This paper describes instruments designed to measure the tone-volume of the piano. The tone-volume depends on the amount of kinetic energy which the player spends on striking the key. The kinetic energy is in direct proportion to the mass set in motion and is the square of the velocity. The mass set is always constant and the energy depends only upon the velocity of the hammer. Thus it is possible to analyze the tone-volume by measuring the velocity of the hammer. Electronic digital devices generally have great advantages over analog ones in the field of physical measurement. The velocity is converted to the time interval by photo-transistors and flip-flop. The signal opens the gate, through which flows a clock pulse of 100KHz. This is transmitted to an electronic pulse counter consisting of 4 decimal counting units. The number of pulses counted by this counter is proportional to the inverse of the velocity, and is compared with 5 preset values to activate one of 6 magnetic counters. Each magnetic counter indicates the number of the swing-stroke, of which the velocity belongs to a previously classified group. This equipment makes it possible to measure the intensity of the tone and its distribution precisely. A velocity of 10m/sec. corresponds to an "ff" tone, with a precision of about 1.25%. The precision increase with decrease of the tone-volume.