HCI Sounds Notes

Hwa Chong Institution Sec 4 Physics 1 Name: _________________________________________ Class: ___________ Date: ______________ Sec 4 Physics (IP / SSMT) Topic 16: Sound I can… By the end of this topic, you should be able to:  relate the loudness and intensity of a sound wave to the square of its amplitude  relate the pitch of a sound wave to its frequency  define ultrasound and give one use of ultrasound  know that a medium is required in order to transmit sound waves and the speed of sound is fastest in solids, then liquids and then air.  describe the production of sound by vibrating sources  describe the longitudinal nature of sound waves in terms of processes of compression and rarefaction  describe how the speed of sound can be determined using reflection of sound wave s  describe how the reflection of sound may produce an echo use it for measuring distances  *describe and explain the Doppler Effect and apply the concept in new situations Basics of Sound 16.1 SOUND PRODUCTION A sound wave is produced by a vibrating / oscillating source placed in an elastic medium. The medium can be any solid, liquid or gas. Usually, it is air. Questions: (a) What is the significance of the statement “Usually, it is air.”? (b) What is it meant by an elastic medium? Which of the following media are elastic; mud, steel, air, or water? (a) For human beings, the best medium for hearing sound is air. (b) Do not mistake elasticity for “ stretchiness”. It simply means the tendency of the particles in the medium to return to their original positions. In humans, our voice production process can be simplified as follows. The diaphragm pushes air from the lungs through the vocal folds in the larynx, the air pressure sets the elastic voca l folds into vibration. This process is called voicing. A periodic train of air pulses is thus produced which moves up the vocal pipe. Hence, the vibrating vocal folds act as the source of the sound waves produced. Image retrieved from http://www.singintune.org/voice-production.html

Hwa Chong Institution Sec 4 Physics 2 16.2 SOUND TRANSMISSION IN AIR Sound is a type of mechanical wave . This means that a sound wave can only pr opagate through a medium. Sound cannot travel through a vacuum. Sound wave in air is an example of a longitudinal wave. This means that energy in a sound wave travels in a direction parallel to the direction of vibration of the particles in the medium. Check you understanding Question: What are compressions and rarefactions? Undisturbed air molecules. If we zoom in more closely, we will observe the molecules to be in constant random motion, in agreement with the Kinetic Model of Matter. Arrangement of air molecules as a sound wave travels through the air. This is a snapshot in time. The picture on the left shows what it means by compressions and rarefactions. (a) When the door is opened, the door will push the molecules away from their initial positions and into their neighbours. The neighbouring molecules, in turn, push into their neighbours and so on, like a compression travelling along a spring, until the curtain flaps out of the window. A pulse of compressed air has travelled from the door to the curtain. This pulse of compressed a ir is called a compression. (b) When we close the door, the door pushes some air molecules out of the room. This produces an area of low pressure behind the door. Neighbouring molecules then move into it, leaving a zone of lower pressure behind them. We say th is zone of lower -pressured air is rarefied. Other molecules farther away from the door, in turn, move into the rarefied regions, and a disturbance again travelled across the room. This is seen by the curtain which flaps inward. This time the disturbance is a rarefaction.

Hwa Chong Institution Sec 4 Physics 3 Representing sound waves in air Instead of drawing dots to represent the air molecules, we can also draw lines to represent the air layers. (i) Layers of air in undisturbed positions. (ii) A compression C is produced as the vibrating source moves rightwards. (iii) A rarefaction R is produced as the vibrating source moves leftwards. (iv) After a while, a series of compressions and rarefactions is set up in the air. A straight source of vibration produces plane waves. 16.3 SOUND DETECTION BY HUMAN The ear lobe receives incoming sound waves and directs them along the canal (about 3cm) towards the ear drum, called the tympanic membrane. The compressions and rarefactions of the longitudinal sound waves cause the ear drum to vibrate. These vibrations are picked up by three bones in the middle ear. These bones act as a lever system for force and pressure amplifications of about 25 times at the oval window. C R C R C R C R C R C

Hwa Chong Institution Sec 4 Physics 4 Vibrations at the oval window cause pressure waves to be formed in the fluid of the inner ear housing the cochlea tube. Inside the cochlea tube, the pressure waves are picked up by the sensory cells that in turn produce neural impulses that are carried by the auditory nerves to the brain. Hence sound is heard. The length of the ear canal causes the human ear to be most sensitive to a sound of frequency of 3,000 Hz. The range of audibility of the human ear is between 20Hz to 20,000 Hz. Sound with very low frequencies which are not audible to the human ear is cal led infrasound, while sound with very high frequencies above 20 kHz is called ultrasound. The way the ear works is that sound waves vibrate the eardrum, just inside your ear. That send s waves through a fluid inside a narrow tube called the cochlea, which in turn vibrates tiny hairs which are tuned to the different pitches of the sound. Information from the vibration of the hairs stimulates nerves which send the signals to the brain for p rocessing. The characteristics of sound include pitch and loudness. You can also determine direction and distance from what you hear. Sounds provide information about the environment around you. The way the ear works is similar to the way a microphone work s, where sound vibrates a diaphragm, which causes electrical signals to travel through a wire to a circuit card for processing. Class Activity: Playing with a sound frequency generator Using a laboratory function generator, it is possible to generate sounds of various frequencies to test your hearing frequency range. However, if we are not too particular about accuracy and precision, we can use an app on the smart phone to achieve similar effects. Download one and play! Like the visible spectrum to our eye, there is also an optimum sound frequency range which to our ear is sensitive. Play with the app to find out.

Hwa Chong Institution Sec 4 Physics 5 16.4 Representing Sound with Graphs (Longitudinal Wave) Pressure vs. Distance Graph Sound waves produced in a hollow pipe of air. At a compression, the air pressure is higher than the normal air pressure where the air is undisturbed. At a rarefaction, the air pressure is lower than the normal air pressure. Pressure – distance graph for the sound wave produced in the hollow tube above Check Your Understanding On the pressure-distance graph above, label: 1. the regions of compression with C and regions of rarefaction with R. 2. the wavelength and pressure amplitude of the sound wave. C C C C R R R R  Pressure amplitude

Hwa Chong Institution Sec 4 Physics 6 3. The figure below shows the (exaggerated) positions of water molecules at an instant whe