Click here to purchase the entire book in PDF format. What's the difference? Up to now in the book, we've been talking about physical characteristics of sound - things that can be measured with equipment. We haven't thought about what happens from the instant a pressure wave in the air hits the side of your head to the moment you think ``I wish that dog in the neighbour's yard would stop barking'' or ``Won't someone please answer that phone!?'' This section is about exactly that - how does a change in air pressure get translated into you brain recognizing what and where the sound is and, even further, what you think about the sound. This process is separable into three different fields of research: The first, physiological acoustics is the study of how the pressure wave that arrives at the side of your head is modified by the shape and surface characteristics of your body, and how that modified sound wave gets converted into electrical impulses that are sent to your brain. The second, psychoacoustics, is the study of things like hearing thresholds (what is the softest sound you can hear), loudness (i.e. how much louder does a sound have to be when you say that it's ``twice as loud''), just noticeable differences (i.e. how much louder does a sound have to be before you can tell that it's louder), masking (we'll talk about this later), and localization (how can you tell where a sound is coming from when you can't see the source). If any of these things are new to you, don't worry, we'll cover them in this chapter. The third, perception is different. This is the study of how you perceive the sound to be - is the stereo mix wider or narrower, is the sound of the harpsichord bright or dark, does the car's engine sound powerful or wimpy. In some cases, these may not be measurable qualities of the signal coming from the sound source. (For example, you can't buy an ``engine powerfulness'' plug-in for a sound pressure level meter - not yet at least.) It makes sense to think of these three in chronological order - that is to say, we can't talk about how you perceive the sound until we know what sound is in your brain (psychoacoustics) and we can't do that until we know what signal your brain is getting (physiological acoustics). Consequently, this chapter is loosely organized so that it deals with physiology, psychoacoustics and perception, in that order.