Heredity

Summary

Summary
 
Key terms
Sound Waves: A sound wave arises because of a repeating pattern of high pressure and low pressure regions moving through a medium. It is sometimes referred to as a pressure wave. Sound wave travels as longitudinal wave in nature. These waves are incapable of traveling through a vacuum.
Intensity of sound: Sound intensity is defined as the sound power per unit area. The usual context is the measurement of sound intensity in the air at a listener's location.
Resonance: When the frequency of an external force matches the natural frequency and standing waves are set up, giving rise to waves of larger amplitude.
Echo: A reflected sound that can be distinguished from the original sound, which usually arrives 0.1 sec or more after the original sound.
Reverberation: An apparent increase in volume caused by reflections, usually arriving within 0.1 second after the original sound.
Vibration: A back and forth motion that repeats itself.
Beats: Rhythmic increase and decrease of volume from constructive and destructive interference between two sound waves of slightly different frequencies, is known as a beat.
Superposition: Superposition is a word used to describe what happens when one wave is superimposed – ‘sat on top of’ – another.
Doppler Effect: An apparent shift in the frequency of sound or light due to relative motion between the source of the sound or light and the observer.
Music: The sound of music has a different character having periodic tones or musical “notes”. The line that separates music and noise however is thin and subjective.
Noise: Sounds made up of groups of waves of random frequency and intensity.
Musical scale: A series of musical notes of different frequencies, arranged in such a manner that their fundamental notes have definite ratios, is called a musical scale.
Acoustics: The study of sound properties is called acoustics. To have the intensity of sound almost uniform in the hall, the walls and the ceiling may be curved in proper fashion. It is often advantageous to place highly reflective surfaces behind the stage to direct sound out to the audience.
Ultra sound: Sound waves too high in frequency to be heard by the human ear; frequencies above 20,000Hz.
Key concepts
  • Sound Wave: Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing. Sound also travels through plasma. These waves are produced by bodies vibrating at frequencies lying between the range of 20Hz and 20,000Hz, perceived by the human ear. This is known as the audible range of frequency. In terms of wavelength, this range extends from about 17m to 1.7cm. Mechanical waves of frequency lesser than 20Hz or greater than 20,000Hz are inaudible and are respectively called infrasonic and ultrasonics.

    The seismic waves are infrasonic waves. Bats, dolphins and submarines make use of ultrasonics to find their way in the dark. A sound wave is an example of a mechanical wave. Sound waves are incapable of traveling through a vacuum. A sound wave arises because of a repeating pattern of high pressure and low pressure regions moving through a medium. It is sometimes referred to as a pressure wave. Sound wave travels as longitudinal wave in nature. Sound wave behaves as a transverse wave in solids. Through gases, plasma and liquid the sound travel as longitudinal wave. Through solids it can be transmitted as transverse as well as longitudinal wave.
  • Speed of Sound waves: The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at 20 °C (68°F), the speed of sound is 343.2 meters per second (1,126ft/s). This is 1,236 kilometers per hour (768mph), or about one kilometer in three seconds or approximately one mile in five seconds.
  • Intensity of Sound waves: Sound intensity is defined as the sound power per unit area. The usual context is the measurement of sound intensity in the air at a listener's location.
    Intensity = Energy/ (Time * Area) = Power/ Area
  • Threshold of Hearing: The absolute threshold of hearing (ATH) is the minimum sound level of a pure tone that an average ear with normal hearing can hear with no other sound present. The absolute threshold relates to the sound that can just be heard by the organism. The absolute threshold is not a discrete point, and is therefore classed as the point at which a response is elicited a specified percentage of the time. The threshold of hearing is frequency dependent and it has been shown that the ear's sensitivity is best at frequencies between 1kHz and 5kHz.
  • Reflection of Sound waves: Reflection is the change in direction of a wave front at an interface between two different media so that the wave front returns into the medium from which it has originated. Common examples include the reflection of light, sound and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Echoes are produced due to the Reflections of sound from hard surfaces. Reverberation is a phenomenon that occurs when the original and reflected sounds are so close to be heard separately, then the original sound seems to be prolonged. The echo method can be used for determining the speed of sound in air. Speed of sound in air   v = Total distance travelled / Time Interval.   Reflection of Sound waves are used in Radars and sonars to analyze the shape and size of the object located at a distance from the source point.
  • Refraction of Sound waves: Sound waves bend or refract when parts of the wave fronts travel at different speeds. Refraction of sound waves is most evident in situations in which the sound wave passes through a medium with gradually varying properties. Refraction of sound waves are used to study what lies below the surface of the sea as the sound can travel through, and bounce off, different seafloor sediments in very different ways.
  • Natural Frequency: Natural frequency is the frequency at which a system naturally vibrates once it has been set into motion. In other words, natural frequency is the number of times a system will oscillate (move back and forth) between its original position and its displaced position, if there is no outside interference. The actual frequency at which an object will vibrate is determined by a variety of factors. Each of these factors will either effect the wavelength or the speed of the object. Any alteration in either speed or wavelength will result in an alteration of the natural frequency.
  • Fundamental frequency and Harmonics: The most fundamental harmonic for a string is the harmonic associated with a standing wave having only one anti-node positioned between the two nodes on the end of the string. This would be the harmonic with the longest wavelength and the lowest frequency. The lowest frequency produced by any particular instrument is known as the fundamental frequency. The fundamental frequency is alternatively called the first harmonic of the instrument. Each natural frequency which an object or instrument produces has its own characteristic vibrational mode or standing wave pattern. These patterns are only created within the object or instrument at specific frequencies of vibration; these frequencies are known as harmonic frequencies, or merely harmonics.
  • Vibrations of Air column and its modes: The disturbance in the source, causes the air within the tube to vibrate with a variety of frequencies, but only frequencies that correspond to standing waves will persist.
  • Modes of Vibration:



