20 SOUND – MEASUREMENT, RELATED TERMS AND UNITS

S. Visalakshi Rajeswari

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  1. Introduction

The interplay of light and sound in the environment is a constant feature and man’s efforts to control them to suit his pursuit remains a sustained endeavour. It is clear that their interplay in the ambient atmosphere may not lend for control. Nevertheless, the interior of a building offers various opportunities for man to show his imagination and intelligence to create dramatic effects. To be successful, knowledge on the nature of the two factors is imperative. The previous sessions would have given an insight on light as a medium and introduced one to the concept of sound. This session covers sound as an exclusive parameter, displaying its multifarious qualities, an understanding of which is a requirement in the control of the factor. These aspects should be considered in designing.

  1. Objectives

By the end of the session one will be able to:

  • Differentiate the types of waves
  • Identify the characteristics of sound waves for better comprehension of the concept
  • Know how sound characteristics are measured and their respective units
  • Gain knowledge on sound trans mission and their behavioral as pects
  1. Concept of waves

The mere thought of a wave reminds anyone primarily about the waves that are formed in the ocean, a body of flowing water etc. People don’t realize light and sound also travel in waves. Since they are invisible the concept has to be understood to believe the fact. Again, one should not confuse sound waves with the type of waves that are formed in the ocean. As stated earlier, similarity between sound and light ends with the fact that they travel in waves, because light waves travel as a different kind of wave. Light and water waves form transverse waves, while sound waves travel only as longitudinal waves.

 

The nature of wave formation caused by their shake in a specific direction differentiates the two kinds of waves. A wave shaking perpendicular to the movement direction is a transverse wave, unlike longitudinal waves which move parallel to movement direction

 

3.1   Points to ponder

 

The factors stated above teach one that for forming sound waves two aspects are absolutely necessary; one a source and the next a medium . Vocal chord in the human throat is the part from which human voice originates. Vibrations from the vocal chords actually produce the voice. When a piper plays an organ pipe, sound is produced due to the vibrations of the air columns.

  1. How do sound waves move in air?

To learn about the method of wave movement in air understanding a few terms related to sound propagation will be helpful. Important terms related to sound wave propagation are sine wave, sinusoidal curve, peaks, length, amplitude and frequency

 

4.1 Frequency Spectrum of Sounds

 

Except basic sine waves, sounds are comprised of many different frequency components vibrating simultaneously. The unique frequency permutations it contains results in the particular characteristics of a sound.

Sinusoidal waves or curves showing different frequencies; from lowe r (orange) to higher(pink) The horizontal line depicts time.

Sound waves are simply described as sinusoidal plane wave motions explained through their specific characteristics, like wave length, frequency and velocity. Wave motion in this context refers to the ability to transfer sound across an area.

  1. Characteristics of sound waves

  5.1 Wave length: denotes the distance which is a characteristic of the particular wave. It indicates the distance travelled by the wave in one time period, i.e. T second. The symbol is ‘λ’, measured in units of meters (m).

5.2 Velocity: Measures the distance covered by sound in a second in a known direction, in meters per second. In air, the increase is directly proportional to the increase in temperature or humidity.

5.3 Period: defines how long it takes for one wavelength to pass a fixed point. To be specific it is the time required to complete one vibration. One could also turn this around and say how many waves go by in one second. The symbol used is T. The unit is seconds (s)

5.4 Frequency of sound: scientifically represents the pitch of a sound wave – high or low. It indicates just how often the waves occur during a particular period of time, that is, the number of vibrations made by the particle in one second. It is denoted by (n) Frequency = Velocity     / wavelength

 

5.5 Amplitude: of a wave is denoted by (a). The units are meters (m).

 

5.6 Wave shape: Whenever music (a note) is played using a tuning fork, a sine wave (an undulating hilly pattern) is shown by the oscilloscope. The same note if played on a trumpet will show the wave to look more or less in a zigzag pattern (usually called a saw-tooth wave); on a flute, as triangular waves and on a clarinet, blown hard, brings out waves showing square forms. Here the instrument pumps energy into the external world. The control practiced in pumping energy decides the shape of the waves. To be specific it relates to the way it can vibrate and in the process make the air within or outside it to vibrate in consonance. This is one of the major things that make instruments produce sounds that differ from one another.

 

5.6.1 Frequency ranges or Bands: This pertains to the energy contained in sound. Bass is the term used to refer to sound containing a lot of low frequency energy. Human beings can hear best in the 250–4000 Hz frequency band. It is explained as midrange, while treble is the term used to refer to high- frequency energy beyond the midrange. This is supposed to add a crispness or brilliance to a sound (figure).

 

Do not confuse this with amplitude. Band speaks of energy contained in sound, but amplitude describes the energy sound waves carry. Actually amplitude is described as high if they carry larger amount and low if less. An integration of the energy of all the various pressures from peak to peak is known as sound energy, measured in units of watts/ sq.cm

 

5.6.2 Application

 

Ø  Sound pressure is used in research focusing on hearing, auditory research and sound measurement in industry.

