How much frequency is needed to make a tennis ball levitate, using only sound? I am planning to make a device that will catch a tennis ball using sound waves to slow down its motion, and make it levitate. But how strong should the sound waves be? Please tell me if there are other specifications than the frequency that I should worry about.
Ex. Details :
i Force = That of a weakly thrown baseball
perhaps.
ii By strong, I mean how much power is required or what frequency is necessary to push the ball and hold it mid air.
iii. The device generating these sound waves is a hand-held one.
 A: This is utterly, utterly implausible.  Let's do an order of magnitude estimate to see why.
The overpressure involved in a sound wave is related to its decibel level by
$$
L_p = 20 \log_{10} \left( \frac{p}{p_0} \right)
$$
where $p_0 = 20$ micropascals.  So let's suppose that you use a 100-dB sound wave (which is about as loud as a car horn at a distance of about 15 feet, and would cause hearing damage after about 15 minutes of exposure.  Solving the above equation, we find that a 100-dB sound wave corresponds to overpressure of about 2 pascals, or 2 newtons per square meter, or 0.0002 newtons per square centimeter.
A 5-cm-diameter ball has a cross-sectional area of 20 square centimeters.  So the best you could hope for, using sound waves that would damage your hearing, would be a force of about 0.004 newtons.  Any object that could be levitated by this sound wave would have to have a weight of less than this.  Since $g \approx 10^{-2}$ newtons per gram, any object with a mass above 0.4 grams would be too heavy for a sound wave to levitate.
(This is ignoring the fact that a sound wave is also an oscillating phenomenon, and if the force on the object from the sound wave was upwards one moment it would be downwards the next.)
A: Not quite what you describe, as it is not quite "sound", but you can use an air blower (or maybe your vacuum cleaner can be reversed). For example, according to the data sheet of a $129 cordless handheld air blower (chosen at random), the blowing force is 10N, much more than the weight of a tennis ball (about 0.56N), so you can choose a cheaper blower.
What is important, the tennis ball will be stably levitated in an upward directed air flow of the blower due to the Bernoulli effect.
