Physics Study Notes: SOUND

Reflection

• The bouncing back of sound when it strikes a hard surface is called a reflection of sound.
• The laws of reflection of light are also obeyed during reflection of sound.
• The working of megaphone, soundboards and ear trumpet is based on the reflection of sound.
• The repetition of sound due to the reflection of sound waves is called an echo.
• The persistence of hearing on human ear is 1/10th of a second.
• The minimum distance from a sound reflecting surface to hear an echo is nearly is nearly 17 m.
• Soundproof rooms are made of two layers of walls having the vacuum between them.
• Reverberation arises due to multiple reflections of sound.
• While designing an auditorium for speech or musical concerts, one has to take proper care for the absorption and reflection of sound.
• Time taken by reverberant sound to decrease its intensity by a factor of 106 is called reverberation time.

Refraction

• When a sound wave moves from one mechanical medium to another mechanical medium, it shows deviation from the original path of the incident wave. The phenomenon is called refraction. It is due to the difference is the speed of sound in media.

Diffraction

• When sound waves originated by a vibrating source, they spread in the medium and if the medium is homogeneous, this leads to bending of sound waves around the edges. Which is known as diffraction.
• The sound waves diffracted broadly and one can easily hear the voice of another person.

Musical Scale

• In theory of music, a musical scale is a set of musical notes by the frequencies of which are in simple ratios to one another. Sa, re, ga, ma, pa, dha, ni is one such scale called the diatonic scale. The interval sa-sa is called an octave (8).

Noise Reduction in Recording Media

• Five types of noise reduction system exist in recording media as discussed below
• Dolby A noise reduction system, intended for use in professional recording studios. It provided about 10 dB of broadband noise reduction.
• Dolby B was developed to achieve about 9 dB noise reduction primarily for cassettes. It was much simpler than Dolby A and therefore less expensive to implement in consumer products.
• Dolby C provides about 15 dB noise reduction.
• Dolby SR (Spectral Recording) system is much more aggressive noise reduction approach than Dolby A. Dolby SR is much more expensive to implement than Dolby B or C, but it is capable of providing up to 25 dB noise reduction in the high-frequency range.
• Dolby S is found on some Hi-Fi and semi-professional recording equipment. It is capable of 10 dB of noise reduction at low frequencies and up to 24 dB of noise reduction at high frequencies.

SONAR
• SONAR stands for Sound Navigation and Ranging. It is used to measure the depth of a sea, to locate the enemy submarines and shipwrecks.
• The transmitter of a sonar produces pulses of ultrasonic sound waves of the frequency of about 50000 Hz. The reflected sound waves are received by the receiver.
Human Ear
• We are able to hear with the help of an extremely sensitive organ of our body called the ear. There are three parts of the human ear.
• The outer ear is called pinna. It collects the sound from the surroundings. The middle ear transmits the amplified pressure variations received from the sound wave to the inner ear.
• In the inner ear, the pressure variations are turned into electrical signals by the cochlea. These electrical signals are sent to the brain via the auditory nerve and the brain interpret them as sound.