Reflection of Sound: Echo, Ultrasound, Pitch and Loudness,

Echo

Sound which has been reflected off hard, flat surfaces such as a large wall or a distant cliff is called an echo.



Reflection of sound waves off of surfaces is affected by the shape of the surface. Flat or plane surfaces reflect sound waves in such a way that the angle at which the wave approaches the surface equals the angle at which the wave leaves the surface.Hence, Laws of reflection of light apply to sound as well.


Audibility

The word 'audible' refers to being able to be heard. The range of frequencies which a person can hear is known as range of audibility.

Sound frequencies between 20 and 20,000 Hz can be heard by people.
As people get older, the higher frequencies become more difficult to hear.


Ultrasound and Infrasound

Sound with a frequency lower than 20 Hz is called infrasound.
These very low frequency sound wavescan be given off by volcanoes and meterorite explosions.Infrasound is used by some large animals for communication.Whales can communicate over hundreds of miles using infrasound.

Sound with a frequency higher than 20,000 Hz is called ultrasound.Ultrasound echoes are used in Scanning and Range and Direction Finding.Ultrasound in liquids can be used to clean precious or delicate items because the compressions and rarefactions
will shake dirt and unwanted material free without the risk of damage being caused by handling the item.


Pitch and Loudness

The loudness of sound depends on the amplitude of the wave.The bigger the amplitude, the louder the sound.

The pitch of sound (how high the note is)depends on the frequency of the wave.
The higher the frequency, the higher the pitch.

Sound is a longitudinal wave and so it is difficult to show the amplitude and frequency on a diagram. A microphone can be used to change the sound wave into an alternating current which can be displayed as a transverse wave on a CRO.This makes it easier to show the effect of amplitude and frequency on loudness and pitch.

Below is a picture of a sound wave which has been changed into alternating current by a microphone and displayed on a CRO.



If the sound is made louder and with a higher pitch, the shape of the wave changes as shown below.



The amplitude has got bigger because the sound is louder. The frequency has increased
(there are more complete waves in the same time) because the sound has a higher pitch.

Sound

Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving. If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. The motion of the particles is parallel (and anti-parallel) to the direction of the energy transport. This is what characterizes sound waves in air as longitudinal waves.



A vibrating tuning fork is capable of creating such a longitudinal wave. As the tines of the fork vibrate back and forth, they push on neighboring air particles. The forward motion of a tine pushes air molecules horizontally to the right and the backward retraction of the tine creates a low-pressure area allowing the air particles to move back to the left.


How is sound produced?

Sound is produced by anything that vibrates or changes air pressure rapidly. It could be a bell vibrating, a vocal cord, the sound board of a guitar, etc. The vibrations cause molecules of air to vibrate, and send the vibrations out in all directions. The vibrating air molecules reach your ear and cause your eardrum to vibrate. You perceive the vibrations as sound.


How does Sound Travel?

Sound is transmitted through gases, plasma, and liquids as longitudinal waves, also called compression waves. Through solids, however, it can be transmitted as both longitudinal waves and transverse waves.

Matter in the medium is periodically displaced by a sound wave, and thus oscillates. The energy carried by the sound wave converts back and forth between the potential energy of the extra compression (in case of longitudinal waves) or lateral displacement strain (in case of transverse waves) of the matter and the kinetic energy of the oscillations of the medium.

Because of the longitudinal motion of the air particles, there are regions in the air where the air particles are compressed together and other regions where the air particles are spread apart. These regions are known as compressions and rarefactions respectively. The compressions are regions of high air pressure while the rarefactions are regions of low air pressure. The diagram below depicts a sound wave created by a tuning fork and propagated through the air in an open tube. The compressions and rarefactions are labeled.



The wavelength of a wave is merely the distance that a disturbance travels along the medium in one complete wave cycle.A longitudinal wave consists of a repeating pattern of compressions and rarefactions. Thus, the wavelength is commonly measured as the distance from one compression to the next adjacent compression or the distance from one rarefaction to the next adjacent rarefaction.

Since a sound wave consists of a repeating pattern of high-pressure and low-pressure regions moving through a medium, it is sometimes referred to as a pressure wave.



