Dhwani - Sound

Sound is an aspect of vibrating objects. Science, in both ancient and modern context recognizes that the sounds we hear in the natural world are produced by vibration. It is perceived through the act of hearing only. It is special to earth as we are surrounded by a medium called atmosphere consisting of  various gases which help in propagation of the sound. Sound cannot be transmitted or heard in vacuum, like in space. 

A vibrating body produces disturbances in the surrounding air that causes the sound to be experienced through the act of hearing. Over time, we tend to associate various sounds with various objects both living (sounds of humans and animals) and non-living (sounds of machines or objects)

Vibration

Scientists affirm that sound is produced because of vibration. Vibration is a form of disturbance in an object which occurs in uniform or non-uniform oscillation. Vibrations are felt in day to day life and are considered both desirable, like in the case of the vibrating mobile phone or undesirable as in sitting inside a truck with the engine vibrating or as felt on ground and buildings during an earthquake.

Vibration is produced by striking any object with force. Sound cannot be generated without vibration. Also a strong vibration is required for sound generation. Hence when any object is struck, it starts to vibrate and these vibrations are passed on to the surrounding air. In consequence the air around the object starts to vibrate.

We can see the aspect of vibration in string instruments like the Tanpura, Sitar, Veena, Guitar. When the strings of a well-tuned Tanpura or Sitar is plucked, the string starts to vibrate by moving up and down from its position and starts produces sound. This process continues till the string loses energy to vibrate. 

Similarly, If we place a small amount of sand on the skin of a Tabla and strike the Tabla we see that the grains of sand start to move or dance. This is proof that the skin of the Tabla vibrates and produces sound. The initial force used in plucking the string or striking the Tabla will induce vibrations. The vibrations will gradually reduce and become zero after sometime.

All forms of vibration do not produce sound. There is a threshold limit after which sound is produced. When the strings of a Tanpura are slackened, it does not produce sound. As and when it is tightened, there comes a point when we start to hear sweet sound. On tightening, further, the sound becomes clearer and brighter. On tightening, further, the string can break too. 

When we pluck the string of a Tanpura, the sting moves up first, then comes back to its original position and goes down an equal extent it went upwards and comes back to original position. This is referred to as one oscillation of the string. The string will oscillate many number of times, usually too fast to observe individual oscillations. Generally vibrations are measured as frequency or the number of oscillations per second.

Sound is also measured as a frequency and the unit of measurement is called Hertz or Hz which denoted the number of oscillations per second.

For e.g. When a sound has a frequency of 240 it means there are 240 oscillations per second.

The Human voice, a sound, is also produced by vibration of the Vocal Cords.

Propagation

Once the vibration is induced in an object, it starts to affect the surrounding medium. any object is either surrounded by gas, liquid, solid or Vacuum. As mentioned before, in vacuum, the vibration cannot be felt or heard from afar unless we go near the object and touch it. In the case of a vibrating object surrounded by gas, liquid, solid; the surrounding medium starts to vibrate in the same frequency. This aspect of transfer of the vibration from one body to another is called propagation. 

Sound can travel in gas, liquid, and solid. The vibration is propagated in the form of waves. When we throw a stone in a still water of a pond then waves are generated from the point of contact of stone with the water. 

In solids and liquids, sounds propagation happens through transverse waves which is perpendicular to the direction of propagation. 

A transverse wave is a moving wave that consists of oscillations occurring perpendicular (or right angled) to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y–z plane. Light is an example of a transverse wave. With regard to transverse waves in matter, the displacement of the medium is perpendicular to the direction of propagation of the wave. A ripple in a pond and a wave on a string are easily visualized as transverse waves. Transverse waves are waves that are oscillating perpendicularly to the direction of propagation. If you anchor one end of a ribbon or string and hold the other end in your hand, you can create transverse waves by moving your hand up and down. Notice though, that you can also launch waves by moving your hand side-to-side. This is an important point. There are two independent directions in which wave motion can occur. In this case, these are the y and z directions mentioned above. There are also antinodes, also known as crests and troughs, which are found in all waves. Transverse mechanical waves are also called "t-waves" or "shear waves".

In gases, the sound is propagated in longitudinal waves. In longitudinal waves the displacement of the medium is parallel to the propagation of the wave.  

