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In this space we have discussed how to best incorporate text, numbers, images, and video into the work of students and teachers in school. This week we consider sound: voices, music, and effects that can help in the process of teaching and learning. Whether it's a sixth-grader's recital of the Gettysburg Address to accompany a slide show of images, or a music teacher's online identify-the-composer quiz, or the sound of ocean waves serving as background for the web site of a Cape Cod school, sound can be an important part of the educative enterprise. This article will get you and your students started in using sound in your computer projects.

The article takes you through three steps:

• locate the source of the sound
• use the appropriate sound-editing software, and
• save in a suitable file format.

Sounds for your project, whether voice, music, or effects, may come from a variety of sources.

• Voices are best recorded from scratch, directly into the sound-editing software on the computer. While voice can be imported from audiotape, better results are possible with live recording.
• Music may be recorded live in the classroom, and is also available on the Internet as MP3 files. (Many of these music performances cannot be used on a commercial or professional Web site or computer project without the permission of the owner. But for a classroom project, the law permits you and your students to use these files freely.).
• Sound effects can be recorded live at the computer. Many publishers sell CDs with hundreds of sound effects and musical clips that can be used freely. Similar effects files can be downloaded from the Internet, edited, and employed in a student project for the classroom.

No matter what the source, your sounds should be imported into a sound-editing program, tested, compressed, and saved in a format suitable to your purposes. Many such programs exist, and you may use the sound editor of your choice to prepare the files for this work. But you may find it easiest to use Audacity, an open-source sound editor that is available for Windows, Linux, Unix, and Macintosh OS X operating systems, in English, French, and other languages, at no charge.

You may download Audacity at http://audacity.sourceforge.net/

While you are there, download also the LAME MP3 Encoder Library, which you will need if you want to export your sound in the compressed MP3 format.

The sound editor uses the computer's built-in microphone and audio input and output hardware. More sophisticated sound recording and editing can be accomplished with specialized hardware and software, such as that from Digidesign. Significantly more expensive, these tools are designed for professional recording studios.

All of the tools work in the same way. The sound is first imported into the editing software, through the microphone, or from another device through audio inputs, or by direct transfer from a compact disc. Once imported, the editor adjusts the sound as necessary, shortening it, changing its volume, adding effects, amplifying one track while attenuating another, and so forth as necessary. The file is then compressed and saved in a form suitable for your project.

The sound you hear from a compact disc was sampled at a rate of 44 kilohertz. That means a sample of sound was collected 44 thousand times each second. Each sample contains 16 bits of information about the pitch and volume of the sound at that point in time. This information is recorded as a number, one number for each sample, very much like the numbers that represent the colors of each pixel in an image file. This process of sampling is called digitization. Each track of the CD consists of a long series of numbers. When these numbers are converted back into sound and played very fast (44,000 numbers a second) they sound to your ear just like the original performance.

The problem is that each of these tracks contains a huge number of numbers. A typical track on a CD takes up 25 megabytes. That's about 26 million bytes, or 210 million bits of information. To send one such uncompressed track through the World Wide Web over a 56k modem would take about an hour, longer than most users are willing to wait to hear the music on your interface.

To reduce the size of your sound files, you will compress the sound as you save the file, to reduce the file size as much as possible while at the same time maintaining as much sound quality as possible. For this project, you will use the MP3 compression scheme.

We'll begin by recording your voice, using the Audacity sound-editing software. Follow these steps.

1. Make sure you have a microphone and headphone connected to your computer.
2. Launch Audacity, and watch its recording window appear. It will look like the illustration below, but without the waveform.
3. Test the input volume of your microphone, by clicking the arrow next to the microphone icon and choosing Monitor input.
4. Speak into the microphone, and watch the recording level appear in the red bar. If you get no indication, check your microphone connections and drivers.
5. Click the round red record button at the top of the window.
6. Speak into the microphone.
7. Click the square brown stop button to stop the recording.
8. Watch the waveform of your voice appear in the audio track below.
9. To hear the recording, click the triangular green play button.
10. Record another sample, until you get a good level and a clear recording.

Next, we'll import a sound from an existing file. Follow these steps:

1. Locate a sound file on your computer. This can be an .mp3, .aif, .wav, or .au file. Download such a file from the Web if you need one.
2. Create a new Audacity window, by choosing File --> New from the menubar.
3. Choose Project --> Import Audio from the menubar.
4. Navigate to the sound file you want to import and click Open.
5. Watch the waveform appear.

Once a sound is visible in the Audacity window, you may edit it. You may select a portion of the sound by clicking and dragging over its waveform. The standard cut, copy, and paste commands work for sounds in Audacity just as they work for words in Word. You may also select a portion of the sound and apply an effect to it, by choosing Effect from the menubar.

Want to combine your voice with some background music? Simply record or import the music into one track, then choose Project --> new Audio Track from the menubar, then record your voice into the second track. When you play it back, you'll hear both tracks combined.

It will take some time to record and edit the sounds you need for your interface design project but it is worth the effort for the effect and the experience. When the sound is complete, move on to the next step of compressing and saving the sound.

Once edited, you may compresses the sound with the sound editing software. For this project, you should save your sound using the MP3 codec. A codec is the algorithm used to compress the sound file. It stands for compressor-decompressor. A file that is compressed by the publisher of the Web site must be decompressed by the viewer. Most computers have the plug-ins they need to play back MP3 files.

To save your sound in the MP3 format from Audacity, choose File --> Export as MP3 from the file menu. Assign a filename that will work on the web (no spaces, no special characters), with the .mp3 filename extension.

Once saved in the MP3 format, the sound can be played on its own, or imported into web pages, Word documents, or PowerPoint slide shows.