Transmission of sound

To produce vibrations that become sounds, a mechanical device (the source) must first receive an input of energy. Next, the device must be in contact with a medium that will receive the sound energy and carry it to a receiver. If the device is not in contact with a medium, the energy will not be transferred to a receiver, and there will be no sound.

Thus, three basic elements for transmission and reception of sound must be present before a sound can be produced.

They are

(1) the source (or transmitter),
(2) a medium for carrying the sound (air, water, metal, etc.), and
(3) the detector (or receiver).

A simple experiment provides convincing evidence that a medium must be present if sound is to be transferred. An electric bell is suspended by rubber bands in a bell jar from which the air can be removed.

An external switch is connected from a battery to the bell so the bell may be rung intermittently. As the air is pumped out, the sound from the bell becomes weaker and weaker. If a perfect vacuum could be obtained, and if no sound were conducted out of the jar by the rubber bands, the sound from the bell would be completely inaudible.



Observation:

You could see the bell being struck, but you could hear no sound because there was no medium to transmit sound from the bell to you.Sound could not be transmitted through a vacuum.When the air is admitted again, the sound is as loud as it was at the beginning.

Results:

This experiment shows that when air is in contact with the vibrating bell, it carries energy to the walls of the jar, which in turn are set in vibration. Thus, the energy passes into the air outside of the jar and then on to the ear of the observer. This experiment illustrates that sound cannot exist in empty space (or a vacuum). Hence, any medium which has particles that can vibrate will transmit sound.

Now let's look at another example in which the third element, the detector, is missing. You see a source (such as an explosion) apparently producing a sound, and you know the medium (air) is present, but you are too far away to hear the noise. Thus, as far as you are concerned, there is no detector and, therefore, no sound. We must assume, then, that sound can exist only when a source transmits sound through a medium, which passes it to a detector. Therefore, in the absence of any one of the basic elements (source, medium, detector) there can be NO sound.

Measuring Speed of Sound

Speed of sound varies according the type of medium it travels through.
Sound travels fastest in solids and slowest in gases.

Equipment needed to measure the speed of sound

You will need the following items in order to measure the speed of sound:

1 50M tape measure (50 metres is very long so if you haven't got one, make a measure by getting a ball of string and measuring out 50 one metre sections. Mark each one with a bit of coloured tape.)

1 stopwatch

1 big flat outdoor wall that produces a good echo

1 friend


Measure the speed of sound

1) Use the tape to measure a distance of 50 metres from the wall.

2) Now clap your hands and check you can clearly hear an echo from the wall. Make sure the echo isn't coming from other walls in the area. The time it takes sound to run 100 metres is the time difference between when you clap and when you hear the echo.



3) However measuring that single short burst of time is difficult.

4) Now clap repeatedly in time with the echo, so that you can only hear your own clap (the echo is masked by your next clap)

5) So we can now measure the time it takes to clap 10 times. Start the stopwatch at the first clap and end it when you hear the echo of the 10th clap.

6) Find the Average value, that is the average speed of sound.

(We now know how long it takes for sound to travel 1 kilometre. A little bit of maths will let us convert that into metres per hour or miles per hour.)

Latent Heat

Latent Heat


Latent Heat (enthalpy) is the "hidden" heat when a substance absorbs or releases heat without producing a change in the temperature of the substance, eg, during a change of state.

Latent Heat of Fusion(L_f)


Latent Heat (enthalpy) of Fusion is the heat absorbed per mole when a substance changes state from solid to liquid at constant temperature (melting point).

The unit for L_f is Joule(J).


Specific Latent Heat of Fusion(l)

The specific latent heat is the amount of energy required to convert 1 kg (or 1 lb) of a substance from solid to liquid (or vice-versa) without a change in the temperature of the surroundings – all absorbed energy goes into the phase change.

Equation:

Q = ml

where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU),

m is the mass of the substance (in kg or in lb), and

L is the specific latent heat for a particular substance (kJ/kg or in BTU/lb); substituted as L_f to represent as the specific latent heat of fusion

Latent Heat of Vapourization(L_v)

Latent Heat (enthalpy) of Vaporization (vaporisation) is the heat absorbed per mole when a substance changes state from liquid to gas at constant temperature (boiling point).

The unit for L<sub>v is Joule(J).


Specific Latent Heat of Vapouriation(l)

The amount of energy required to convert 1 kg (or 1 lb) of a substance from liquid to gas (or vice-versa) without a change in the external temperature is known as the specific latent heat of vaporization for that substance.

