Formats, File Sizes, and More Settings for Audio
As with any other multimedia, the biggest challenge arises from capabilities, bandwidths, memory, and wireless connection speeds that are uneven and unpredictable across different devices. The good news is that unlike some other common multimedia elements (such as Flash), the iPhone/iPad is a whiz at playing the industry standard audio format: MP3.
MP3 doesn’t provide the best compression codec (the mathematical formula that reduces the size of an audio file). In fact, music engineers groan and rub their temples when forced to listen to the sound of highly compressed MP3s playing on tiny speakers. But MP3 is a universal standard just because of the number of people who have used it for many years.
Thus, your biggest decision is how much to compress your MP3 to ensure that a mobile user can play it. Essentially, you compress an audio file in an audio editor by setting the bit rate for the file. Bit rate — a measure of the file’s audio quality — is the number of bits of digital information per second that are decoded and turned into a sound.
In much the same way that reducing color depth in images represents a trade-off between quality and file size, so too does bit rate work for audio files. Bigger is usually better — to a point. The following list explains the differences in bit rate for the desktop and mobile web:
On the regular desktop web, bit rates range from 96 to 320 Kbps. Professionals consider 96 kilobits per second (Kbps) adequate for a file that contains human speech, such as an interview or a monologue.
Until recently, 128 Kbps was considered standard for music, and the vast majority of songs sold on iTunes were at this bit rate; however, sites such as MOG are making a name for themselves by offering music at 320 Kbps or higher for true audiophiles.
For the mobile Web, aim for 64 Kbps or lower. The giant music-streaming service Pandora has found that 64 Kbps is the highest practical bit rate to deliver over wireless connections because of bandwidth and dropout constraints. However, as 3G networks become more stable, and as 4G networks get rolled out, expect that number (and the quality of the audio) to start going up dramatically.