The amplitude of a sound wave can be quantified in at least three ways:
None of these quantities are used much, however. In fact, the first two are unusually difficult to measure directly. For typical sound waves, the maximum displacement of the molecules in the air is only a hundred or a thousand times larger than the molecules themselves. Any resulting density fluctuations are equally miniscule and very short lived. (The period of sound waves is typically measured in milliseconds.)
Pressure fluctuations caused by sound waves are much easier to measure (animals have been doing it for hundreds of millions of years with ears), but the results of such measurements are rarely ever reported. Instead, amplitude measurements are almost always used as the raw data in some computation. When done by an electronic circuit — like the circuits in a level meter — the resulting value is called the intensity. When done by a neuronal circuit — like the circuits in your brain — the resulting sensation is called the loudness.
Briefly, the intensity of a sound wave is a combination of its rate and density of energy transfer. It is an objective quantity associated with a wave. Loudness is a perceptual response to the physical property of intensity. It is a subjective quality associated with a wave and is a bit complex. As a general rule the larger the amplitude, the greater the intensity, the louder the sound. Sound waves with large amplitudes are said to be "loud". Sound waves with small amplitudes are said to be "quiet" or "soft". (The word "low" is sometimes also used to mean quiet, but this should be avoided. Use the word "low" only to describe sounds that are low in frequency or pitch.) Loudness will be discussed at the end of this section.
By definition, the intensity of any wave is the timeaveraged power it transfers per area through some region of space. The traditional way to indicate the timeaveraged value of a varying quantity is to enclose it in angle brackets. These look similar to greater and less than symbols but are taller and less pointy. That gives us an equation that looks something like this…
I =  ⟨P⟩ 
A 
The unit of intensity is the watt per square meter — a unit that has no special name.
For simple mechanical waves like sound, intensity is related to the density of the medium and the speed, frequency, and amplitude of the wave. This can be shown with a long, horrible, calculation. Jump to the next highlighted equation if you don't care to see the sausage being made below.
Start with the definition of intensity. Replace power with energy (both kinetic and elastic) over time (one period, for convenience sake).
I =  ⟨P⟩  =  ⟨E⟩/T  =  ⟨K + U_{s}⟩/T 
A  A  A 
Since kinetic and elastic energies are always positive we can split the timeaveraged portion into two parts.
⟨P⟩ =  ⟨E⟩  =  ⟨K + U_{s}⟩  =  ⟨K⟩  +  ⟨U_{s}⟩  
T  T  T  T 
Mechanical waves in a continuous media can be thought of as an infinite collection of infinitesimal coupled harmonic oscillators. Little masses connected to other little masses with little springs as far as the eye can see. On average, half the energy in a simple harmonic oscillator is kinetic and half is elastic. The timeaveraged total energy in then either twice the average kinetic energy or twice the average potential energy.
⟨P⟩ =  2⟨K⟩  =  2⟨U_{s}⟩  
T  T 
Let's work on the kinetic energy and see where it takes us. It has two important parts — mass and velocity.
K = ½mv^{2}
The particles in a longitudinal wave are displaced from their equilibrium positions by a function that oscillates in time and space.
Δx(x,t) = Δx_{max} sin  ⎡ ⎣ 
2π  ⎛ ⎝ 
ft −  x  ⎞⎤ ⎠⎦ 
λ 
Take the time derivative to get velocity.
v(x,t) =  ∂  Δx(x,t) = 2πƒΔx_{max} cos  ⎡ ⎣ 
2π  ⎛ ⎝ 
ft −  x  ⎞⎤ ⎠⎦ 
∂t  λ 
Then square it.
v^{2}(x,t) = 4π^{2}ƒ^{2}Δx^{2}_{max} cos^{2}  ⎡ ⎣ 
2π  ⎛ ⎝ 
ft −  x  ⎞⎤ ⎠⎦ 
λ 
On to the mass. Density times volume is mass. The volume of material we're concerned with is a box whose area is the surface through which the wave is traveling and whose length is the distance the wave travels.
m = ρV = ρAx
In one period a wave would move forward one wavelength. In the volume spanned by a single wavelength, all the bits of matter are moving with different speeds. Calculus is needed to combine a multitude of varying values into one integrated value. We're dealing with a periodic system here, one that repeats itself over and over again. We can choose to do our calculation at any time we wish as long as we finish at the end of one cycle. For convenience sake let's choose time to be zero — the beginning of a sinusoidal wave.





