Monday, August 13, 2012

Sound - A Short View of Definition and Behavior


Sound is a mechanical vibration of a medium in the hearing range.

A medium can be practically anything: a gas, a liquid or a hard material. Most commonly we perceive sound propagation in the air, where it travels with the speed of 343 m/s at the room temperature (20°C). The speed varies with temperature, but in the human habitat's thermal range, that's not a big variation (a couple of percent).

Some aircraft can reach the speed which is higher than the speed of sound (343 m/s= 1,235 km/h), also called the supersonic speed. Some other phenomena in the nature can reach that speed, such as meteorites entering the Earth's atmosphere, There are also scientific speculations about the fact that some dinosaurs were able to move their tail with the ultrasonic speed, generating intimidating sounds.

The speed of sound is different in different media: for example, in water, this parameter is 1,484 m/s, while for iron it is 5,120 m/s, again, both at the room temperature.

Basic parameters

The basic shape of a pure uniform oscillation is a sine wave. This is what we use as a basic signal shape in sound, too, especially in electronics. This is the shape that we use for most measurements to test the behaviour of sound equipment.

This signal is defined by two parameters:

amplitude (A), which is directly connected with the loudness, therefore how loud or quiet the sound is, and
frequency (f), which determines the pitch of the sound - it tells us how many times per second a sound vibrates.
The signal makes its full turn when it equally passes positive and negative half-period creating a full cycle.

The time needed for this signal to terminate a full cycle is called a period (t0) or wavelength and it is measured in seconds. Frequency determines how many periods are completed in one second. The unit for frequency is 1/s or, more commonly, Hertz (Hz).

For example: if a signal oscillates with a hundred periods in a second, that's 100 Hz.

From the definition "sound is a mechanical vibration of a medium in the hearing range", we have already covered a part of a medium, but "what is the hearing range"?

The hearing range is the range between the lowest and the highest audible frequency. For a human, this is between 20 Hz and 20 kHz (20,000 Hz). Different animal species have different hearing ranges. For example, a mouse can hear from 1 kHz to 100 kHz and a chicken from 125 Hz to 2 kHz.

Now we have covered the whole definition of sound: we know what a medium is and how the speed of sound behaves in it, and we know what the hearing range is.

Dynamics

Previously we have said that a sound/signal is defined with two parameters: frequency and amplitude. As there is a definition for the hearing range in frequency, there are also limits in amplitude: what is the quietest sound we can hear and what is the loudest sound we can bear?

The amplitude range for a human hearing is defined between 0 and 120 dB, where 0 dB is the "loudness" of breathing, and 120 dB is a threshold of pain.

dB is not really a unit, but a logarithmic way to define a ratio between a reference signal and a measured signal.

0 dB means that the ratio between a reference and a measured signal is 1:1, while 120 dB is 1:1,000,000, therefore we can't bear the sound that is a million times louder than our breathing. Another interesting ratio is 6 dB (1:2).

Just to give you a real life example of what dB means as a noise:

0 dB - minimal perceivable sound
30 dB - whisper
60 dB - conversation
98 db - hand drill
115 dB - loud rock concert
120 dB - pain threshold
140 dB - jet engine
180 dB - death of hearing tissue
In music notation, we use two extremes for the loudness definition: ppp (pianississimo, which equals to whispering) and fff (fortississimo, equal to yelling).

Super Storms


The name Hurricane Sandy did not do justice to the overachieving, late-season cyclone that spread death and destruction from Jamaica to New England. The media has taken to calling it "Superstorm Sandy" in order to describe a once-in-a-lifetime meteorological event.


Once in a lifetime will be more than enough for anyone who dealt with the storm's vicious winds, heavy rains, absurd October snow drifts (yes, snow from a system that was born in the tropics), towering ocean waves and record storm surges. The grim figures are not in, but it seems likely that Sandy claimed close to 100 lives, perhaps more. Having seen the video of the storm's aftermath, I suspect property damage will rival or surpass Hurricane Katrina's destruction of New Orleans and the Gulf Coast.

If nothing else, Sandy probably ensured, at least for a while, that people will pay more heed to mere Category 1 hurricanes, whose winds of 74 to 95 mph often do not command the respect paid to more impressive "major" hurricanes in Categories 3, 4 and 5. When a storm strikes a densely populated area, or one with a lot of trees or buildings that cannot withstand high winds and high water, there is no such thing as a "minor" hurricane.

I did not personally experience Sandy's full force. I was in Florida when it brushed us on its way northward through the Bahamas. We had a blustery, rainy evening in Fort Lauderdale, and that was about it. I lived through the rest of the storm vicariously, checking in frequently with two daughters who live in Manhattan (the upper part of the borough, where the streets stayed dry and the lights stayed on), our firm's New York staff and our office in Scarsdale, which lost power briefly and internet access for a day, but which we kept closed for three days for safety reasons. My New York home went dark and is likely to stay that way for some time to come.