Ø   Sound energy is used for research in acoustics

 

6.Resonance: Every object has a natural frequency at which they will vibrate when disturbed. Unless and until disturbed many objects do not vibrate. For instance a sound is created by a metal or wood when dropped down. In static position they don’t vibrate and therefore don’t create sound. Resonance occurs when the natural frequency of an object coincides with the frequency of any vibrations applied to the object.

 

6.1 Points to ponder

 

Try to play the same musical note in two different instruments, perhaps in the same volume. They can sound completely different. How can that be if they’re producing the same sound waves? It is quite obvious: they are not producing the same sound waves! Scientists use an oscilloscope to see the difference. It is an electronic graph-drawing machine, resembling a cathode-ray television, which basically can show pictures of how waves look like. Naturally, anybody will get a doubt; “do the graphs differ”? Yes, they are different in their wave shapes. Everybody enjoys music. The best way to explain the concept is by referring the same to music.

  1. Measure ment of sound intensity

Human perception of sound seems to be logarithmic (a measure expressing a ratio between two sound pressures). The range of sound pressures which can be heard by the human ear is very great, and it is convenient to express these differences in a logarithmic scale. The unit normally used is the bel, which is a logarithm of a ratio of ten. But the most common and convenient measure of sound intensity is the decibel (dB), where the bel is divided into ten parts. Therefore a decibel is equal to 1/ 10 of a bel.

 

A pressure of 0.0002 dyne per square centimeter has been adopted as the basic pressure, from which the decibel scale starts. This is roughly equivalent to the threshold of hearing at 1000 c/s. So, threshold of hearing is equal to 1000 c/s and the pressure will be 0.0002 micro bar or dynes/ sq.cm.

 

7.1 Apparatus used

 

Measurement of the intensity of sound is generally made in industry by means of a Sound Level Meter.

8. Transmission of sound

 

It means the sound energy transmitted through the walls. When sound is produced in a room, it traverses outwards as waves of spherical pattern and strike the boundaries of the room. Such waves undergo reflection, absorption and transmission in varying amounts. The frequency of sound and the characteristics of the walls of the room (wall thickness, weight, material used, nature of surface) decide the amounts of the three processes. These explain the process of sound transmission. The concern here is whether it can be continuous. Can there be losses? Yes; there is every possibility to incur losses in enclosed buildings, especially houses with different rooms.

 

8.1 Trans mission loss: A wall or any other barrier will cause loss of sound energy, which is defined as transmission loss. Whenever sound is transmitted from a source to an adjoining area or room, such hindrances can entail reduction in sound energy, causing the phenomenon of transmission in loss. By measuring the sound levels on either sides of a wall transmission loss can be calculated. For instance it the sound levels in the interior and exterior of a wall of a room are 48dB and 8dB respectively, then transmission loss of the wall will be 40 dB

 

8.3 Self – check exercise

 

Find out the transmission losses that occur in the various rooms in your house. Compare losses that incur with (i) load bearing and partition walls and (ii) materials used for construction.

 

8.4 Points to ponder in application

 

When designing an interior these major aspects have to be borne in mind. Transmission losses can be varied depending on the:

  • Sound insulation required in the room. Larger the loss, greater the sound insulation
  • Materials used for construction. Losses vary with building materials used
  • Methods of construction adopted. Values for losses tend to differ with methods used
  • Frequency of the sound produced

It is evident therefore that sound transmission has an influence on the behavior of sound in an interior.

 

9.Behavior of sound in an enclosed space

 

As stated in the previous session, sound from the outside cannot be controlled, but sound produced in an enclosed space can be controlled. For this, one needs to know how sounds behave in an interior. Terms related to this concept can be broadly delineated as sound reflection, echo and reverberation. Let’s see what they mean?

 

9.1 Sound reflection: When sound is reflected it obeys the law of reflection of all waves. The angle, here, per se is measured between the path of the sound wave and the reflector. The angle of reflection differs with the surfaces of reflection. Concave surfaces concentrate sound waves in certain areas, while convex surfaces disperse the waves. On parallel smooth surfaces sound waves move as they do from the source of sound.

 

9.2 Echo: An echo is defined as a long delayed reflection; a sound that is reflected and heard for more than 0.1 seconds succeeding the actual one. This happens whenever a sound wave when reflected reaches the aural processes and almost simultaneously (within a time interval of about 1/7 second) another sound wave follows it as a repetition. A delayed but stronger reflection causes the echoes to blur and confuse the original sound, because the sound arriving first will be louder. So, the definition needs to be revisited for fine-tuning. Subsequent to reception of a direct sound if a strong reflection is received (by a 1/20t h of a second interval), echoes occur.

 

9.2.1 Learn more

 

Echo also occurs when the shape of the reflected surface is curved and smooth. To overcome echoes, designing proper room shapes, its surfaces and use of suitable resilient materials in the interior is suggested. Covering the long distance walls and high ceilings with absorbent materials can be helpful.

