Elenco Snap Circuits® Deluxe Sound & Light Combo - Manual (Page 19)

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Sound in Our World
Some of a sound wave’s energy can reflect off
walls or objects and come back to you.
Normally you don’t notice these reflections
when you are speaking because not all of the
energy is reflected, and the delay is so short
that your ears can’t distinguish it from the
original sound, but sometimes (such as in a
very large open room) you can hear them -
these are echoes! You hear an echo when a
lot of the energy of your voice is reflected back
to you after a noticeable delay. The delay time
is the distance (to the reflection point and
back) divided by the speed of sound. Most
people cannot distinguish reflected sound
waves with delays of less than 1/15 of a
second, and perceive them as being part of
the original sound. Echoes can be simulated
electronically by replaying a recorded sound
with a small delay and at reduced volume. See
project 10 and others for examples.
In project 195, if your speaker is too close to
your microphone then the echo sound can be
picked up by the microphone and echoed
again and again until you can’t hear anything
else. The same thing can occur in telephone
systems, and these systems sometimes have
echo-cancelling circuitry to prevent problems
(especially in overseas calls, where the
transmission delay times may be longer).
Engineers developing sensitive audio
equipment need to make very accurate sound
measurements. They need rooms that are
sealed from outside sounds, and need to
minimize the measured signal’s reflections off
the walls/ceiling/floor. Specialized rooms have
been designed for this, called anechoic
chambers. These chambers are virtually
soundproof and have specially shaped
materials (usually made of foam) on the walls
to absorb sound waves without producing any
echoes. These chambers simulate a quiet,
open space, allowing the engineers to
accurately measure the equipment being
tested.
Everything has a natural frequency, its
resonance frequency, at which it will vibrate
more easily. When sound waves strike an
object at its natural frequency, the object can
absorb and store significantly more energy
from the sound waves, as vibration. To help
understand this concept, think of a playground
swing, which tends to always swing back and
forth at the same rate. If you push the swing at
the ideal moment, it will absorb energy from
you and swing higher. You don’t need to push
the swing very hard to make it go high, you just
need to keep adding energy at the right
moment. In project 198 (Sound Energy
Demonstration), the frequency is tuned to the
speaker’s natural frequency, making it vibrate
noticeably.
Resonance is an important consideration in
the design of musical instruments, and also in
construction. If high winds blow on a tall
building or a bridge at the structure’s resonant
frequency, vibrations can slowly increase until
the structure is torn apart and collapses.
A cone can help you project your voice. A cone
keeps the sound waves (air pressure variations)
together longer, so they don’t spread out so
quickly. Long ago, people who had trouble
hearing used an ear trumpet, which helps
collect sound waves. A person would speak into
the wide end of the ear trumpet, and the trumpet
makes the sound louder at the listening person’s
ear. Electronic hearing aids have replaced ear
trumpets. Doctors use a stethoscope to hear
inside patient’s bodies. A stethoscope uses a
cone-like structure to collect sound waves; then
passes them into the doctor’s ear.
Sound waves reflecting off a wall
Anechoic chamber
Small pushes at the right
moment will make the swing
go higher.
SCC-350_Manual_Part_A.qxp 7/25/14 2:39 PM Page 19