Acoustics has to do with transmission, reception and clarity of sound within a given space. While we are concerned with improving acoustics within the classroom, these principles and guidelines apply to other environments as well.
Good acoustics enhance speech intelligibility and that helps everyone's comprehension. For the following groups of students, good acoustics can mean the difference between success and failure:
- Hard-of-hearing (HOH) students
- Very young children whose language skills are still developing
- Children learning to read
- People learning English as a second language
- People with learning disabilities and attention deficit problems
How clearly we understand speech depends on many environmental factors.
Five KEY environmental issues are:
- Loudness of background noise
- Loudness of the speech
- Signal-to-noise ratio (SNR)
- Amount of reverberation (echoes)
- Distance between the speaker and the listener.
1) Background Noise (also called "noise") is sound we do not want to hear. Background noise covers-up or masks sounds we do want to hear. Noise may include; people talking, chairs scraping, traffic sounds, the hum of a refrigerator, ventilation systems, adjoining rooms and hallways, computers and other electrical equipment, sinks, toilets, cafeteria, outdoors and other sound sources.
2) Loudness of the Speech Signal has an affect on our ability to understand the speaker. But if you ratchet up the volume, is it always enough? Even a speaker with a powerful voice will get drowned out if the competing noise is too loud. In order to determine speech intelligibility, we need to know both the volume of the speech and the volume of the noise. And that's where the signal-to-noise ratio comes in.
3) The Signal-to-noise ratio (SNR) compares the loudness of the speech to the loudness of the background noise (in decibels). In an ideal environment, the speech signal should be at least 20 dB ABOVE the noise.
If the noise and the person speaking are at the same loudness level, the SNR is zero. If the noise is 5 dB louder than the person speaking, the SNR is -5 dB. If the teacher is speaking at 60 dB and the noise level is 50 dB, the SNR is 10 dB. Importantly, the SNR will vary from point to point within a room. The SNR may be 15 dB in the front row, and -5 in the back of the room! The louder we raise the signal above the noise, the greater the SNR, and the easier it is to hear the signal.
In a classroom, a student with normal hearing may need an SNR of at least 12 to 15 dB for reasonable comprehension. People with hearing loss are at a real disadvantage. They may need an SNR of 20 to 25 dB or more.
To improve the signal-to-noise ratio:
1) Raise the speech signal.
2) Lower the noise.
3) Decrease the distance between the listener and the speaker.
4) Reverberations are echoes. If you've ever tried carrying on a conversation in a stairwell, you know that reverberation can make it impossible to understand what someone is saying. Stairwells have lots of hard, flat surfaces that reflect sound. Sound waves ricochet wildly in all directions like a steel ball flying around a pinball machine. Since hard surfaces dont absorb sounds, the sounds take much longer to decay and fade out.
When sound waves hit a wall or a hard, reflective surface, sound literally bounces off the surface.
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, these actions can occur at the same time.
Reverberation negatively impacts speech clarity for two reasons:
- Reverberations raise the noise level by causinga buildup of sound in the room consisting of the original sound and its reflected sounds.
- Since reflected sounds arrive at our ears after the original sound, theydistort and smear the total sound. Our brain can filter out a lot of noise, but it has considerable difficulty with distorted words since they more closely resemble the original sounds.
Importantly, excessive reverberation doesn't distort sounds equally. Unfortunately, it does more damage to consonants than vowels.
Reverberation Time (RT) is essentially the time it takes for a sound to fade away.
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. For people with hearing loss, anything more than one-half second RT can cause comprehension problems.
How to determine if you have excessive reverberation?
- 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 fade, 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 these 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.
Distance from the speaker also impacts speech comprehension. The further we move from the sound source, the weaker the sound becomes and the greater the probability that noise and reverberation will interfere with it. To understand the impact of distance . . .
Consider - As the distance doubles, the sound intensity decreases by six decibels. If a student is four feet away from the teacher, and the volume is 60 dB, a student eight feet away would receive the sound at 54 dB, at 16 feet it would be 48 dB and at 32 feet the sound would drop to 42 dB. The student in the back of the room is hearing the words at about 1/4 the volume as the student in the front.
Assistive Listening Devices (ALD's): Help for real-world listening situations:
ALD's are electronic devices, other than hearing aids, that help you hear better in difficult listening situations. Some ALD's help you hear more clearly while others alert you to something that requires attention.