  • Beats: Beats are the periodic and repeating fluctuations heard in the intensity of a sound when two sound waves of very similar frequencies interfere with one another. Beat pattern would be consistent with a wave, which varies in volume at a regular rate. Beat Frequency is given by f beat = fa – fb
  • Diffraction of Sound waves: The phenomena observed when waves passes the edge of an obstacle and are associated with a deviation of the waves from rectilinear propagation upon interaction with the obstacle. Because of diffraction, waves bend around obstacles, penetrating into the region of the geometric shadow. The possibility of hearing the voice of a person around the corner of a house is due to the diffraction of sound waves.

    The reception of radio signals in the long–wave and medium–wave bands far beyond the limits of direct visibility of the radiating antenna is due to the diffraction of radio waves around the surface of the earth. Diffraction depends significantly on the ratio between the wavelength λ and the size of the object that causes diffraction. Diffraction is observed most distinctly in those cases when the size of the obstacles being rounded is commensurate with the wavelength. long–wavelengths are able to diffract and carry farther than the short–wavelengths. Low–pitched (high wavelength) sounds always carry further than high pitched (low wavelength) sounds.
  • Interference of Sound waves: When two coherent sound waves superpose in certain region interference takes place. For waves to be coherent, the phase difference between the two waves should not change with time. This requires that the wavelength of the two waves must be the same. Furthermore to have a better contrast between the maximum and minimum intensities, the two waves producing interference and should propagate in the same direction. If the two interfering waves have a displacement in the same direction, such type of interference is called constructive interference. Destructive interference is a type of interference that occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction.
  • Principle of Superposition: When two waves interfere, the resulting displacement of the medium at any location is the algebraic sum of the displacements of the individual waves at that same location.
  • Doppler Effect: The difference between the frequency of a wave (as of sound or light) as measured at its source and as measured by an observer in relative motion. The Doppler effect can be used to determine the relative speed of an object by bouncing a wave (usually a radar wave) off the object and measuring the shift in the frequency of the wave. This technique is the basis of Doppler radar, as used in traffic control and navigation systems. The Doppler effect is also known as the Doppler shift.If the source and the observer are getting farther apart, the observed frequency is lower than the source frequency.

    In the case of light waves, the phenomenon is known as red shift. The amount of red shift in the spectra of stars is used in astronomy to determine how quickly the Earth and those stars are moving apart. If the source and the observer are getting closer together, the observed frequency is higher than the source frequency. In the case of light waves, the phenomenon is known as blue shift.
  • Different cases of Doppler effect:

  • Shock Waves: A supersonic plane travelling in air with a speed greater than the speed of sound (supersonic speed) creates pressure disturbances in the air resulting in the formation of three – dimensional shock waves.
  • Bow waves: A boat when moves across a lake's surface faster than the wave (it produces), outruns those waves and overlap at the edges, due to the speeding boat a "bow wave" is created which goes behind the boat in a distinctive "V". This "bow wave" is because waves bunch up or pile up on each other before they can move away. The width of the bow wave depends on the speed of the source; the faster the source travels, the narrower the wave becomes.
  • Comparison between Music and Noise:

