Biomedical Engineering Reference
In-Depth Information
2 Sources and Filters
To know that under certain conditions air is capable of propagating pressure
(or density) perturbations is the first step in our understanding of sound.
However, we still need to discuss a couple of issues in order to continue our
presentation on birdsong. On one hand, we are interested in the mechanism
used by the bird in order to generate these perturbations. On the other hand,
we need to know what happens to these perturbations in the space between
where they originate and the open air. Before we analyze the structure of the
avian vocal organ, it will be useful to have a general picture of the process of
the generation of sounds and the way in which they are filtered. This chapter
is dedicated to describing these phenomena.
2.1 Sources of Sound
2.1.1 Flow, Air Density and Pressure
We can begin our discussion by thinking about sources of sound we are famil-
iar with. Examples could include a siren, a flute or the sound that is produced
when we blow air between two sheets of paper [Titze 1994]. What do these
processes have in common? What physical phenomenon is at play in these
cases? As we saw in the previous chapter, sound is a pressure (or density)
perturbation that propagates in a medium - in the case we are interested in,
the atmosphere. How can we generate such a perturbation?
In order to describe the process by which sound is generated, we must
introduce the concept of flow. It is not a complex concept: we constantly refer
to tra
c flow, the flow of a liquid through a pipe, etc. A flow (of something)
is the amount (of that thing) that passes through a surface in certain time.
For example, the flow of cars passing through a tollgate on a highway is the
number of cars that go through the gate in a certain interval of time. The
concept has implicit in it the existence of an area and a velocity. The flow
of cars can increase because more cars pass through the gates in the interval
of time (i.e., the “velocity” is increased). But let us imagine that the gates
are already letting as many cars pass per unit of time as possible: we can
also increase the flow by increasing the number of gates (i.e., by increasing