First of all, a sound source radiates power and this translates into sound pressure, where sound power is the 'cause' and sound pressure is the 'effect'. However, we must always keep in mind that sound cannot travel in a vacuum, simply because sound propagates as a wave, which creates "compressions" and "rarefactions", as it interacts with the particles of the medium. If there is no medium, then there are no particles and, of course, there is no wave-particle interaction and, consequently, no sound is produced. This specific fact was proven by Robert Boyle (17th century), a researcher who demonstrated through multiple experiments, that a bell ringing in a vacuum cannot be heard. We must mention here that sound travels at different speeds through various media. Sound can be divided into 2 types: simple and complex. Imagine a simple tone as a sine wave composed of a single frequency, which is in fact the simplest oscillation that can occur in nature. This means that it cannot be further analyzed into simpler swings. The complex sound, on the other hand, is made up of a fundamental frequency (which is the lowest and usually carries with it most of the intensity) and some other frequencies called overtones. When these overtones are integer multiples of the fundamental (including the fundamental), then we speak of harmonic overtones. Let's now examine the sound qualities. Sound characteristics can be classified into two categories: objective and subjective qualities. Objective qualities are related to the physical properties of sound, such as intensity, frequency and frequency spectrum, which can be measured through the use of specific instrumentation. On the other hand, the term 'subjective' refers to the... center of the paper... r sound pressure level) are the same. This is demonstrated by the so-called Fletcher & Munson curves. These curves are based on the work of Fletcher and Munson at Bell Laboratories in the 1930s, or rather, we could say that they are refinements made more recently by Robinson and Dadson. These were made by asking people to judge when pure tones of two different frequencies had the same intensity. Since this is a very difficult judgment to make and the curves represent the average results of many subjects, they should be considered as general indicators rather than as a prescription for what a single individual might feel. As you can see in the table below, the 60 phon curve crosses the dB scale at 60 per 1KHz. To be able to perceive for example 40 Hz as the same volume as 1 KHz, we will need about 20 dB more power (the 60 phon curve meets 40 Hz at about 82 on the Y axis).