14 Recording Studio Design with light traffic in a small town will tend to produce a sound pressure level (SPL) of around 60 dBA, whereas a loud rock band may produce around 120 dBA (dB and dBA will be discussed later in the chapter). The sound pressure difference between 60 and 120 dBA is one thousand times, but it is self-evident that a loud rock band is not one thousand times louder than light traffic. If we use the 10 dB concept then 70 dBA will be twice as loud as 60 dBA, 80 dBA four times as loud (2 × 2), 90 dBA eight times (2 × 2 × 2), 100 dBA 16 times (2 × 2 × 2 × 2), 110 dBA 32 times (2 × 2 × 2 × 2 × 2), and 120 dBA 64 times as loud (2 × 2 × 2 × 2 × 2 × 2). The concept of a loud rock band being 64 times as loud as light traffic is more intuitively reasonable, and in fact it is a good approximation. The concept of 1 dB being the smallest perceivable level change only holds true for pure tones. For complex signals in mid frequency bands it has been shown that much smaller level changes can be noticeable. Indeed, Dr Roger Lagadec, the former head of digital development at Studer Inter- national, in Switzerland, detected in the early 1980s audible colouration caused by amplitude response ripples in a digital filter at levels only just above ±0.001 dB. However, whether he was detecting the level changes, per se, or an artefact of the periodicity of the ripples, may still be open to question. Perhaps it is therefore important to note at this early stage of the chapter that many so-called facts of hearing are often wrongly applied. Tests done on pure tones or speech frequently do not represent what occurs with musical sounds. Traditionally it has been the medical and communications industries