Physical characteristics of sound Amplitude – The measure of displacement of the air pressure wave Frequency – Represents the number of periods in a second. – and is measured in hertz (Hz) or cycles per second.

Psychological characteristics of sound (I) Sound has three defining characteristics: 1- loudness – how loud (intense) the sound appears 2- pitch – can be said to be simply the pitch of a sound is determined by its frequency 3- timbre – the nature of the sound (e.g. distinctive timbre of instruments) All sounds have a loudness, – but many are unpitched timbre is often used as a catch -all term to describe those aspects of the sound not captured by loudness and pitch.

Psychological characteristics of sound (II) Direction – Sound is normally generated by some source and normally there are lots of concurrent sources – and each source has some location – so that the sound from it is perceived to come from some direction

Basically, brain identifies source of a sound on the basis of differences in intensity and phase between the signals received from the Left (L) and Right (R) ears Earlier & louder L R

Psychological characteristics of sound (III) Distance – we can often also tell (roughly) the distance of the sound source – this comes partly from the loudness of the sound, and partly from other characteristics of the sound Physical correlates of distance are – reflections and spectral shape

Psychological characteristics of sound (IV) Associations – many sounds have associations – these may be obvious (and usable)… breaking glass scream door slamming – or may be personal (and different for each individual)

Nyquist Theorem The sampling frequency determines the limit of audio frequencies that can be reproduced digitally. One of the most important rules of sampling is called the Nyquist Theorem, which states that the highest frequency which can be accurately represented is less than one-half of the sampling rate. So, if we want a full 20 kHz audio bandwidth, we must sample at least twice that fast, i.e. over 40 kHz. If we don't, bad things happen. Here's our example sine wave

Now if we sample at twice the sample frequency, i.e Nyquist Rate, we start to make some progress. An alternative way of viewing thr waveform (re)genereation is to think of straight lines joining up the peaks of the samples. In this case (at these sample points) we see we get a sawtooth wave that begins to start crudely approximating a sine wave

Nyquist rate -- For lossless digitization, the sampling rate should be at least twice the maximum frequency responses. Indeed many times more the better.

MP3ACC.mp3.m4a,.m4b,.m4p,.m4v,.m4r,.3gp,.mp4,.aac File extension Audio only Handles MPEG – 1 Audio Layer 3Advanced Audio CodingOriginal Name Lossy Compression Algorithm standard for audio filesPopular because of iTunes, and iPods. However, not as popular as MP3 Popularity MP3 offers lower quality than AAC at the same bit rate. AAC offers better quality than MP3 at the same bit rate, even though AAC also uses lossy compression. Quality