This is a little bit like trying to explain color to a person who has never been sighted. It is also like trying to answer "Have you stopped beating your wife"!

Generally, a question like this is asked on a test in a music appreciation test, and the correct answer is given either in the book or lectures of that course. As you will see, taking notes in class and outlining your book's chapters is valuable, because there is no valid single answer to this question.

The piano is a complex instrument which comes in many forms and has many things which affect it's timbre (tone color). It is a tuned percussion instrument, in that, like orchestra bells, there are many pitches each of which is produced by a felt hammer being struck against strings (which acoustically act much like metal bars.) It is a pitched instrument like the organ in that a keyboard allows selecting which pitches to emit at any one time. It is a stringed keyboard instrument, in many ways like the Harpsichord, but still very unlike it. It is a chimera, in that one of the pedals (the center, "Una Corde" pedal) changes the way the tone is produced and radically changes the sound. And it is affected by the touch and talent of the player.

'Timbre' refers to the 'color' of a musical sound. As such, it doesn't take into account the initial sound (percussive) when a note is produced. It is affected by the material of the strings, the tension they are under, the quality and condition of the felt hammers which strike the strings, whether both or all three of the doubled (tenors) or tripled (trebles) strings are struck at once, or (Una Corda pedal held down( only one string of a doubled- or tripled- set is struck.

Acoustically, we can talk about timbre as the power of the partials (overtones) in relation to the Fundamental and each other. Usually, in music, we talk about 'color words' when discussing timbre. For instance, an oboe (a small double-reed instrument with a high pitch range) is perceived as having a 'bright' timbre; a bassoon (a large double-reed instrument with a low pitch range) is perceived as "mellow", a clarinet (a single-reed instrument with a very wide pitch range, but a cylindrical bore, which produces a sound that lacks power in the even harmonics) has a timbre which changes so much through it's range that different terms have been assigned to them: the lowest range is "Chalumeau", often described as "boxy" or "hollow", the notes using the keys closest to the mouthpiece is "Throat range" and often sounds muffled compared to the ranges on either side, the next range is called "clarino", because of its piercing tone (like that of a clarino trumpet), and the highest, which is classified as "shrill and piercing" is called altissimo, from Italian for "highest". Experts in playing these instruments can affect the tone of each register and knit them into a feeling that they are well-related from lowest to highest note.

The "tone" of the piano is enimigmatic: physically, in 'normal' operation, each key fires a hammer at one, two or three strings (tuned to the same pitch). The lower strings are single, long and fat. Different materials are used (steel core with brass overwinding) to get the desired pitches in reasonable lengths. The 'tenor' notes have doubled strings, with smaller-diameter overwraps of brass. The 'treble' strings are tripled, three steel strings per note, with no overwrap. Where more than one string is present, differences in tuning between the strings can greatly affect the timbre, from a relatively 'warm' sense of warble (beating) between them to raucous jangling.

The speed of the hammer when it hits the strings also affects tambre: a gentle touch on the key causes a much less 'strident' tone than a sharp stroke.

Additionally, the piano is tuned to 12-tone Equal Temperament, usually abbreviated ET12. Violins, trombones and voices (and other instruments played by experts who are both aware of and experienced in tuning their supposedly fixed-pitch instruments 'on-the-fly') will often tune intervals exactly based on harmonic matching, so that the chords are 'beatless'. There is an acoustic affect from this (called Ringing Chords in Barbershop music) which increases the power of the overall sound of a chord. Pianos get no benefit from this affect. However, chords played on the piano tend to sound more 'alive' than on, say, an organ using a single rank of pipes. Also, the mechanism allows a wide variety of tone from soft to piercing, depending on how the key is struck (unlike both the harpsichord and organ).

Because of the variation of materials, tensions and number of strings per note, designers of pianos pay particular attention to the "scale": the weight of each string, whether single, double or tripled, to produce an "even" evolution of the timbre from one end of the piano to the other. It is often not an easy job.

"The Piano" has many meanings: the Concert Grand piano has a 12' long (or 9') scale, meaning that the piano must be slightly longer to support these strings. The spinet, embodied in the Baldwin Acrosonic, has very short strings by comparison, in a short box designed to 'fit' into small appartments. In the first case, the strings sound quite different from the second. Additionally, upright pianos rarely have an "una corda" pedal, so that particular tone is not universal to all pianos.

And, just to complicate things, the tone of the piano can be perceived as being very 'sharp' and 'cutting' or 'mellow and flowing' depending on whether the player is pounding out staccato (short) accented chords or long strings of arpeggios (broken chords).

So. What is the timbre of "a piano"? There is no single answer: in the most general of terms, it is often described as "warm" and sustained (although the organ is much more sustained, as it has a constant stream of air to keep the vibrations going while the key is pressed, and a piano tone dies out over time.) It can be made to sound "bell like", but this is from the 'shape' of the sound rather than its timbre. It can sound as if it is tuned percussion, melodic instrument or a whole orchestra of diverse tone, depending on the player, the techniques employed, etc.

Add to this the 'effects', such as "una chorda" (one string is struck, the others are vibrated into action by the first, and vibrating power passes back and forth between the strings in a note until they all become synchronized. This gives a quieter beginning to the note, and a softer, more mellow sound with very very long sustain), or "Tack-down" (the sound associated with Barrel-house style). And if the sustain pedal is held down, each note played causes many others to vibrate sympathetically. On a grand piano, the left pedal allows "latching" any dampers held off the string by a key so that those strings remain free to vibrate. Some modern composers have used this (latch notes 'open', then pound on either random keys or play sharp, short chords related to the harmonics of the'open' strings, then let the dampers cut off the played notes, while the latched dampers allow the harmonics of the 'open' strings to linger.)

A harp can be small and held partly on one's lap. The first video is a clip from Austrailia's Got Talent showing a small handheld lap harp and a later performance with a floor version.

Here is a second video with full views of full-size harps: