Piano World Home Page Forums Our Most Popular Forums Piano Tuner-Technicians Forum A theoretical question

It is summer and things are slow. I have a hypothetical question for all you budding piano designers and engineer types out there.

Suppose you had two identical pianos. I know it is impossible but this is a theoretical question. Both instruments had all aspects the same with the exception of the soundboard. While the soundboards were made from the same “board selection” of tight-grained spruce, the difference was in topography.

Piano A’s soundboard was with a more-or-less standard 5mm thickness throughout. Piano B was made with 30-40% more mass. It is 9mm thick directly under the treble bridge and tapers out toward the edges where it too was 5mm. Needless to say the bridges would need to be modified to maintain the string line.

Would the addition of this mass affect tone quality and if so, in what way?

Would the location/shape of the taper make any difference, if so what difference?

Would a theoretically identical board 9mm thick at the treble side, gradually tapering to 5mm on the bass side perform similarly? If not, how would it differ?

Steve Cohen

This is one of those "features" who's benefits are more for giving a salesman something to talk about than it is a tonal benefit to the piano. Some of the best sounding old Steinways you'll find do not have a taper in the board. It's only one part of a chain of design decisions one has to make when designing a new scale. Since your example said everything else in the piano is the same, merely switching from a nontapered board to a tapered board and leaving everything else alone would fall under the heading of "sales hype", one of those things factories do to give the salesmen something to talk about to make the customer think their piano has some added benefit over some competitor.

In fact, if the only thing that is changed is the soundboard, and the overall design of the piano was designed for a nontapered board, it's possible for a tapered board to do more harm to the sound of the piano than good. Most likely though, it wouldn't do much of anything. It's sort of like putting duplexes in a piano smaller than 6'. It accents the high frequencies in a piano that doesn't need the highs accented, and makes the poor little thing ring like a screaming banshee. But it's nice and shiny piece of chrome that a salesman can point to and talk about, and the typical customer can look at and think "how pretty. And the salesman said I should have one". Then techs end up having to mute them out so the piano will sound good, and so we can tune it.

There's a lot of this kind of stuff going on these days in Korean and Chinese built pianos as they try to market a link in the customer's mind to "German engineering". But hey, you can spread chocolate icing on a shoebox and make it look like a cake too, right?


Chuck

I am not so sure that changing the shape and thickness of the board is just sales hype. Mass and stiffness effect how well the board transmits certain frequencies. Stiffness effects how much off the vibrational energy from the strings is transferred to the board, which effects sustain. Stiffness is frequency dependent - some fresquencies are dumped faster than others, which controls how the tone quality changes as the note decays. A more massive soundboard transmits lower frequencies better than higher frequencies, leading to more fundamental. Less massive soundboards transmit higher frequencies better, leading to more upper harmonics and less fundamental. Varying the mass across the soundboard will change the frequency content of the notes across the scale, which could be a way to achieve a desired balance across the scale.

Not to say that a tapered soundboard necessarily sounds better than a non-tapered soundboard. But I have to believe that the same pianos, one with a tapered soundboard and another with a non-tapered soundboard, would have to sound different. I guess how much of a difference there would be is debateable.

I do know that these parameters have a huge effect in speaker design. For example, I have two guitar amplifier speakers that have exactly the same components and design, except that one has a slightly more damped than the other, achieved by putting two folds in the surround instead of one. One might think that the difference was only marketing hype with the goal of selling more speakers. However, they really do sound significantly different from each other, even with the same guitar and amplifier. Even a major magazine reviewed the speakers (and a bunch of others) and found them to sound different, finding one to sound better than the other.

The more I think about it, the more I thionk that soundboard has to contribute greatly to the basic sound quality of a of a piano.

Ryan

One of the fundamental mistakes people make when thinking of a soundboard is to think of it as a speaker, and draw comparisons. A soundboard is not a speaker. It is a transducer. And, it is only one part of the sound body. I agree with Chuck - if the rest of the soundbody was designed to work with a nontapered board and the only thing you change is the board, little if any significant benefit is derived.

