You need flash (get it here) and a pair of speakers or headphones to get the perfect sounds from these 'tuning forks'.
These tuning fork sounds were professionally recorded from real tuning forks placed onto an amplified sounding board.
The "tuning fork" was invented by John Shore, English musician to the royal court in 1711 and had a pitch of A423.5. The numbers underneath the musical notes represent the Hertz (Hz) or cycles per second of the sound wave made when the fork is vibrating.
Standard Pitch or Concert Pitch is a universal frequency or note that all musical instruments are tuned to. Today's standard pitch is A440 or C523.3 and this enables musicians to play instruments together in harmony, without clashing pitches. A tuning fork is normally used to set the pitch. Pitch pipes and electronic tuning forks can also be used, but are not as common.
Oddly, some people are gifted with 'perfect pitch' and can remember exactly what a certain pitch should be without using a tuning fork or any other kind of reference. Many of those same people say it's not always a gift because a lot of music is played slightly out of tune and for those with perfect pitch it sounds horrible!
When struck, the two prongs of the tuning fork vibrate uniformly, producing a very clean soundwave. The pitch (or frequency) of the sound wave is determined by the length of the prongs. If you analyse the sound wave produced from a tuning fork, you will find that it is a perfect sine wave formation. For some interesting scientific animations of tuning fork soundwaves, visit this page.
Tuning forks are not only used for music. They are also used for science, medicine (hearing and vibration treatment) and electronics. In fact, the computer you're using right now has probably got several tiny tuning forks inside it, vibrating thin slices of quartz crystal to keep the processor's clocks ticking at the right intervals.