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Correct option is  B) a, b and d

Decibel is the unit for measuring the intensity of sound. It is denoted as dB.

The unwanted sound in our surroundings is called sound pollution. Causes of sound pollution:

1. Sounds of traffic

2. Sounds by motor bikes and their horns 

3. Sounds in construction sites 

4. Sounds at industries 

5. Sounds at mines 

6. Sounds during explosions and bursting of crackers 

7. Sounds from mixer/grinder, washing machines and motors, etc.

The subjective property of sound related to its frequency is-

Pitch

Due to reflection of light a sound created in a big hall will persist until it is reduced to a value where it is no longer audible.

This persistence of audible sound due to the successive reflections from the surrounding objects even after the source has stopped to produce that sound is called reverberation.

There should not be excessive reverberation. To reduce reverberation, the roof and walls of the auditorium are generally covered with sound- absorbent materials like compressed fibre -board, rough plaster or draperies.

Wavelength : 

(a) In sound waves the combined length of a compression and an adjacent rarefaction is called its wavelength. 

(b) The distance between the centres of two consecutive compressions or two consecutive rarefactions is also called its wavelength. 

(c) It is denoted by the Greek letter lamda λ. Its SI unit is metre.

Frequency : 

(a) No. of complete waves produced in one second or number of vibrations per second is called frequency. 

(b) Number of compressions or rarefactions passed in one second is also frequency. 

• Frequency of wave is same as the frequency of the vibrating body which produces the wave. 

• The SI unit of frequency is hertz (Hz). The symbol of frequency is v (nu). 

• 1 Hertz : One Hz is equal to 1 vibration per second. 

• Bigger unit of frequency is kilohertz kHz = 1000 Hz.

Sounds of frequency higher than 20,000 Hz are called the ultrasonics. We cannot hear the ultrasonic sounds.

Difference between Echo and Reverberation:

The distance between two consecutive compressions or two successive rarefactions is called the wavelength of a sound wave.

If the particles of the medium vibrate perpendicular to the direction of the wave, then the wave is called a transverse wave.

The maximum variation in density or pressure from the mean value is called amplitude.

The number of oscillations of the density of the medium at a place per unit time is called the frequency of the sound wave.

Speed of Sound in Various Mediums 

(i) Speed of sound depends on the nature of material through which it travels. It is slowest in gases, faster in liquids and fastest in solids. 

(ii) Speed of sound increases with the rise in temperature. 

(iii) Speed of sound increases as humidity of air increases. 

(iv) Speed of light is faster than speed of sound. 

(v) In air, speed of sound is 344 ms^-1 at 22ºC.

Frequency and time period are related as 

T = 1/u 

or 

o = 1/T

The relation between the speed of a sound wave, wavelength, and frequency is v = uA.

(a) its amplitude.

The distance by which a point on the wave, such as a compression or rarefaction travel in unit time is called the speed of the sound wave.

Ultrasounds mean the sounds having a frequency greater than 20,000Hz. 

The uses of ultrasound are: 

1. They are used in sonography. 

2. They used in radars.

(d) i, ii & iv only

(i) The sound will become loud on increasing the amplitude.

(ii) The sound will become shrill on increasing the frequency.

We can feel the vibration of table membrane when we hit it to produce sound.

The Right option is : (a) 0.02 s

(a) Hearing organ

The voice box is also known as Larynx.

The sound in the audible range is called Sonic.

Sound cannot travel through Vaccum.

If the frequency of the vibrations is less than 20 Hz, then the human ear cannot perceive them. Such vibrations are called infrasonic vibrations. If the frequency of vibrations is between 20 Hz to 20,000 Hz, then they can be perceived by the human ear. Such vibrations are called sonic vibrations or sound vibrations. It has been further found that higher the frequency, the sharper or shriller is the effect of sound on the ears. However, if the frequency of vibrations is above 20000 Hz, then the human ear cannot perceive them. Such vibrations are called ultrasonic vibrations or ultrasonics.

The number of vibration/oscillation per second is called the frequency of the wave. It is represented by the symbol v (nu). Its S.I unit is Hertz (Hz).

The S.I unit of frequency is in Hertz.

Speed of sound is least in Gas (air).

As we know that the unpleasant sounds are called noise. High pitched noise is usually more annoying than low pitched noise. Noise can damage the ears, cause of tiredness and loss of concentration. If it is very loud, it may even result in sickness and temporary deafness. Some commonly heard noise are those made by flying air craft, moving vehicles, greatly amplified music, and machinery including domestic appliances.
Such unwanted noise can be reduced by:

1. Fitting vehicles with silencers.
2. Using sound insulating material at home such as carpets, curtains, planting trees between houses and roads.
3. Wearing ear protectors which minimise noise effects.

Sound Navigation and Ranging.

Frequency: The number of vibrations or oscillations per second is called frequency.

Time period: Time required to complete one oscillation or vibration is called time period.
Following is the relationship between frequency and time period.

Time period = 1/Frequency

SONAR- Sound Navigation and Ranging.

(a) Frequency

The time taken by one complete oscillation in the density of the medium is called time period. It is denoted by the capital letter T. Its S.I unit is second.

(c) Frequency = 1/time period

Frequency υ = 400 Hz 

Time period T =?

Time period T = 1 / u = 1 / 400 = 0.0025 s

The time taken to complete one oscillation of the density of the medium is called the time period of the sound wave.

The loudness of a sound depends on the amplitude of vibration of the vibrating body producing the sound.

A sound wave is characterized by its amplitude and frequency. Depending upon the (amplitude and frequency of the sound wave, the following two characteristics of sound : 

(1) Loudness, and 

(2) Pitch.

Loudness : 

The loudness depends on the amplitude of the sound wave. 

• Loudness is the measure of the sound energy reaching the ear per sec. 

• Greater the amplitude of sound wave, greater is the energy, louder the sound; short is the amplitude, less is the energy, soft is the sound. 

• Loudness is measured in decibel ‘dB’.