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Asia Noise News

How Sound Wave Work Underwater?

Do You Know?

Acoustics is not only about sound propagation in the air, but also its propagation in the water. The study of sound propagation and how it behaves in water is called underwater acoustics. Underwater acoustics is a branch of science, and it has become a technology that has been used since World War I. Even before that, in 1490, Leonardo da Vinci has stated his theory in an article “if you stop your ship in the ocean and you put one side of a long tube into the water, then put your ear in the other side, you’ll listen the sound of the ship from great distance.” This indicates that underwater acoustics technology is already known for long time. 

In World War II, in military cases, underwater acoustics was used as a communication platform to channel information through the water. In 1925, underwater acoustics was used to measure ocean depth based on sound waves obtained — one of its usability is to find the plane crashed into the bottom of the sea. Time went by and many technologies developed and researches were performed.

One of the applications which can also be used for fisherman is fish-finder navigation tools. These tools can be used for fishers to find schools of fish in the ocean. We can also know the distance and the position of the school of fish from the ship based on the frequency range of the sound propagated.

In the industry, underwater acoustics has been applied to determine the presence of oil and gas in the sea. The method used is quite effective and efficient. In disaster management, early detection of a tsunami from the sea has been developed based on the propagation of infrasound detected from the seabed. In recent years, one technology that has attracted interest in many studies is the Autonomous Underwater Vehicle (AUV). AUV is an unmanned underwater vehicle, where the AUV can identify underwater biology and physics. The use of AUV can be the best choice in identifying the shape conditions of coastal waters because it can be operated in the long run. Besides, the use of AUV can also avoid damage to coral reefs and marine ecosystems.

The necessity for underwater research is quite high, especially for countries with vast oceans, such as Indonesia. Underwater acoustic research is needed in mining operations, observations on coral reefs, offshore oil exploration, and sea accidents.

The speed of a wave is the rate at which vibrations move through the medium. Sound moves at a faster pace in water and with long-distance than in air because the mechanical properties of water differ from the air. We know that the speed of sound wave propagation in the air is between 333 m/s and 340 m/s, the speed of sound waves in water is four times faster than the speed of sound in the air. The speed of sound waves in water ranges from 1500 m/s to 1520 m/s. We know that sound propagation occurs because of the ups and downs of particles in a medium. At sea, the deeper the depth of the sea, the higher the pressure. High-pressure water particles will be compressed so that they continue to propagate the sound without losing much energy. Besides, the density in water is higher than the density in the air. This causes the sound can travel fast and far away in the water. Unfortunately, the speed of sound in seawater is not a constant value. It varies by a small amount (a few percent) from place to place, season to season, morning to evening, and with water depth. Although the variations in the speed of sound are not large, they have important effects on how sound travels in the ocean. However, the temperature in seawater also affects the speed of sound waves, warm water travels faster and farther than colder water. 

There are three layers in the sea, based on its temperature, namely mixed water, thermocline, and deep water. In the thermocline, temperature decreases rapidly from the mixed upper layer of the ocean to much colder deep water. In the thermocline, the speed of sound waves decreases with the depth of the sea. In the layer below the thermocline, the temperature becomes constant again, and the pressure increases. In this layer, the speed of the sound waves again increases with the depth of the sea.

Temperature ⇢

As we know, wavelength is inversely proportional to frequency.

As can be seen in the equation above, the lower the frequency the longer the wavelength. Therefore, a 20 Hz sound wave is 75 m long in the water whereas a 20 Hz sound wave in air is only 17 m long in the air. Generally, the sensor used to capture underwater sound is a hydrophone or underwater microphone.

Decibels as a unit of sound pressure is the ratio between the pressure measurement and the reference pressure. Note that the reference pressure in the air with water is different. Therefore, 150 dB of sound in water is not the same as 150 dB of sound in air. In air, the reference pressure is 20μPa while in water the reference pressure is 1μPa. Based on the Sound Pressure Level equation, the conversion value of dB in air to water is

The characteristic impedance of water is about 3600 times that of air then

Therefore, the air to water conversion factor is

For example, if the sound of a jet engine in the air is 135 dB then the water is 197 dB in water.

