What differences between Piano and Guitar make them sound different?

[Liam McGillivray]

What construction differences between Piano's and Guitar-family instruments contribute to their different sound?

I'm especially interested in the bass notes. Here is what the bass notes on an upright piano sound like:
https://www.youtube.com/watch?v=X1bPQ2z5pec

Very different from the same notes on a bass guitar (acoustic or electric). The piano has a more wobbly, brighter sound. I used to not like the sound of the bass notes on the piano because of this, but now I'm making an instrument for noise music (with a similar range to a bass guitar), and I would like to have a more piano-like sound than a typical bass guitar. It's probably going to be a guitar-like instrument, but it would be nice if I could somehow get a more piano-like sound.

It seems like you get a more wobbly sound on a guitar/bass when you tune down the strings; make them looser. But from what I've read, piano's actually have high tension on their strings. So why do they have such a wobbly sound on their bass strings?

I suppose one reason for their [i]brighter/i] sound is that the strings are struck rather than plucked. But I suspect that there's more to it than that. I found that striking the strings on a guitar with a blunt hard object does give a brighter sound than plucking it with a pick or finger, but not as bright as a piano. Maybe it's because of the more rigid frame that piano strings are mounted on. Piano strings are mounted on a heavy cast iron frame. This is probably more rigid than the neck of a guitar. Maybe the bendiness of a guitar dampens the upper frequencies.

Is there anyone with a decent understanding of acoustics, and the construction of both piano's and guitar-family instruments that can explain why they have a different sound?

[David King]

The piano has a lot of compromises in the bass range starting with strings that are much too short for the lowest notes. This is why a concert grand is a 9 foot piano and a parlor grand is a mere 5'6" or less. The ideal piano would use strings that were all the same gauge and at the same tension and the bass strings would be 30 feet long or more. The short bass strings have to be nearly 1/4" thick to get the tension high enough to have any volume. That thick string has a lot of discordant overtones which are what you hear.

That cast iron piano frame is there for both mass and rigidity. A guitar only has 6 strings at about 30-40 pounds of tension while a piano has 288 strings each at tensions of 160-200 pounds.

[Barry Daniels]

Rather than listing the differences between a piano and a guitar, I would try to list the similarities. I don't think there are very many. About as different as they can be.

[JC Whitney]

Amazon sells single piano bass strings for about $20... lots of potential for experimentation.

[David King]

Part of a piano's sound comes from the fact that the hammer strikes each string at the 7th harmonic node in an effort to eliminate that particular harmonic from the sound. Plucking a guitar at that point should do something similar.

[Alan Carruth]

Much of the 'wobbly' sound comes from the fact that pianos use more than one string per note. Rossing published an article many years ago on this in 'Scientific American'.

If you have two strings attached to the same bridge it turns out to be impossible to tune them to exactly the same pitch. They form a coupled system. Suppose you tune one string to a certain pitch, and then try to tune the other. As the second string approaches the pitch of the first, the first one will start to vibrate, stealing energy from the second. They will be moving out of phase: when one goes up the other will be going down. As they get closer in pitch they will actually 'push' each other apart in frequency: one will attain its maximum amplitude at a slightly lower pitch than the target frequency, while the other will be higher than you want. If you're tuning the second string upward, the one you already tuned to the desired pitch will end up a little higher, and the second one will be the lower of the pair. There's not much difference in the frequencies: the separation you end up with is a function of the band width of the string vibration, which is really narrow.

When you play the note the two strings will start out in phase because they don't have much choice. The bridge is being driven by both strings in the same direction. Since each one is running at a slightly different pitch they will fall out of step gradually. At some point one of them will be out of phase with the other, pushing the bridge 'up' while the other string pushes it 'down'. If the two have the same amplitude the bridge will feel no net force and stop moving. This doesn't last long, as the strings keep falling out of step. Most of the time the bridge/soundboard is pulling energy from both strings, so the sound is loud, and dies off at a fairly high rate. This is called the 'prompt sound'.

It's virtually impossible to make a hammer strike both strings with exactly the same force; one of them is going to start out moving more than the other. Eventually it will run out of energy and stop. But not for long. At this point it starts to steal energy from it's brother string, and will do so until both are going at about the same amplitude. They're still out of step, though, so the sound still comes and goes. Since the strings are often moving out of phase at this point they don't drive the bridge as strongly, so the sound is softer. At the same time, since they're not pushing as hard the soundboard isn't extracting the energy as fast, so the sound dies off more slowly. This is the 'after sound'.

The more strings you have tuned to the same note, the higher the peak of the prompt sound, and the faster it dies out. With more strings, otoh, the after sound starts off at a higher level, tends to last longer and to be at a more uniform level.

So the key to getting that particular sound is to use multiple strings per course tuned to the same pitch.

[Liam McGillivray]

I'm sorry for being so late to reply to these.

Part of a piano's sound comes from the fact that the hammer strikes each string at the 7th harmonic node in an effort to eliminate that particular harmonic from the sound. Plucking a guitar at that point should do something similar.

Thank you. That is good information to know.

Much of the 'wobbly' sound comes from the fact that pianos use more than one string per note. Rossing published an article many years ago on this in 'Scientific American'.

...

So the key to getting that particular sound is to use multiple strings per course tuned to the same pitch.

Thank you for all this. But the "wobbly" sound I'm reffering to is in the bass notes. Don't the bass notes have only one string?

Also, would all this work if the second string was sympathetic, or do they both need to be struck to have this characteristic?

[Barry Daniels]

On my baby grand the only bass strings that are single is the very bottom octave. The bass strings above that have double strings.