Longitudinal waves, also known as "L-waves", are waves in which the displacement of the medium is in the same direction as, or the opposite direction to, the direction of travel of the wave. Mechanical longitudinal waves are also called compressional waves or compression waves because they produce compression and rarefaction when traveling through a medium

A single-frequency sound wave traveling through air will cause a sinusoidal pressure variation in the air. The air motion which accompanies the passage of the sound wave will be back and forth in the direction of the propagation of the sound. In the areas of increased pressure is known as compression and the area of decreased pressure is known as rarefaction. 

Propagation of sound is affected by

1. density of and pressure in the medium. This relationship, affected by temperature, determines the speed of sound within the medium.

2. The motion of the medium itself, e.g., wind. Independent of the motion of sound through the medium, if the medium is moving, the sound is further transported. 

3. The viscosity of the medium. This determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible.

When we are singing, or playing an instrument we produce a variety iof sounds. some sounds, like those of kids are high pitched or shrill and those of adult men sound low pitched. Pitch is a term used to denote frequency of the sound. Pitch is not just an experience of the ears, but also a characteristic of the object generating it. The Pitch of sound depends upon frequency of the oscillating object. When the frequency increases, the pitch increases and when the frequency decreases, the pitch decreases. Frequency is measured in Hertz or Hz. It is defined as the no of oscillations per second.

The below physical aspects are also important when discussing the oscillating frequency of any object.

Length of the Object

In simple terms, the frequency of sound emanating from a string is directly related to the Length of the string. Length of a string determines the time of oscillation and the frequency. As the length increases, the amplitude of oscillation increases and frequency reduces. As the length of the string decreases, the amplitude of oscillation reduces and the frequency increases. The relation between Amplitude of oscillation and length of string is not constant. As the force of strinking the string reduces, the time spent in oscillation reduces and the amplitude of oscillation reduces. But the frequency will not vary. Hence the Length of the string and frequency are inversely related. If a particular length of string produces a sound of 400 Hz, by reducing the length in half, we get a sound of 800 Hz

Thickness of the Object

When we study oscillations based on the thickness of the string, Thinner strings produce bright and higher frequency of sound, whereas thicker strings produce a dull and lower pitched sound.

Density of Object

The frequency also depends on the material of the object. If we take the example of the strings, we use Steel and brass strings. Even though they can be identical, the frequency produced will change. The reason being steel strings are lighter than the brass strings. 

Weight of Object

As the weight of the string increases, the frequency produced reduces. Scientist Mersenne had found out that if the weight increased four fold the frequency produced reduced by half. The weight of a string can increase by two methods, the thickness and density. The brass strings in comparison with steel strings are more dense hence brass strings are heavy and steel ones are light.

Tension in the Object

Every string has its own tension limits within which it will vibrate without breaking. Some tension is necessary to produce a clear and bright sound. On increasing the tension, the pitch of the sound produced increases and on reducing the tension, the pitch reduces. Very high tension can cause the string or skin to break in instrument.

Summary

1. The length of the string and frequency are inversely related. When length of the string is halved, the frequency doubles. When the length of the string is 1/3rd then the frequency triples.

2. Based on the tension in the string the frequency will vary proportional to the square of rate of increase in tension. If the tension increases by 2,3, or 4 times then the frequency will increase by 4,9,or 16 times. 

3. Based on the thickness of the string the frequency will vary inversely. When the thickness doubles, the frequency reduces by half

4. Based on the weight of the string, the frequency changes inversely to the square root of rate of weight increase. When the weight increases by 4 or 9 times, then the frequency reduces by ½ or 1/3rd 

Sound can be produced softly or loudly. When we sing softly, it is heard for a short distance. When we sing loudly, it is heard over a long distance. In music, sound can be obtained softly or loudly depending upon the force of the striking. This quality of sound is called amplitude. It is not affected by the frequency of the sound. The amplitude of sound for loud sound is larger compared to softly produced sounds. When the strings of the tanpura is plucked lightly, the amplitude of vibration is less and the sound produced is of low volume. Similarly, when the string is plucked with force, the amplitude of vibration increases producing a louder sound. Larger the displacement of the body or air from the mean position, larger the amplitude and larger the resulting sound. Large amplitudes are generated by high energy sources.

The speed of travel of sound will differ with each medium. The velocity of sound depends on

1. Density of medium

2. Pressure in medium

3. Temperature of medium

4. Movement of medium

5. Viscosity of medium

At Zero degrees temperature, speed of sound is 332 m/s in Air. And in hydrogen the speed of sound is 1233 m/s. the reason being hydrogen is more elastic. If the elasticity of the medium is high, then the speed of sound increases accordingly. Since metal is denser, the velocity of sound is high in it. The velocity of sound is fastest in metals followed by liquids and then in gas. 

