A Grand piano is considered better than an upright piano. Aesthetically I could understand why, but why does the orientation of the strings matter so much sound-wise?
It's all about the size, and therefore the length of the strings and the size of the vibrating surface of the wooden soundboard.
Even a baby grand at ~5 feet is longer than a typical upright is tall. A concert grand at 7-10 feet is much, much longer.
I can't do any better than what Wikipedia says, so I'm going to quote wholesale:
All else being equal, longer pianos with longer strings have larger, richer sound and lower inharmonicity of the strings. Inharmonicity is the degree to which the frequencies of overtones (known as partials or harmonics) sound sharp relative to whole multiples of the fundamental frequency. This results from the piano's considerable string stiffness; as a struck string decays its harmonics vibrate, not from their termination, but from a point very slightly toward the center (or more flexible part) of the string. The higher the partial, the further sharp it runs. Pianos with shorter and thicker string (i.e. small pianos with short string scales) have more inharmonicity. The greater the inharmonicity, the more the ear perceives it as harshness of tone.
Inharmonicity requires that octaves be stretched, or tuned to a lower octave's corresponding sharp overtone rather than to a theoretically correct octave. If octaves are not stretched, single octaves sound in tune, but double—and notably triple—octaves are unacceptably narrow. Stretching a small piano's octaves to match its inherent inharmonicity level creates an imbalance among all the instrument's intervallic relationships, not just its octaves. In a concert grand, however, the octave "stretch" retains harmonic balance, even when aligning treble notes to a harmonic produced from three octaves below. This lets close and widespread octaves sound pure, and produces virtually beatless perfect fifths. This gives the concert grand a brilliant, singing and sustaining tone quality—one of the principal reasons that full-size grands are used in the concert hall. Smaller grands satisfy the space and cost needs of domestic use.
Really tall uprights do exist, but they're not very common. They are sometimes referred to as upright grands.
Of course, whether inharmonicity is good or bad is purely subjective. That classic "pub piano" sound, fits perfectly with some kinds of music. It's the sound of an upright, and probably couldn't be replicated on a concert grand.
The action makes a grand better.
In a grand, gravity helps the key return so you can play faster stuff. In an upright, the action goes sideways and has to be helped out by, umm, don't quote me on this, springs I think?
And the sound, too.
The sound is more of a subjective thing. But in almost all cases, a long enough grand will sound better due to the wonderful bass sound. Also they open more directly to the room so the string/soundboard sound gets to you more directly (plus the room reflections).
Uprights have to bounce the string/soundboard sound out the back against the dang painted drywall (your wall).
Also the pedals.
Grand pianos have a middle pedal called a Sostenuto pedal, which captures keys which are depressed and lets them ring while keys played after the pedal can play without being held on. Uprights on the other hand have a middle pedal that serves as a "practice pedal" which essentially mutes the whole instrument. Usually this middle pedal is missing or non-functional on an upright.
Adding to the above, this is the reason why upright pianos went from having strings vertical to being overstrung. This means the strings ,particularly the lower ones, are diagonal across the soundboard.It makes them longer, with the above advantages, but still not as long as those in a grand. 36" on the bottom string of one of my uprights, compared with 42" on my baby grand.
I used to think that a baby grand piano was superior to an upright piano but this is definitely not the case. The reason most people think this is the poor quality of most upright pianos.
A 1930s Blüthner upright piano sounds nearly as good as a Blüthner 7ft grand piano, with a fantasatically sonorant bass. A Blüthner baby grand of the same era definitely sounds somewhat “dead” in the bass, indeed has somewhat of a toy like sound compared to a proper piano with long strings.
Comparing same build quality uprights and grands, the answer is mainly the string length and soundboard area are larger on a decent grand. Uprights are limited to a maximum height (these days) of around 132cm but grands are available up to 280cm and even bigger. The longest bass strings must fit into this size, and longer is better (less inharmonicity caused by thickness being a substantial fraction of length in the small uprights), so the sound is purer. Larger soundboard means better matching to the string, so the decay time is longer on a good grand. The una corda pedal on a grand (when properly adjusted) is usually much more effective than the half-blow pedal on an upright.
