Date: Wed, 07 Nov 2001 16:33:46 -0600
From: Ron Nossaman <RNossaman@KSCABLE.com>
Subject: Re: Tuned front duplexes
>So, to me, at least, the question is unanswered. What constitutes
>'projection' anyway?
That would be an arbitrary perceived volume at X distance, X being
projection, but not otherwise quantified by any specific proximity
measurement. I hope that clears it up some. Now I'm not much on large hall
acoustics, being more of a dimly lit small airless enclosure kind of guy,
but doesn't low frequency sound carry farther than higher frequencies?
Isn't a harmonic spectrum with lots of low frequency partials going to
"project" farther back into the cheap seats than something with a lot of
high frequency noise - like for instance clangy trebles or those front
duplex noises that enhance projection? Can't I hear the bass line from
Spike's stereo, through the house walls, while he's still three blocks
away, but can't hear his high range stuff until he opens the car door in
the driveway? No wonder the most common phrase I hear regarding piano tone
is "But just listen to that bass"! Well yea, that's bone conductance. So I
wonder why even harder hammers and more painful trebles still don't get to
that back row.
-----
Ron,
A bit of a slow response on this. I thought I used to know
some of this stuff but found I needed to consult a text.
The first acoustics text I laid my hands on had so many
double integrals and Bessel functions that my eyes started
to glaze over so I had to go in search of a more rudimentary
text (amazing the things that one could do in college that
one can't do now). Let me preface all of this with the
caveat that I am not an acoustician and I may be skewing
or misinterpreting some of this. If there is an acoustician
out there please correct me.
As I read it acoustic power goes up with frequency at the
same amplitude of vibration. I believe perceived loudness
is closely related to power (I don't have a psychoacoustics
text handy). I believe a high frequency tone with the same
energy as a low frequency tone will sound louder. A tone
with a preponderance of high frequency information with the
same energy as a tone with a preponderance of low frequency
information will sound louder. How the power is dissipated
over distance relative to frequency I'm still not clear on.
More study will be required. But, transmission through the
air from a vibrating source is different than transmission
through a barrier (such as the body of a car) or what the
acousticians like to call a partition. I'm not sure that
the reason you hear the bass tones from the rap music in the
passing car is because lower tones 'carry further'. One
thing is, I believe, clear from my reading. Higher frequency
tones are much more directional than low frequency tones.
Everyone in the hall can hear the low frequency information
but you have to be in the right seat to hear the high
frequency information. This could perhaps explain the
results from Richard's not very scientific 'test' between
the Yamaha and the Steinway. Perhaps the 'testers' were in
the wrong seats. If my reading is correct the Yamaha, which
I assume would have a preponderance of higher partials in
its tone, would have more perceived loudness if you are in
the right spot, than the Steinway, which I assume would have
more energy concentrated in the lower partials. The
corollary to this is that the variation in perceived loudness
of the Yamaha is going to be greater than for the Steinway
as you move around the hall. If you're in the right spot
to get the high frequency information the Yamaha will sound
louder ('project more') but if you're in the 'wrong' spot
you'll only be getting the low frequency information, and
since the Steinway presumably has more of that, the Steinway
will sound louder. The percentage of the hall that is the
'right spot' for getting the high frequency information is
I believe small. I believe this also means that the
perceived tone quality of the Yamaha will have more
variation as you move around the hall. Since the tone
is made up of lower frequency information and higher
frequency information, if you're in the spot to get the
high frequency information the perceived tone quality
will be different than if you are in a spot not to receive
as much high frequency information. So this means to me:
1. The Yamaha is potentially louder ('projects better')
than the Steinway but this potential is only realized in
a few spots in the hall.
2. The Steinway is not as potentially loud as the Yamaha
if you're in the right spot but in a majority of the hall
it will be perceived as louder ('project better').
3. The Steinway will have less variation in perceived
loudness and tone quality than the Yamaha around the hall.
If I were a designer I'd like the conclusions about the
Steinway better than the conclusions about the Yamaha, but
that's just me.
What does this mean to tuned duplexes?
For the front duplex, if the presence of the duplex adds
high frequency information or causes the string to vibrate
in such a way that it contains more high frequency
information then it seems you would be increasing the
potential projection. If we could just 'add on' the front
duplex then we would, I think, be increasing projection
potential. In some places in the hall we would realize
this potential, but in most places in the hall we wouldn't.
However, we can't just 'add on' the duplex. The duplex
gets its energy from the string. So, once again, by
concentrating more of the vibrational energy in the upper
partials we have increased projection potential in a few
places in the hall. But we have done this at the expense
of lower frequency information in the string. So in most
of the hall we have sacrificed projection. I think we have
also contributed to more loudness and tonal variations
around the hall as well which I don't think of as a good
thing. The difference between a tuned and untuned duplex
is unclear to me. Both would be putting string energy into
higher partials. Perhaps the tuned duplex, by interacting
with the speaking length causes the string to want to vibrate
more strongly in the partial corresponding to the duplex and
concentrate more of its energy in this partial. In that
case the tuned duplex would make the phenomenon that we're
discussing here more pronounced. The conclusion that I
draw is that as a designer, if I want better projection in
a majority of a hall, and want to minimize tonal and
loudness variations around the hall, the front duplex should
be eliminated altogether. I think the pianists would thank
us as well since they wouldn't have to listen to annoying
noises that go along with a front duplex.
As to the rear duplex. I would draw the same conclusion.
Eliminate it. Unfortunately we don't have that option.
There has to be a back scale in order for the bridge to move.
So we really have the option of a tuned or untuned system,
although with either system the duplex could be felted out.
Either system is putting high frequency information into
the sound. So the same things apply here that were talked
about above. It's hard for me to see that there would be
any difference between a tuned and untuned system as far
as projection is concerned. Sustain is another issue not
talked about here, but without looking into it further I
can't see why a tuned system would give better sustain. I
believe there will be a difference in tone quality or color
with the tuned and untuned systems. I'm not sure which I
would choose if the two were side by side, but I'm not
convinced it would be the tuned system. I think an argument
could be made in favor of felting out rear duplex. Based on
the things talked about above if you want less variation in
perceived tone quality around a hall you might be better off
not adding this additional higher frequency information.
Also, if you believe a beautiful piano tone contains more
fundamental relative to upper partials then you might want
to felt out these additional 'partials'. I suspect all of
this is a matter of taste. Some people will prefer one
system and some another. Using different systems would
allow us to have differences among our pianos which in my
opinion would be a good thing.
Phil
---
Phillip Ford
Piano Service & Restoration
1777 Yosemite Ave - 215
San Francisco, CA 94124
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