Name ___________________________________________________________________
16.8 DECIBELS
decibel (dB) – measurement unit used when comparing two ___________________________.
Simplest method would be to compute the ratio of the intensities.
Why isn’t a ratio used? _____________________________________________________________________
=
Example:
I = 8 x 10^-12 W/m^2
Io = 1 x 10^-12 W/m^2
Find the decibels:
Decibel is dimensionless.
An intensity level of zero decibels does not mean that the sound intensity I is zero; it
means ________________________________.
The brain interprets a sound as loud or soft depending on the ___________________ of
the wave.
Greater intensities give rise to _________________________ sounds.
However, the relation between intensity and loudness is not a simple
proportionality, because doubling the ______________________ does not double the
____________________.
___________dB is the smallest change in loudness that an average listener with normal
hearing can detect.
Example 9: Comparing Sound Intensities
Audio system 1 produces an intensity level of 1 = 90.0 dB, and system 2 produces
an intensity level of 2 = 93.0 dB. The corresponding intensities (in W/m^2) and I1
and I2. Determine the ration I2/I1.
To double the loudness of a sound, the intensity must be increased by more than a
factor of two.
A 70dB intensity level sounds about twice as loud as a 60dB level.
16.9 The Doppler Effect
When a truck approaches you the pitch is ___________ and as it passes and moves
away the pitch suddenly _____________.
Who first discovered that the sound changes as an observer moves toward or away
from a stationary source of sound? __________________________________
Explain the Doppler effect: _________________________________________________________________
Moving Source:
Explain how sound changes when the sound source is moving _________________________
(16.11)
Explain Doppler shift: _______________________________________________________________________
Example 10: The Sound of a Passing Train
Moving Observer:
(16.13)
An observer moving away from a stationary source moves in the same direction as
the sound wave and, as a result, ____________________________________________________________
(16.14)
Example 11: An Accelerating Speedboat and the Doppler Effect
General Case:
Source and observer both moving:
(16.15)
Explain why there is a plus and minus sign _______________________________________________
Homework:
70. The security alarm on a parked car goes off and produces a frequency of 960 Hz.
The speed of sound is 343 m/s. As you drive toward this parked car, pass it, and
drive away, you observe the frequency to change by 95 Hz. At what speed are you
driving?
71. You are riding your bicycle directly away from a stationary source of sound and
hear a frequency that is 1.0% lower than the emitted frequency. The speed of sound
is 343m/s. What is yoru speed?
72. You are flying in an ultra-light aircraft at a speed of 39m/s. An eagle, whose
speed is 18m/s, is flying directly toward you. Each of the given speeds is relative to
the ground. The eagle emits a shrill cry whose frequency is 3400 Hz. The speed of
sound is 330m/s. What frequency do you hear?