Thursday, 17 November 2016

Hearing - I'm picking up good vibrations

My mother lost the hearing in her left ear when she was in her late thirties through Meniere's disease. It is a nasty condition that affects the inner ear, causing pressure, dizziness and nausea. The symptoms usually pass: Mum was not so lucky. It also seems to have reduced her sense of smell. Three years ago she was also diagnosed with macular degeneration affecting both eyes. The deterioration of her sight has been a terrible blow. She is losing her ability to read, to enjoy watching television and seeing faces. Now at 96, she depends on me more than ever.

We struggle with her hearing aid. If the volume is high it amplifies the background noise.  When we watch a programme together it is too loud for her, so the volume is off and I read the text too but often, because she can't see it, she switches off part way through. When we are in the car she loves to listen to classical music on the radio. It seems that the resonance of non-electronically produced music is easier to hear. I think she feels the vibration through the bones of her face. Thinking about this led me to do some research about bioacoustics. Please don't switch off just yet. You may find this interesting.

"Dolphins possess a highly sophisticated auditory system and a keen capability for echolocation. Signals are emitted in the form of high intensity, short duration, broadband exponentially decaying pulses. The frequency spectra of echolocation signals used by many dolphins are dependent on the output intensity of the signals and not on any fine tuning by the animals. When the output intensity is low, the center [sic]frequency of the click tends to be low. As the output intensity increases, the center frequency also tends to increase. The pulses propagate from the dolphin's melon in a relatively narrow beam, and echoes are received via the lower jaw, with a slightly wider beam.

Echo- locating dolphins can detect targets at ranges of approximately 100 plus meters, depending on the size of the targets. Target discrimination experiments have shown that dolphins can discriminate the shape, size, material composition and internal structure of targets from the echoes. The broadband, short duration properties of the signal allow the echoes to have high temporal resolution, so that within the structure of the echoes a considerable amount of information on the properties of the target can be conveyed.

A brief comparison between the bat and dolphin sonar system will also be made. Bats typically emit much longer signals and a wider variety of different types of signals than dolphins. Signals used by some bats are suited to detecting Doppler shift, whereas the dolphin signal is designed to be tolerant of Doppler effects." Whitlow W.L. Au (1997).  Echolocation in dolphins with a dolphin-bat comparison. Bioacoustics 8(1-2): 137-162

So what about bats then? You are going to ask - aren't you? Bats are not blind; in fact they can see almost as well as humans. But to fly around and hunt for insects in the dark, bats make calls as they fly and listen to the returning echoes to build up a sonic map of their surroundings. The bat can tell how far away something is by how long it takes the sounds to return to them. These calls are usually pitched at a frequency too high for adult humans to hear naturally. Human hearing ranges from approximately 20Hz (cycles per second) to 15 to 20 kHz (1000Hz) depending on age. In comparison, some bats can hear sounds up to 110 kHz in frequency. By emitting a series of often quite loud ultrasounds that either sweep from a high to low frequency or vary around a frequency, bats can distinguish objects and their insect prey and therefore avoid the object or catch the insect. Individual bat species echolocate within specific frequency ranges that suit their environment and prey types.

Many moons ago I took my children on a bat walk organized by the Wyre Rangers in Dolphinholme, (strange coincidence). We waited until twilight to see thousands of bats fly from the roof of a house and helped to estimate their numbers visually.  It was certainly not accurate science. The little beggars fly in circles. Later we went in the dark to stand close to a pond with bat-detectors and hears the clicks picked up by their inaudible high-pitched emissions as they dipped to catch bugs. We can only hear them using a type of hearing aid. 

Mum struggles to hear my voice.  It seems that she is tuned to a lower frequency and her comprehension is far greater when listening to deeper male voice.  Or perhaps even at 96 she prefers male company?  She certainly warms to the bass voice. As do I...

When we met,
I admired your shape,  
I heard your voice
And something deep
began to resonate.

When we share
The dark, warm night,
I hear your smile,
And map your contours
with sweet kisses.

When we are apart,
I send my longing song,
So your vision finds
And overflows my
listening heart.


 Have a good week.  Thanks for listening. Adele


Anonymous said...

Very interesting blog - great photograph, loved the poem.