Clean up the constants.
1  (ρA)(4π^{2}ƒ^{2}Δx^{2}_{max}) = 2π^{2}ρAƒ^{2}Δx^{2}_{max} 
2 
Then work on the integral. It may look hard, but it isn't. Just visualize the cosine squared curve traced out over one cycle. See how it divides the rectangle bounding it into equal halves?
The height of this rectangle is one (as in the number 1 with no units) and its width is one wavelength. That gives an area of one wavelength and a halfarea of half a wavelength.
λ  
⌠ ⌡ 
cos^{2}  ⎡ ⎣ 
− 2π  x  ⎤ ⎦ 
dx =  1  λ 
λ  2  
0 
Put the constants together with the integral and divide by one period to get the timeaveraged kinetic energy. (Remember that wavelength divided by period is speed.)



That concludes the hard part. Double the equation above and divide by area…



One last bit of algebra and we're done.
I = 2π^{2}ρƒ^{2}vΔx^{2}_{max}
Does this formula make sense? Check to see how each of the factors affect intensity.
factor  comments 

I ∝ ρ  The denser the medium, the more intense the wave. That makes sense. A dense medium packs more mass into any volume than a rarefied medium and kinetic energy goes with mass. 
I ∝ ƒ^{2}  The more frequently a wave vibrates the medium, the more intense the wave is. I can see that one with my mind's eye. A lackluster wave that just doesn't get the medium moving isn't going to carry as much energy as one that shakes the medium like crazy. 
I ∝ v  The faster the wave travels, the more quickly it transmits energy. This is where you have to remember that intensity doesn't so much measure the amount of energy transferred as it measures the rate at which this energy is transferred. 
I ∝ Δx^{2}_{max}  The greater the amplitude, the more intense the wave. Just think of ocean waves for a moment. A hurricanedriven, wallofwater packs a lot more punch than ripples in the bathtub. The metaphor isn't visually correct, since sound waves are longitudinal and ocean waves are complex, but it is intuitively correct. 
Don't forget to breathe, then discuss intensity and pressure amplitude. Amplitude is measured in meters [m] while pressure amplitude is measured in pascals [Pa] or more commonly millipascals [mPa] or most commonly micropascals [μPa]. Intensity is proportional to the square of pressure amplitude.
I =  ⟨power⟩  =  ⟨F⟩v  = ⟨pressure⟩v 
A  A 
For simple harmonic motion…
v_{max} = 2πƒΔx_{max}
Dimensional analysis game…
…
The range of audible sound intensities is so great, that it takes six orders of magnitude to get us from the threshold of hearing (20 μPa) to the threshold of pain (20 Pa).
Given a periodic signal of any sort, its level (β) in bels [B] is defined as the base ten logarithm of the ratio of its intensity to the intensity of a reference signal. Since this unit is a bit large for most purposes, it is customary to divide the bel into tenths or decibels [dB]. The bel is a dimensionless unit.
β[dB] = 10 log  ⎛ ⎝ 
I  ⎞ ⎠ 
I_{0} 
 
 

β[dB] = 20 log  ⎛ ⎝ 
P  ⎞ ⎠ 
P_{0} 
By convention, sound has a level of 0 dB at a pressure intensity of 20 μPa and frequency of 1,000 Hz. This is the generally agreed upon threshold of hearing for humans. Sounds with intensities below this value are inaudible to (quite possibly) every human.
Notes
It would be equally reasonable to use natural logarithms in place of base ten, but this is far, far less common. Given a periodic signal of any sort, the ratio of the natural logarithm of its intensity to a reference signal is a measure of its level (β) in nepers [Np]. As with the bel it is customary to divide neper into tenths or decinepers [dNp]. The neper is also a dimensionless unit.
β[dNp] = 10 ln  ⎛ ⎝ 
I  ⎞ ⎠ 
= 20 ln  ⎛ ⎝ 
P  ⎞ ⎠ 
I_{0}  P_{0} 
The neper and decineper are so rare in comparison to the bel and decibel that is essentially the answer to a trivia question.
Notes and quotes.
intensity (dB)  source 

