Page 161 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Exposure to Computer Disciplines

Notes vibrates, the surrounding air is disturbed causing changes in air pressure that are called sound
waves. When sound waves arrive at our ears they cause small bones in our ears to vibrate. These
vibrations then cause nerve impulses to be sent to the brain where they are interpreted as sound.
7.5.3.1 How is Sound Recorded?

Sound waves can be transduced (converted to another form) using a microphone. A microphone
is similar to the human ear in that it has a diaphragm which vibrates in response to changes in
air pressure. The movements of the diaphragm within an electromagnetic field cause changes
in electrical voltage. These voltage changes can be directed to a tape recorder which alters the
magnetic particles on the tape to correspond to the voltage changes. A “picture” of the sound then
exists on the tape. When you press play on the tape recorder, the “picture” is read back as a series
of voltage changes which are then sent to a speaker. The voltage changes cause an electromagnet
within the speaker to push and pull on a diaphragm. The movement of the diaphragm then causes
air pressure changes which our ears interpret as the original sound. This process is known as
analog recording because the picture of the sound on the tape is analogous to the original changes
in air pressure caused by the sound event.

Usually we represent sound visually as a waveform. The height is called the amplitude and
represents volume. The distance between cycles is called the period or wavelength. The number
of cycles per second is called frequency and is interpreted by our ears as pitch. Frequency is
measured in Herz (Hz) or kilohertz (kHz).
Figure 7.5: Waveform Sound

The waveform above is a simple sine wave. Typical sounds are more complex in appearance. Here
is a waveform of a short spoken phrase. Note the frequent changes in wavelength, amplitude,
and frequency.
Digital recording differs from analog recording in that the “picture” of the sound is created by
measuring the voltage changes coming from the microphone and assigning numbers to each
measurement. The term “sampling” is used to describe the process of measuring an electrical
signal’s voltage thousands of times per second at a given level of precision (resolution). The
number of measurements per second is called the “sampling rate” and is expressed as kilohertz
(kHz). A rate of 11,000 measurements per second is thus designated as 11 kHz. Sampling rates
range from 5 kHz to 48 kHz with higher rates being used for the best quality recordings. The
frequency range of a digitized sound is limited to one-half the sampling rate. Since
humans can hear frequencies in a range of 20 herz to about 20 kiloherz, it is necessary
to sample at more than 40 kiloherz to capture the full range of frequencies perceptible
to the human ear.
The number of measurements per second, however, is only part of the picture. The degree of precision
within each measurement is also important. This is known as “sampling resolution”. Sampling
resolution is used to divide the total range of the electrical voltage into discrete parts. Common sampling
resolutions in use today are 8-bit and 16-bit. Sampling at 8-bits divides the voltage into 256 parts

154 LOVELY PROFESSIONAL UNIVERSITY