The normal human ear is an exquisitely sensitive instrument. It can detect sounds as quiet as a piece of tissue paper falling to those as loud as a commercial jet taking off directly overhead. The chart below shows the loudness of several common sounds, along with the effects of exposure to noise at various levels.
The loudest sounds that can be perceived are an astonishing one million times louder than the softest sounds. We may talk about the difference in loudness, but it is more accurate to describe loudness in ratio terms. By saying a million times as loud, we are already using ratios.
It is convenient to convert the ratios to decibels, or dB. The value in dB as a function of the ratio is given by (warning – a very small bit of math that you may safely ignore):
For every factor of 10, the loudness increases by 20dB. If the ratio is 100, we say 40dB. And for the whole range of 1,000,000, we have 120dB.
While a normal ear responds to this enormous range of sound, it cannot do so all at once. When you are at a U2 concert, you won’t be able to hear your watch ticking. Yet you can hear that same watch while sitting in a quiet room.
The Input Dynamic Range, or IDR, of a cochlear implant sound processor is the ratio between the loudest and softest sounds that it will present at any given time. Cochlear uses the term Instantaneous Input Dynamic Range, or IIDR, for this.
An IDR of 60dB, for example, will present sounds with a loudness range of 1000:1. If you do score tickets to that U2 concert, that 60dB will shift up towards the louder end of the range. You will hear the Edge’s screaming guitar, Bono’s vocals, and maybe, just maybe, the person standing next to you.
When you come home from the concert, you will be able to discuss it with your friends, and hear the clock ticking during the pauses in conversation. The circuitry or software that controls the overall volume for different situations is called Automatic Gain Control, or AGC.
If the clock ticking were to hamper your ability to understand the conversation, your audiologist may choose to lower the IDR. In the extreme, the IDR may be lowered significantly, at which point you will only be able to hear sounds within a limited range of loudness at any given time. This may help you to understand speech better, but at the expense of discarding most of the environmental sounds that allow you to feel immersed in the situation.
On the other hand, speech isn’t all the same loudness either. The difference between vowels and soft consonants is around 30 or 40dB. Lowering IDR too much can decrease speech comprehension, even without any background noise.
If you perform well at your current IDR setting, you may want to see if a higher IDR works even better for you. If you find that background noise makes it more challenging for you to understand speech, a lower IDR may help improve your performance.