now apparent that viewers perceive sound levels differently; what may be annoying for some is not of concern to others."

The Federal Communications Commission in the USA ban TV stations from boosting the average volume of commercials but, because of the average, commercials can be produced with louder parts being "averaged" by the quieter parts.

Automatic gain control (AGC) was invented in the old days of radio when signals waxed and waned as the ionosphere moved up and down. When signals were weaker the amplifiers amplified them so they stayed at constant amplitude (but the background noise got louder). This is still observed today for distant radio stations, and sometimes when an aircraft flies just overhead the sound and vision go wonky briefly, due to interference between the direct signal and the TV wave reflected by the metal of the aircraft. Your TV may have the ability for AGC to be applied to help reduce the sound from commercials.

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