sound

Audible range of frequency for humans

20Hz to 20,000Hz

Ultrasonic frequencies

Frequencies above 20,000 Hz

Infrasonic frequencies

Frequencies below 20 Hz

Amplitude

The maximum displacement of a particle of a medium on either side of its mean position.

Time Period

The time taken by the particle of medium to complete one vibration.

Frequency

The number of vibrations made by the particle of the medium in one second.

Wavelength

The distance travelled by the wave in one time period of vibration of particle of a medium.

Wave velocity

The distance travelled by the wave in one second.

Velocity - frequency - wavelength relation

V = f * wavelength lambda

frequency time period relation

f = 1/T

Mechanical waves

A wave that transfers energy through a medium

Longitudinal waves

A wave that travels with compressions and rarefactions and in which particles move in the direction of movement of the wave.

Speed of longitudinal wave in gaseous medium

V = root of[(ratio of two specific heats*pressure)/density]

When a wave travelling in a medium enters another medium, what quality of the wave doesn't change?

Frequency

Reflection of sound waves

When sound waves, upon striking a hard surface, return back in the same medium.

Echo

The sound heard after reflection from a distant obstacle after the original sound has ceased.

Conditions for echo to be heard.

1. In air, reflecting surface must be at a minimum distance of 17m from the listener.

2. The reflecting surface has to be larger than the wavelength of the sound.

3. The intensity of sound should be such that reflected sound on reaching the ear is audible.

Velocity distance time relation for reflection of sound

v = 2d/t

Useful properties of ultrasonic waves

1. They can travel undeviated through large distances.

2. They can be confined to a narrow beam

3. They are not easily absorbed in a medium.

Sound ranging

The process of detecting obstacles by using the reflection of ultrasonic waves.

Natural vibrations

The periodic vibrations of a body in the absence of any external force on it, are called natural vibrations. Only occur in vacuum because medium particles provide resistance which reduces the amplitude over time.

Natural frequency of simple pendulum

1/2pi * root[g/l] where g is acceleration due to gravity and l is length of pendulum

Relation between natural frequency of air column of flute and length of air column

inversely proportional

Frequency of a note produced by plucking a string on a musical instrument can be increased by?

1. Decreasing the length of string

2. Decreasing the radius of string

3. Increasing the tension in the string

Damped vibrations

The periodic vibrations of a body of decreasing amplitude in presence of a resistive force

Forced vibrations

The vibrations of a body which take place under the influence of an external periodic force acting on it

What are the forces acting upon a body executing forced vibrations

1. The restoring force

2. The frictional (resistive or damping) force

3. The external periodic force or driving force

Natural vs Forced vibrations

1. In natural vibration, there is no resistive force. In forced vibrations, there is resistive force as well as an external periodic force.

2. The frequency of natural vibration depends on the size and shape of the body. The frequency of forced vibrations is equal to that of the applied force.

3. Frequency in natural vibration remains constant. In forced vibrations, the frequency changes with change in the frequency of applied force.

Resonance

A special case of forced vibrations that occurs when the frequency of externally applied periodic force is equal to the natural frequency of the body, in which case the body begins to vibrate readily with an increased amplitude.

Forced vibration vs Resonance

1. Vibrations of a body under an external periodic force of frequency different to the natural frequency of the body are known as forced vibrations. Vibrations of a body under an external periodic force where the frequency of the applied force is equal to the natural frequency of the body are called resonant vibrations.

2. Forced vibrations have smaller amplitude while resonant vibrations have a very large amplitude

3. The vibrations of the body are not in phase with the applied force in forced vibrations while in resonance, they are in phase.

Resonance in machine parts

In a vehicle or machine, it is possible that a some parts have a natural frequency of vibration that is the same as the frequency of the to-and-fro motion of the piston. At this particular speed, the resonant parts start vibrating vigorously and cause a rattling sound. If the parts are not attached properly, they can also dislodge and drop out.

Resonance in a stretched string and sound box of musical instruments

A vibrating string by itself does not produce a very prominent sound. Therefore all stringed musical instruments are provided with a hollow sound box. The instruments are so constructed that the column of air inside the sound box has the same natural frequency of the stretched strings, and thus when the strings are plucked, the air column starts vibrating with the same frequency and large amplitude, creating a loud sound.

Resonance in a bridge

When a troop of soldiers has to cross a suspension bridge, they are asked to break their steps. This is done so that the frequency of the periodic marching doesn't become equal to the natural frequency of the bridge, which would cause the bridge to vibrate with a large amplitude and possibly collapse.

Relation between length and frequency of wire

inversely proportional

Loudness

Characteristic of sound by virtue of which a loud sound can be distinguished from a faint one, both having the same pitch and quality. It depends in the amplitude of the wave.

Factors affecting loudness

1. Loudness is proportional to the square of amplitude

2. Loudness varies inversely as the square of distance from the source

3. Loudness depends on the surface area of vibrating body. Larger area sends forth a greater amount of energy

4. Loudness depends on the density of the medium 5.Loudness depends on the presence of resonant bodies.

loudness vs intensity

Loudness is perceived: qualitative Intensity is not: quantitative

1 dB

the increase in level of loudness when intensity increases by 26%

Pitch

The characteristic of sound by which a sharp (shrill) sound can be distinguished from a grave (flat) sound when loudness and quality are the same. Depends on frequency

Pitch vs. Frequency

Pitch is perceived and is subjective while frequency is a measurable quantity

Quality or timbre

The characteristic that distinguished two sounds of the same loudness and pitch but emitted by two different instruments, on the basis of differences in the wave forms. It depends on the wave form.

Principal vibration

The vibration of lowest frequency and maximum amplitude.

Quality of musical sound depends on

the number of subsidiary notes and their relative amplitude, as well as the principal note.

Music vs Noise

Differences are subjective. Music consists of regular, periodic and smooth vibrations while noise has irregular, harsh vibrations. The sound level of music is low while that of noise is high. Music has a regular wave form while noise has an irregular waveform.

Safe limit of sound level

80dB