Chaptor-3 

The Nature of Sound

As per the modern scientific aspect, sound can defined as follows.

Sound is a longitudinal, mechanical wave.Sound can travel through any medium, but it cannot travel through a vacuum. There is no sound in outer space.Sound is a variation in pressure. A region of increased pressure on a sound wave is called a compression (or condensation). A region of decreased pressure on a sound wave is called a rarefaction (or dilation).The sources of sound:

• vibrating solids
• rapid expansion or compression (explosions and implosions)
• Smooth (laminar) airflow around blunt obstacles may result in the formation of vortices (the plural of vortex) that snap off or shed with a characteristic frequency. This process is called vortex shedding and is another means by which sound waves are formed. This is how a whistle or flute produces sound. Also the aeolian harp effect of singing power lines and fluttering venetian blinds.

What are the different characteristics of a wave? What are the things that can be measured about waves? Amplitude, frequency (and period), wavelength, speed, and maybe phase. Deal with each one in that order.

amplitude, intensity, loudness, volume

Amplitude goes with intensity, loudness, or volume. That’s the basic idea.

frequency, pitch, tone

• Typical sounds produced by human speech have frequencies on the order of 100 to 1000 Hz.
• The peak sensitivity of human hearing is around 4000 Hz.

speed of sound

The speed of sound depends upon the type of medium and its state. It is generally affected by two things: elasticity and inertia. Elasticity

Acoustic Thermometry of Ocean Climates (ATOC)

• in water, sounds below 1 kHz travel much farther than higher frequencies
• “shipping noise is loudest in the 30 to 200 Hz range [lowest piano note to middle of cello]”
• “blue and fin whales are the loudest sound in the 17 to 30 Hz range”
• “In pre-industrial times, the low frequency range of 15 to 300 Hz in which most of the baleen whales sings was the quietest part of the sound spectrum, nestled between the subsonic ramblings of earthquakes and the higher pitched rattle of wind, waves and rain.” Bob Holmes. “Noises Off.” New Scientist. 1 March 1997: 30–33.

echoesscraps

• As with any wave the speed of sound depends on the medium in which it is propagating.
• Sound generally travels faster in solids and liquids than in gases.
• The speed of sound is faster in materials that have some stiffness like steel and slower in softer materials like rubber.
• Factors, Which Affect the Speed of Sound in Air.
• The speed of sound in air is approximately 330 m/s (about 1,200 kph or 700 mph).
• The speed of sound in air is nearly the same for all frequencies and amplitudes.
• It increases with temperature.
• Determining the Distance to a Lightning Bolt: Sound waves take approximately 5 seconds to travel 1 mile. Using this information, it is possible to measure one’s distance from a lightning bolt. Begin counting immediately after you see the flash. Every five seconds counted is roughly equivalent to one mile of distance.

frequency & wavelength

The frequency of a sound wave is called it pitch. High frequency sounds are said to be “high pitched” or just “high”; low frequency sounds are said to be “low pitched” or just “low”. Humans are generally capable of hearing sounds between 20 Hz and 20 kHz (although I can’t hear sounds above 13 kHz). Sounds with frequencies above the range of human hearing are called ultrasound. Sounds with frequencies below the range of human hearing are called infrasound. 

Notes on the hearing of various animals

• ultrasound
  • ?
• infrasound
  • Elephants, whales, hippos, rhinoceros, giraffe, okapi, and alligator are just a few examples of animals that create infrasound.
  • Some migratory birds are able to hear the infrasonic sounds produced when ocean waves break. This allows them to orient themselves with coastlines.
• bat
  • bats use ultrasonic frequencies of around 100,000 Hz to navigate
• dolphin
  • ?
• whale
  • ?
• dog
  • Dogs can hear sound as high as 40,000 Hz.
• cat
  • Cat’s hearing range between 100 and 60,000 Hz
• mice and rats
  • Mice can hear frequencies between 1,000 and 100,000 Hz.
  • Rat has hearing range between 1,000 and 90,000 Hz.
• elephant
  • Elephant’s hearing range between 1 and 20,000 Hz.
  • An elephant is capable of hearing sound waves well below our the human hearing limitation (approximately 30 Hertz). Typically, an elephant’s numerous different rumbles will span between 14 and 35 Hertz. The far-reaching use of high-pressure infrasound opens the elephant’s spatial experience far beyond our limited capabilities.
  • Silent Thunder, Katy Payne
• rhinoceros
  • Rhinoceroses use infrasonic waves of about 5 Hz to signal to each other
• pigeon
  • Pigeon can detect sounds as low as 0.1 Hz.
  • Mel Kreithen, Cornell University
• insects
  • Grasshopper can hear up to 50,000 Hz.
  • Noctuid moth has a hearing range between 1,000 and 240,000 Hz.
• miscellaneous
  • Drum Fish: Collects underwater sound vibrations with an air bladder. The signals are then send from the air bladder to the “weberian apparatus” in the middle ear and then to the inner ear. Hair cells in the inner ear respond to the vibration and transmit sound information to the fish brain.
  • Snakes have no external ears. Therefore, they do not hear the music of a “snake charmer”. Instead, they are probably responding to the movements of the snake charmer and the flute. However, sound waves may travel through bones in their heads to the middle ear.

