This applet demonstrates beats, the result of combining two waves of slightly different frequencies. Ideally, you will be able to notice the slight difference between the two sounds so that when you play the combined wave, you'll know that there will be beats.
Here are step by step instructions for using the Beats Applet:
Choose a frequency level (between 437 and 443 Hz) for Frequency #1.
Choose a different level for Frequency #2.
Click the "Hear It" button underneath the bottom box by "Combined Wave" and listen for the beats. You should hear a "wah-wah" sound -- that's the beat.
Next time, listen to each separate wave before listening to the combined wave. Choose 440 Hz for Frequency #1 and 441 Hz for Frequency #2. Click the "Hear It" button underneath Frequency #1 and listen carefully to its pitch. Then, click the "Hear It" button under Frequency #2 and try to detect a difference from the first one. You'll find that it is nearly impossible to hear a difference. Musicians face this problem all the time when tuning their instruments to a reference pitch. By listening to both sounds at the same time, the musician can tell if the separate sounds are the same. If there is a beat, the frequencies are not the same. If there is no beat, they are the same and the musician's instrument is in tune.