    Music Noise
    Music is the art of arranging and combining sounds in order to create a harmonious melody. Noise is an unwanted sound that is usually very loud and meaningless.
    Music is pleasing to the ears. Noise is an unpleasant sound.
    Music has frequencies and wave lengths that are harmonious. Noise has irregular wave form and wave length and has low frequency.
  • The following properties of sound can distinguish a musical sound from noise:

    • Intensity or loudness should not change suddenly
    • Pitch or the frequency should not be very high and should not change suddenly
    • Quality or timbre of sound should be consistent.
  • Musical Scale: If two musical instruments have the same intensity and frequency, it is the quality that distinguishes one from the other. Each frequency is called a note. A series of musical notes of different frequencies, arranged in such a manner that their fundamental notes have definite ratios, is called a musical scale. A scale that has eight notes is called an octave. Ratio between frequencies of two notes is called an interval. In musical sound, an interval is never less than one.

  • Musical Instruments: It is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument. There are different types of musical instruments, like, (a) Wind instruments which produce sounds by means of vibrating air columns; (b) Percussion instruments such as tabla, mridangam, drums and cymbals, where a two–dimensional membrane or elastic surface is struck to produce sound; (c) Electronic musical instruments use electrons to generate the signals that make up musical sounds. All these Musical instruments are set into vibrational motion at their natural frequencies when a person plays them. Each natural frequency of the object is associated with one of the many standing wave patterns by which that object could vibrate.
  • Human Ear: The human ear can be divided into three parts. The external ear consists of the physical ear and an air canal. The chief purpose of this part is to collect sound waves and direct them to the middle portion of the ear. The human ear is capable of detecting sound waves with a wide range of frequencies between 20 Hz to 20,000 Hz. Any sound with a frequency below the audible range of or less than 20 Hz is known as an infrasound. Any sound with a frequency above the audible range or more than 20,000 Hz is known as an ultrasound.
  • Ultrasound: Ultrasound waves are high energy sound waves with extremely small wavelengths due to which they can penetrate deep into the matter. They travel in straight lines without bending around the corners. Ultrasound is used in industries to detect cracks in metal blocks without damaging them. Of all the applications of industrial ultrasonic testing, flaw detection is the oldest and the most common.
  • Fourier Analysis: All periodic waves may be broken down into constituent sine waves of different amplitudes and frequencies. The mathematical operation for doing this is called Fourier analysis. The principle of superposition is evident as their individual waves combine to produce the new wave form. Our ear performs a sort of Fourier analysis automatically.
  • Compact disc: A compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio. There are different types of Cd's like bootable CD, Enhanced CD, Vinyl CD, Photo CD, SVCD, etc., which store the digital information using lasers.
  • Acoustics: The study of sound properties is called acoustics. Reflection of sound in a room makes it sound lively and full. Following factors are to be considered while designing the acoustics of a hall:

    • The intensity or the loudness should be same at all points
    • Reverberation should be avoided
    • Echo or direct sound reflections from surfaces should be avoided
    • Interference or focusing should be avoided
    • Resonance effects should be avoided
    • The Auditorium must be made soundproof so that no outside noise enters it.
key formulae
  • Speed of sound in air = 343 m/s
  • Speeds of sound (v) :
    • In a solid :

      where
      γ = Young's modulus
      ρ = Density
    • In a fluid :

      where
      B = Bulk modulus
      ρ = Density of the fluid
    • In a gas :

      where
      γ = Ratio of heat capacities
      R = Universal gas constant
      T = Temperature in K
      M = Molar mass
    • At normal atmospheric pressure, v(in m/s) = 343 + 0.6 T
  • Intensity :
    (dB) = 10 log (Ps / Pr)
    where
    Pr = reference sound level (normally 10–12 W/m2)
    Ps = level of any other sound to be measured
  • Modes of vibration :
    • In an open tube
      fn = v/λn = (v/2L)n where n = 1, 2, 3,.....
    • In a closed tube
      fn = where n = 1, 3, 5,....
  • Beat frequency :
    fbeat = fa – fb
  • Doppler effect :
    • The relation between the detected frequency (f ') and the emitted frequency (f) is given by

      where
      v = speed of sound in air
      v0 = speed of the observer
      vs = speed of the source relative to air
      Note:
      When the source and observer move towards each other, i.e., vs is positive and v0 is negative.
      When the source and observer move away from each other, i.e., vs is negative and v0 is positive.
      When the source moves away from the observer and the observer moves towards the source. i.e., vs is negative and v0 is negative.
      When the observer moves away from the source and the source moves towards the observer, i.e., v0 is positive and vs is positive.