Written by:

Adetia Alfadenata

Acoustic Engineer

Geonoise Indonesia

support.id@geonoise.asia

Reference:

  • Urick, Robert J.1983.” Principal of Underwater Sound/3rd Edition”.McGraw-Hill Book Company
  • Nieukirk, Sharon.” Understandig Ocean Acoustic”.NOAA Ocean explorer Webmaster
  • Singh H, Roman C, Pizarro O, Eustice R. Advances in High Resolution Imaging from Underwater Vehicles. In: Thrun S, Brooks R, Durrant-Whyte H, editors. Robotics Research. vol. 28 of Springer Tracts in Advanced Robotics. Springer Berlin Heidelberg; 2007. p. 430–448
  • Pike, John.  “Underwater Acoustic”. Diakses secara online melalui https://fas.org/man/dod-101/sys/ship/acoustics.htm
  • Discovery of Sound in the Sea.”How does sound in air differ from sound in water?” diakses secara online melalui https://dosits.org/science/sounds-in-the-sea/how-does-sound-in-air-differ-from-sound-in-water/
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Noise and Vibration Product News

NoiseCompass, Noise monitoring with direction

Categories
Noise and Vibration Product News

NoiseCompass, Noise monitoring with direction

Noise monitoring with direction

One of the greatest challenges with unattended noise monitoring is to ensure that the monitored site really is the source being measured.

What is making the noise?

Is it the construction site, a nearby railway or an aircraft?

Read more about Noise Compass.

Categories
Asia Noise News Building Accoustics

The Colors of The Noise

Sound is a collection of random signals that have certain physical characteristics that depend on the sound source. One of the physical characteristics of sound can be seen from the spectrum formed. There is a lot of noise that can be distinguished based on the spectrum character, such as White Noise, Pink Noise, Brownian Noise, Blue Noise, Violet Noise, Gray Noise, and others. In general, what is often used is White Noise, Pink Noise, and Brownian Noise both in measurement and audio testing.

Many people are very familiar with White Noise, usually, the static sound from the Air Conditioner that delivers us to sleep by disguising background noise is always considered White Noise even though technically what we hear from the Air Conditioner fan rotation is not White Noise. Many of the sounds we associate with White Noise are actually Pink Noise, Brownian Noise, Green Noise, or Blue Noise. In the world of audio engineering, there are various types of noise colors with their own unique spectrum, this is produced to give a rich impression on music arrangements, relaxation, and so forth. So, this article will explain that static noise is not always White Noise.

Here are some sound colors that are quite familiar and often discussed in the world of audio engineering:

  1. White Noise

The most commonly mentioned noisy color in everyday life is White Noise. White Noise is called “White” as a symbolization of a white light containing all frequencies evenly or flatly in mathematical calculations. It is said mathematically because, in reality, it is not perfectly flat. The White Noise calculation pattern is evenly distributed if it is calculated using the following equation:

So in the case of White Noise, the signal power becomes:

The resulting spectrum is in the form of a constant straight line like the following graph,

Keep in mind that the graph shown is a logarithmic function and not a linear function where the frequency range at high frequencies is wider than the frequency range at low frequency. Here is a White Noise that can be heard:

https://soundcloud.com/betabayu-santika/betabayus-white-noise
2. Pink Noise

Proportionally the pink noise spectrum is seen to decrease on a logarithmic scale but it has equal power in bands that are proportionally wide. This means that pink noise would have equal power in the frequency range from 40 to 60 Hz as in the band from 4000 to 6000 Hz. Since humans hear in such a proportional space, where a doubling of frequency (an octave) is perceived the same regardless of actual frequency (40–60 Hz is heard as the same interval and distance as 4000–6000 Hz), every octave contains the same amount of energy and thus pink noise is often used as a reference signal in audio engineering. The spectral power density, compared with white noise, decreases by 3 dB per octave (density proportional to 1/f ). For this reason, pink noise is often called “1/f noise”. Some people associate pink with red and white where pink is brighter than red and fainter than white so that it is described as a decreased spectrum with values close to a ~ 1. Mathematically, Pink Noise can be calculated using the formulation below:

The depiction of the curve produced by Pink Noise is as follows:

Pink Noise will heard like the following audio file below,

https://soundcloud.com/betabayu-santika/betabayus-pink-noise
3. Brownian Noise (Red Noise)

Brownian Noise color has several names, some people call it Brown Noise, Brownian Noise, or Red Noise. Brownian was discovered by Robert Brown, the inventor of Brownian Motion (Random Walk or Drunkard’s Walk) where the Noise produced by Brownian Motion is the same as Red Noise / Brown Noise. Described as a red light that is darker than Pink and White, the spectrum formed has the characteristic of a sharp decrease that exceeds a decrease in Pink Noise (1 / f2 or a decrease of 6 dB per octave). Visually the Red Noise value is the boundary of the Pink Noise, together with the White Noise, so the spectrum curve formed is as follows:

Brownian Noise will sound like the following audio file  below:

https://soundcloud.com/betabayu-santika/betabayus-brown-noise
4. Blue Noise (Azure Noise)

If Red Noise and Pink Noise have a decreased character, then Blue Noise is the opposite. Blue Noise has an uphill spectrum curve characteristic that is inversely proportional to Pink Noise. Blue noise’s power density increases 3 dB per octave with increasing frequency (density proportional to f ) over a finite frequency range. In computer graphics, the term “blue noise” is sometimes used more loosely as any noise with minimal low-frequency components and no concentrated spikes in energy. This can be a good noise for dithering. Cherenkov radiation is a naturally occurring example of almost perfect blue noise, with the power density growing linearly with frequency over spectrum regions where the permeability of the index of refraction of the medium is approximately constant. The exact density spectrum is given by the Frank–Tamm formula. In this case, the finiteness of the frequency range comes from the finiteness of the range over which a material can have a refractive index greater than unity. Cherenkov radiation also appears as a bright blue color, for these reasons.