Other improvements like the repetition levers in a good grand piano action, and longer key length possible in a big grand (impossible in an upright as it would then not fit through a normal door) also allow more precise playing by a virtuoso, but are lesser issues than string length and soundboard area. Hardly anyone seriously uses the middle sostenuto pedal, so that also is not a big issue.
If we are not comparing same quality, it is perfectly possible for a good upright to be better than a poor grand. The Yamaha U3 (older model) or SU132 (newer model) is a good example of an excellent design that will see off many small and ill-designed grands. If you are just starting out and can't afford at least a Yamaha C3 grand, a Yamaha U3 or SU132 is a good choice.
Sound quality of a piano strongly depends on the instrument's size and strings alignment. Longer strings give out better (richer) sound due to lower inharmonicity. One major factor that makes grand pianos sound better is gravity. Because of the horizontal alignment of the strings, most parts of the instrument would have to move up and down. Eventually brought to rest by gravity. However, in the upright piano the string action is verticall and requires springs, and over time they tend to acquire imperfections. For same reasons, the upright has some limit in the speed and control, as the note can be repeated only after key comes back to the top. In the horizontal piano, the same key can be repeated after about 1/3 of the key’s way to the top. This is as simple as it can be put without getting too technical. If you are very interested in seeing all of the machinations of these wonderful inventions, I would suggest observing the transparent piano in action and see all the intricate components working together that go into creating the sound.
As with all things....it depends. A high-quality professional upright made by one of the top companies is an excellent piano indeed. I have played on some truly wonderful upright pianos. Schimmel makes uprights that have a touch and sound that will rival many very good grand pianos, and does Sauter. In a home setting, an upright piano can have just the right amount of volume and tone.
Grand pianos sound much better because they are better. :-)
Just kidding. Grand pianos are constructed a bit differently as has been noted, but the biggest thing is the sound is project up and out from the instrument and not towards the user with a lot of wood in one's face. An upright piano also has its action in front of the strings unlike a grand with the action underneath and striking the strings from behind. This also affects the sound, making an upright sound much different.
On an upright piano, the pedals work differently. The upright's una chorda pedal works by placing the hammers closer to the strings to make the piano play softer. This is a totally different mechanism where the hammers are moved over and strike two strings, rarely one string, on a grand piano. The tres chorda pedal on both works in a similar fashion with the string dampers are lifted off the strings to allow the strings to vibrate. The sostenuto pedal is a rare beast on an upright. They do exist but only on the really expensive ones. Many uprights that do have a middle pedal, are fitted with a practice pedal which puts a thin sheet of felt between the hammers and the strings. This creates a muted sound so the piano isn't as loud and intrusive, and allows for much later evening practice.
Overstringing does not increase the length of the strings. What does is create harmonics in the open part of the strings. Up until the early 20th century, even grand pianos were straight strung, with a mix of both straight strung and overstrung instruments around. Erard was still making straight strung concert grand pianos right up until the 1920s.
Here's a place that's worth visiting should you be able to. This is a collection of antique pianos that can be played on and heard. I have play on all these pianos listed on this website.
The main site is http://frederickcollection.org/
Just to chime in with one more advantage of grands over uprights: they hold tune better. The floors of concert halls or living rooms might seem really solid, but they're actually carrying vibrations pretty frequently - vibrations from footsteps, passing trucks, etc.
Picture a vibration propagating across the floor. When it meets an upright piano, that piano's case is resting on four points on the floor. As the vibration reaches the piano, one of those points is going to start moving before the others, which means the case will flex microscopically. And that flex transfers to the sound board, the pin block, the harp - you're basically shaking the strings slightly, and they might slip a bit.
Contrast that with a grand, which rests on the floor on just three points. When the vibration reaches the piano, the piano tilts a bit - but it doesn't flex. Three points of contact allow it to float along with the vibrations. Less shaking of the strings, less slipping of the pins, and better tune overall.