FM Systems for School Aged Children

FM Systemsfor School Aged Children Linda Thibodeau, Ph.D. Advanced Hearing Research Center Callier Center for Communication Disorders University of Texas at Dallas Copyright by Linda M. Thibodeau 2005

Supportive information May be obtained at the Website of Linda Thibodeau www.utdallas.edu/~thib Copyright by Linda M. Thibodeau 2005

Welcome to this lecture on FM Systems! I hope you will find this information interesting and rewarding as you learn of the significant differences that can be achieved with FM Systems! Copyright by Linda M. Thibodeau 2005

OVERVIEW 1. Rationale for use of FM Systems 2. FM Systems 3. FM Evaluation Procedures 4. Use of FM Systems Copyright by Linda M. Thibodeau 2005

Let’s start with the Rationale for using FM Systems. Copyright by Linda M. Thibodeau 2005

1. Rationale for use of FM Systems A. Signal-to-Noise Ratio B. Typical Classroom Noise C. Audio Demonstration of Classroom Noise Copyright by Linda M. Thibodeau 2005

Signal-to-Noise Ratio A major problem for all persons with hearing loss is….Listening in Noise ! The problem is described by the signal-to-noise ratio or SNR. The amount of “Signal,” what you WANT to hear . . .compared to. . . “Noise”, ANY sound you don’t want to hear. Copyright by Linda M. Thibodeau 2005

In a large group, distance and noise can be a problem. Copyright by Linda M. Thibodeau 2005

This results in a poor signal-to-noise ratio. Copyright by Linda M. Thibodeau 2005

i.e. the intensity of the noise at the listener’s ear is greater than the signal of the speaker. Copyright by Linda M. Thibodeau 2005

A poor S/N ratio is -10 dB • the noise is 10 dB more intense than the signal 80 dB 70 dB Copyright by Linda M. Thibodeau 2005

A good S/N ratio is +20 dB • the signal is 20 dB more intense than the noise 70 dB 90 dB Copyright by Linda M. Thibodeau 2005

So...ideally we want to have a positive S/N ratio and a constant signal level regardless of distance between the speaker and the listener. Copyright by Linda M. Thibodeau 2005

We can accomplish this by placing a microphone on the speaker and delivering the sound directly to the listener. Copyright by Linda M. Thibodeau 2005

B. Typical Classroom Noise • Optimal noise level • for a class with students with hearing loss would be • 30-35 dBA, • yet the typical level is about • 60 dBA. • Average conversational speech is about 60 dBA, which means the SNR is often 0 dB! • (ASHA, 2005) Copyright by Linda M. Thibodeau 2005

The optimal SNR for teaching students with hearing loss would be +15 dB, • (the teacher’s voice is 15 dB more intense than the background noise). • However, the typical SNR in classrooms is only about • +5 to – 7 dB! • (ASHA, 2005) Copyright by Linda M. Thibodeau 2005

Effects of Noise on Speech Recognition Finitzo-Heiber and Tillman (1978) measured the effects of noise and reverberation on speech recognition scores with 8 to 12 year old children with Normal Hearing and Hearing loss . Copyright by Linda M. Thibodeau 2005

In a soundbooth: No Noise, Little Reverberation Children with Normal Hearing scored 95% - Only missed 5% of words Children who were Hard of Hearing scored = 83% - Only missed 17% of words Copyright by Linda M. Thibodeau 2005

In a situation similar to a typical classroom: Normal Hard of Hearing Hearing SNR of +12 dB: 83% 60% SNR of 0 dB: 48% 28% These results suggest that children with hearing loss may only be hearing about one fourth of speech in a typical noisy classroom. Copyright by Linda M. Thibodeau 2005

C. Audio Demonstration of Classroom Noise • Click on each loudspeaker to hear: • Typical Classroom Noise • A teacher reads a story in a classroom with a typical SNR • Same teacher reads a story in • an optimal SNR provided by • using an FM System Copyright by Linda M. Thibodeau 2005

Now let’s look at the operation and design of FM Systems in our second section of the lecture. Copyright by Linda M. Thibodeau 2005

2. FM Systems A. Introduction to FM Technology B. Transmitters/Microphones C. Receivers/Sound Transducers D. Cochlear Implants and FM Systems Copyright by Linda M. Thibodeau 2005

A. Introduction to FM Technology FM Systems use a frequency-modulated signal to transmit the speech through the air similar to an FM Radio station. The system involves a microphone and transmitter on the speaker and a receiver for the listener with some type of sound transducer. Copyright by Linda M. Thibodeau 2005

All FM systems have these parts- Microphone Transmitter Receiver Sound Transducer Copyright by Linda M. Thibodeau 2005

FM Transmission Process 3) Voltage fluctuations produce fluctuations of electric and magnetic fields around the antenna at a specific frequency 1) Speaker talks into microphone which generates rapid fluctuations of voltage 2) Voltage fluctuations are passed to a transmitting antenna 5) Electromagnetic waves produce fluctuations in voltage at a receiving antenna 4) Electric and Magnetic field fluctuations travel as electromagnetic waves 6) The voltage fluctuations are amplified 8) The voltage fluctuations in that frequency range are sent to the acoustic transducer 9) The transducer converts the voltage fluctuations into sound waves to go to the ear 7) A filter selectsthe frequency of interest Copyright by Linda M. Thibodeau 2005

Transmitting Frequencies • Typically each transmitter is assigned a frequency, • also called a channel. These may be coded by numbers, • colors, or letters. The channel component may be • called an oscillator. • Examples: • Number Color Letter Trans. Freq. • #1 Red/Gray A 72.000-72.025 • #2 Brown/Gray B 72.025-72.075 • There may be as many as 40 different channels used • in a school. Copyright by Linda M. Thibodeau 2005