ultrasound

• animal echolocation
  • microchiropterans a.k.a. microbats: carnivorous bats (not fruit bats or flying foxes)
  • cetaceans: dolphins, porpoises, orcas, whales
  • two bird species: swiftlets and oilbirds
  • some visually impaired humans have learned this technique

• sonar (an acronym for sound navigation and ranging) including
  • bathymetry
  • echo sounding
  • fish finders
• medical ultrasonography (the images generated are called sonograms).

infrasound

• avalanches: location, depth, duration
• meteors: altitude, direction, type, size, location
• ocean waves: storms at sea, magnitude, spectra
• severe weather: location, intensity
• tornadoes: detection, location, warning, core radius, funnel shape, precursors
• turbulence: aircraft avoidance, altitude, strength, extent
• earthquakes: precursors, seismic-acoustic coupling
• volcanoes: location, intensity

Human hearing

·                 locating the source of sound Phase differences are one way we localize sounds. Only effective for wavelengths greater than 2 head diameters (ear-to-ear distances). a.k.a. Intera-ural Time Difference (ITD) Sound waves diffract easily at wavelengths larger than the diameter of the human head (around 500 Hz wavelength equals 69 cm). At higher frequencies the head casts a “shadow”. Sounds in one ear will be louder than the other. a.k.a. Inter-aural Level difference (ILD) ·         The human ear can distinguish some … 1400 different pitches An understanding of sound is rein-sting about forecasting of nature using sound. A most popular discussion was over after the Tsunami wave hits. Sixth Sense?There were many stories after the December 26th 2004 tsunami of animals vacating the danger areas for higher ground hours before the deadly ‘Harbor Wave’ struck with such divesting effect.Flocks of birds, elephants, buffalo, antelopes etc. all flew and stampeded for higher and safer ground. Dogs refused to go for their run on the beach. Hares and rabbits had disappeared. In the aftermath of the tsunami’s destruction, survivors were amazed at how few dead animals there were amongst the debris. In some parts not a single dead animal was found! All this was in areas where human fatalities were numerous and where cars and fishing boats had been flung into treetops.There are no definite answers to this phenomenon but as you would expect, many theories. One theory is that animals have “sixth sense” although met with great skepticism by scientists. There is thought that elephants have extra senses in their feet that can sense vibrations and even recognize different types of vibration. There is of course the fact that animals in general can hear frequencies that humans cannot. Animals also pick up on natural signs developed over thousands of years and this may give them alert signals. Humans are distracted by many material objects that have no interest whatsoever to the animal kingdom. Birds in particular are constantly adjusting to environmental changes, and perhaps their distress signals alert other creatures. Elephants are known to lay their trunks on the ground when an airplane or truck generates large seismic noise as if to feel it.The truth is that nobody knows for certain. But the fact is that, to a very great extent, animals escaped the 2004 tsunami.Wise ElephantsIn Khao Lak, 50 miles north of Phuket along Thailand’s western coast, a dozen elephants giving tourists rides began trumpeting hours before the tsunami struck the shorelines.About the time the 9.0-magnitude quake fractured the ocean floor. An hour before the wall of waves slammed the resort area, the elephants reportedly again grew agitated and began trumpeting in a distressed manner. Just before disaster struck, they fled for higher ground — some breaking their chains to flee. Flamingos that breed this time of year at Point Calimere sanctuary on India’s southern coast left for safer forests well before the tsunami hit. One herd of elephants reportedly cleared a path in Banda Aceh, in order to make their way to higher ground. Sensitive to ground vibrations, elephants may have detected the undersea quake long before the tsunami hit. At the hard-hit Yala National Park in Sri Lanka, stunned wildlife officials reported that hundreds of elephants, leopards, tigers, wild boar, deer, water buffalo, monkeys and smaller mammals and reptiles had escaped unscathed. And while large turtles have been found dead in