To analyse the acoustic system and the possible sounds made by the whistle bottle, laboratory experiments were carried out. The work began with the construction of a replica bottle which, after modelling, drying and firing, was 72% of the size of the original. The construction process is described in Table 2. This object would correspond to Crespo's phase 5 of the evolution of the whistle bottle.
During manufacturing, clay slabs were placed on spherical supports of synthetic material, which served as molds; similar to the technique used within ancestral practices, known as the 'false lathe' to form the vessel (Ortiz 1981).
Table 2. Process of manufacturing replica of bottle whistle in laboratory
1. The process started with the identification of the components and taking measurements of the original bottle.
2. Templates were made using a marker on a plastic film, which made it possible to map the shape of the different surfaces. The scale templates were then moved to a clay that was plate 8.0mm thick.
3. Each ellipsoidal body was constructed by joining two curved slabs (half spheres), which were previously formed on styrofoam spheres. The other components were subsequently modelled according to the original bottle. The bird was modelled by hand and then hollowed.
4.
The components were assembled in leather condition (semi-wet), and an opening was made in the upper part of the ellipsoidal vessels. The neck of the bottle and the body of the bird, without the head, were placed very carefully.
5. On the body of the bird, at the base of the neck, a platform was made in which a small circular hole, an air duct, was created. Then the hollow sphere 'resonator' with a hole was added. During assembly, it was important to maintain the same humidity in all the components, otherwise they could contract and move and affect the sound. Tests and acoustic recordings were made, verifying the similarity with the original.
6. Before fixing the head to the body, details of the eye and beak were made. Eight holes were made around the head: two circular holes that make up the eyes; two more circular holes in the back; two in the form of a slit on the sides; one in the form of slit on top; and one inverted U-shape at the base of the beak. Acoustic tests and recordings were again carried out, verifying the similarity with the sound of the original bottle.
7. Once the object was dry, it was fired in an electric kiln at 1040 degrees Celsius. Final acoustic tests were made by aerodynamic and hydraulic action with the respective recordings being analysed in the acoustic laboratory. It was verified that the acoustic structure, as analysed in the original bottle, made the same detected sound variations.
8. The replica and the original.
5.1 Laboratory results
The size of the internal resonator, as well as the diameter of the holes (air duct and resonator hole), together with the remote or near placing of the resonator in relation to the interior wall of the bird's head, constitute the formal variables that have acoustic implications. Because the quality and variety of sound depend on the combination of these variables, it is presumed that the original potters would have possessed this awareness and knowledge.
When selecting the type of whistle, the Chorrera culture potters wished to represent the sounds emitted by a bird, so to this end a resonator of approximately 15mm was used with a hole of 3mm in diameter and the 3mm diameter air duct that generated sharp sounds. This sound can vary in strength or disappear altogether if the resonator is very close to the surrounding walls of the head. Adequate space is needed for the production and diffusion of the sound through the holes in the head - the same ones used in fingering, because when they are obstructed the acoustic tones vary in some way.
A different sound is produced, but of lower intensity, when the bottle is moved with water inside and the hole in the neck is covered. The sound is similar to the chirping of some chicks. Blowing directly produces a high-pitched sound that imitates the trilling of a bird that can be modulated by closing the finger holes in the head.
5.2 Description of the acoustic phenomenon
The pierced head of the bird is the sound-emitting device, inside which is the whistle. The whistle itself is a fairly simple system. The hole of the resonator sphere is in front of the air duct and angled approximately 60°. The jet of air that comes out collides with the edge of the entrance hole of the resonator sphere and there it oscillates causing an edge tone that will come into resonance with the cavity of the sphere. This is how a whistle, a globular flute, works. However, this sound source is complicated owing to the fact that the whistle is inside a perforated cavity (the head of the bird) which reacts as an external resonator and, due to the effect of the feedback, imposes its own influence, making the system more complex with the ability to emit several sounds at once. Discussing the operation of the acoustic system, de Arce (2015, 74) considers that this remarkable function requires a very precise organology, reacting to a weak air pressure and delicate changes of dynamics, a specialization that is reflected in the construction of the bottles.
Based on the laboratory results, it was determined that the sound effect varies sequentially or successively when used in conjunction with the eight perforations in the head of the bird. This may have allowed the performer to pluralise the sound by fingering or with obstructive movements produced by the fingers near the head of the bird.
The sound in the object is produced by the jet of air that reaches the whistle. There are three possible ways to activate the acoustic system: 1) by insufflation or blowing directly, 2) by a manual action that generates the internal movement of the water within the bottle (hydraulic effect), and 3) a combination of the previous two (Video 1).
Three methods for activating the acoustic system.
In the first case, the sound is instantaneous and can vary according to the intensity with which the performer sends the air into the bottle through the neck. The air passes from the entrance chamber to the exit chamber, and leaves through the holes, creating the sound of the whistle. By blowing directly, it is possible to reach higher insufflation pressures, which imply the emission of other sounds. In addition, the musical performance (intensity and sound height) can be varied by manual action when covering and uncovering the so-called finger holes that are found in the head of the bird. These sound effects occur more easily when the object is resting on a table.
The second way to produce sound is through the hydraulic handling caused by manual movement of the bottle when it contains a certain amount of water, with movements from the handle towards the bird. This allows the liquid inside to move from one side to the other, exerting pressure on the air contained in the direction of the air duct located on the platform at the base of the neck of the bird. This air is accelerated and compressed in the duct, due to the reduction in the diameter of the outlet chamber. The air that comes out of the duct collides with the edge of the entrance hole of the resonating sphere that is aligned with it; producing one or more sounds similar to the whistling of a bird. It is an edge tone, functioning just like a flute. When the bottle is tilted back, the water returns to the first chamber, it sucks air back through the duct through the holes in the bird's head (eyes, ears, beak) and replaces the air previously expelled from the exit chamber in the first movement. This continues with each tipping motion.
The sound depends on the movement of the water; when it is directed towards the neck of the bottle or first chamber it sucks the air from the outside through the air duct (a kind of "in-breath", if we relate it to a human action). The intensity of the sound depends on the thrust of the water in motion. Another way to produce sound, also through hydraulic action, is by covering the neck of the first vessel with the thumb and making movements, thus producing a low-intensity sound similar to the whistling of birds.
In the third way of activation, the neck of the bottle is blown with water inside, which produces much longer sounds that are combined with the sounds produced by the movement of water. Modification of the sound by blocking the holes with fingers can also be used.
According to the replica laboratory tests, it is estimated that the 'best' volume of water that enters the interior of the bottle through the neck is approximately 50% of the internal capacity. More than this amount shortens the sound and the water easily spills from the duct. If it is less than 50% there is no air pressure and there is almost no sound (see Video 1).
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