Measuring sound for loudness

John Heraty, a BBC broadcast technology trainer, on measuring sound for loudness

The 2012 Proms saw BBC audio levels being measured in a new way for the first time in 80 years. John Heraty, a BBC broadcast technology trainer, looks at what has changed in sound monitoring technology and what this means for BBC audiences.

Peak loudness was previously measured by a simple wooden box called a Peak Programme Meter (PPM). This was simple but limited in its range and ability to measure loudness, as it only had a small gap between peak signal and distortion (known as headroom). New digital transmission systems have much larger ranges, and have extra headroom engineered into them.

When we listen to the radio or watch TV, our ear averages out the different frequencies it encounters – bass, treble and mid-range – but a PPM is not capable of making this differentiation. John explains how a loudness meter measures sound more accurately, by reflecting the way the ear hears different frequencies. He also looks at the True Peak Meter, used to check that there is no clipping or distortion on the digital signal.

Another reason for improving audio monitoring is audience complaints about varying loudness across different television programmes. A lack of smooth transition between - for example - a tense, quiet crime drama and a loud, lively talent show meant viewers were turning up their volume only to be blasted with sound when one programme ended and a trailer for another began. Now all BBC programmes are mastered to conform to a consistent average loudness, so that the viewer can set the volume once and then leave the remote control alone for the rest of their viewing.