Learning about the science behind treating hearing loss can be daunting. There are so many terms, concepts and measurements – it’s often hard to understand what each of them really means for your hearing, and how these concepts influence the way you treat your hearing loss.
Here we will try to explain one of those concepts, a sound processor’s maximum power output (MPO), or maximum output force level (the technical term for bone conduction devices), with a simple analogy: the ceiling of a house. Understanding MPO can give you insight on why your audiologist chooses or recommends a certain sound processor for your individual hearing needs, and this knowledge can empower you to ultimately make a choice that best suits you.
Different people have different levels of hearing loss, measured in an audiogram. Your level of hearing loss will determine your need for amplification. The fitting range is the most common measurement for any given sound processor. Fitting range is a guide to understand the level of hearing loss a device can address. When audiologists determine which sound processor would provide the most benefit to you, they compare your hearing loss to the sound processor’s fitting range. For example, the Cochlear Baha 5 Sound Processor has a fitting range that covers hearing loss to 45 decibels (dB) SNHL.1
All sound processors have limits on how much power they can provide, and this limit is the most important contributor to a device’s fitting range. A device’s power limit is known as the maximum power output, or MPO. In a chart measuring a device’s output (see illustration below), the MPO appears as a line graph showing the maximum power (in dB) it can provide across the hearing (frequency) range – from deeper sounds like the hum of traffic to higher-pitched sounds like a bird’s song. Now imagine the MPO line takes the shape of a house’s ceiling.
The height of the ceiling is important: Just as a taller person needs a high ceiling to be comfortable, a person with a high level of hearing loss needs a high MPO level to hear sounds comfortably.
Cochlear and other manufacturers make sound processors with different levels of amplification to “fit” your individual hearing loss, as the person in this illustration “fits” into their house.
However, just as its peak height (the highest level of power provided) is important, the shape of the MPO line – the way it slopes or curves across the chart – is also important. This shows how the sound processor delivers power across a range of the most important lower to higher frequencies. The average MPO is calculated across a number of frequencies, which illustrates the shape of this line.
The MPO on some devices delivers significantly more power to middle-range frequencies than the lower or higher frequencies important for speech. For these devices, the MPO chart looks more like a house with a very pointed ceiling and short walls. A tall person may stand comfortably in the centre, but they can’t walk over to the window without bumping their head on the ceiling. In these sound processors, at the lower and higher frequencies important for speech, sounds hit the “ceiling” of the sound processor’s power limits, distorting them.
Ideally, a device with a high average MPO allows you to clearly hear sounds across the frequency range – in the same way that a house with a gently sloped ceiling allows you to move around freely inside, all the way to peer out each window without bumping your head.
As most people have hearing loss across a wide range of frequencies, a peaked MPO means the sound processor may not amplify sound along the whole frequency range where they need it. The person wearing this sound processor actually hears sounds less clearly across these important frequencies.
Two devices may have a similar fitting range but can differ widely in average MPO. Ideally you should experience clear sounds, even moving across lower and higher frequencies, without hitting the “ceiling” distorting the sounds you hear. This is what a high average MPO delivers.
To find out whether you could benefit from a more powerful device, or to learn more about your treatment options, use this tool to find a clinic near you.
1 Flynn M. (2015) Smart and Small – innovative technologies behind the Cochlear Baha 5 Sound Processor. Cochlear Bone Anchored Solutions AB, 629761.