Biomedical Engineering Reference
In-Depth Information
1
0.5
0
-0.5
-1
0
time(s)
1.2
Fig. 1.2.
A sound wave, as recorded by a microphone. A mechanical part within
the microphone (for instance, a membrane or a piezoelectric crystal, capable of
sensing tiny air vibrations) moves when the sound pressure perturbation reaches
the microphone. The movement of this mechanical part induces a voltage in the
microphone's circuit, which is recorded as a function of time. In this zoomed-out
view of the recording, we can see hardly any details of the oscillation; instead,
however, we could certainly draw the “envelope”, which is a measure of how the
sound amplitude changes with time
time. In this way, we can visualize how the pressure in the vicinity of the mi-
crophone varies as the recording takes place. In this figure, we have displayed
52 972 voltage values separated by time intervals of 1
/
44 100 s (i.e., a total
recording time of 1
.
2 s). The inverse of this discrete interval of time is known
as the
sampling frequency
, in this case 44 100 Hz. The larger the sampling fre-
quency, the larger the number of data points representing the same total time
of recording, and therefore the better the quality. This record corresponds to
the song of the great grebe (
Podiceps major
) [Straneck 1990a].
1.2 Frequency and Amplitude
1.2.1 Periodic Signals vs. Noise
We now have the elements that we need to move forward and to present other
elements important for the description of sound records. A sound source pro-
duces a signal that propagates in the air, generating pressure perturbations
in the vicinity of a microphone. What do the time records of different sounds
look like? In Fig. 1.3, we have two records corresponding to different sounds.
The first one corresponds to what we call “noise” (for example, we might
record the sound of the wind while we wait for the song of our favorite bird).
The second record corresponds to what we would identify as a “note”, a sound
with a given and well-determined frequency. In fact, this record corresponds
to a fraction of the great grebe's song (3
/
1000) s long. The first characteristic
