• hard-of-hearing (HOH) students
• very young children whose language skills are still developing
• children learning to read, because they need to clearly hear the letter sounds (especially the softly-spoken consonant sounds).
• students learning English as a second language
• students with learning disabilities and attention deficit problems

1. level of background noise
2. distance from the speaker
3. loudness of the speech (signal strength)
4. signal-to-noise ratio (SNR)
5. amount of reverberation (echoes)

To improve the signal-to-noise ratio:

1) raise the signal  
2) lower the noise  
3) decrease the distance to the speaker  
4) do a combination of all three

The easiest way to raise (improve) the SNR is to get closer to the sound source or bring the sound source closer to you.

5)   Reverberations 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 sound.

What happens when sound waves hit a wall or an object in a room?

The 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.)

Reverberations are the arch enemy of speech clarity for two reasons:

1) 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.

To 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.

How 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.)

How 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.

How can you determine if reverberation in your classroom is excessive?

Four 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.

2.  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.

3.  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.

4.  Acoustical engineers can measure the reverberations using sophisticated (and expensive) equipment.

Reverberation time depends upon the volume of the room and the surface material of the room and the objects within.

 To reduce the reverberation time of the room:
a) Decrease 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.

By 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.

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How to Reduce the Noise in Classrooms

To Improve the acoustics within a classroom:

1. Pinpoint the sources of the noise.
2. Keep noise out of the room.
3. Prevent noise from arising within the room.
4. Decrease reverberations (Reverberation Time) and noise levels
5. Increase signal strength (increase speech volume and/or decrease distance to speaker).

1)   Pinpoint the sources of the noise.

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.

This 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".

2)  Keep noise out of the room.

Noise can enter the room from outdoors, from adjoining classrooms, and from hallways.

• Relocate 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:

  1. 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.)
     
     
  2. Install 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.
     
     
• If walls are sheetrock (gypsum board) with an air space between them, fill the space with sound absorbing foam.
  
  
• Look 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.
  
  
• Fix cracks in walls. More noise can enter through a crack 1/16th of an inch wide than through the entire wall.
  
  
• Check 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.
  
  
• Doors

  • 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.
  • Check that the door closes snugly in the frame. If it doesn't, sound can get through. Replace/install weatherstripping.
  • Noise can also enter through the gap under the door. Install weatherstripping or a drop seal.
    
    
• Consider carpeting in the hallways and "softening up" hard, flat surfaces.
  
  
• Repair windows. Make sure windows are tight-fitting when closed and that rubber gaskets are in good condition and not missing from any areas.
  
  
• Use double-pane windows to keep out noises from the outside.
  
  
• Hang heavy curtains to cut down on outside noise.

3) Prevent noise from arising within the room.

It 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, for instance).
  
  
• Regular maintenance of HVAC (heating, ventilation, and air conditioning), fluorescent lights, computers, printers and other electrical devices.
  
  
• 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.

4)  Decrease reverberations (reverberation time) and noise levels

Excessive 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.

• Tips on installing paneling:

  • Place panels on at least 20% to 25% of the wall.
  • Place panels mainly in the middle of the wall. Do not place them in the lower two feet of the wall because few sound waves strike that area.
  •  Leave some space between each panel. Keep edges exposed. One panel should not touch another because that reduces its efficiency. Use a checkerboard or "shotgun" pattern.
  • Stagger the panels. Do not have one panel directly facing the panel on the opposite wall. This can produce "flutter" echoes, a pinball effect that occurs between parallel walls.
  • Use panels with a noise reduction coefficient (NRC) of .75 or higher.

• 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 hard-surfaced windows.
  
  
• Flat, hard-surfaced objects (bare walls, metal filing cabinets) raise the noise level and the reverberation time.
  • Place 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)
  • Mount acoustical panels (see above).
  • Remember -- bare walls are a "no-no."

5)   Increase speech volume and/or decrease distance to teacher.

To 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 in half.
  
  
• Consider 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.)
  
  
• For 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.

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Part 5:  Electronic Devices

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