The change of temperature also results in difference in speed of sound and instruments become out of sur. In cold, the voice sounds weak and in normal temperature it sounds very bright. Also, when we sing in the direction of wind, it is heard for a long distance, but when sung in opposing direction of wind, it is heard only for a short distance. 

Sound wave also gets reflected as light waves do. Bouncing back of sound wave from the surface of solid or liquid is called reflection of sound. Reflection of sound follows the Laws of Reflection of light waves. This means the angle of incident wave and reflected wave to the normal are equal. For reflection of sound a polished or rough and big obstacle is necessary. Reflection of sound is used in many devices. For example, megaphone, loudspeaker, bulb horn, stethoscope, hearing aid, sound board etc. 

Sound boards are used to send the sound towards audience in big hall or auditorium. This works on the basis of laws of reflection of sound waves. Sound board is a big concave board and is set in such a fashion behind the stage that speaker is at the focus. Sound coming from speaker falls over sound board and gets reflected towards the audience. As a result, the audience sitting in the hall even at far distance from the speaker can clearly hear what the speaker is saying. Additionally, the ceiling of the auditorium is also made curved so that it also acts like sound board. The curved surface of the ceiling reflects the sound waves and facilitates better hearing.

In Large halls without any sound proofing, sound undergoes multiple reflection and leads to what is called reverberation. Sound becomes too blurred and distorted to be heard in big concert halls because of reverberation. This can often lead to annoyance. To overcome this problem, sound absorbent materials, such as curtains, plant fibre, compressed fireboard, carpets, etc. are used in the auditorium. These materials absorb undesired reflected sound and reduce reverberation.

When sound waves enter from one medium to another medium, there is a change in the speed and direction of sound. This is called refraction of sound.

If the air above the earth is warmer than that at the surface, sound will be bent back downward toward the surface by refraction.

Sound propagates in all directions from a point source. Normally, only that which is initially directed toward the listener can be heard, but refraction can bend sound downward. Normally, only the direct sound is received. But refraction can add some additional sound, effectively amplifying the sound. Natural amplifiers can occur over cool lakes.

Early morning fishermen may be the persons most familiar with the refraction of sound. If one is near a lake before dawn, just as the sun rises over a cool lake, one can even hear the voice of someone from the opposite side of the lake. The cool water keeps the air near the water cool, but the early sun has begun to heat the air higher up, creating a "thermal inversion". The fact that the speed of sound is faster in warmer air bends some sound back downward toward you - sound that would not reach your ear under normal circumstances. This natural amplification over cool bodies of water is one of the few natural examples of sound refraction.

The bending of waves around small obstacles and the spreading out of waves beyond small openings is known as Diffraction of sound. Important parts of our experience with sound involve diffraction. The fact that you can hear sounds around corners and around barriers involves both diffraction and reflection of sound. Diffraction in such cases helps the sound to "bend around" the obstacles. The fact that diffraction is more pronounced with longer wavelengths implies that you can hear low frequencies around obstacles better than high frequencies.

You may perceive diffraction to have a dual nature, since the same phenomenon which causes waves to bend around obstacles causes them to spread out past small openings. This aspect of diffraction also has many implications. Besides being able to hear the sound when you are outside a door in a concert hall, this spreading out of sound waves has consequences when you are trying to soundproof a room. Good sound proofing requires that a room be well sealed, because any openings will allow sound from the outside to spread out in the room - it is surprising how much sound can get in through a small opening. Good sealing of loudspeaker cabinets is required for similar reasons.

The long wavelength will diffract more efficiently than the more directional, short wavelength sounds of the higher pitched instruments.

In certain circumstances, when two sounds are played together, the resulting sound is nil. Two traveling waves which exist in the same medium will interfere with each other. If their amplitudes add, the interference is said to be constructive interference, and destructive interference if they are "out of phase" and subtract. Patterns of destructive and constructive interference may lead to "dead spots" and "live spots" in auditorium acoustics.

Interference of incident and reflected waves is essential to the production of resonant standing waves. Interference has far reaching consequences in sound because of the production of "beats" between two frequencies which interfere with each other.

