d. Echo. An echo is the repetition of a sound caused by reflection from a surface. To be an
echo, the reflected sound must be 1/20 of a second or longer behind the original sound.
e. Reverberation. A reverberation is the persistence of sound within an enclosure after the
original sound has ceased. Reverberation may also be considered as a series of multiple echos that
merge into a single continuous sound.
Learning Event 4:
USE SOUND MEASUREMENT
The decibel (db). Even though the ear "hears" a range of frequencies from 15 to 20,000 Hz (the
audio spectrum), it hears the range of frequencies from 500 to 6,000 Hz better. Also, the intensity or
power of a sound must be increased 10 times before the ear notices a doubling of the sound loudness.
Because of these two situations, scientists developed a logarithmic measurement based on tens called a
a. Engineers developed the db or decibel to measure frequencies (especially single frequencies)
where the signal does not fluctuate, but remains constant. The sounds heard each day consist of
complex waves (two or more single frequencies). Therefore, technicians developed another unit of
sound measurement called the VU or volume unit.
b. The volume unit equals the db in that the mathematics to figure both the VU and db remains
the same. The VU measures complex waves with their continuous fluctuations and not the single
constant frequencies as measured by the decibel.
c. Because a volume unit approximately equals a db, people often use the two terms
Percentage of Modulation. Equipment manufacturers have established a maximum power or
signal amplitude which may go through the equipment without being affected.
a. One hundred percent modulation of a recording system equals this maximum power or signal
amplitude which may be applied without being distorted by the audio equipment. Any percent (%)
below 100% modulation equals a percentage of the total or maximum signal amplitude which can be
applied to the equipment. When a VU meter reads 40% modulation, then only 40% of the normal
maximum signal amplitude travels through the equipment. If the VU meter reads 50% modulation, then
50% of the maximum signal amplitude travels through the equipment, and so on.
b. Since 100 percent modulation equals the maximum power or signal amplitude to be applied
to the equipment, the 100% modulation becomes a reference by which db reference levels are
established. Zero (0) db equals 100% modulation or the maximum signal amplitude (power) which may
be applied to the equipment.