Did you know that there’s a radar gun hidden inside your Nokia phone? Well, not exactly – it’s not going to help the police to catch speeding drivers. But with one of the five applications sold under the name Speedhero, any of around 100 Nokia handsets can measure the speed of airborne tennis balls, footballs (of the soccer variety, that is), volleyballs, hockey pucks, and whiffle-like floorballs. (Floorball, U.S. readers may wish to know, is an indoor hockey game played mainly in Europe – lots of action, but no body checks or brawling.)

Speedhero is available from Entetrainer Ltd. in five versions, each one tuned to a specific sport but all calculating speed by measuring the time delay between two distinct sounds. Using the phone’s built-in microphone and audio-analysis capabilities, the app listens first for an initial strike of the ball or puck and then, a moment later, for the object’s impact against a hard wall. With the phone situated at a known distance between player and wall, the time between those two sounds yields a good measure of the object’s average speed.

And guess what: It really works. We tried out Speedhero Tennis at a local court and it clocked our tennis serves just fine – and those of a younger member of the family, too. We had to take the app’s word, of course, for the speeds it displayed, but all in all, we were quite impressed.

First, following the app’s informative instructions, we placed our Nokia N97 handset on the pavement at a spot equidistant between the serving position and a tennis practice wall. Adults and juniors, serving at speeds as high as 161 mph (259 km/hr.), are to stand 39 ft. (11.9 m) from the wall; children, serving in the 18 to 87 mph range, stand 21 ft. away. One can toggle between these two distances, and between metric and imperial units, on the app’s Settings screen.

When activated, the app emits a loud whistling sound, indicating that it’s now time to hit, or kick, the ball. Besides giving it your best shot, you must make sure to hit the wall directly, with no bounces in-between. Immediately after it detects the ball’s impact on the wall, Speedhero displays the ball’s speed in large, easy-to-read digits and also announces the figure through the phone’s speaker. (And for added ego-boost, it plays the sounds of a cheering crowd.) In Settings, you can adjust the volume of these sounds as well as the app’s sensitivity to sound.

Behind the scenes, the app keeps a running tally of your serves and can display best and average speeds for all sessions and the current one – and this for multiple players, too. Furthermore, Entetrainer, the publisher, invites players to upload their results to its central server or to a dedicated Facebook page, where they can maintain a personal “Player Card.” Naturally, this requires registration.

Want to see the app in action? This video from our friends at Nokia Conversations shows Speedhero for football (or, soccer, if you live in the U.S.):

How accurate are these speed measurements? Entetrainer says the app’s results differ by just 2%, at most, from those of true radar-based speed guns. Not bad, all things considered. But the answer to the obvious question looming over the five different Speedhero apps is No. As the company states in their online FAQs, Speedhero Tennis cannot measure the speed of, say, ice hockey pucks. The reason, it claims: Each app relies on its own sport-specific algorithm to analyze impact sounds and these differ enough to foil its “thinking.” (This may explain the difference in prices between Speedhero apps, too. Tennis is $2.99, Ice Hockey $4.99) We didn’t bother to test this claim, but Speedhero Tennis does respond, we discovered, to the sounds of hand claps, finger clicks, and other hand-made sounds.