the debris along the shore of Indonesia’s devastated Aceh province, the tsunami’s impact on wildlife was “limited,” said Frank Momberg, coordinator for emergency response in Aceh for the conservation group Fauna & Flora International.Tales of animals behaving strangely before the quake and of wildlife escaping to safety abounded in the wake of the tsunami, raising questions about what these members of the animal kingdom knew that humans didn’t — and what, if anything, can be learned from it? Seismologists have sophisticated instruments that can measure quake factors during and after the fact, but experts admit no one can predict exactly when one will happen. Some scientists say certain animals have a kind of sensory hard-wiring that can detect earthquakes ahead of time, which one day might be replicated in man-made instruments.Reports of animals’ “sixth sense” in detecting hurricanes, earthquakes, tsunamis and volcanic eruptions long before the earth starts shaking go back centuries. Rats racing from buildings, sparrows taking flight in flocks, dogs howling incessantly: It’s an impressive track record, though anecdotal. After the 2004 tsunami, a Danish man staying in Ao Sane Beach, north of Phuket, wrote on a Danish Web site: “Dogs are smarter than all of us. . . . They started running away up to the hilltops long before we even realized what was coming.”Looking for proof.Scientists are shy on a subject that, for obvious reasons, is difficult to replicate in a laboratory.There are always explanations and theories that mitigate the mystery of the anecdotes. In the case of this tsunami, says Ken Grant, project coordinator at the Humane Society International Asia office in Bali, Indonesia, a lot of animals escaped simply because they tend to live inland in the forest. Nevertheless, some scientists are looking for explanations of why some species behave strangely before natural catastrophes, by correlating the animals’ sensory abilities with microscopic and invisible sensory stimuli.“I don’t know if I’d call this a sixth sense so much as a better sense,” Grant says. “Most animals know that when the ground starts to shake something is wrong.”Small Changes, Big Hints.Animals’ sensory physiology — super-sensitive to sound, temperature, touch, vibration, electrostatic and chemical activity and magnetic fields — gives them a head start in the days and hours before natural calamities. “It appears a lot of animals have sensory organs that detect these micro-tremors and micro-changes that we cannot possibly monitor,” says George Pararas-Carayannis, a former University of Hawaii oceanographer and geophysicist who leads the Tsunami Society.“It’s a sensitivity that we humans don’t have. But animals through millions of years of evolution have developed it, and that’s how they have been able to survive as a species. It is run or perish,” says Pararas-Carayannis, author of the 2001 book “The Big One: The Next Great California Earthquake — Why, Where, and When It Will Happen.”

 Do Indigenous People Sense Disaster?

Indigenous peoples on some of the Indian Oceans remotest islands also faired well in surviving the tsunami. There was great fear that many would have been totally wiped out by the destructive wave. But in many cases the opposite was true. The instincts and knowledge of nature of the tribes had sent many fleeing for the safety of the forests and higher ground.As one of the first Coast Guard helicopters with relief supplies for the tsunami victims slowed over the Indian Andaman and Nicobar islands to assess the damage, a lone tribesman sent a message from below: leave us alone. The lone Sentinelese man stood naked on the beach and shot a bow-strung arrow at the helicopter.The Sentinelese are one of five indigenous tribes on the Indian archipelago, and one of perhaps hundreds affected by the massive waves of last month’s tsunami. The tribes’ stories are vignettes of survival amidst massive destruction. In some cases, the disaster foraged ties to urban neighbors, but in others it highlighted the tribes people’s unique intuitive ties to nature that urban dwellers seem to lack.As the tsunami’s death toll topped around a quarter of a million people, just one of the 200 Moken living on Thailand’s South Surin Island perished in the tsunami, and the ancestors of an ancient South Indian island tribe all survived when their king instructed them to rush up nearby mountains.