Equation:</sub>

_{Q = mL}

where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU),

m is the mass of the substance (in kg or in lb), and

L is the specific latent heat for a particular substance (kJ/Kg or in BTU/lb); substituted as L_v as specific latent heat of vaporization.

Thermal Properties - States of Matter

Phase Transition of Water

Melting and Solidification



This graph shows the phase diagram of water.

At the beginning we have ice at -20 ºC. Ice gain heat in the interval of points A and B, and its temperature becomes 0 ºC that is the melting point of it. We have only ice in the 1st region.

As you can see between the points B and C, temperature of the mass does not change, because its state is changing in this interval. Gained heat is absorbed and spent on breaking the bonds of molecules. 2nd region includes both water and ice. .

After melting process completed, in the 3rd region there is only water and temperature of water starts to increase. When the temperature of the water becomes 100 ºC, it starts to boil and evaporation of it speeds up.

In region 4 our mass exists in two state, water and steam. After completion of evaporation, all water converted to the steam and in region 5 we have only vapor of water.

The graph given below shows the relation of temperature vs. heat of water vapor that lost heat.



This graph shows the condensation of water vapor which lost heat.

As it seen from the graph, steam lost heat and its temperature decreases at 100 ºC, at this temperature it condensate and becomes water, after heat lost it reaches at temperature 0 ºC and starts to freeze. Finally, if it continues to lost heat, their temperature also continues to decrease.

Evaporation




Evaporation is a type of vaporization of a liquid, that occurs only on the surface of a liquid.

Evaporation is a type of phase transition; it is the process by which molecules in a liquid state (e.g. water) spontaneously become gaseous (e.g. water vapor). Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed to a significant volume of gas. Vaporization and evaporation however, are not entirely the same processes.

Evaporation requires thermal energy from the surroundings.Thermal energy from our bodies helps the water on our skin to evaporate taking away thermal energy from the surface of your skin helping us cool down.

Evaporation

1.Occurs at any temperature

2.Slow Process

3.Takes place only at the liquid surface

4.No bubbles are formed in the liquid

5.Temperature may change

6.Thermal energy is supplied by the surroundings


Boiling

1.Occurs at fixed temperature

2.Quick Process

3.Takes place throughout the liquid

4.Bubbles are formed in the liquid

5.Temperature remains constant

6.Thermal energy is supplied by an energy source



How does evaporation take place?

We know that the molecules are never at rest. They can have slight translational and vibrational motions about their mean positions.

They thus posses some amount of kinetic energy. Sometimes the molecules collide with each other and exchange their kinetic energies. Thus at any given time, the kinetic energy of a few molecules may become quite large and they can escape from the surface. The probability of escape from the surface is larger for molecule 1, as its cohesive force holding it back to the liquid is less than that experienced by molecule 2. This is how evaporation takes place.




Factors affecting Rate of Evaporation

Temperature

The rate of evaporation increases as the temperature of a liquid is increased, as it is an endothermic process. For example, a glass of hot water evaporates more rapidly than a glass of cold water.

Surface area

The larger the exposed surface area of the liquid the greater is the number of molecules escaping from its surface.

Humidity of Surrounding Air

If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.

If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.

Flow rate of air

This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is the result of the boundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.

Pressure

Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves.

Strength of intermolecular forces

The ease of evaporation of a liquid is related to the strength of the attractive forces between the molecules in the liquid. In polar liquids cohesive forces are strong while in non-polar liquids the cohesive forces are very weak and the molecules escape easily. For example, ether evaporates more rapidly than ethyl alcohol while ethyl alcohol evaporates quicker than water.

Presence of impurities

Impurities affect the vapour pressure of a liquid appreciably. The non-volatile impurities lower the vapour pressure of a liquid. If the liquid contains impurities, it will have a lower capacity for evaporation.

