9How sound reaches to us ?1. Sound waves are collected by the outer ear (or pinna).6.The auditory nervetakes the signalsto the brain.4. The small bones (ossicles) amplify the vibrations.2. The waves travel along the ear canal.3. The waves reach the eardrum and make it vibrate.5. The cochlea turns these into electrical signals.9
10Range of HearingHuman beings can hear sound frequencies between 20 Hz and 20 KHz.Sound whose frequency is less than 20 Hz is called infrasonic sound. Animals like dogs, elephants, rhinoceros, whales etc. produce and hear infrasonic sound.Sound whose frequency is more than Hz is called ultrasonic sound. Animals like dolphins, bats, rats porpoises etc. produce and hear ultrasonic sound.Bats use reflection of ultrasonic sound waves to detect an obstacle or its prey.
11Comparing hearing ranges Which animals hear the lowest and the highest frequencies?100,00010,0001,000100101frequency(Hz)Frequency low (Hz) high (Hz)HumanDogBatElephantMouseCatDolphinhumandogbatelephantmousedolphin
1210.4 Sound need medium to travel The bell-jar experimentvacuumpump on
1310.6 Order of magnitude of speed of sound in different medium The speed at which it travels though a material will therefore depend on how close together the molecules are, and so upon the density of a material - the more tightly packed the molecules, the more quickly vibrations can be transferred from molecule to molecule, i.e. the faster the sound can travel though the material.
14Speed of sound in different materials The speed of sound is approximately:340m/s in air - gas molecules are relatively far apart, and the transfer of vibrations (sound) is relatively slow1400m/s in water - liquid molecules are much closer together, and transfer of vibrations is much more rapid6000m/s in steel - solid molecules are even closer together, and transfer of vibrations is even more rapid
15Sound in different materials Sound needs a substance through which to travel because it travels by making particles vibrate.Which state of matter does sound travel fastest through?Sound waves travel fastest through solids.The particles in a solid are closer together than in a gas, and more tightly bound than in a liquid. This means vibrations are more easily passed from particle to particle, and so sound travels faster.15
20The louder the sound, the greater the amplitude. What is the difference between the sound wave of a quiet sound and a loud sound?quiet soundloud soundThe loud sound has taller waves.The louder the sound, the greater the amplitude.
21What is the difference between the sound wave of a low pitch sound and a high pitch sound? The high pitch sound has a shorter wavelength,so more waves are visible. It has higher frequency waves.
22Frequency PitchAmplitude LoudnessLoudnessThe loudness of a sound increases with the amplitude of the sound wave.loudquiet
23The pitch increases with frequency. low pitchhigh pitch.
24Match the description to the oscilloscope pattern: BCD1. Low-pitched sound, very loudD2. Loud, high-pitched soundC3. Medium sound with medium pitchB4. Quiet, high-pitched soundA
25EchoIf we shout or clap near a reflecting surface like tall building or a mountain, we hear the same sound again. This sound which we hear is called echo. It is caused due to the reflection of sound.To hear an echo clearly, the time interval between the original sound and the echo must be at least 0.1 s.Since the speed of sound in air is 344 m/s, the distance travelled by sound in 0.I s = 344 m/s x 0.1 s = mSo to hear an echo clearly, the minimum distance of the reflecting surface should be half this distance, that is 17.2 m.ReverberationEchoes may be heard more than once due to repeated or multiple reflections of sound from several reflecting surfaces. This causes persistence of sound called reverberation.In big halls or auditoriums to reduce reverberation, the roofs and walls are covered by sound absorbing materials like compressed fiber boards, rough plaster or draperies.