Amplification Systems help you hear more clearly. Examples include the headsets worn in movie theaters and houses of worship and amplified telephones.
Alert (Notification) Systems notify you when some event occurs such as the ringing of the doorbell or phone, or a smoke alarm.
I. AMPLIFICATION SYSTEMS:
These devices enhance and amplify sound and bring it "directly" to your ears. Like hearing aids, ALD's make sounds louder but unlike most hearing aids, ALD's generally amplify only the sounds you want to hear. ALDs are placed at or near the sounds you want to hear. In essence, an effective ALD amplifies the sound directly from the sound source and places it directly into the ear. Therefore, ALDs . . .
- Eliminate or reduce background noise
- Overcome the problem of distance, putting the speaker right next to you
- Increase the clarity of the words
- Provide clearer sound in noisy environments than hearing aids.
In addition, they are generally easy to use, require no custom fittings and are less expensive than most hearing aids.
A. Amplified Phones
Amplified phones can strengthen the incoming signal more than thirty times (up to 50 decibels louder). Most amplified phones can boost higher frequencies, which is where most hearing loss occurs. This results in greater speech intelligibility because the consonants which are audible (can be heard) become clearer.
Many phones contain large buttons for the visually impaired, extra-loud ringers with visual indicators (flashing lights) and speed dial buttons. Some will even amplify the outgoing voice in case the hard-of-hearing person has a soft or faint voice.
All phones are hearing aid compatible. To understand what this really means, we need to shed some light on the mysterious "T-coil." T-coil stands for telephone coil. It is a tiny coil of wire that fits in the hearing aid. About 30% of the hearing aids in the USA have T-coils. You have one if your hearing aid has a "T" setting. T-coils are good to have because they enhance the clarity of phone conversations and can also help in certain public settings. (See "Induction Loop System" below.) Of course, the T-coil does add to the price of the hearing aid, and for that reason many people choose not to have the T-coil option.
When the switch on the hearing aid is set to "T", the regular microphone is turned off. However, when current runs through the speaker in your telephone handset, it generates a magnetic field that, in turn, generates (induces) a current within the T-coil of the hearing aid. The signal is usually very clear and since the hearing aid microphone is not being used, there is no feedback and no amplification of room noise.
B. Large-Area Amplification Systems
Auditorium-style amplification systems are designed to help multiple users hear better in large area situations such as:
- houses of worship
- movie theaters
- lecture halls / auditoriums
- outdoor gatherings
All large area systems are wireless. In wireless systems, the sound is picked up from the person speaking and is transmitted through the air as invisible electromagnetic waves where it is received, amplified, and delivered directly to the listener.
The listener is free to move around since they are not wired to the sound source. There is no restriction on the number of people receiving the signal as long as they are within range.
Receivers are compact, about the size of a "Walkman" or a deck of cards, while transmitters vary in size. Transmitters for large areas are about the size of a toaster oven. Personal systems and TV listening systems use smaller transmitters.
There are three large area amplification systems:
1) The FM system uses the same type of FM radio wave as a regular radio. It is probably the most common and versatile system in use today. FM systems consist of a small transmitter and a small portable receiver. The receivers and transmitter are tuned to the same frequency ("station"). The transmitter usually sits on the stage or is connected to the sound system in the sound room. The signal can reach people more than 300 feet away.
FM systems can be used in adjoining rooms without interfering with one another if the transmitters and receivers in each room are set to their own unique frequency.
Portable and easy to set up and use
Works equally well indoors and out
Large area coverage
Penetrates most walls and other physical obstacles. Disadvantages:May be subject to outside interference (police, fire, pagers)
Spillover into adjacent areas permit eavesdropping 2) Infrared Light system uses the same type of signal as your TV's remote control. These invisible light waves are just below the visible spectrum. The receiver, often a headphone, has a little "window" that catches the light waves and converts them back into sound. This window must be accessible to the light. It can not be covered up or kept out of sight.
Large area systems are commonly used in movie theaters. Lightwaves do not penetrate walls so transmitters in adjoining theaters will not interfere with one another. Large area infrared light systems are more difficult to set up than other systems. The transmitters must be set at the correct angle and may require more than one transmitter, so the system can be more expensive as well.