The modes of vibration associated with resonance in extended objects like strings and air columns have characteristic patterns called standing waves. These standing wave modes arise from the combination of reflection and interference such that the reflected waves interfere constructively with the incident waves. An important part of the condition for this constructive interference for stretched strings is the fact that the waves change phase upon reflection from a fixed end. Under these conditions, the medium appears to vibrate in segments or regions and the fact that these vibrations are made up of traveling waves is not apparent - hence the term "standing wave".

In music, the Naad which is considered musical must possess the important qualities listed: (1) Sound must be audible (2) melodious (3) stability or sustenance of sound.

When a sound does not have sustenance power, then it is not useful in music. This aspect of sustenance depends on echo – Goonj or prathidhwani. Greater the sustenance power more useful it becomes in music. 

Echo is the reflection of soundwave off a distant surface causing sound to be heard distinctly after the original sound has stopped. When we speak out loud in a large hall or near mountains, immediately after we stop the same voice is heard once or twice. This effect is known as echo. Usually, echo is heard when the reflecting body is at distances more than 50 Ft.

When the sound is heard after 1/10th second from the original sound, the sound is called echo. If the sound is heard below 1/10th second, then it is not echo as it will merge with the original sound giving it sustain. In open air, sound travel at the speed of 350 m/s. hence the reflector should be about 35 m away from the listeners.

Generally, it is considered that a Dhwani which has more echo is more suitable for music. This is because, with the development of echo, the specialties of the sound are also developed. The sound of the bells in a temple or church are more pleasant and bring out the feeling of bhakti compared to the sound of striking a tin box. 

The structural making of Indian instruments of taal like mridandam, pakhwaj are based on the principle of goonj and hence it is given paramount importance in music.

There is a single opinion in the scientific community on the generation of echo or goonj. 

While tuning a tanpura, the taar is tightened to the desired swar and using a cloth string placed between the taar and bridge, goonj is generated at some point while moving the thread along the taar. At this point the resulting sound from the tanpura is loud with various anuswars being produced by the resulting echo and harmonics. 

When a Tabla skin is tightened to a higher swar and kept during the monsoons, if there is loud thunder is heard, the skin of the Tabla might tear and rip apart. The reason being the sound of thunder creates an echo which is received by the Tabla. This creates vibration which cannot be restrained by the skin of Tabla and it finally tears apart.

When a group of musicians are performing a closed room, the loud voice and force of rhythm instruments tend to create vibration in the glass of the doors and windows. At some frequency, the overall sound tends to increase when the frequency of glass and the sound produced are matching. When the intensity of the sound increases, the chances of the glass breaking also increase.

When the sound of the singer and audience bounce back and are heard together, then the resulting sound becomes hazy. Generally, the audience sit 40 ft away from the musician. But when they are in a big hall or closed room, the echo can be very disturbing. The sound reaches the ears after 1/10th of a second. 

Large auditoriums are built with such angles that the echo is not generated. Halls which have flat floors tend to produce echo. Glossy and hard surfaced floors tend to produce a lot of echo. Soft floors tend to produce lighter and pleasing echos. The walls of halls are made in such a way that they don’t reflect sound but absorb it by using special materials and designs.

Hence it can be summarized that a small level of echo is useful in music. And anything greater only tends to disturb or destroy it. 

When a singer sings in a big hall and even after he stops singing, the sound continues to be heard for some time. This effect of Naad is called as reverberation. This effect is produced by the bouncing of sound against the four walls repeatedly. 

The prolongation of sound due to multiple reflection within a closed space is called reverberation. A reverberation, or reverb, is created when a sound or signal is reflected causing many reflections to build up and then decay as the sound is absorbed by the surfaces of objects in the space – which could include furniture, people, and air. This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude, until they reach zero amplitude. 

In music, the concept of reverberation is especially important and useful too. Both the singer and listeners get benefited with the aspect of reverb as it adds beauty to the sound. It also causes less strain for the singer to produce sound. Hence, with less force the sound becomes more dominant. Reverb gives consistency to each swar the musician sings. 

When the reverberation is high in any room, then it becomes detrimental for music. To reduce the reverb various changes are done to the walls to absorb the sound and reduce the reverb. The following material are used to reduce reverb – Heeraclith, Celotax, Cardboard, Glass wool, thick cloth, heavy curtains, cushioned chair, wavy walls and roofs, wood shaving with plaster.

The time up to which the reverb is heard is known as time of reverberation. When the time is higher, then the clarity of music is disturbed. 