Regular readers know that I take a strong interest in the science, history, economics and politics of weather. It has been part of my makeup as long as I can remember. As a little boy one summer in New York's Catskills, I noticed, during a heavy rain, that the field outside our bungalow was so foggy that I could not see the trees on the far side. I slipped out of the house and walked across the field to see what it was like to be in the fog. I remember being surprised when I looked back and saw that, from my new vantage point, our bungalow had now vanished in the mist.

I also remember that my mother was not amused when I returned, utterly drenched, from being AWOL.

When I was a teenager, Hurricane Agnes swept up the East Coast. The winds gusted into the Bronx off Long Island Sound, and I had a lot of fun marching around in the rain. (Apparently, I was a slow learner.) But Agnes was not fun for people in the Appalachian hills of New York state and Pennsylvania, where the storm stalled and dumped feet of rain that caused devastating floods in places like Elmira and Corning, N.Y. My family happened to drive through those towns weeks later, during a summer vacation trip to Canada, and I saw house after house where people's lives had been emptied into the street, in the form of ruined belongings piled on the curb. That was when I realized that bad weather is not always fun.

Agnes was just a tropical storm by the time it reached the Northeast, but it was still the costliest U.S. storm in history to that date.

I can personally remember at least a dozen "once in a lifetime" weather events, though I did not experience all of them firsthand. Hurricane Camille, a Category 5 storm that presaged Katrina's later assault on New Orleans, was just a news story to me, though a college friend later described cowering all night in his family's car inside their Louisiana garage. Camille struck in 1969, a few months after the "Lindsay blizzard" crippled both Queens and the political hopes of New York City's mayor, John V. Lindsay. That was the last big snowstorm of my childhood.

April 3-4, 1974, brought a massive outbreak of tornadoes in the midsection of the country. I remember that event simply by the name "Xenia, Ohio," which was a small city that was nearly leveled by one of the twisters. I thought I might never see another such outbreak, but in April 2011, my staff and I were in Atlanta when an even worse cluster of twisters roared across the Deep South and Midwest. Some of us were stranded for a couple of days, but we escaped injury or damage, though many others were less fortunate.

We had "super storms" in the 1970s, too. In January 1978, a monstrously powerful low pressure system marched up the western flank of the Appalachians to the Great Lakes with blinding snow and 100-mph winds. I was living in Montana and was used to blizzards by then, but I was duly impressed. Less than two weeks later, a comparable storm struck New York City and New England. Though this was a powerful extra-tropical cyclone, meteorologists noted that its intensity gave it an eye-like structure reminiscent of a hurricane. People died in their cars along Interstate 95 when snow blocked the exhaust pipes as the occupants kept their engines running for warmth.

The 1980s were a fairly quiet time, if you discount the freak East Coast blizzard of April 7, 1982, which put down a foot of snow in New York City and nearly two feet in Albany, N.Y., where I was then working. The decade ended with Hurricane Hugo smashing into Charleston, S.C., and with a Christmas Eve snow and ice storm that stretched from Jacksonville, Fla., to the North Carolina coast. My family and I managed to get stuck in that storm when our car broke down in Jacksonville just as everything started to freeze. It was the first white Christmas on that stretch of coastline in a century.

Storms don't only happen in the eastern United States, of course. Others of equal severity happen all over the world with regularity. They may be "once in a lifetime" events, but they don't happen to be part of my personal lifetime. One that comes to mind is the Columbus Day storm that struck the Oregon coast with wind gusts up to 138 mph in 1962. Another is Hurricane Gilbert, a beautifully formed (meteorologically speaking) Category 5. It was a terrible disaster in the Caribbean and Mexico, killing more than 400 people in 1988.

The early 1990s brought a plethora of remarkable weather: the "Perfect Storm" of 1991, Hurricane Andrew in 1992, the "Storm of the Century" in 1993. The Perfect Storm had much in common, meteorologically, with Superstorm Sandy, as it was a merger of a tropical system with a strong extra-tropical cyclone. But the Storm of the Century was more like Sandy in the way it was predicted days in advance; disasters were declared and the Emergency Broadcast System was activated in many areas long before the first snowflake fell.

Many other storms have come and gone since then. Some have names we will long remember, like Katrina and Wilma and, now, Sandy; some have pedestrian monikers that fade into the history pages, like the Blizzard of 1996. We're getting better at naming our weather systems. Who is going to forget Snowmageddon?

Each of these weather phenomena was unique, so I suppose each could be called a "once in a lifetime" event. It is highly unlikely, though not impossible, that I will live to see another storm tide in New York Harbor like the one Sandy produced; it took just the right combination of wind speed, direction and duration, timing at high tide and phase of the moon to send water cascading through the Financial District's streets and into the Brooklyn-Battery Tunnel.

But extreme weather events are not rare and are hardly once in a lifetime experiences. People older than me have other memories, like the Labor Day hurricane that hit the Florida Keys in 1935, or the Long Island Express hurricane that sent a wall of water up Narragansett Bay and inundated downtown Providence, R.I., in 1938.

Nature regularly reminds us that she has the power to reshape our lives and our memories, no matter how we name the reminders.