improve the signal-to-noise ratio:
raise the signal
3) decrease the distance to the speaker
4) do a combination of all three
easiest way to raise (improve) the SNR is to get closer to
the sound source or bring the sound source closer to you.
are echoes. If you've
ever tried carrying on a conversation in a stairwell, you
know that severe reverberation can make it almost impossible
to understand what someone is saying. Here's what's happening.
Stairwells have lots of hard, flat surfaces that reflect,
rather than absorb sound. Sound waves ricochet wildly in all
directions like a steel ball flying around a pinball machine.
Since the surfaces are not absorbing the sounds, the sounds
take much longer to decay and die out. These reflected sounds
arrive at our ears at multiple times after we hear the direct
happens when sound waves hit a wall or an object in a room?
illustrations below show sound being 1) reflected,
like a rubber ball hitting a wall 2) absorbed
and 3) diffused, with much weaker reflections
scattered in all directions. (In reality, all three occur
at the same time but one predominates.)
are the arch enemy of speech clarity for two reasons:
Reverberations raise the noise level; they cause a
buildup of sound in the room consisting of the sum of the
original sound and its reflected sounds.
2) Since reflected sounds arrive at our ears after the original
sound, they distort and smear everything in their
wake. Our brain can filter out a lot of noise, but it
has much more difficulty with distorted words since they more
closely resemble the original sounds.
make matters worse, excessive reverberation doesn't distort
the words equally; it does more damage to the consonants than
it does to the vowels. We have seen that consonants are more
important to speech intelligibility than vowels (Hearing
loss, audiograms, and speech intelligibility), yet are
much weaker. The vowels, having more energy, bounce around
the room longer, smearing and covering up the hapless consonants.
can we tell if the reverberations are excessive? We measure
the reverberation time, the RT. Reverberation Time
(RT) is basically the time it takes for a sound to
die down. (Technically, it's the time required for a sound
to drop by 60 dB.)
much reverberation can we tolerate before we lose speech comprehension?
Careful testing has shown that most people with normal hearing
can tolerate an RT of up to one second without serious loss
of comprehension. But for a person with hearing loss, anything
more than one-half second RT can cause comprehension problems.
can you determine if reverberation in your classroom is excessive?
ways to determine if your classroom has excessive reverberation:
1. Clapping test. When the room is empty and
relatively noise-free, clap your hands loudly (one time)
at different points in the room. If you hear a ringing sound
or if the clap takes more than 1/2 second to die down, you
may have excessive reverberation.
Speech intelligibility test. Lists of words
and nonsense syllables are read aloud by a speaker or played
on a tape recorder while listeners record what they hear.
The percentage of correctly-heard words is an indication
of speech intelligibility in a given room. Your school audiologist
can provide more information on cost and exactly how to
conduct such tests.
Computer software evaluations. Programs in
which you enter the dimensions of the room and the surface
nature of furniture and objects contained within, can tell
you whether reverberation is a problem.
Acoustical engineers can measure the reverberations
using sophisticated (and expensive) equipment.
time depends upon the volume of the room and the surface
material of the room and the objects within.
reduce the reverberation time of the room:
the volume (hang a suspended ceiling, for example).
b) Increase the absorption by "softening up" the
room. Reduce and break up the hard, flat surfaces in the
room and add soft objects.
breaking up the surfaces that reflect sound, with objects
that absorb and diffuse the sound, the clarity will increase.
The next section will provide more detail.
to Reduce the Noise in Classrooms
Improve the acoustics within a classroom:
- Pinpoint the
sources of the noise.
- Keep noise
out of the room.
- Prevent noise
from arising within the room.
- Decrease reverberations
(Reverberation Time) and noise levels
- Increase signal
strength (increase speech volume and/or decrease distance
Pinpoint the sources of the noise.
can enter the classroom from the outdoors, from adjoining
classrooms, from hallways, and from within the room. To pinpoint
the major noise offenders, you'll want to start with an unoccupied
classroom with all equipment off (including heating and air
conditioning). Listen for any noises, pinpoint the exact sources,
and one-by-one start turning on the a/c, heating, and the
various pieces of equipment in the room, while listening at
each stage. If the adjoining spaces are unoccupied, turn on
a radio to simulate the noise coming from those areas.
process is described fully in an excellent recent article
by Mike Nixon, an acoustical engineer,
"Assessing the acoustics in your child's classroom: A
guide for parents".
Keep noise out of the room.
can enter the room from outdoors, from adjoining classrooms,
and from hallways.
class away from noisy areas such as shop and cafeteria.
- Many rooms
have suspended ceilings where the interior walls do not
meet the true ceiling. Noise in one room can go up and
over the wall to the adjoining classroom. Acoustical tiling
will absorb some sound and lower reverberation, but does
not really act as a barrier to sound waves.
