Sustain and volume are closely related to one another. These qualities — the direct results of the ability of the soundboard, bridges, rim, and bracing (the piano's "belly") to amplify the sound of the strings — create the ability of the piano to project sound to the rear of the hall.
For the purpose of selecting an instrument, the timbre is the least important tonal element, and should be considered last in the process. Most important is that the sustain and volume of the instrument must be assessed separately from the timbre, in order to differentiate between the sound-producing capacity of the belly, on the one hand, and the hammer voicing, on the other.
Sustain and Volume Tests
Here are a few tests that can be done by a pianist on any piano, in a concert hall or home, to evaluate the instrument's sustain and volume. They take very little time and will quickly narrow down the selection. To sharpen your ears and get you up to speed on the procedures, it's best to practice these tests on several pianos prior to the selection.
Play a note in each section of the piano at mezzo forte, holding the key down to keep the damper off the string. Keep a record of how long each note sustains. You may frequently run across references to how long this sustain should be. However, since every acoustical environment is different, you will get different results in different rooms. When choosing a concert grand, most likely you'll be trying out several instruments in the same dealer showroom or factory selection room. In that case, you'll simply compare each piano's sustain times with the rest. This test will be less conclusive if each instrument is in a different acoustical environment, but it's still worth doing as long as you take those differences into account when assessing the results.
Try another sustain test on the same notes: Pluck a string with the key depressed, so that the damper is off the string. (So as not to transfer body oils or sweat, I usually wear a latex glove for any tests that involve touching a string.) Test several notes in each section. At this point it will be instructive to compare the sustain when a string is plucked to the sustain when the same note is struck by the hammer. Technicians use this test to determine whether the source of a tonal problem is in the belly or the hammer. If the sustain is long when the string is plucked but not when it's struck, it means that the instrument has the capacity for good sustain, and that through the use of advanced voicing techniques, it may be possible to lengthen the sustain with hammer to match that of the string when plucked. But if the plucked sustain is short, then the instrument's capacity for sustain is likely limited, and there's probably little the technician can do to improve it.
Why is sustain so important? Legato playing, and the illusion of a "singing" quality in the instrument, lie within this parameter. If the soundboard can't sustain, notes sound staccato, and the connection of notes in legato is much more difficult to achieve. Although sustain is important in each register of the piano, most critical is the mid-treble section, in the fifth and sixth octaves from the bottom, for two reasons. First, this is the register in which, in most music, the singing melodic line is written. Second, for technical reasons related to soundboard design, this is tonally the weakest area of most soundboards. In my experience, concert grands can usually be made huge and beautiful in the bass, midrange, and high treble, but it's the fifth and sixth octaves that set apart the great pianos from those that are merely very good. To test this element, be sure to bring music that includes a soft, slow, melodic movement.
The volume of each instrument can be evaluated by holding the sustain pedal down and slapping the treble side of the piano's case. (You won't harm the piano structurally by doing this, but be careful not to damage the finish.) You can go from piano to piano and do this test quickly, but be sure to slap each instrument with the same amount of force. The louder pianos will make themselves evident by the greater din they produce.
In the middle of the keyboard, slowly hold down a C-major chord without letting the hammers strike the strings — just raise the dampers off the strings. Then, with the keys still depressed, forcefully strike and release a low-C octave in the bass. As the sound of the bass notes die away, you'll hear the original C chord singing sympathetically, even though those notes' hammers never struck the strings. Do this on each piano. Again, the louder pianos will become apparent through the volume of the resulting sympathetic sound.
The loudness of a string's sound when plucked is also an indicator of the piano's potential for volume. In the earlier sustain test in which strings were plucked, make note of any marked differences in volume among the instruments being tested. Be sure to use the same plucking force on each string.
Timbre, Hammers, and Voicing
As mentioned earlier, the timbre of the piano is the least important element of the tone for you to consider, in large part because it is the most variable and temporary, and the most easily changed through hammer voicing.