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# Ultrasound

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We cannot hear all of the sound that enters our ear. Our ears are built to hear only a certain range of frequencies (or wavelengths) of sound waves and we will not register others. An everyday example of this can be seen in the use of dog whistles, which make an extremely high-pitched noise that dogs can hear but we cannot. This is because dogs have ears that are sensitive to higher frequency sound waves. These sound waves are called ultrasound - they have a higher frequency than we can hear.

## Ultrasound definition

Ultrasound is sound with frequency above the human hearing range - we say that it is too high-pitched for humans to hear. Ultrasound has a frequency above 20 kHz. Someone with perfect hearing has a range of hearing from 20Hz up to 20 kHz, although most people have a hearing range that is within these extreme values. Humans are particularly sensitive to frequencies in the range 2000 Hz to 5000 Hz.

## Ultrasound examples

Although we cannot hear ultrasound, it can be very useful in many different ways:

### Ultrasound examples: Sonar

Sonar - also known as echolocation is a method for finding the distance to objects and surfaces that are difficult to reach or measure. The time that it takes for the ultrasound to bounce back from the object can be used to find the distance.

Fishing boats use ultrasound waves to find the position of fish beneath them, istockphoto

For example, fishing boats sometimes carry sonar equipment so that they can determine the depth of water that they are travelling through. Ultrasound pulses are emitted from the boat by a transmitter. The time taken for the pulses to reach the seabed and reflect back to the boat is measured using a detector (the ultrasound waves could also be directed at a shoal of fish in order to see how far away they to aid in catching them). The ultrasound signal is converted into an electrical pulse and the time taken between releasing the waves and the largest value of the electrical signal can be used along with the speed that the waves move through the water to find how deep the water is.

A large fishing boat is travelling along somewhere in the Atlantic ocean and wishes to know what the depth of the water is. They send a pulse of ultrasound waves towards the seabed and they get a peak in the electrical signal in the detector$12seconds$later. How deep is the ocean at this point (the speed of ultra sound waves in water is approximately$1500\mathrm{m}/\mathrm{s}$)?

We know the time that the ultrasound waves were travelling for and also their speed in water so we can find the distance travelled by using the following equation:

$s=vt$$s=vt$

Or in words,

$\mathrm{Distance}=\mathrm{speed}×\mathrm{time}$

in which$s$is the distance,$v$is the speed and$t$is time.

The values for the speed and time given in the question can be used to find the distance:

$s=vt=1500\mathrm{m}/\mathrm{s}×12\mathrm{s}=18000\mathrm{m}$.

However, the ultrasound waves have to travel to the seabed and back up to the boat, so the depth d will be equal to half of the total distance travelled:

$d=\frac{s}{2}=9000\mathrm{m}$

### Ultrasound scans

Ultrasound can also be used for foetal scanning - also known as pre-natal scanning. It enables doctors to check the sex of the babies before they are born. It can also be used to see if a baby is in good health and to find what position it is in.

Ultrasound scans are used to produce images of babies and foetuses while still in the womb, Wikimedia commons

The process of foetal scanning involves moving a probe over the mother's stomach, which sends ultrasound waves into her body. Ultrasound waves are partially reflected whenever they reach a boundary between two different materials. In this case, some of the ultrasound signal will be sent back when it reaches the baby's body, which is a different material to the fluid surrounding it. The probe then detects the returning ultrasound radiation. It can be moved around to find how long it takes for the ultrasound to return at different points, which can be used along with the known speed of the waves in the fluid to produce an image of the baby.

## Ultrasound uses

In addition to the examples mentioned above, ultrasound has many other uses both in medicine and industry.

### Ultrasound uses: Medical applications

Ultrasound is not just used for people having babies, it can also be used for medical imaging purposes. For example, people who who are having some kind of problem with their internal organs, such as damaged lungs or kidneys.

You may have already learned about how X-rays can be used to identify internal problems with the body. There is much less risk of damage to your internal organs when using ultrasound as opposed to X-rays.

### Ultrasound uses: Quality control

Ultrasound is also used in industry to check for irregularities in materials, such as metal castings. This involves sending ultrasound pulses through the material, when the waves reach a crack in the structure of the object, they will be partially reflected. This means that there will be a small pulse in the electrical signal detected that arrives at an earlier time than the time it would take for the waves to travel to the end of the object and back. This can be used along with the speed of the ultrasound waves in the material to find the exact position of the irregularity.