Now let’s look more closely at the Transmitter/Microphone features…. Copyright by Linda M. Thibodeau 2005

B. FM Transmitters/Microphones Descriptive Categories 1) Microphone Type 2) Microphone Location 3) Channel Options Copyright by Linda M. Thibodeau 2005

FM Transmitters/Microphones Microphone Type: The microphone characteristics can vary. Omni-directional Microphone- picks up sound from all around Directional Microphone- picks up sound primarily from the top Multi-DirectionalMicrophone- picks up sound from all around OR focused area Copyright by Linda M. Thibodeau 2005

FM Transmitters/Microphones Microphone Location: All Transmitters are worn on the body, but the microphone placement may vary. On the Body On the Head On the Lapel On the Cheek (also called Boom mic) Copyright by Linda M. Thibodeau 2005

FM Transmitters/Microphones Channel Options on the transmitters: Single, but fixed-Oscillator channel set at the factory and cannot be changed Multi-Frequency Manual frequency selection-Oscillator can be removed and changed to another frequency Wireless frequency selection-Channel is changed digitally Copyright by Linda M. Thibodeau 2005

Wireless Channel Changeability 1) Direct Frequency Synchronization- Pressing a button on the transmitter sends a signal to the receiver to synchronize the channel Ex. Phonak MLxS with Campus S 2) Automatic Frequency Synchronization- Walking near a plate on the wall that contains transmitter changes the channel to a preset number Ex. Phonak MLxS with Wall Pilot Copyright by Linda M. Thibodeau 2005

Automatic Frequency Synchronization-Students wearing Phonak MLxS walking past Wall Pilot as they enter classroom to get synchronized to the correct frequency for thatclass. Copyright by Linda M. Thibodeau 2005

OTHER OPTIONS - Transmitters Programmable-Allows the channels to be set digitally via a connection to a computer. Audio Input Jack-An input jack for the audio signal from another source such as a tape player, VCR, or computer. No FM/Low Battery Lights-Alerts the users that the batteries or FM switches should be checked. Talk Over-Allows the microphone to be active even when the transmitter is connected to another audio source such as VCR Mute Switch-Allows the microphone to be deactivated so conversations can be private. Copyright by Linda M. Thibodeau 2005

Now let’s look more closely at the Receiver/Sound Transducer features…. Copyright by Linda M. Thibodeau 2005

C. FM Receivers/Sound Transducers Descriptive Categories 1) Type-Relationship to Personal Amplification/Cochlear Implant 2) Receiver Location 3) Channel Options 4) Sound Transducers Copyright by Linda M. Thibodeau 2005

FM Receivers/Sound Transducers 1) Type-Relationship to Personal Amplification/Cochlear Implant There are two main types with respect to interacting with Personal Devices: Basic-Does not interface with Hearing aid or Cochlear Personal-Does interface with Hearing aid or Cochlear Implant Copyright by Linda M. Thibodeau 2005

FM Receivers/Sound Transducers 2) Receiver Location: Some FM Receivers interface with a personal hearing aid worn at the ear? (i.e.one that the student wears throughout the day) Students and parents generally like this option!!! Some are worn: On the Body On the Ear Copyright by Linda M. Thibodeau 2005

FM Receivers/Sound Transducers 3) Channel options on FM Receivers? Single, but fixed-Oscillator channel set at the factory and cannot be changed Multi-Frequency Manual frequency selection-Oscillator can be removed and changed to another frequency to match transmitter Wireless frequency selection-Channel is changed digitally by pressing a button on the transmitter that is in close proximity Copyright by Linda M. Thibodeau 2005

FM Receivers/Sound Transducers • Sound Transducers Two Basic Categories: Basic System-Child takes off personal hearing aid each day and puts on the FM Receiver that picks up the teacher’s voice AND acts like a hearing aid. Personal System-Child keeps the personal hearing aid on all day and the FM Receiver is interfaced through one of three arrangements. Copyright by Linda M. Thibodeau 2005

FM Receivers/Sound Transducers Basic System Options Button Earphone with Body-worn FM system Ear Level FM Receiver with limited power Soundfield Speaker to set on desktop or mounted on the wall FM Amplifier with Behind the Ear hearing aid Personal System Options Neckloop with Personal Aid on T Switch Direct Audio Input with Personal Aid Silhouette with Personal Aid on T Switch Copyright by Linda M. Thibodeau 2005

Let’s look at some illustrations of these different types of sound transducers! Copyright by Linda M. Thibodeau 2005

Button Earphone with Body-worn FM system Copyright by Linda M. Thibodeau 2005

EduLink by Phonak Basic Ear Level FM Receiver with limited Power For Persons with Normal hearing, Mild loss, Auditory Processing Disorder Copyright by Linda M. Thibodeau 2005

Soundfield Speaker to set on desktop or mounted on the wall Desktop Speaker Wall Mounted Speakers Copyright by Linda M. Thibodeau 2005

FM Amplifier with Behind the Ear hearing aid Copyright by Linda M. Thibodeau 2005

Silhouette with Personal Aid on T Switch Copyright by Linda M. Thibodeau 2005

OTHER OPTIONS - FM RECEIVERS Programmable/Digital-Allows the features of the receiver to be set via a connection to the computer which results in more precise control of the FM signal. Audio Input Jack-Allows a direct electrical connection to a VCR, computer, or tape/CD player. No FM/Low Battery Lights-Warning lights to indicate the switch settings and/or batteries need checking. Microphones-When the FM receiver is not interfaced with a personal hearing aid that has a microphone, a microphone will be needed on the FM receiver to pick up the sounds nearby the student but at a lower level than the teacher’s voice. Copyright by Linda M. Thibodeau 2005

FM Systems for School Aged Children