One alternate way of sending audio information to the brain is to vibrate and / or electrify the skin at different places for each frequency. The frequencies should increase in a line and the higher frequencies should result in a higher vibration and / or electric frequency on a smaller area.

Another way of sending audio information to the brain is to reduce its frequency by a large factor, increase its amplitude and apply it to a bone. Maybe have different frequency bands going to different bones; probably sending the higher frequencies to a smaller bone with a higher amplitude.

Yet another way is to visually display the seperate frequencies either on a small screen or with virtual reality gear. Relatively little screen area is needed for the sounds to be recognizable even though they are now visual.

These methods would probably work best in combination, all three together should be enough to enable the deaf to recognize warnings and maybe understand clear speech.
 
 

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