Reverberation is the most important factor in the acoustics of rooms which are to be used for rendition of music. Reverberation is frequency dependent: the length of the decay, or reverberation time, receives special consideration in the architectural design of spaces which need to have specific reverberation times to achieve optimum performance for their intended activity.

Reverberation occurs naturally when a person sings, talks or plays an instrument acoustically in a hall or performance space with sound-reflective surfaces. The sound of reverberation is often electronically added to the vocals of singers in live sound systems and sound recordings by using effects units or digital delay effects. If there is no reverberation, then the sound stops immediately after it is produced. Hence there is lot of effort on the vocalist or instrumentalist while performing in open spaces. 

One must notice that when music is played in mandra saptak, the reverberation is high compared to music played in taar saptak.

Uses of Reverberation:

1. Singer or musician must employ less efforts in music production and the resulting sound is heard for a longer duration.

2. The musician or singer can hear his own music due to reverb and hence it is possible to sound melodious and stay in sur.

3. When singer is performing Alaap, the reverb tends to merge all the swars with one another and the resulting sound is very melodious 

4. Due to the reverb, the music or raga is hangs in the atmosphere like a background music enhancing the overall rasa experience.

Resonance is a natural phenomenon in which one vibrating body can induce vibrations in another body without any physical contact at specific frequencies known as the system's resonant frequencies or resonance frequencies. One can observe this with two tuning forks of same frequency. when one vibrating tuning fork is placed near another tuning fork, the second fork starts to vibrate. At resonant frequencies, small periodic driving forces can produce large amplitude oscillations, due to the storage of vibrational energy.

Resonance is a process by which the relatively small acoustic energy delivered by sonorous bodies like vocal cords, strings may be greatly augmented. It is resonance that gives all musical instruments the tonal value or timbre they possess. 

Acoustic resonance is a phenomenon that consists of a given acoustic system amplifying a sound whose frequency matches one of its own natural frequencies of vibration (its resonance frequencies). An acoustically resonant object usually has more than one resonance frequency, especially at harmonics of the strongest resonance. It will easily vibrate at those frequencies and vibrate less strongly at other frequencies.

In any instrument, there are two types of vibrations

1. Free vibrations

2. Induced vibrations

When a string vibrates, and causes another string to vibrate without direct contact, this form of vibration is known as resonance. Every material has its own natural frequency. Hence when any sound having the same frequency of the material is played, then the material too starts to vibrate in response to the sound. 

String resonance occurs on string instruments and adds beauty to the overall sound from the instrument. Strings or parts of strings may resonate at their fundamental or overtone frequencies when other strings are sounded. 

When two tanpuras are tuned to the same swar and when one is played, we can notice that the other tanpura string will also start to play. For example, an A string at 440 Hz will cause an E string at 330 Hz to resonate, because they share an overtone of 1320 Hz (3rd overtone of A and 4th overtone of E). String resonance is a factor in the timbre of a string instrument.

Sympathetic strings or resonance strings (Taraph) are auxiliary strings found on many Indian musical instruments. They are typically not played directly by the performer (except occasionally as an effect), only indirectly through the tones that are played on the main strings, based on the principle of sympathetic resonance. The resonance is most often heard when the fundamental frequency of the string is in unison or an octave lower or higher than the catalyst notes, although it can occur for other intervals, such as a fifth, with less effect. Instruments such as Sarangi, Dilruba, Esraj, Surbahar, Sitar, Sarod, Vichitra Veena utilize this concept of using auxiliary strings.

When the main strings are plucked in a string instrument, the Taraph strings which are tuned to a given raga, based on the swar played would start vibrating sympathetically based on the concept of harmonics. It acts like a loudspeaker. 

The swar to which a taraph is tuned to, maintains its relations with other swars.

Similarly, every instrument has its own resonant frequency depending on its size and shape. When a Tabla or Tanpura is tuned to a particular swar, the resulting sound is much resonant than in other pitches. 

The principle of Jawari in a Tanpura is the best example of Resonance. The strings act as the vibrator and the tumba acts as a resonator. When the strings are struck, they start to vibrate. By adjusting the cotton thread in the bridge, we are changing the length of string and hence the frequency, when any frequency matches with the resonant frequency of the tumba, the resulting sound is rich and resonant.

In the western world, resonance was first talked about by galileo. He says all bodies have a natural frequency and cannot be vibrated in another medium.