Category Sound

            The contributions made by the Ancient Greeks to the world of acoustics are unparalleled.

            One of them is the amphitheatre of Epidaurus, designed by Polykleitos the Younger, around the 4th century BC. The theatre had a tripartite structure, that is, an orchestra, auditorium, and stage building. There were also 34 rows of limestone seats that could accommodate 14,000 people.

            The acoustics of the place was such that a performer standing on the open-air stage could be heard in the back rows, almost 60 metres away.

            Experts attribute this rare feature to the arrangement of seats. According to them, the stepped row of seat structure was perfectly shaped so as to act as an acoustic filter. The seats suppressed low-frequency sounds that formed the major component background noise like the murmur of a crowd. This in turn, reflected high-frequency noises of the performers off the seats, and back toward the seated audience.

            We have read that most humans can hear sounds between 20 and 20,000 hertz. Ultrasounds are those above this limit or specifically, above 20,000 hertz. They are not different from the normal sound in terms of physical properties. But the only difference is that they can be heard and produced by only a few animals like bats, moths, dolphins etc. and not by humans. In other words, the range of ultrasound begins where our sonic range ends.

            The uses of ultrasound can be seen in electronic, navigational, industrial, medicinal and security applications. Let’s look at a few examples in brief.

            In some cases, ultrasound is used to detect objects or to measure distances. They are also helpful in testing products and structures.

            In addition to these, ultrasound is used in the field of medicine to view the internal organs of our body.

            There are quite a lot of animals that make use of ultrasound for purposes like navigation, communication, catching preys, avoiding obstacles etc. Let’s look at a few examples.

            Marine animals like dolphins and toothed whales are very famous for their sonar, which employs sounds from 250 Hz to 220 kHz.

            Then, there are the bats that have a variety of ultrasonic ranging techniques. They enable the mammals to detect preys and avoid obstacles, even in thick darkness.

            There are also many insects that have excellent ultrasonic hearing abilities. For example, like moths, beetles, lacewings, praying mantis etc. They use their skill to listen to echo locating bats. Upon hearing a bat, they make plans to escape being caught.

            Another group of animals that are responsive to ultrasonic sounds is mice. The next in the category of animals that can perceive high frequencies are dogs and cats.

             Sonar is the short form for Sound Navigation and Ranging. It is an ultrasonic system used in ships and other vessels for navigation, and locating objects underwater.

            As we know, sound waves travel faster than light through water. However, ordinary sound waves cannot travel longer distances, only ultrasonic waves can. Due to their high frequency and short wavelength, ultrasonic waves penetrate water to very long distances and it is this feature that is utilized in sonar.

            Let’s see how this works, in the case of a submarine. While deep in water, the vessel finds its way by sending out pulses of ultrasound and listening to the echoes. It is just like the phenomenon of echolocation in bats. Depending on the time it takes for the echoes to come back, the navigator of the vessel can figure out if there are any ships, submarines, or other obstacles nearby.

            This technique is also used by ships to calculate how deep the waters are by firing sound beams straight downward.

           The importance of ears is something we are all aware of. This organ is the receiver of sound in the human body and plays a very important role in communication.

           Structurally, the ear is made of three sections- the outer ear, the middle ear, and the inner ear. The outer ear is the visible external part which consists of the pinna and the ear canal. It gathers sounds and sends them to the middle ear through the ear canal.

           The middle ear is an air-filled cavity that turns sound waves into vibrations. It is separated from the external ear by the eardrum, a thin, cone-shaped piece of tissue. Past this drum, there are three small, but important bones in the middle ear. Collectively known as the ossicles, they are the malleus, incus and stapes.

           The third and final part of the ear is the inner ear. It consists of a tiny organ called the cochlea that converts the vibrations from the middle ear to nerve impulses. These impulses then travel to the brain, from where it gets converted as sound.  

            We already saw how our ears collect sounds from outside and conduct it to the inner parts. The sound waves are at first collected by the outer ear and passed through the ear canal. It then causes the eardrum to vibrate. Subsequently, these vibrations are transmitted to the cochlea by the tiny bones of the middle ear.

             Cochlea is a snail-shaped structure filled with fluid situated in the inner ear. An elastic partition runs from the beginning to the end of the cochlea, splitting it into an upper and lower part. This partition is called the basilar membrane because it serves as the base, or ground floor, on which key hearing structures sit.

              Once the vibrations cause the fluid inside the cochlea to ripple, a travelling wave forms along the basilar membrane. Hair cells sitting on top of the basilar membrane ride the wave. The movement of hair cells eventually results in the formation of electrical signals.

              The auditory nerve carries this electrical signal to the brain, which turns it into a sound that we understand.