Maximum Power Output and Maximum Force Output Available to Baha® Patients
AudiologyOnline: What is MPO?
Fredrik Breitholtz: Maximum Power Output or MPO is the maximum output that an amplifier can produce and is related to the microphones, power supply, and sound producing components of that amplifier. In bone conduction devices, this term is Maximum Force Output or MFO.
AudiologyOnline: What is the difference between MPO and MFO?
Fredrik Breitholtz: For bone conduction systems, Maximum Force Output or MFO is measured in dB SPL re: one micro newton of force. Maximum Power Output or MPO in traditional hearing aids is measured in dBSPL re: 20 dA Pa (deca-Pascals).
AudiologyOnline: Why is MPO or MFO important to patient’s outcomes?
Fredrik Breitholtz: MPO/MFO is important as it sets the dynamic range that the sound processor can provide to the patient. There are two types of dynamic range: that of the amplifier in a sound processor and that of human hearing. In an amplifier, the dynamic range is defined as the difference between the smallest amplified intensity and the loudest output of the amplifier. Amplification near or above the MPO/MFO causes distortion in the signal. For human hearing, the dynamic range is defined as the difference between the softest sound heard and sounds that are uncomfortably loud.
Normal hearing individuals can have dynamic ranges of 100 dB or more, meaning the softest sound they can hear and the point at which sound becomes uncomfortably loud can range up to 100 dB. Sensorineural hearing loss and conductive hearing loss may affect this dynamic range in different ways.
With sensorineural hearing loss, the patient’s thresholds are increased (meaning sound has to be louder before they can hear it), but their loudness tolerance is either the same or can be decreased, leading to an overall reduction in dynamic range. This must be addressed when fitting amplification, usually by compressing the amplifier’s dynamic range to accommodate the patient’s impaired range of hearing.
With conductive hearing loss, the patient’s thresholds are increased, as it is with sensorineural hearing loss, but their loudness tolerance is also increased, mostly preserving the dynamic range.
Therefore, while someone with sensorineural hearing loss may require more gain for soft sounds and less or no gain for loud sounds, someone with conductive hearing loss will require similar gain for soft, moderate and loud sounds (i.e. linear amplification). This means someone with a conductive component to their hearing loss may require a much higher output from amplification than someone with sensorineural hearing loss and therefore a sound processor with a large dynamic range.
AudiologyOnline: What does “headroom” mean?
Fredrik Breitholtz: Headroom is related to the concept of dynamic range and is defined by the difference in decibels between the gain for a specific input signal and the Maximum Force Output of the amplifying system. Maximum gain can never exceed MFO. Amplification near or above the MFO causes distortion, so having lots of headroom in the amplifier is desirable.
AudiologyOnline: How much MFO is available in the Baha 5 family of sound processors?
Fredrik Breitholtz: The Baha 5, Baha 5 Power and Baha 5 SuperPower Sound Processors have MFO measured with a 90 dBSPL input on a Baha Connect System, which results in MFO at 116, 115, and 125 dBSPL re: one Micro Newton of Force, respectively.
Figure 1. Average MPO available to the patient.
Figure 2. MPO data from Baha 5 Power, SuperPower and Ponto 3 SuperPower. “User setting” values as presented in Genie Medical version 2016.1.
AudiologyOnline: That seems like a lot of power in the Baha 5 Power and SuperPower! How do you get that much gain?
Fredrik Breitholtz: All Baha 5 sound processors are based on the BCDrive™ technology. BCDrive™ technology makes it possible to design more powerful transducers, while keeping a similar size or even reducing the size of the transducer. Incorporating better internal design of the moving components allows the transducer to produce more powerful vibrations.
Along with BCDrive technology, the split design of the Baha 5 SuperPower – where the input microphones are in a separate housing from the actuator– provides isolation between the vibrating mass of the actuator and sound input to the microphones. This allows for more usable gain before mechanical interactions between these two components. This is where acoustic feedback may occur.
AudiologyOnline: Is the MFO the only factor that will define the Dynamic range of a sound processor?
Fredrik Breitholtz: No, quite often feedback is a limiting factor to the amount of gain that the audiologist can actually prescribe to a patient. A good feedback management system is therefore vital once the MFO is sufficient.
AudiologyOnline: How do you work to limit feedback in the Baha 5 SuperPower?
Fredrik Breitholtz: First, the Baha 5 sound processors use a very efficient dual-track feedback management system that constantly detects changes in the feedback performance and deploys methods to counter feedback via phase cancellation. Complementing this, the Baha 5 SuperPower uses a split design where the transducer is separated from the microphones by a cable. This means that vibrations from the transducer are not mechanically transmitted to the microphones in the device. With very powerful sound processors, the vibrations that are sent to the bone can actually be radiated back to the processor, resulting in a particular kind of feedback that is referred to as skull radiation. Skull radiation is a function of the skull acting as a sounding board to radiate acoustic energy. In some cases, these vibrations can be handled by the feedback management system. However, at high amplification levels, these vibrations may become a limiting factor. With the Everywear™ wearing options, the audiologist can adapt how the patient wears the sound processor. By shifting the microphones to sit under the ear, the skull radiation is largely overcome and more gain can be prescribed for the patient. If this is not sufficient to reduce feedback, the processor can be separated further from the transducer and worn as a clip on, which will totally overcome feedback as a limiting factor further increasing the dynamic range.
The combination of design concepts and improved technology have allowed Cochlear Bone Anchored Solutions to produce a SuperPower sound processor that has the most Maximum Force Output in its class. This product gives clinicians an additional resource to fit up to a pure tone average of 65dB SNHL in mixed conductive hearing losses.
For more information, visit http://pronews.cochlearamericas.com/mpo-available-to-the-patient/ or the Cochlear Expo Page on AudiologyOnline.