 

9.3 Echelon effect: Provision of a set of metal railings and regular spacing of reflected surfaces are responsible for this effect, where they tend to produce a musical note due to regular succession of echoes of sounds. This further adds to the confusion in hearing and effective audibility

 

9.4 Reverberation: Sounds produced in a building tend to stay after it is produced and reaches the listener many times. To be specific the first will be directly from the source followed by those reflected from the internal building components, namely, walls, ceiling and floor of the interior. What the listener receives is series of sounds of diminishing intensity. Part of the energy is lost at every step of reflection, an important point to remember in space designing. Such prolonged reflection of sound from the walls, floor and ceiling of the room is referred to as reverberation.

 

If the room is carpeted and is finished well with furnishings, these soft fabrics will absorb sound energy quickly and thereby muffle the sound spontaneously. In an empty room especially, the reflections and subsequent reverberation are found to sustain longer.

 

9.4.1 Reverberation time: is the duration for which the sound persists. It is explained as the duration taken for the sound to record below the minimum audibility measured from the instant when the sound ended sounding. This is defined as the duration for a sound to die by 60dB from its actual level. It depends upon the distance between the surfaces of the room, the absorption of those surfaces and the frequency of the sound. The optimum reverberation time depends on the volume of the room, size of the hall, loudness of the sound, the kind of music and the types of sound for which the hall was built. W. C. Sabine, Professor of Physics, Harvard University had done extensive research on the topic. Sabine’s formula is used to help in estimation.

 

9.5 Factors affecting efficiency in hearing in an interior

9.5.1 Reverbe ration: Loss of clarity results when reverberation is large. The opposite causes inadequacy in loudness. It can be controlled by:

 

  • Providing functional fenestrations; Windows and ventilators made to facilitate opening and closing to help maintain optimum reverberation time.
  • Decorating the walls with hangings for sound absorption
  • Hanging heavy draperies and curtains
  • Covering walls with absorbent material liners
  •  Ensuring full capacity of audience in theatres and conference halls
  •  Using floor coverings, especially carpets
  •  Laying floors with acoustic tiles

9.5.2 Loudness: Both sound frequency and the wave’s amplitude decide the loudness. Human beings do not exhibit equal sensitiveness in hearing at all frequencies. In the frequency range between 2 kHz and 5 kHz an individual’s ear is most sensitive, but least sensitive at low or at extremely high frequencies. Provisions made for great absorption to manage reverberation time, may at times affect the intelligibility of hearing. To ensure satisfactory hearing, sufficientloudness across the entire room is a must. In such cases attempts to increase loudness can be made.

 

While constructing one can opt for low ceilings in rooms; Providing false ceiling at alower height in spaces where many people assemble can be helpful especially when one needs to reflect sound energy towards the audience

  •  In theatres and auditoriums and home theatres other cosmetic corrections can also be tried like

      Ø  Using large sounding boards behind the speaker and facing the audience

  Ø   Keeping large polished reflectors exactly above the speakers

  Ø   Using amplifiers to provide additional sound energy

 

9.5.3 Focusing s urfaces: During construction of buildings and designing individual spaces, designers generally make two mistakes. Firstly, they are found to introduce focusing surfaces like a concave, spherical, cylindrical or parabolic feature in the walls and / or ceilings in buildings, especially in the interiors. It can be a curved wall, a domed ceiling or a cylindrical structure around a spiral staircase. Whatever may that be, their introduction produces concentration of sound into particular regions while in some other part of the room no sound reaches at all, creating islands of silence or poor audibility. The major aim in sound transmission is uniform distribution of sound in the room. The second mistake designers make is in providing extensive reflecting surfaces in the room. When sound waves strike them, the reflected and the direct sound waves together tend to form stationary wave system resulting in bad and uneven sound intensity distribution system. This jeopardizes the entire aim of providing uniform distribution of sound energy in a room. Designers have to adopt a few methods to curtail these conditions.

  • Avoid curved surfaces in construction
  • Cover Curved surfaces if at all provided with absorbent ma terials,
  •  Provide low ceiling
  •  A paraboloidal surface, if at all provided in large halls, can be converted into a reflecting surface by arranging with a speaker at the focus. This will ensure sending a uniform reflected beam of sound in the hall

     9.5.4 Resonance : In some buildings window panes, less – rigid walls, sections of wooden parts like a shutter are prone to vibration and they create peculiar sounds. If frequencies are similar to other audio frequencies in the room, this will result in resonance. A major cause of resonance in an interior is enclosed air. Suitable dampers have to be used to contain such occurrences.

     9.5.5 Noise: produced from the exterior or produced within the room/ building has a great impact on people and their actions. Being defined as unwanted sound, it can be jarring or displeasing to the inmates. Three types of noises (air – borne, structure – borne and inside) called extraneous noise have been identified which prove to be troublesome for lis teners. Sound insulation or sound proofing refers to the methods adopted to prevent their transmission, the type of which differs and depends upon the type of noise to be treated.

  1. Summary

This session was meant to instill in the minds of the learners the concepts related to sound and noise and their interplay in designing comfortable interiors affording peaceful ambience. Knowledge on the terms related to sound help in comprehending them and identifying the basic differences. In the process it also dispels ambiguity of terms which are generally misconstrued, say, sound energy and sound pressure. Awareness on these aspects prove beneficial in designing acoustically perfect interiors.

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Web links

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