... triangles.2.1
Then again, Pythagoras also believed in reincarnation and thought that if you were really bad, you might come back as a bean, so the Pythagoreans (his followers) didn't eat meat or beans... However, the topic of legumic reincarnation will remain outside the scope of this book. 
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...).2.2
Let's just chat for a minute why it's $\frac{1}{\sqrt{2}}$. Look back at Figure 1.7 and imagine that the wheel has rotated $45^{\circ}$, then draw a right triangle where the hypotenuse is a line connecting the centre of the wheel with the dot on the edge, one side is a line dropping straight down from the dot, and the third side is a horizontal line extending to the right from the centre. Since the angle of rotation is $45^{\circ}$, the triangle is isoceles, meaning that the two short sides are of equal length. We also know from Section 1.2 that $a = \sqrt{b^{2} + c^{2}}$. Since $b=c$, then $a = \sqrt{2 b^{2}}$. As we said a couple of paragraphs earlier, the radius of the circle (and therefore the length of the hypotenuse) is 1. Therefore $\sqrt{2 b^{2}} = 1$. Therefore $\sqrt{2} \sqrt{ b^{2}} = 1$. Therefore $\sqrt{ b^{2}} = 1 / \sqrt{2}$. Therefore $b = 1 / \sqrt{2}$. 
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... up.2.3
Okay, okay, so there were no committee meetings. What actually happened was a little more complicated, involving a soap-operatic dispute between three Italian mathematicians named Fontana, Cardano and Bombelli that lived around 1500.[Clegg, 2003] 
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... Leonhard2.4
Historical side note: Leonard Euler lived at the court of King Frederick the Great, King of Prussia, in Potsdam for 25 years (as did Voltaire and La Mettrie). This is the same King Frederick that took flute lessons from Joachim Quantz and who bought 15 newfangled ``piano-forte's'' (better known to us as a ``piano'') from Silbermann because he thought that this new instrument showed some promise. However, King Frederick's most important contribution to the history of music is that he composed the theme that J.S. Bach used to create the ``Musikalisches Opfer'' or ``Musical Offering.'' 
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... nothing.2.5
For those of you that are interested in the intersection of mathematics with philosophy, you might want to read about a Greek philosopher named Zeno and his idea that movement was impossible.[Clegg, 2003] For example, in order to walk to the store, you have to walk half the distance first. Before you get that far, you have to walk half that distance, and before you get that far, you have to walk half that distance, and so on and so on. If you continue with this logic, then you have to start by making an infinitely small distance, and since $1 / \infty = 0$ then you never move in the first place. This makes it difficult to buy milk. 
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... sign.2.6
In fact, it intentionally looks like a capital letter S since it was originally meant to stand for summa - the Latin word for sum[Clegg, 2003] 
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... now...)3.1
If numbers that look like $20 * 10^{-6}$ give you a headache, check out Chapter 1.10. 
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... kPa)4.1
A Pascal is a unit of pressure (Unless, of course, you live in the United States and insist on using a different measurement system than the rest of us.). How much pressure? Well, if there was no gravity and you had a 1 kg block of steel and you pushed it with a pressure of 1 Pa, the block would accelerate at 1 meter per second per second. So, if you pushed for 1 second and stopped, the block would be moving 1 m/s. If you pushed for 2 seconds it would be moving 2 m/s and so on... This is one way to think of a Pascal. The other definition is that 1 Pascal equals the force of 1 Newton (abbreviated N) distributed over 1 square metre. The problem with this definition is that we have to worry about what a Newton is... Not surprisingly, a Newton is the amount of force it takes to accelerate 1 kg at a rate of 1 meter per second squared. 
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... cubed)4.2
There is a lot of confusion regarding the units for acoustic impedance. You will see books talking about an acoustic ohm. The problem with this unit is that it has different values depending on what system you're using. Usually, 1 acoustic ohm = 1 N s / m$^5$, but this is not necessarily the case, so be careful[Morfey, 2001]. 
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... boundary4.3
a line perpendicular to the surface and intersecting the point of reflection 
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...phons. 6.1
Note: I got an email from Bert Noeth, a professor teaching sound and acoustics in Belgium who tells me that, in Europe, phons are called isophons. 
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... difference6.2
This is a fancy term meaning ``the difference in the times of arrival of the signals at your two ears'' 
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... idea9.1
In fairness, I actually stole this response from Mohandas Ghandi. While visiting London, he was once asked ``Mr. Ghandi, what do you think of western civilization?'' Ghandi responded, ``I think it would be a very good idea.'' 
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... mathematician10.1
Paul Dirac was actually half-Swiss and half English. He was born in 1902 in Bristol, England and died in 1984 in Tallahassee, Florida, USA. He worked with Heisenberg and Niels Bohr among others. His principal claim to fame was that he reconciled Quantum Mechanics and Einstein's Theory of Relativity. 
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