Ultrasound equipment can be used to detect cracks in a pipe, zetec

Ultrasound is used in medicine for a variety of different medical problems. This is because it has some key advantages over other medical techniques:

• Ultrasound can be used to form images of internal organs without the need to operate on patients. This can save a lot of time and money and prevents the risks associated with invasive surgery.
• The equipment used for ultrasound is inexpensive compared to other types of medical imaging equipment. It is also quite easy to transport and operate.
• As mentioned above, ultrasound is not very harmful to internal organs (or for foetuses when it is used for foetal scanning). It does not cause any harm to the tissue in comparison with other techniques that require the patient to be exposed to ionising radiation such as X-rays which can sometimes be very damaging to cells in the body.
• Ultrasound can be used to capture images of soft tissues which will not show up when using other medical imaging techniques such as X-rays. This is because the pictures produced come from how ultrasound is partially reflected at the boundaries between materials and even if the electrical signal produced is small, the boundary can still be identified. On the other hand, X-rays work on the basis of forming a picture by passing X-ray radiation through the body and forming an image from the points where no radiation passes through (for example if it is blocked by a bone), so soft tissues will not be identified.

## Ultrasound - Key takeaways

• Ultrasound is sound with a frequency above 20 kHz - this is the upper limit of the range of sound frequencies that humans can hear.
• Ultrasound is very useful in finding where objects and surfaces are that cannot be easily reached or cannot be measured directly.

• Images of babies can be made by the use of ultrasound foetal scanning. This involves directing ultrasound waves at the foetus and using the intensity of reflected waves at different points to form an image.

• There are many advantages of using ultrasound rather than other medical techniques as the equipment of cheaper, operations are not needed and ultrasound is much safer than many other methods.

Ultrasound is sound with frequency above the human hearing range.

Ultrasound has many applications, such as locating the position of fish below a fishing boat and also foetal scanning.

The advantages of ultrasound are varied. One of the main advantages of ultrasound is that it can be used for non-invasive medical imaging. It can also be used to visualise objects on the other side of physical barriers such as thick walls. Ultrasound has a number of properties that mean that it is useful for probing materials and gaining useful information from them by studying the reflected signals.

Ultrasound refers to acoustic waves that cannot be heard by humans. Therefore there aren't really many distinctions to make between different ultrasound except perhaps their sources and their particular frequencies. In medical imaging, however, different names are given to the various uses of ultrasound.

The frequency of ultrasound is any frequency above approximately 20 kHz.

## Final Ultrasound Quiz

Question

What is ultrasound?

Ultrasound is sound with frequency above the human hearing range.

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Question

What is the range of frequency of human hearing?

20 Hz - 20 kHz

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Question

What range of frequency sounds are humans particularly sensitive to?

2000 Hz - 5000 Hz

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Question

What is echolocalisation?

It is a method for finding the distance to objects and surfaces by the use of ultrasound.

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Question

Can dogs or humans hear higher frequencies?

Dogs

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Question

Which of these options is not a use of ultrasound on a fishing boat?

Finding the distance to the shore

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Question

Why can humans not hear the sound coming from a dog whistle?

The frequency is outside our range of hearing

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Question

Ultrasound is dangerous for human tissue. Is this statement true or false?

False

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Question

Is a foetal image created using ultrasound or X-rays?

Ultrasound

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Question

A fishing boat sends an ultrasound pulse towards a shoal of fish and it takes 1 s to return (ultrasound waves travel at 1500 m/s through water). How far away are the fish?

750 m

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Question

Is there a lowest frequency that humans can hear?

Yes, 20 Hz is the lower limit of our frequency range

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Question

Does everyone human have a full range of hearing?

No, it depends on the conditions of the ears

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Question

An ultrasound pulse is sent through a metal beam. The time taken to pass through the entire beam and back in known. How will this time vary if there is a crack along the path?

It will decrease

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Question

Can ultrasound be used for scans of internal organs?

Yes

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Question

Does ultrasound pass through soft tissue?

Yes

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