What Do We Know About Sound Waves So Far?• Sound is a longitudinal wave that moves through air, or another substance.• In general, the more dense a substance is, the faster the sound waves move.• Hes Megatrons favourite Decepticon.
Speed of Sound• The speed of sound (in m/s) in air is based on the temperature by the equation... v = 331 + 0.6 TC where TC is the temperature in oC• This is an approximation. It doesn’t work for temperatures that are very high or low.
• For a typical room temperature of about 22oC, this equates to a speed of about 344 m/s or 1240 km/h. v = 331 + 0.6 (22) = 331+13.2 = 344 m/s
• You may have heard of jet speeds rated in mach numbers.• Mach 1 is defined as the speed of sound in air... any number other than 1 is a multiple of this.
• For example, the maximum speed of an F/A- 18 Hornet fighter jet, (the Canadian military standard), is mach 1.7, or 580 m/s.
• The fastest jet (on record) is the SR-71 Blackbird, with a top speed of mach 3.3.
• The space shuttle orbits Earth at about 8000 m/s, or mach 23.
• The terms subsonic and supersonic just mean less than, or greater than, the speed of sound.
The Beoing 474 “Jumbo Jet” is subsonic (mach 0.75)
The Concorde is supersonic (mach 2.2)
The Doppler Effect• The Doppler Effect occurs any time a sound with a (fairly) consistent frequency is emitted from a moving object.• If the object is moving relatively slowly compared to the speed of sound, then the Doppler Effect is barely noticeable, but if you start to get near, or above the speed of sound it can be dramatic.
• Since the sound waves given off move outwards in spheres as the source moves, the circles wont have the same centers.• This causes the circles to be closer together than in front of the sound source (shorter wavelength and higher frequency) and farther apart (longer wavelength and lower frequency) behind the source.
Sheldon: Big Bang Theory• "I don’t care if anybody gets it, I’m going as the Doppler Effect. If I have to, I can demonstrate. Neeeoooow."
Redshift / Blueshift of Light• This is the Doppler Effect for visible light as opposed to sound.
Sonic Booms• An extreme example of the Doppler Effect occurs when the source of the sound is moving at, or above, mach 1.• At mach 1, all sound waves given off by the object build up in front of it, since the waves themselves are moving at the exact same speed as the object. The longer the sound is given off, the more waves build up in front of the object. This creates a tremendous compression in the air known as a sonic boom.
Jet at Mach 1.0
• Objects moving faster than the speed of sound leave a cone shaped bundle of sound waves behind them, which is called a mach cone.
Bullet at Mach 2.45
Sound Intensity• The sound intensity (loudness) of a sound is measured in W/m2 or decibels (dB).• These are 2 very different scales, but both measure the same thing.
• When an object makes a sound, it gives off sound energy in a sphere.• As the sound gets farther from the source, that sphere gets bigger, and the surface area of the sphere increases proportionally to the radius of the sphere.• Since the same amount of total sound energy is in the sphere regardless of its size, the amount of energy per unit area (surface area) must drop as the radius gets larger.
• This follows a typical inverse square law, which basically means if you double the radius (or distance from the source) you go down to 1/4th of the intensity per unit area. Triple the radius and you go down to 1/9th of the intensity per unit area, and so on.• We have an amount of energy per second (J/s) going through a unit area, we measure sound intensity in W/m2 (watts per m2).
• It has been determined that for most people, the lowest sound intensity that we can hear is about 10-12 W/m2. The decibel system is based on comparing a sounds intensity with this base value.• The decibel system is not linear - its logarithmic. If you dont understand logarithms, dont fear... Ill make this part as painless as possible. :)
• Basically, you take the intensity of the sound, and divide it by the intensity of the quietest sound we can hear. Express your answer as a power of ten.• For example, say the sound youre talking about has an intensity of 10-5 W/m2, which is about the intensity of sound most people would watch TV at.
• Step 1: Divide the sound intensity by the quietest sound we can hear.• Step 2: Take the exponent on the 10... thats the number of bels (B) of the sound. So the TV is 7 bels, or 7 B.• Step 3: Take that and multiply it by 10 to get it in dB, (the prefix "d" means "deci", or 1/10th).• Our TV is 70 dB.
• So, say that we fire a rifle, and find that it produces a sound with an intensity of 20,000 W/m2. Step 1: Divide this by 10-12 W/m2 Step 2: Take the log of our answer. log (2 x 1016) = 16.3 (Note: 2 x 1016 = 1016.3) Step 3: Multiple by 10 16.3 B = 163 dB So the sound intensity of our rifle was 163 dB.
• By definition, the quietest sound that the average person can hear is 0 dB.• It is possible for some people and animals to hear quieter sounds (negative dB), but the average person cant detect them.
dB Scale Points of Reference• 0 dB the softest sound an average person can hear• 10 dB normal breathing• 20 dB whispering at 5 feet• 30 dB soft whisper• 50 dB rainfall• 60 dB normal conversation• 110 dB shouting in ear• 120 dB thunder
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