Pre-requisites to FM Fitting
It is not recommended to fit an FM system on the same day as hearing instruments, due to the large amount of information that the client must absorb, unless it is a case of extreme reduced speech recognition.
Fitting
During a hearing aid fitting, there may be differences between the frequency response/gain/output displayed on the programming screen and the actual frequency response/gain/output measures at the level of the eardrum, even when using the same unit (dB SPL). For this reason, it is important to verify that the fitting is accurate with electroacoustic measures.
When an FM is coupled to a hearing aid, verification becomes even more critical because it is possible that the addition of the FM may further alter the frequency response/gain/output. You can not make the assumption that hearing aid response via hearing aid microphone applies to the hearing aid response via the FM microphone. Electroacoustic verification of the hearing aid must be made with and without the FM receiver activated to ensure that the addition of the FM system is ‘acoustically transparent’.
The 2cc coupler measures work well for verifying the response of the hearing aid but is based on a standard ear canal volume for the average adult ear. Because ear canal volume changes from person to person, a more precise measure would be real-ear or probe microphone measures. For this reason, Phonak recommends the use of real-ear measures for verification purposes but acknowledges that time and equipment may limit verification technique to 2 cc coupler measures.
Although the sound quality of FM signals are dependent upon the hearing instruments they are connected to, there are other factors that must be considered when measuring their performance. The most important difference is that the input level of the FM microphone is more intense than the hearing instrument microphone because of its placement close to the mouth.
The typical FM microphone input (at 6-8 inches or 15-20 centimeters) produces an overall level of speech that is approximately 80-85 dB SPL (ASHA, 2000). The typical hearing aid microphone input (at 3-6 feet or 1-2 meters) produces a conversational level of speech that is approximately 65 dB SPL (ASHA, 2000). Therefore, it is recommended that the FM input be 15-20 dB SPL greater than the hearing aid input to reflect realistic signals reaching both microphones (ASHA, 2000). It is noteworthy to point out, however, that the outputs resulting from these inputs may vary because of: 1) compression in either the FM and/or hearing aid microphones; 2) the combination of multiple sound sources (for FM+M mode); and 3) other signal processing characteristics.
The second issue relates to the different microphone input possibilities, i.e. the FM microphone may be omni-directional, directional, or super directional.
The basic goal in fitting is that the remote microphone should give an output 10 dB higher than that produced by the input into the hearing instrument microphone. There are three different microphone configurations that should be evaluated:
- Hearing instrument only
- FM + hearing instrument
- FM only
Because there are three types of output that can be generated at the eardrum of the FM user, we recommend three electroacoustic measures for each of the active microphone settings: hearing aid only (M); for the FM only (FM); and for the hearing aid plus FM (FM+M) setting.
- Hearing aid only (M)
- FM only (FM)
These microphone settings may be measured sequentially.
- FM plus hearing aid (FM+M)
In this setting, both microphones are active simultaneously. Currently, electroacoustic procedures for verifying outputs must be done in sequential fashion because it the contributions from simultaneous microphone inputs cannot be reliably analyzed separately.
In 2000 ASHA has published a guideline for fitting and monitoring FM systems. It provides step by step directions for using a 2cc coupler. Appendix B outlines three rationales for verifying simultaneous inputs: 1) equal output criterion; 2) equal gain criterion; and 3) 10 dB advantage criterion. With the equal output criterion, there is no FM advantage. With the equal gain approach, there is the greatest FM advantage but there is concern that ambient noise levels may interfere with the processing of speech. The 10 dB advantage criterion is a procedure in which the output of the FM microphone is 10 dB greater than the hearing aid microphone, an acceptable starting point for adjusting FM gain. However, the 80dB input to the remote FM microphone will be operating under compression (typically 72 dB SPL kneepoint). This procedure may cause underestimation of actual SNR.
American Speech-Language-Hearing Association (2000). Guidelines for Fitting and Monitoring FM Systems. ASHA Desk Reference.
Phonak has considered the limitations of each strategy and has developed the Phonak Offset Protocol (POP). The POP is recommended for because it is a procedure that verifies simultaneous inputs with sequential methods (typical clinical equipment) below the compression threshold of the FM microphone.