Advantages:Light doesn't penetrate walls (no spillover)
Not subject to interference from radio waves. Disadvantages:Transmitters and receivers must be unobstructed
Can be used indoors only
High intensity lights and direct sunlight can interfere with signal
Systems for large areas are more expensive and more difficult to set up. 3. Magnetic or Induction Loop system operates on a basic principle of physics that when electricity runs through a wire, it creates a magnetic field. In the induction loop system, a wire is laid around the perimeter of a room or activity area (like a museum exhibit). The transmitter, instead of sending the sound directly through the air as invisible waves, first sends it through the wire, creating a magnetic field that fills the area within the perimeter of the wire. This signal can be picked up by a hearing aid with a T-coil or by a portable receiver.
Once set up, the induction loop system is ideal for anyone with a T-coil in their hearing aid. No additional receiver is necessary.
Easy to operate
No additional receiver necessary for T-coil wearers
Pickup areas can be precisely defined. Disadvantages:Subject to electrical interference from power sources and electrical equipment
High installation costs
"Dead" areas may exist within loop. C. Personal Amplification Systems
Personal systems are used in the home, in the restaurant, the car and the classroom, as well as outdoors. They are ideal for one-on-one conversations, small group discussions, and teacher/student interactions.
There are two types of personal systems commonly in use: the Personal FM (wireless) and the Personal Hardwired.
1) Personal FM systems operate like FM systems described above, only they are much more portable. Personal FM is perfect for one-on-one communication and is typically used in classrooms. The teacher wears a lapel microphone wired to a pocket-sized transmitter, carried on his person. He can move about freely and everything he says is transmitted through the air and heard clearly by students equipped with receivers. The student's receiver usually comes equipped with its own microphone so, if he wishes, he can hear other students in the immediate area.
The DAI (Direct Audio Input) cord is a wire that runs from the receiver to a boot attached to a hearing aid or a cochlear implant. A boot is a little adapter that snaps onto the hearing aid to give the DAI cord something to plug into. The DAI cord can also connect directly to computers, tape recorders and other audio sources. FM receiver boots snap onto some hearing aids and can receive signals directly from FM transmitters. No other receiver is necessary. This is the ultimate in portability.
2) Personal Hardwired system. Hardwired devices use a direct electrical connection. It is a self-contained unit the size of a "Walkman" to which you connect a headset, earbud or neckloop. The unit is placed closer to the person or group speaking and can usually pick them up quite clearly.
D. TV Listening Systems
TV listening systems are typically infrared wireless systems that transmit sound from a TV (or other sound source such as a stereo or computer) to a receiver, via light energy. A small microphone is placed over the speaker, or a cable plugged into the "audio out" socket on the TV leads to a transmitter placed near the TV. Like other wireless systems, the sound is converted to radio waves, infrared light, or magnetic energy, transmitted through the air, and picked up by the receiver.
The headset usually has its own volume control. The headset can be turned up loud, while the TV's volume is set at a comfortable level for other listeners.
II. Alert Systems:
Alerting ALD's convert sounds into visual stimuli or vibratory stimuli. Alerting systems may simply amplify the sound so it can be heard. Some examples are:
Lamps can be plugged into special receivers so that whenever the phone rings, the lamps flash. In this case, the signal is transmitted through the house wiring. You might also choose to place a small doorbell/transmitter at the front door. When somebody rings the bell, the transmitter sends an FM radio signal to receivers in various rooms which cause lamps to flash on and off.
Alternatively, instead of flashing lamps, you might prefer your wrist watch or beltclip receiver to vibrate. If you can't hear the alarm clock, you could put a vibrating bedshaker under the mattress or pillow, or use a clock with a built-in strobe light.
If you want to hear the oven timer go off or be sure the baby is o.k., plug in a transmitter with a built-in microphone into a nearby outlet. When it "hears" a sound, it will send the signal through the house wiring to all receivers.
For the ultimate in effective low-tech, try the door-knock signaler. Simply hang it on the inside of your motel or dorm door. When someone knocks, a bright light goes off.
There are also base stations (master consoles) available which receive signals from different types of transmitters and indicate, via lighted icon, which event is occurring.
To learn more about these systems and other assistive hearing devices please visit the authors website at www.HDHearing.com.