Worked Examples on Heat Capacity and Specific Heat Capacity

Examples

1. Calculate the amount of heat needed to increase the temperature of 250g of water from 20^oC to 56^oC.

   q = m x C_g x (T_f - T_i)
   m = 250g
   C_g = 4.18 J ^oC^-1 g^-1 (from table above)
   T_f = 56^oC
   T_i = 20^oC

   q = 250 x 4.18 x (56 - 20)
   q = 250 x 4.18 x 36
   q = 37 620 J = 38 kJ

2. Calculate the specific heat capacity of copper given that 204.75 J of energy raises the temperature of 15g of copper from 25^o to 60^o.

   q = m x C_g x (T_f - T_i)
   q = 204.75 J
   m = 15g
   T_i = 25^oC
   T_f = 60^oC

   204.75 = 15 x C_g x (60 - 25)
   204.75 = 15 x C_g x 35
   204.75 = 525 x C_g
   C_g = 204.75 ÷ 204.75 = 0.39 J^oC^-1 g^-1

3. 216 J of energy is required to raise the temperature of aluminium from 15^o to 35^oC. Calculate the mass of aluminium.
   (Specific Heat Capacity of aluminium is 0.90 J^oC^-1g^-1).

   q = m x C_g x (T_f - T_i)
   q = 216 J
   C_g = 0.90 J^oC^-1g^-1
   T_i = 15^oC
   T_f = 35^oC

   216 = m x 0.90 x (35 - 15)
   216 = m x 0.90 x 20
   216 = m x 18
   m = 216 ÷ 18 = 12g

4. The initial temperature of 150g of ethanol was 22^oC. What will be the final temperature of the ethanol if 3240 J was needed to raise the temperature of the ethanol?
   (Specific heat capacity of ethanol is 2.44 J^oC^-1g^-1).

   q = m x C_g x (T_f - T_i)
   q = 3240 J
   m = 150g
   C_g = 2.44 J^oC^-1g^-1
   T_i = 22^oC

   3240 = 150 x 2.44 x (T_f - 22)
   3240 = 366 (T_f - 22)
   8.85 = T_f - 22
   T_f = 30.9^oC

Thermal Properties of Matter

Heat Capacity

What is Internal Energy?

Internal energy is defined as the energy associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale.

For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic . But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole.



If the temperature of a substance rises, it is due to an increase in the average kinetic energy of its particles only.

What is heat capacity?

Heat capacity (usually denoted by a capital C, often with subscripts) is the measurable physical quantity that characterizes the amount of heat required to raise a body's temperature by 1K or 1*C.

Amount of Thermal energy needed depends on Object's mass.

Equation for Heat Capacity:



where,
Q - thermal energy needed in Joules(J)
T - Temperature change in K or *C

The unit for Heat Capacity(C) is J/Kg

Specific Heat Capacity

Specific Heat Capacity (c) of a substance is the amount of heat required to raise the temperature of 1g of the substance by 1oC (or by 1 K).

The units of specific heat capacity are J ^oC^-1 g^-1 or J K^-1 g^-1

Equation for Specific Heat Capacity:





where,
Q= Heat supplied to substance,
m= Mass of the substance,
c= Specific heat capacity,
T= Temperature rise.

Thermocouple Thermometer

What is a Thermocouple?

The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires, e.g., one chromel and one constantan, coupled at the probe tip (measurement junction) and extended to the reference (known temperature) junction.

The temperature difference between the probe tip and the reference junction is detected by measuring the change in voltage (electromotive force, EMF) at the reference junction.

The absolute temperature reading can then be obtained by combining the information of the known reference temperature and the difference of temperature between probe tip and the reference.

How does a thermocouple thermometer work?



If two junction are at different temperatures (e.g. one junction is placed in a hotter region than the other), a small voltage is produced. The greater the difference in temperature, the greater the voltage produced across the ends of the two junctions.If one junction is kept at a fixed temperature such as 0*C, then the other junction can be used as a tiny probe to measure the other temperature.

e.m.f produced is inversely proportional to the temperature difference between reference junction and the probe.

Advantages

- ability to measure a large temperature range
- very useful in a variety of circumstances in many different industries
- respond rapidly to changes in temperature and can be used at high temperatures at which other sensors would be destroyed.
- very versatile,robust,fairly accurate
- small mass
- made rugged, simply constructed and are immune to shock and vibration
- can be made very small, fitting into applications that other temperature sensors
- measure the temperature, as well as be coupled with certain circuits in order to discontinue the heating process once the wanted temperature is achieved.

(This can play an important part in safety measures, thus thermocouples can be considered important safety )