Phonak FM offset protocol (PDF, 68 kB)
Recommended Protocol for Fitting FM device with Real-Ear Measures
A real ear analyzer is used to verify that the real ear insertion gain (REIG) of the current hearing aid response was appropriate when compared to an NAL-R insertion gain target. Next the hearing aid and the FM receivers are set to the FM only mode (disable the hearing aid microphone). The FM transmitter is placed 6 inches from the real ear analyzer speaker that delivered a 65-dB SPL composite signal and REIG was measured. The REIG measured in the FM only mode was greater than the REIG measured through the hearing aid due to the distance from the real ear analyzer speaker to the FM transmitter being only 6 inches whereas the distance from the real ear analyzer speaker to the hearing aid was 1 meter. The purpose of this verification was to assure that the FM system was providing a smooth and appropriate frequency response when coupled to the hearing aid.
Pre-fitting Preparation
FM transmitter must be charged prior to fitting.
- SmartLink/ZoomLink/EasyLink contain a lithium-ion battery with fast-charge capability. A fully exhausted battery takes around 2 hours to recharge to 80% of capacity.
- Campus S – New batteries require at least 12 hours charging time.
- Make sure you have appropriate audio shoe to couple the hearing instrument to the FM receiver. Coupling the universal FM receiver (MLxS) to a patient’s hearing instrument (other than Phonak) may require manufacturer modifications such as placing a hole in the battery door to enable proper coupling to the audio shoe.
- When ordering the ML8S for Perseo/Claro and ML9S for Savia, Savia Art,, Eleva, eXtra, Una and VERVE, make sure that the ML8S/ML9S are ordered in the same color as the hearing instrument.
- Determine adequate functioning of client’s hearing instrument(s).
- Perform listening check with hearing instrument and FM units to make sure FM signal is received.
Fitting
Analog and Computer programmable hearing instruments (any model) (PDF, 19 kB)
Phonak Digital hearing Instruments: (PDF, 18 kB) Savia, eXtra, Eleva, Perseo or Claro
FM Desktop Fitting Guide
Post-fitting Verification
Real Ear Measurements (PDF, 15 kB)
Complete testing in the office and assure FM receiver is properly connected and system is working as expected.
Examples for testing the FM functions:
- Walk out of the room wearing the transmitter and see if patient can hear you.
- Perform a listening check to verify that the hearing instrument microphone is muted when hearing instrument is set to FM mode only.
- Bring the patient to a noisy place (i.e., cafeteria or street) or simulate a noisy place with recordings in the office.
- While using FM+M, confirm that the patient feels the FM input is loud enough and FM benefit is substantial compared to the HA-only condition.
- If you determine that adequate FM advantage is not achieved, you will need to program the MLxS.
Programming FM
Phonak’s latest generation of FM devices featuring synthesized technology (MicroMLxS, MLxS, ML9S, MyLink, EduLink, iLink, MicroLink Freedom) offers:
- programmability for FM gain
- default frequency in the FM receivers
- selection of accessible frequencies in the transmitters (Campus SX, WallPilot, SmartLink, ZoomLink and EasyLink).
Programming requires hardware (computer, HI-PRO, programming interface and FM Successware v 3.6.
Using FM Successware
1. When you get to the fitting screen, select the hearing instrument that you are using
2. If you are fitting FM or a non-Phonak hearing instrument, choose “Other” (or call your Phonak representative regarding how to customize this list further)
3. WWhen you open up the Advanced FM features button, you will see a gain slider bar for FM ONLY and FM+M positions. They will be set at default levels. You can customize them as necessary to achieve the desired level of FM advantage. Increasing the gain of the receiver will result in greater difference between the hearing instrument response and the FM response. For most instruments, the level of the FM only input can be adjusted separately from the FM+M input.
- If you are fitting FM on a Supero instrument, there will only be one slider for adjustment because the MLxS must remain in the single dot position at all times. The Supero switch is used to access FM and FM+M as hearing instrument programs.
- If you are fitting FM on a Perseo or Claro, you must select ML8S rather than MLxS in the instrument menu of the FM programming software. In the fitting screen you will then see one gain slider for adjustment because the hearing instrument switch is used to access FM and FM+M as hearing instrument programs.
- If you are programming the EduLink, it is possible to change the maximum output of the receiver. The device is initially set for normal hearing listeners at 95 dB SPL but the MPO can be increased through programming to 101 dB SPL.
4. Save settings in the end fitting screen with the hearing instrument and FM in the FM programming interface (toaster). The hearing instrument must have a fresh battery and both the hearing instrument and receiver must be set up for FM or FM+M.
5. Re-evaluate using real ear and/or subjective measures.
Phonak FM offset protocol (PDF, 68 kB)
SNR Advantage, FM Advantage and FM Fitting (PDF, 417 kB)
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