Sound Projection and double lids

[John Musselwhite]

At 11:51 AM 25/06/96 -0400, Stephen B. wrote:

>John Musselwhite wrote:
>> .... one of the worst things you can
>> do to a grand piano's sound (under most conditions) is to take off the lid.
>> Next to that is to put it on carpet.
>>
>Common practice with early pianos in public performance was to remove
>the lid. At a recent concert Bilson played a piano solo with the lid
>in typical 20th C position, then a Mozart concerto with the lid
>completely removed (this was a modern version of a c1800 piano). With
>the lid off the sound was noticeably better and distinctly louder.
>This was also a closed bottom instrument.

In a closed-bottom instrument you shouldn't have the same sort of phase
cancellation problems since the rear waves are either contained or focused
through a "port" in the soundboard. If you were to imagine a guitar with a
"lid" on it it might give you an idea of what such a lid might do.

>>> .. soundboard would seem to produce the same signal from both sides
>>> (top and bottom) but would they not be 180 degrees out of phase?
>>
>> They are indeed. Part of the function of the case is to bring the top and
>> bottom of the board into phase by the time the sound reaches you.
>>
>Not sure what you mean by this John.

The case and lid act as somewhat of a "baffle" or separator between the top
and bottom of the soundboard (or the front and rear of any transducer).  If
it wasn't there at all, there would be more phase cancellation in the lower
end.  Those who have had the misfortune of tuning a Helpenstill electric
grand may have noticed the effect. For those who haven't it's more-or-less a
"strung back" (by Kimball?) with a keyboard, and the case is removeable and
made of a light material of some kind. Tuning the bass is more fun than...
well, it's a toss-up between tuning a CP-70 or rope burns... and welts on
flesh is almost a more appealing choice some days!

>Some of the theories that have been posted in this acoustic
>discussion neglect to consider that reflective surfaces can
>introduce phase changes (hard vs soft).

As far as a concert grand's soundboard is concerned, once you are about 10
feet or so from it the phase is irrelevent since it's low-frequency energy
has either been reinforced or dispersed by then. "Phase" is a property that
refers to timing  and the phase changes in the hall in various locations
become important because of the delay of the reflections of certain
frequencies. This is different than the phase of the soundboard itself. Once
you are past 1/4 of the length of the lowest wave the actual phase of the
transducer/soundboard doesn't mean quite as much as far as speaker
technology goes..

>Also, for the greatest sound
>intensity we should all sit on the bass side of the piano to the left
>of the damper rail...there is more to acoustics than just sound
>intensity.

But is "intensity" what you are listening for or is it "balance"?  The
position of the piano on the stage itself makes a big difference. There has
been some discussion of various reflective surfaces here already such as the
portable band shells made by Wenger or the beautiful system they use at the
Centennial Auditorium in Saskatoon Saskatchewan, consisting of
proscenium-high wooden backdrops on casters with curved wooden "tormentors"
flown from the grid and all painted "chroma-key blue" for a spectacular
visual and auditory effect.

As for the best place to listen, a lot depends on your own hearing and the
acoustics of the hall almost as much as the piano itself I would think.

>Some open questions that relate to lids etc.:

If I may give you my responses... <g>

>1) What is the acoustic difference between closed and open bottom
>   instruments? Graf, f.i. made essentially the same piano with and
>   without bottom, changing c 1830. Does the closed bottom
>   instrument create a resonance effect?

A closed bottom ensures that any rear-waves are cancelled out because they
are "contained", so the forward waves are all that matter. I would suspect
there is a resonance effect only if there is a way for the sound to escape,
either through a tuned port or through the soundboard like many stringed
instruments.

>2) What is the function of the oversoundboard with which *most*
>   first-half 19th C. Viennese pianos were fitted. How does the

I'm not really familiar with that, however I have seen guitars made with two
backs with a soundpost between them. C.F. Martin made such an instrument,
but it wasn't overly successful. Valuable, yes but the efficacy of two backs
was questionable enough that the instruments are extremely rare.

>If (or when?) my new research proposal gets funding to proceed I hope
>to investigate these (and other) questions. There is certainly room
>for some exciting discoveries still to be made in piano acoustics.

I'm not sure how much "room" there actually IS. Not that I would put a
damper on research or anything, but even though they put a man on the moon
before they figured out exactly why a piano does what it does, a lot of
things have been tried and rejected already.

>I think looking back to early pianos will be quite revealing.

It will be of course. As has been observed on previous occasions, in the
"old days" everything was done by rule-of-thumb, but everyone's thumbs were
different sizes and some people seemed to be all thumbs when it came to
scaling, design and execution. Many things were tried from the simplest to
those which would intrigue Rube Goldberg, and their advantages and
disadvantages should be re-assessed in light of modern materials and techniques.

			John

John Musselwhite, RPT
Calgary, Alberta Canada
musselj@cadvision.com