The curve produced by Blue Noise is as follows:

Blue Noise will sound like the following audio file  below:

https://soundcloud.com/betabayu-santika/betabayus-blue-noise
5. Violet Noise (Purple Noise)

If Blue Noise is the opposite of Pink Noise, then Violet can be categorized as the opposite of Red or Brownian Noise. This can be seen from the addition of the power density of Violet Noise which is 6 dB per octave with increasing frequency value. The proportional density of Violet Noise or often also called Purple Noise is f2 over a finite frequency range. Violet Noise is also known as differentiated white noise, due to its being the result of the differentiation of a white noise signal.

The curve produced by Violet Noise is as follows:

Violet Noise will sound like the following audio file  below:

https://soundcloud.com/betabayu-santika/betabayus-violet-noise
6. Grey Noise

Gray Noise is a randomized White Noise that is correlated with the same psychoacoustic noise curve or can be said to be an inverse A-weighting curve, with a specific frequency range that gives the impression or perception that this sounds equally loud at all frequencies. This is in contrast to standard white noise which has equal strength over a linear scale of frequencies but is not perceived as being equally loud due to biases in the human equal-loudness contour.

The curve produced by Grey Noise is as follows:

Grey Noise will sound like the following audio file  below:

https://soundcloud.com/betabayu-santika/betabayus-grey-noise

Written by:

Betabayu Santika

Acoustic Design Engineer

Geonoise Indonesia

Beta@geonoise.asia

 

Sources:

Pics: Noise Curves By Warrakkk – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19274696

Hartmann, William M. Signals, sound, and sensation. Springer Science & Business Media, 2004.

“Federal Standard 1037C”. Institute for Telecommunication Sciences. Institute for Telecommunication Sciences, National Telecommunications and Information Administration (ITS-NTIA). Retrieved 16 January 2018.

Lau, Daniel Leo; Arce, Gonzalo R.; Gallagher, Neal C. (1998), “Green-noise digital halftoning”, Proceedings of the IEEE, 86 (12): 2424–42, doi:10.1109/5.735449

Joseph S. Wisniewski (7 October 1996). “Colors of noise pseudo FAQ, version 1.3”. Newsgroup: comp.dsp. Archived from the original on 30 April 2011. Retrieved 1 March 2011.

 

Categories
Asia Noise News Building Accoustics Noise and Vibration Product News

The Nano-guitar String that Plays Itself

Scientists at Lancaster University and the University of Oxford have created a nano-electronic circuit which vibrates without any external force.

Using a tiny suspended wire, resembling a vibrating guitar string, their experiment shows how a simple nano-device can generate motion directly from an electrical current.

To create the device, the researchers took a carbon nanotube, which is wire with a diameter of about 3 nanometers, roughly 100,000 times thinner than a guitar string. They mounted it on metal supports at each end, and then cooled it to a temperature of 0.02 degrees above absolute zero. The central part of the wire was free to vibrate, which the researchers could detect by passing a current through it and measuring a change in electrical resistance.

Just as a guitar string vibrates when it is plucked, the wire vibrates when it is forced into motion by an oscillating voltage. This was exactly as the researchers expected.

The surprise came when they repeated the experiment without the forcing voltage. Under the right conditions, the wire oscillated of its own accord.

The nano-guitar string was playing itself.

Lead researcher Dr Edward Laird of Lancaster University said: “It took us a while to work out what was causing the vibrations, but we eventually understood. In such a tiny device, it is important that an electrical current consists of individual electrons. The electrons hop one by one onto the wire, each giving it a small push. Usually these pushes are random, but we realized that when you control the parameters just right, they will synchronize and generate an oscillation.”

So what note does the nano-guitar play?

“The nanotube is far thinner than a guitar string, so it oscillates at much higher frequency — well into the ultrasound range so no human would be able to hear it.

“However, we can still assign it a note. Its frequency is 231 million hertz, which means it’s an A string, pitched 21 octaves above standard tuning.”

The nano-oscillator could be used to amplify tiny forces, such as in novel microscopes, or to measure the viscosity of exotic quantum fluids. These experiments will be pursued in a new laboratory that Dr Laird is setting up in the Physics Department at Lancaster, supported by a €2.7M grant from the European Union.

Credit: https://www.lancaster.ac.uk/news/the-nano-guitar-string-that-plays-itself

Written by: Phawin Phanudom