Simple Science.
How to measure the depth of the ocean.
Sound.
Sound is vibration of something. We hear sound because our ears are capable of feeling vibrations in the air. Every
sound must have a ‘source’ – something that causes the air to vibrate – an explosion, a musical instrument, a door slamming.
In order to appreciate the fact that there is a sound, we need a ‘receiver’, something that is capable of intercepting and
interpreting the vibrations in the air – our ears, a microphone. And in order to get the sound from one place to another,
we need a ‘medium’ between the source and the receiver – the most common medium for humans is the air, but also
includes solid items like masonry, glass, plastic etc. (Ever heard the conversation in the next door hotel room? – fire sirens
through a plate glass window?)
The sound is transmitted through the air by the molecules of air essentially banging together, moving backward and forward.
A molecule of air right next to the source is pushed outward by the source, and bangs into its neighbour, its neighbour in
turn bangs into its neighbour, etc.. And each one of those neighbours bangs back at the molecule that banged
into it in the first place. The molecules of air move back and forth. Imagine one of those educational toys with the 5 or 6
metal balls all in a line dangling from a frame, where you draw one ball aside, and let it go. The ball at the far end springs
outward. It’s something like that. Now, place you ear at the far end of the toy, and set it in motion. You will ‘receive’ a smart
smack in the ear!
This back and forth motion does not happen instantaneously. It takes time for each molecule to move from where it was
to go bang into its neighbor, then turn around and bang into the one that knocked into it in the first place. Watching fireworks
from the distance – you see the beautiful effect and then a little time later you hear the explosion. (Light travels very
fast – almost instantaneously.) Sound travels at about 1,100 feet (340 meters) per second in air.
A similar scenario holds true for sound in the ocean, although the speed of sound is considerably faster, at about
4,800 feet (1,500 meters) per second.
One of the earliest uses of sound in the ocean was in calculating the depth of the ocean.
If we make a noise in the ocean, the sound will travel outwards in all directions (in 3 dimensions). If the noise is of short
duration, like a ‘crack’ or a ‘beep’ or a ‘ping’, (as opposed to ‘rolling thunder’), the molecules of water will vibrate for just a
few vibrations, and then settle back to rest, having passed their vibrations on to their neighbor. The sound will be travelling
outwards like an ever-expanding balloon, at 4,800 feet every second.
Sound is vibration of something. We hear sound because our ears are capable of feeling vibrations in the air. Every
sound must have a ‘source’ – something that causes the air to vibrate – an explosion, a musical instrument, a door slamming.
In order to appreciate the fact that there is a sound, we need a ‘receiver’, something that is capable of intercepting and
interpreting the vibrations in the air – our ears, a microphone. And in order to get the sound from one place to another,
we need a ‘medium’ between the source and the receiver – the most common medium for humans is the air, but also
includes solid items like masonry, glass, plastic etc. (Ever heard the conversation in the next door hotel room? – fire sirens
through a plate glass window?)
The sound is transmitted through the air by the molecules of air essentially banging together, moving backward and forward.
A molecule of air right next to the source is pushed outward by the source, and bangs into its neighbour, its neighbour in
turn bangs into its neighbour, etc.. And each one of those neighbours bangs back at the molecule that banged
into it in the first place. The molecules of air move back and forth. Imagine one of those educational toys with the 5 or 6
metal balls all in a line dangling from a frame, where you draw one ball aside, and let it go. The ball at the far end springs
outward. It’s something like that. Now, place you ear at the far end of the toy, and set it in motion. You will ‘receive’ a smart
smack in the ear!
This back and forth motion does not happen instantaneously. It takes time for each molecule to move from where it was
to go bang into its neighbor, then turn around and bang into the one that knocked into it in the first place. Watching fireworks
from the distance – you see the beautiful effect and then a little time later you hear the explosion. (Light travels very
fast – almost instantaneously.) Sound travels at about 1,100 feet (340 meters) per second in air.
A similar scenario holds true for sound in the ocean, although the speed of sound is considerably faster, at about
4,800 feet (1,500 meters) per second.
One of the earliest uses of sound in the ocean was in calculating the depth of the ocean.
If we make a noise in the ocean, the sound will travel outwards in all directions (in 3 dimensions). If the noise is of short
duration, like a ‘crack’ or a ‘beep’ or a ‘ping’, (as opposed to ‘rolling thunder’), the molecules of water will vibrate for just a
few vibrations, and then settle back to rest, having passed their vibrations on to their neighbor. The sound will be travelling
outwards like an ever-expanding balloon, at 4,800 feet every second.
