Range of Hearing | Sound

curved soundboard may be placed behind the stage so that the sound, after reflecting from the sound board, spreads evenly across the width of the hall (Fig 12.15). Megaphone Horn Fig 12.12: A megaphone and a horn. Fig. 12.14: Curved ceiling of a conference hall. In these instruments, a tube followed by a conical opening reflects sound successively to guide most of the sound waves from the source in the forward Sound board direction towards the audience. 2. Stethoscope is a medical instrument used for listening to sounds produced within the body, chiefly in the heart or lungs. In stethoscopes the sound of the patient's heartbeat reaches the doctor's ears by multiple reflection of sound, as shown in Fig. 12.13. Source of sound Fig. 12.15: Sound board used in a big hall. uestion 1. Why are the ceilings of concert halls curved? Fig.12.13: Stethoscope 3. Generally the conference halls and cinema halls are ngs of concert halls, 12.4 Range of Hearing curved so that sound after reflection The audible range of sound for human beings extends from about 20 Hz to 20000 Hz (one = one cycle/s). Children under the age of reaches all corners of the hall, as shown in Fig 12.14. Sometimes a Hz five and some animals, such as dogs can hear up to 25 kHz (1 kHz = 1000 Hz). As people grow older their ears become less sensitive to higher frequencies. Sounds of frequencies below 20 Hz are called infrasonic sound or 12.5 Applications of Ultrasound Ultrasounds are high frequency waves. Ultrasounds are able to travel along well- defined paths even in the presence of obstacles. Ultrasounds are used extensively in industries and for medical purposes. Ultrasound is generally used to clean parts located in hard-to-reach places, for example, spiral tube, odd shaped parts, electronic components etc. Objects to be cleaned are placed in a cleaning solution and ultrasonic infrasound. If we could hear infrasound we would hear the vibrations of a pendulum just as we hear the vibrations of the wings of a bee. Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales and elephants produce sound in the infrasound range. It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before the main shock waves waves are sent into the solution. Due begin which possibly alert the animals. Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Ultrasound to the high frequency, the particles of dust, grease and dirt get detached and drop out. The objects thus get thoroughly cleaned. is produced by dolphins, bats and porpoises. Moths of certain families have very sensitive hearing equipment. These moths can hear the high frequency squeaks of the bat and know when a bat is flying nearby, and are able to escape capture. Rats also play games by producing ultrasound. Ultrasounds can be used to detect cracks and flaws in metal blocks. Metallic components are generally used in construction of big structures like buildings, bridges, machines and also scientific equipment. The cracks or holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. Ultrasonic waves are allowed to pass through the Hearing Aid: People with hearing loss may need a hearing aid. A hearing aid is an electronic, battery operated device. The hearing aid receives sound through a microphone. The microphone converts the sound waves to electrical signals. These electrical signals are amplified by an amplifier. The amplified electrical signals are given to a speaker of the hearing aid. The speaker converts the amplified electrical signal to sound and sends to the ear for clear hearing. metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of the flaw or defect, as shown in Fig. 12.16. Defect or flaw uestions 1. What is the audible range of the average human ear? 2. What is the range of frequencies associated wvith Metal Block (a) Infrasound? (b) Ultrasound? Fig 12.16: Ultrasound is reflected back from the defective locations inside a metal block. Ultrasound Detectors