This is a tough problem to fix. Two possibilities exist:
- If possible,
raise the wall to meet the true, structural ceiling.
(This may not be practical due to vents and electrical
cables in the airspace.)
a barrier on top of the acoustical ceiling using R-11
fiberglass insulation, vinyl barrier, or noise barrier
tiles. To get the most "bang for the buck,"
you may need to extend only 5' or 6' out from the
wall instead of covering the entire ceiling. Seek
the advice of manufacturers of acoustical materials
or an acoustical engineer.
walls are sheetrock (gypsum board) with an air space between
them, fill the space with sound absorbing foam.
for air ducts between classrooms. If they are not being
used, close them off or fill them with sound absorbing
material. If they are being used, perhaps they can be
lined with absorbent material.
cracks in walls. More noise can enter through a crack
1/16th of an inch wide than through the entire wall.
to see if sound is entering from the bottom of the wall.
The wall may not sit firmly on the floor, leaving a very
thin gap between the wall and the floor. If so, remove
the baseboard and caulk the gap to seal off the sound.
- Get a
solid-core door. It keeps a lot more sound out than
a hollow-core door. (A hollow-core door is basically
hollow --you can tell by tapping-- and sound can pass
right through it.
that the door closes snugly in the frame. If it doesn't,
sound can get through. Replace/install weatherstripping.
can also enter through the gap under the door.
Install weatherstripping or a drop seal.
carpeting in the hallways and "softening up"
hard, flat surfaces.
windows. Make sure windows are tight-fitting when closed
and that rubber gaskets are in good condition and not
missing from any areas.
double-pane windows to keep out noises from the outside.
heavy curtains to cut down on outside noise.
noise from arising within the room.
is most important to keep noise from arising in the first
place. Prevention is easier and less costly than control.
- Thin carpeting
on floor subdues scraping sound of chairs and desks and
shuffling of feet.
- Rubber tips
on chair and desk legs also cut down on the scraping sound.
You can also use tennis balls (with slits cut in them)
in lieu of rubber tips. Use weatherstripping to prevent
surfaces from banging against each other (lowering desktops,
- Regular maintenance
of HVAC (heating, ventilation, and air conditioning),
fluorescent lights, computers, printers and other electrical
- Keep machines
turned off when not in use, especially overhead projectors.
The cumulative effect can be significant.
- Place machines
on sound absorbent pads.
- Minimize the
time students are retrieving materials through careful
planning and clear directions.
- Lower the
reverberations within the room. (Please see next section.)
Excessive reverberation increases the general noise level.
(reverberation time) and noise levels
reverberations (longer reverberation time) distort words and
add to the general noise level.
- Place acoustical
paneling on the side walls and particularly the back wall.
This will prevent the teacher's voice from reflecting
off the back wall and up towards the front.
- If you have
high ceilings, install suspended ceilings with sound absorbent
tiles. This is one of the best things you can do, for
two reasons: it decreases the volume of the room, resulting
in shorter reverberation time, and the tiles will absorb
rather than reflect the sound. Look for tiles with a Noise
Reduction Coefficient (NRC) of .75 or better. (The higher
the number, the better.)
- Heavy curtains
can absorb sound and keep them from reflecting off the
- Flat, hard-surfaced
objects (bare walls, metal filing cabinets) raise the
noise level and the reverberation time.
large, sound-absorbing objects around the room: bookcases
and/or shelving containing objects of different size
and textures, corkboards, fabric partitions, and paper
pad easels. (Note: large objects should be placed
at a slight angle from the wall --10% to 15%-- to
break up the pinball effect of sounds ricocheting
between parallel walls.)
- Put up
art projects: textured sculptures, macrame, mobiles,
and fabric wall-hangings (Check fire codes)
acoustical panels (see above).
-- bare walls are a "no-no."
speech volume and/or decrease distance to teacher.
hear more clearly, we can also move the students closer to
the teacher or make the teacher's voice louder. In either
case, this would have the effect of raising the signal strength.
- Think of rearranging
the furniture so no one is more than 15' or 20' from the
teacher. When someone is speaking, make sure they don't
turn their backs to the students. This cuts the volume
using a sound field system. This is actually a sound equalization
system. The teacher wears a lapel microphone and a portable,
beltclip transmitter. Her voice is broadcast to wall or
ceiling-mounted speakers so everyone can hear her equally
well. (NOTE: If excessive reverberation is a problem,
you should not use this technique as it may make
the reverberations worse. First, get the reverberations
under control. Then consider this type of system.)
students with hearing loss, the teacher can use a personal
FM system in conjunction with a sound field system or
by itself. Wearing a lapel mike and a small, beltclip
transmitter, her voice is transmitted to a portable receiver
worn by the student and pumped directly into his ear.
5: Electronic Devices
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