Sound Intensity and Vibrations

Sound Intensity ▪Rate that energy flows through a given area – Intensity = (ΔE/Δt)= P. area Intensity is Power ÷ area  watt/m 2 Equation for the area of a circle = 4πr 2

Sound Intensity ▪Vibrating objects do work on the air as they push against the molecules. ▪Intensity is the rate of energy flow through an area. – What is “rate of energy flow” called? ▪  E/t is called power (P). – Since the waves spread out spherically, you must calculate the area of a sphere. How? ▪A = 4  r 2 – So, what is the equation for intensity?

Sound Intensity ▪SI unit: W/m 2 ▪This is an inverse square relationship. – Doubling r reduces intensity by ¼. – What happens if r is halved? ▪Intensity increases by a factor of 4.

Intensity and Decibels ▪An intensity scale based on human perception of loudness is often used. ▪The base unit of this scale is the bel. More commonly, the decibel (dB) is used. – 0.1 bel = 1 dB,1 bel = 10 dB, 5 bels = 50 dB, etc. – The lowest intensity humans hear is assigned a value of zero. ▪The scale is logarithmic, so each increase of 1 bel is 10 times louder. – An increase in intensity of 3 bels is times louder.

Click below to watch the Visual Concept. Visual Concept Human Hearing

Threshold of Hearing SourceIntensity Level # of Times Greater Than TOH Threshold of Hearing (TOH)1* W/m 2 0 dB10 0 Rustling Leaves1* W/m 2 10 dB10 1 Whisper1* W/m 2 20 dB10 2 Normal Conversation1*10 -6 W/m 2 60 dB10 6 Busy Street Traffic1*10 -5 W/m 2 70 dB10 7 Vacuum Cleaner1*10 -4 W/m 2 80 dB10 8 Large Orchestra6.3*10 -3 W/m 2 98 dB Walkman at Maximum Level1*10 -2 W/m dB10 Front Rows of Rock Concert1*10 -1 W/m dB10 11 Threshold of Pain1*10 1 W/m dB10 13 Military Jet Takeoff1*10 2 W/m dB10 14 Instant Perforation of Eardrum1*10 4 W/m dB10 16

Intensity ▪Measured in decibels – Relative intensity- relating the intensityof a given sound to the intensity a the threshold of hearing – Dimensionless because it is a comparison unit

Audible Sounds ▪The softest sound humans can hear is called the threshold of hearing. – Intensity = 1  W/m 2 or zero dB ▪The loudest sound humans can tolerate is called the threshold of pain. – Intensity = 1.0 W/m 2 or 120 dB ▪Human hearing depends on both the frequency and the intensity.

Vibration & Resonance ▪Forced vibrations: The tendency of one object to force another adjoining or interconnected object into vibrational motion ▪Sympathetic vibrations: a vibration produced in one body by the vibrations of exactly the same period in a neighboring body

Forced Vibrations ▪Sympathetic vibrations occur when a vibrating object forces another to vibrate as well. – A piano string vibrates the sound board. – A guitar string vibrates the bridge. ▪This makes the sound louder and the vibrations die out faster. – Energy is transferred from the string to the sound board or bridge.

Vibration & Resonance ▪Natural frequency: The frequency or frequencies at which an object tends to vibrate with when hit, struck, plucked, strummed or somehow disturbed ▪Resonance:when one object vibrating at the same natural frequency of a second object forces that second object into vibrational motion.

Resonance ▪The red rubber band links the 4 pendulums. ▪If a blue pendulum is set in motion, only the other blue pendulum will have large- amplitude vibrations. – The others will just move a small amount. ▪Since the vibrating frequencies of the blue pendulums match, they are resonant.

Resonance ▪Large amplitude vibrations produced when the frequency of the applied force matches the natural frequency of receiver – One blue pendulum was the driving force and the other was the receiver. ▪Bridges have collapsed as a result of structural resonance. – Tacoma Narrows in the wind – A freeway overpass during an earthquake

Click below to watch the Visual Concept. Visual Concept Resonance (Frequency)

Human Ear ▪Transmits vibrations that cause nerve impulses Pathway = Outer ear (ear canal)  Eardrum  3 bones (hammer, anvil, stirrup)  Cochlea  Basilar membranes

Question 1 ▪When the decibel level of traffic in the street goes from 40 to 60 dB, how much louder does the traffic noise seem? ▪How much greater is the intensity?

Question 2 ▪Of the following factors: – Intensity – Speed of sound waves – Frequency – Decibel level – Wavelength – Amplitude Which factors change the loudness of the sound? Which factors change when pitch gets higher?

Question 3 ▪ A tuning fork consists of two metal prongs that vibrate at a single frequency when struck lightly. What will happen if a vibrating tuning fork is placed near another tuning fork of the same frequency?