## 2 Acoustic Effects in Built Spaces
### Orchestrating Acoustic Effects
Moreover, it has been discovered that the site could favour particular subsonic frequencies (\(\approx\)10 Hz), which can stimulate relaxation and trance states, favouring the creation and synchronisation of alpha brain waves. Findings in the field of archaeoacoustics reveal that resonant characteristics of ancient structures may have been deliberately designed to exhibit specific acoustic qualities (the resonance frequencies of a room, or _room modes_, are the frequencies in which wavelengths are directly related to the room dimensions). [PERSON] et al. [37] measured the resonance frequencies of five megalithic chambers from the chalcolithic age and one chamber dated to 400 BC; they found the dominant resonance frequency of all the structures to be in the range of 95 to 120 Hz, particularly around 110 Hz. It suggests that the room modes were chosen to enhance male voices during rituals involving chanting. [PERSON] et al. [38] later showed by electroencephalography (EEG) that human brain activity exhibits measurable changes when exposed to frequencies in close vicinity of 110 Hz, similar to those assumed to be associated with emotional processing. These findings can lead to the hypothesis that the room resonances of the investigated chambers had an additional purpose in changing the emotional state of participants of events involving musical performances. Similar intentions have been discovered through studies conducted in the megalithic temples of Malta, in particular, the Hal Saflieni hypogeum. In this temple, the Oracle Room [39; 40] seems to have particular acoustical characteristics. This particularity consists in having a wide resonance at low frequencies, with a reverberation of 16 seconds, but the intelligibility of words becomes distorted over a very close distance. It seems that, therefore, special sound effects may be more suitable for syllables sung in a prolonged fashion, especially if from female voices rather than for tight rhythms. Moreover, the lack of intelligibility but the sustained ability to amplify sounds with components among the medium low frequencies, suggest for this structure and this place a use based on the atmospheres deriving from the sound blend and their unpredictable spatialisation due to the smooth surfaces rather than the clear communication of particular messages. Moving forward a few centuries, a more extensive articulation of mathematical and geometrical knowledge was devised, useful for discriminating the effects caused by surfaces and constructions on sound phenomena. [PERSON] treatise \"De Architectura\", dating from between 40 and 20 BC, was well known in ancient times [41]. As [PERSON] suggests, Greek architects and builders were considered experts of theatre design and their acoustic effects, probably also because of the influence that the Pythagorean school may have had in some intellectual contexts. In fact, [PERSON], the architect of the theatre of [PERSON] (360 BC) [42; 43], was a disciple of [PERSON] [44] who can be considered the main figure promoting the study of acoustics in Greek times. During this period, devices were created, such as the Acoustic Masks [45; 46] that, according to [PERSON], were of surprising efficiency, worked as an impedance adapter for a spokesperson voice, causing a greater emission of sound [47]. [PERSON] is also responsible for introducing the famous acoustical descriptors that have influenced several scholars over the centuries. These consist of architectural solutions defined as _resonantes_, _consonantes_, _circumsonantes_, _and dissonantes_, depending on the integration of the architecture in the local geographic morphology. In addition to these definitions, [PERSON] suggests a complex use of devices called _echeia_ to obtain particular acoustic effects at certain frequencies. These echeia, or pinakes, consist of ceramic pots, now known as Helmholtz resonators, aimed at amplifying specific frequencies.