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The primary goal of sound optimization for limited delivery networks (such as the Internet) is to deliver an acceptable quality without a large file-size "cost." You should be concerned about the file size of your audio clips for several reasons:
Sound files require a large amount of drive space.
Managing large sound files and importing them into Flash can be cumbersome and slow.
Download times for large, elaborate sound clips (even when heavily compressed upon export from Flash) can be detrimental to the appreciation of your Flash project (even if your target audience has what may be considered a high-speed Internet connection).
When you're working with audio clips, it's important to create the shortest audio clips possible. That means trimming off any excess sound that you don't need, especially any blank lead-in or lead-out handles (also called in and out points) at either the beginning or the end of a clip.
| Cross-Reference |
We discuss trimming excess sound briefly in Chapter 15, "Adding Sound," with reference to Flash's sound tools. We explain the topic in greater detail in the archived chapter, "Working with Audio Applications," found at www.flashsupport.com/archive. |
If you plan to have a background music track in your Flash project, it's a good idea to use a small audio clip that can be looped.
We describe looping audio clips in Chapter 15, "Adding Sound."
| Cross-Reference |
Here is a simple formula to determine the file size, in bytes of a given audio clip: Seconds of audio × sample rate × # of channels × (bit depth ÷ 8) = file size |
| Note |
In the preceding formula, the sample rate is expressed in Hz (hertz), not kHz (kilohertz). The bit depth is divided by 8 because there are 8 bits per byte. |
Thus, a 20-second stereo audio loop at 8 bits, 11 kHz would be calculated like this:
20 sec × 11,025 Hz × 2 channels × (8 bits ÷ 8 bits/byte) = 441,000 bytes = 430KB
There are two schools of thought regarding the ideal quality of sound files for import into Flash. These schools are pretty much divided into those who have high-end sound-editing tools and those who don't. In an effort to delineate the best path for each group, we've noted the following:
If you don't have high-end sound tools available, you may be among those who always prefer to start with audio source files of the highest possible quality (16-bit, 44.1 kHz is ideal), and then use the Flash sound settings to obtain optimal compression upon export.
| Cross-Reference |
See Chapter 15, "Adding Sound," for detailed information on the sound export settings for Flash movies. |
If you do have high-end sound tools available, you may prefer to compose most of your clients' music from scratch and may very rarely work with the MP3 format before importing into Flash. You may also disagree with those who advise you to bring sound into Flash at the highest quality before optimizing. This workflow difference may be attributable to the plethora of options that are available to those with high-end sound tools. We know of one sound engineer who converts all of his audio to 16-bit, 22.1 kHz mono files, "with major bass reduction," before importing into Flash.
| Web Resource |
For more information on high-end sound tools, refer to the archived Macromedia Flash MX 2004 Bible (Wiley, 2004) chapter, "Working with Audio Applications," at www.flashsupport.com/archive. As with so many things, individual mileage may vary. |
You should always keep in mind that Flash can retain imported MP3 compression settings only with those MP3 files that will be used for non-Stream sync options. Anytime you set a sound to use Stream sync on a timeline, Flash needs to recompress the file with the Stream export settings found in the Publish Settings dialog box.
Finally, all linked sounds, or those set to export from the Library and played back via ActionScript, are treated as non-Stream sounds. If you use linked sounds, you may find it use ful to import precompressed MP3 files at your preferred bit rate. Flash will not recompress the MP3 file upon export to an .swf file.
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