Think of these arrows as ‘rays’ of sound. There is one ray that is travelling directly downwards towards the bottom of the
ocean, where it will eventually hit the ocean bed. Just like when you throw a ball at a wall, the sound will bounce back,
(just like an echo), and start heading upwards to the surface. And strange as it may seem, when this ray reaches the surface,
it is reflected again, heading back down into the depths. …and so on. (Like when you hit a billiard ball really hard, and it bounces back and forth across the table several times!)
A sound source produces a fixed amount of energy, (loudness), and as this energy spreads out in 3 dimensions it becomes
diluted more and more. (The skin of a balloon gets thinner and thinner as it expands when it is being inflated. The total
amount of latex (energy) hasn’t changed but the thickness (loudness) has diminished.) So the amount of energy (loudness)
that reaches the ocean bed beneath the ship is considerably less than when it started. In addition, when the sound is reflected
at the ocean bed, some energy is lost into the earth. So the sound gets weaker and weaker, until it can no longer be ‘heard’
above the background sounds of the ocean. (.... and the billiard ball finally comes to rest.)
If we have a gizmo, attached to the bottom of a ship, which can produce a short, loud ‘ping’, and we have another gizmo,
also attached to the bottom of the ship, which is an underwater microphone so we can hear the returning echo, and we
have a very accurate stopwatch, we can calculate the depth of the ocean beneath the ship. If we start the stopwatch when
we send out a ‘ping’, and stop the stopwatch when we hear the return sound, we can determine how far the sound has
traveled going to the bottom and back. For example, if the time between the outgoing ‘ping’ and the return echo is 5 seconds,
then the sound has traveled 5 x 4800 feet. This is 24,000 feet. But this is for a 2-way journey down to the bottom and back
up to the surface, so the ocean depth is a half of 24,000 feet, which is 12,000 feet, (which is about 2 ¼ miles or 2,000 fathoms).
The above process has been refined and automated to produce the Depth Sounders that are found in most boats today.
During the late 1940’s - early '50s, SOFAR assisted in the development of the Precision Depth Recorder, which used a tuning fork to
accurately control the rate of outgoing ‘pings’ and the display of the returning echoes on a drum recorder.
Posted Jan. 15th 2014
ocean, where it will eventually hit the ocean bed. Just like when you throw a ball at a wall, the sound will bounce back,
(just like an echo), and start heading upwards to the surface. And strange as it may seem, when this ray reaches the surface,
it is reflected again, heading back down into the depths. …and so on. (Like when you hit a billiard ball really hard, and it bounces back and forth across the table several times!)
A sound source produces a fixed amount of energy, (loudness), and as this energy spreads out in 3 dimensions it becomes
diluted more and more. (The skin of a balloon gets thinner and thinner as it expands when it is being inflated. The total
amount of latex (energy) hasn’t changed but the thickness (loudness) has diminished.) So the amount of energy (loudness)
that reaches the ocean bed beneath the ship is considerably less than when it started. In addition, when the sound is reflected
at the ocean bed, some energy is lost into the earth. So the sound gets weaker and weaker, until it can no longer be ‘heard’
above the background sounds of the ocean. (.... and the billiard ball finally comes to rest.)
If we have a gizmo, attached to the bottom of a ship, which can produce a short, loud ‘ping’, and we have another gizmo,
also attached to the bottom of the ship, which is an underwater microphone so we can hear the returning echo, and we
have a very accurate stopwatch, we can calculate the depth of the ocean beneath the ship. If we start the stopwatch when
we send out a ‘ping’, and stop the stopwatch when we hear the return sound, we can determine how far the sound has
traveled going to the bottom and back. For example, if the time between the outgoing ‘ping’ and the return echo is 5 seconds,
then the sound has traveled 5 x 4800 feet. This is 24,000 feet. But this is for a 2-way journey down to the bottom and back
up to the surface, so the ocean depth is a half of 24,000 feet, which is 12,000 feet, (which is about 2 ¼ miles or 2,000 fathoms).
The above process has been refined and automated to produce the Depth Sounders that are found in most boats today.
During the late 1940’s - early '50s, SOFAR assisted in the development of the Precision Depth Recorder, which used a tuning fork to
accurately control the rate of outgoing ‘pings’ and the display of the returning echoes on a drum recorder.
Posted Jan. 15th 2014