True, there are big differences between loudspeakers and soundboards. Still, they have the same basic function, converting energy into sound waves without amplification. They are even similar in that the soundboard reflects energy back into the strings while speakers reflect electrical energy back into the amplifier (the speaker is both a motor and a generator). And, both act as a filter - some frequencies are passed and others filtered out. The filtering function is based on mass, stiffness, intrinsic characteristics of the wood, and frequency.

I haven't run any numbers or done any experiements along these lines. I'm just thinking about based on my filter classes in school. Somebody like Del Fandrich could probably give a lot better answer to this question.

Transducer is as good a word as any to describe the sound producing aspect of a piano. A transducer converts energy of one form into output energy of another. This is exactly what's happening in a piano. It converts mechanical energy into sound energy. So, lets look at the problem from an energy standpoint.

The energy of a vibrating plate (i.e., the soundboard) consists of three components: kinetic, potential and damping. The first two are often lumped together and it can be shown that they are proportional to the square of the vibration frequency and amplitude of the vibration. Because frequency is related to the system mass and stiffness, soundboards with different distribution of mass and stiffness (thickness) will have different frequency contents and vibration amplitudes and thus will sound different.

The third component of energy - damping energy in a mechanical system exists in three forms: viscous, Coulomb and hysteresis. The first form (viscous) is frequency (and hence mass and stiffness) dependent, the other two are not. Damping not only affects sustain (because it damps out the vibration) but the frequency content of the system (because it tends to damp out the high frequency vibration modes.) Indeed, there are some vibration modes that should be damped out because they tend to create 'white noise' and thus 'corrupt' the sound of the piano, so damping is not all that bad.

So, from a theoretical standpoint, it is not difficult to show that a tapered soundboard will produce a different set of harmonics than a uniform soundboard because the vibration mode shapes of the two soundboards are different. If two pianos are identical except one with a tapered soundboard and one without, the two pianos will sound different. Is one better than the other in terms of tone? It depends on the individual, some may like the sound produced by the tapered soundboard, some may not. Is one better than the other in terms of sustain? The piano with the bidirectional tapered soundboard will more likely have a longer sustain because it is more flexible around the edge where it touches the inner rim due to boundary condition effect. Of course, this conclusion is made assuming ideal conditions, there are other factors that affect sustain, e.g., position of bridges, number and location of ribs, string tension, amount of down and side bearing, material used to construct the piano, etc.

Eric

[ July 24, 2001: Message edited by: EricL ]

I would say that there are differences. By the way Steinway does taper their sound board I saw it at the factory in 1958. New Yamaha's have started tapering their sound board in the treble and it produces a more singing tone

There is always some tapering but not absolutely at the edges , and for what I know going too far (as 5 mm thickness in the high treble) does not product the expected result.

Richard Dain at Hurstwood Farm made lot of experiments on tampering, using agrafes, etc.

In the basses the question is to have the max ability to follow the large strings move.


You may read the writings from Adrien Mamou, http://adrienmamoumani.wordpress.com/ who just pass his doctorate this year, and worked on soundboard theory and reality for many years now.

He uses the subject of soundboard construction types, and the effect of downbearing , as principal theme.

My mentor always thought the sound of the Yama G2 was stingy. The board measured .410. He thinned it down and made it sing.

He tapers his boards. The rimm adds too much stiffness to the board. Why not carry the ribs full thickness into the rim??? Certainly there is a reason for having a different structure around the edge of the board. If you had 10 engineers create 10 models of a soundboard, You would have 10 completely different basis for each model. If I was modeling a soundboard, I would treat it like a trampoline with the center section having a different reaction than the edges, which are springs. The strecth of the center canvas has some to do with the launch but the edges are reponsible for the high toss. In that way, my model will differ significantly.

Would a theoretically identical board 9mm thick at the treble side, gradually tapering to 5mm on the bass side perform similarly? If not, how would it differ?

Upright boards are tapered from the top to the bottom.