We are inaugurating just before the winter break our new column on the topics of listening culture, initiated thanks to the collaboration with Architettura Sonora. A series of contents to reflect together on how to make conscious choices in the field of sound diffusion: because a pleasant environment, where you want to come back, where you feel good, is also an environment where sound input plays a leading role.
Arrangement of loudspeakers is seldom the first step in furnishing a room, since it is often necessary first to satisfy more pressing needs related to criteria of safety, ergonomics, aesthetics, style, you name it; even in our homes it is likely that we have first thought about where and how to arrange the furniture, and only later where to place our sound system. Strange as it may seem, this kind of approach has become normal even in the setting up of a large concert, where even one might expect the focus to be more on sound. Musical events are in fact increasingly understood as all-round shows, during which music is only one of many declined arts: great importance has therefore been given to the design of the shape and extent of the stage (especially if the musicians are joined by dance troupes), the arrangement of lights, screens and the now ubiquitous video walls, to show even the most distant spectators what is happening center stage, with the result that the loudspeakers have lost their former dominance and are sometimes placed in the spaces left free by the rest of the equipment. Everyone will have happened, talking about a concert attended, to utter or hear the famous words “too bad, I was a bit far from the stage, I couldn’t see well…”, further confirming how much our society has become increasingly accustomed to the solicitation of the “visual” component at the expense of the sound component.
Returning to the subject of architecture and furnishings, one of the most important aspects is undoubtedly that of light, which, in addition to making an environment more welcoming or determining its function, has also always been used to stir powerful feelings and provoke intense emotions (think of the atmosphere of recollection suggested by the semi-darkness of a Romanesque church). Excluding specific cases such as theaters, recording studios et similia, the sound aspect has generally played a secondary role, thus foregoing both functional and furnishing opportunities: an area characterized by music at a lower volume may be suitable for conversation, in another the choice of a certain type of sound may encourage concentration or rest, and so on, in some ways “doubling” the role of light in defining the meaning of spaces.
Sound pressure field produced by a source 30 cm in diameter as the wavelength changes. Lighter and darker areas are those affected by higher and lower sound pressure levels, respectively. The extension of the sound beam is greatest at low frequencies, where the wavelengths are much greater than the size of the sound source, and decreases with increasing frequency, here the wavelengths are first comparable and then smaller than the size of the source
To approach this idea, however, it is essential to have a clear understanding of both the differences between the operation of light sources and sound sources and the similarities that can be exploited. In this article we will focus on the differences, deferring examination of the analogies to a later installment. Light is electromagnetic radiation, of which our visual apparatus can perceive only the fraction characterized by a wavelength between 400 and 700 nanometers (billionths of a meter), which correspond to blue and red, respectively: the fact that every human-made light source is immensely larger in size than the wavelengths of the radiation it produces is the reason why, by moving a lamp, one can direct light to one place or another. Light can be approximated as a set of rays that reflect and refract according to the laws of geometric optics, with which we are instinctively familiar.
The wavelengths of sound radiation, on the other hand, range from about 17 meters of the lowest frequencies (those of the lowest sounds) to about 17 millimeters of the highest frequencies (high-pitched sounds); since a sound source generally has intermediate dimensions between these two values, its ability to direct sound is by no means equal at all frequencies; if, in fact, at high frequencies the sound source can have a characteristic size as much as 20 times larger than the wavelength and it can therefore be assumed that the acoustic radiation can be approximated as a set of rays similarly to light, at low frequencies exactly the opposite occurs: the source is so small that it appears as a “vibrating dot” at those frequencies, and it can only emit sound in all directions, without being able to govern it. This is why, as you move away from the dance floor of a club, an area toward which all acoustic speakers are directed, the sound becomes darker and less intelligible: the bass is in fact still present, unlike the higher notes and brighter tones.
If full and tonally balanced sound coverage is desired, therefore, it is necessary to ensure that there are always high-frequency sound sources directed toward the listening points. In some situations the goal can be achieved more easily than in others: in large halls or open spaces such as gardens and parks, for example, it is possible to use omnidirectional loudspeakers, which through appropriate design expedients are able to propagate all around the sound even at high frequencies; in more confined places or those characterized by more articulated geometries, on the other hand, different approaches may be successful.
To recap, in design it is important not to neglect the different directivity of a loudspeaker at various frequencies. To do so would be tantamount to arranging the light sources–with eyes closed!