−∞  absolute silence 
−9  world's quietest room (Orfield Labs, Minneapolis) 
00–10  threshold of hearing, anechoic chamber 
10–20  normal breathing, rustling leaves 
20–30  whispering at 5 feet 
30–40  
40–50  coffee maker, library, quiet office, quiet residential area 
50–60  dishwasher, electric shaver, electric toothbrush, large office, rainfall, refrigerator, sewing machine 
60–70  air conditioner, automobile interior, alarm clock, background music, normal conversation, television, vacuum cleaner, washing machine 
70–80  coffee grinder, flush toilet, freeway traffic, garbage disposal, hair dryer 
80–90  blender, doorbell, bus interior, food processor, garbage disposal, heavy traffic, hand saw, lawn mower, machine tools, noisy restaurant, toaster, ringing telephone, whistling kettle Employers in the United States must provide hearing protectors to all workers exposed to continuous noise levels of 85 dB or above. 
090–100  electric drill, shouted conversation, tractor, truck 
100–110  baby crying, boom box, factory machinery, motorcycle, school dance, snow blower, snowmobile, squeaky toy held close to the ear, subway train, woodworking class 
110–120  ambulance siren, car horn, chain saw, disco, football game, jet plane at ramp, leaf blower, personal cassette player on high, power saw, rock concert, shouting in ear, symphony concert, video arcade, loudest clap (113 dB Alistair Galpin) 
120–130  auto stereo, band concert, chain saw, hammer on nail, heavy machinery, pneumatic drills, stock car races, thunder, power drill, percussion section at symphony 
130–140  threshold of pain, air raid siren, jet airplane taking off, jackhammer 
140–150  
150–160  artillery fire at 500 feet, balloon pop, cap gun, firecracker, jet engine taking off 
160–170  fireworks, handgun, rifle 
170–180  shotgun 
180–190  rocket launch, volcanic eruption 
190–200  
+∞  infinitely loud 
Extended quote that needs to be paraphrased.
Magnitude scales are quantitative.With these scales, one measures the size of the earthquake as expressed by the seismic wave amplitude (amount of shaking at a point distant from the earthquake) rather than the intensity or degree of destructiveness. Most magnitude scales have a logarithmic basis, so that an increase in one whole number corresponds to an earthquake 10 times stronger than one indicated by the next lower number. This translates into an approximate 30fold increase in the amount of energy released. Thus magnitude 5 represents ground motion about 10 times that of magnitude 4, and about 30 times as much energy released. A magnitude 5 earthquake represents 100 times the ground motion and 900 times the energy released of a magnitude 3 earthquake.
The Richter scale was created by Charles Richter in 1935 at the California Institute of Technology. It was created to compare the size of earthquakes. One of Dr. Charles F. Richter's most valuable contributions was to recognize that the seismic waves radiated by all earthquakes can provide good estimates of their magnitudes. He collected the recordings of seismic waves from a large number of earthquakes, and developed a calibrated system of measuring them for magnitude. He calibrated his scale of magnitudes using measured maximum amplitudes of shear waves on seismometers particularly sensitive to shear waves with periods of about one second. The records had to be obtained from a specific kind of instrument, called a WoodAnderson seismograph. Although his work was originally calibrated only for these specific seismometers, and only for earthquakes in southern California, seismologists have developed scale factors to extend Richter's magnitude scale to many other types of measurements on all types of seismometers, all over the world. In fact, magnitude estimates have been made for thousands of moonquakes and for two quakes on Mars.
Most estimates of energy have historically relied on the empirical relationship developed by Beno Gutenberg and Charles Richter.
log_{10} E_{s} = 4.8 + 1.5 M_{s}
where energy, E_{s}, is expressed in joules. The drawback of this method is that M_{s} is computed from a bandwidth between approximately 18 to 22 s. It is now known that the energy radiated by an earthquake is concentrated over a different bandwidth and at higher frequencies. Note that this is not the total "intrinsic" energy of the earthquake, transferred from sources such as gravitational energy or to sinks such as heat energy. It is only the amount radiated from the earthquake as seismic waves, which ought to be a small fraction of the total energy transferred during the earthquake process.
With the worldwide deployment of modern digitally recording seismograph with broad bandwidth response, computerized methods are now able to make accurate and explicit estimates of energy on a routine basis for all major earthquakes. A magnitude based on energy radiated by an earthquake, M_{e}, can now be defined. These energy magnitudes are computed from the radiated energy using the Choy and Boatwright (1995) formula
M_{e} = (2/3) log_{10} E_{s} − 2.9
where E_{s} is the radiated seismic energy in joules. M_{e}, computed from high frequency seismic data, is a measure of the seismic potential for damage.