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So finally I bought my first grand, a 1927 M&H. Its sound is rich and solid. And its sustain is amazing --- It seems to be around 35 to 40 seconds on any mid-range note.

However, from the postings on this forum, it is said that a 15-second sustain is considered "excellent". My measurement seems too good to be correct. So I wonder how sustain should be measured. I measured it from the moment I hit the key to the moment I could no longer hear the note (with the sustain pedal pressed of course). Should I stop the clock when the sound volume is below certain level (rather than completely died-out)? If so what is the threshold level?

No sustain pedal, single note from mid range. If you use the sustain pedal then of course it is much longer since all the strings are ringing providing more energy. Also, when you let go of the key, the sustain should go to zero immediately, which is just as important.

I just checked my piano. At middle D, without sustain pedal, 20 seconds, with sustain pedal about 40 seconds. This seems to correspond to your results.

I assume that piano sounds exponentially

http://en.wikipedia.org/wiki/Exponential_decay

You might want to test that

If it is, I assume you can fit the decay curve and then use the mean lifetime tau=1/lambda. I suppose the initial attack does not properly fit the exponential decay and needs to be removed for fitting.

Mean lifetime tau is then the time it takes to reduce the initial volume to 1/e = 0.368 of initial volume.

I assume you just mean for the midrange as far as this, because low notes will ring for a teeny bit when you drop the damper on them.

I think almost any decent piano has plenty of sustain in the midrange so sustain in that area or below is a non issue. I usually listen to sustain starting at around the C above middle C.

Listening to hear how long the sound is audible(until it completely dies out) is also a seriously incorrect approach I think. If a note drops off very quickly but then "sustains" at a very low dynamic level for a long time, then what good is that? Better to listen for the useful part of the sustain and see how long that lasts. Of course, some would disagree on the definition of what's "useful" but at least that approach makes sense.

I would even taken it one step further and assess sustain in terms of how long a melody note can sustain through a held chord of at least 4 notes. Some pianos sustain as a single note, but seem to lose that sustain when other frequencies are involves. Almost like destructive interference. Good pianos don't do this.

I agree completely. As a pianist, the ability of a treble note, in particular, which is part of a continuing melodic line, to be heard through a number of chord changes and still allow the arc of the melody to make sense, is critical. Any other measure of sustain is just window dressing.

From LF's site :

"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."

F#5 (middle of fifth octave) on mine lasts roughly 12 s, F#6 about 6 s.

In addition to measured sustain, I look for pianos that are balanced with similar or well blended decay characteristics between sections.

For example: compare the attack and decay characteristics of notes D18-A25, A61-D66 (and eventually all combinations over the entire keyboard). If I remember correctly, the recommended balance for comfortable nuanced playing of notes which are three to four octaves apart, is the upper note requiring around 3xs the hammer strike force for it to match the decay of the lower note.

Every aspect of design, manufacture, and prep effects the attack-and-sustain characteristics of a piano; which makes it very difficult to measure or view sustain/decay as a single feature. Balancing the right and left hands happens very comfortably on some pianos and is like arm wrestling on others. No two pianos are the same!

Enjoy

F#5 to C#6 is the most difficult and critical area. It is not just sustain. It is how the piano works in music that needs a singing quality. If the sustain is 10 minutes it doesn't matter. The sound only needs to stay around as long as the music demands. More important is actually the quality of the attack, the immediate difference in volume between the attack and the moment after attack, whether or not there is any bloom to the tone after the attack, and then how long does the sound last at a volume that is musically significant.

Like everything else in testing the performance of a piano, it needs to be done in the context of actual music. Chopin has lots of pieces with slow, exposed melodies right smack in that bad area. If the piano allows you to beautifully shape and express a slow and exposed Chopin melody in the F#5 to C#6 range, you definitely can check off a big point on the is this a good piano list.

Its not just overall sustain of the whole tone one is interested in, but sustain of the fundamental pitch of the tone.

Mute 2 of the 3 strings of a unison. Lift the damper either by hand, or hold the key down. pluck the remaining open string.

Now...don't just listen to how long the "whole" sound lasts. Rather listen how long the fundamental pitch lasts. The whole pitch consists of a whole audible series of partials...for this, one wants to only focus on the fundamental pitch's sound.

It can be hard to isolate the fundmental pitch and also isolate when that fundamental pitch drops out . Here's a technique that can help you focus and recognize just the fundamental portion of the plucked pitch. Pluck that 1 open string...pluck...wait 5 seconds...pluck again...wait...etc for a full 5 minutes until you have entered into the tonal twighlight zone. This is the realm where really good techs dwell 24/7. If you patiently do the pluck...wait...you will start to here different pitches within the tone. Focus on the lowest pitch...hopefuilly it will be the fundamental. How long does that fundamental last before it drops out, leaving the other partials still sounding?

The brain takes the partials that are still sounding when the fundamental drops out and "interprets" them to be the fundamental to some degree. However, the actual experience of a sounding fundamental is a very different and visceral experience as opposed to the experience of a psychologically created fundamental.

Most any piano will make a some sound or other for a long time. A piano with sustain has long sustain of the fundamental...At least that's how I approach it in my work.

Jim Ialeggio

The proper way to measure decay (because that is what you are really measuring) is to record the sound and see how long it takes to go from one reference level of volume to another reference point. It should be done on each note of the piano.

Thank you all for your informative replies. It proves that, if you want many answers, different answers, or emtional answers, just post a simple question in this forum.