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Re: AUDITORY Digest - 21 Jan 2005 to 22 Jan 2005 (#2005-14)

In the article below (you'll have difficult time getting  it I suspect)
I measured B for new and "old" guitar strings. The hypothesis was that
as the string ages, B would change.

What I found was that the damping of the high partials changed and that
was the real source of the aging effect.

A second conclusion was that the size of B is perceptually very
important. [A tiny value of B (numbers are in the paper, and I suspect
in many other places as well) leads to a huge perceptual effect.]

A third conclusion was that there are two sets of modes, and the beating
of these harmonics from the two sets of modes, was as important as the
inharmonisity (sp?) effect (the effect of B). These mode sets make the
measurement of B a nontrivial exercise in a real string sound. The
"vertical" and "horizontal" modes are most likely (this was the
assumption of the model I developed in the paper, at least) due to the
difference in the boundary condition at the bridge. Thus in the paper,
the impedance was represented as a matrix, with the two modes separated
by the bridge matrix boundary condition.

A forth conclusion was that there was another effect I didd not nail
down. There is a fast wave that arrives much before the normal wave on
the string, which is easily seen in the impulse response. I have no idea
how important this fast wave is perceptually, but I cant rule it out as
important, other than to say the following. With Mindy Garber (my
student many years later) we did time domain simulations of piano
strings, and we used three strings in parallel, with vertical and
horzizontal waves (for a total of 6 modes at each partial, all beating
against each other), at we found excellent agreement with real sounds.
They were really natural sounds. As far as I know, Mindy never published
this result. She went off to Stanford and started working for Charles
Steel, and I never (well almost never) saw here again.

author = {Allen, J. B.},
   title = {On the aging of steel guitar strings},
   journal = {Catgut Acoustical Society Newsletter},
   year = {1976}

If I get more than 10 requests for this, I'll scan a copy and make it
available on my web site. If you wish to make one of these requests,
send it to the following email address. Please dont spam my regular
email address:


Jont Allen (jba "at" auditorymodels.org)

Automatic digest processor wrote:
There are 2 messages totalling 151 lines in this issue.

Topics of the day:

  1. Definition and Measurement of Harmonicity (2)


Date:    Sat, 22 Jan 2005 11:06:36 -0500
From:    Judy Brown <brown@xxxxxxxxxxxxx>
Subject: Definition and Measurement of Harmonicity

I have a paper measuring this ratio plus calculation on other instruments.

  Brown, J.C.(1994).  ``Measurement of harmonic ratios of sounds
produced by musical instruments,'' J. Acoust. Soc. Am.  95, 2889.

At 05:23 15/01/2005, Reinhart Frosch wrote:

The inharmonicity of piano strings is treated in
section 12.3 of the book "The Physics of Musical
Instruments", by Fletcher and Rossing (Springer,
2nd ed. 1998).

The basic equation for the frequency of the k-th
partial tone is:

f[k] = f[1i] * k * (1 + k^2 * B)^0.5 ;

here, f[1i] is the fundamental frequency of an
idealized string that has the same length, mass
and tension as the real string but is infinitely
flexible (i.e., has no stiffness).

B = 0 corresponds to a string without stiffness
and thus to a harmonic complex tone;
B is an "inharmonicity coefficient".


    Judy Brown            |http://www.media.mit.edu/~brown
    jbrown @ wellesley.edu|http://www.wellesley.edu/Physics/brown/jbrown.html
    brown @ media.mit.edu |E15 483,   MIT, Cambridge, MA 02139
                          |584 Science Cnt, Wellesley College, 02481


Date:    Sun, 23 Jan 2005 10:44:35 +1100
From:    Harvey Holmes <H.Holmes@xxxxxxxxxxx>
Subject: Re: Definition and Measurement of Harmonicity

Jim, Reinhart, Chris and Others,

I think Jim's comment below (under the new thread: Inharmonicity definition
and measurement) is probably what Chris intended, and my contribution was
attempting to show ways of doing this, with measures of degree of
harmonicity that could be derived from the signal itself without reference
to a physical production mechanism (e.g. vibrating strings), interesting
though the production mechanism may be in its own right.

I mentioned two basic harmonicity measures (with variations, including
another one below), but there are many other conceivable measures, such as
those in the survey by W.J. Hess that I quoted, plus a great many
contributions in the speech coding literature of the last 10 or 15 years.

However, I think the term "tonality" should be used with care, since this
and several similar words (below) have a number of different but related
meanings, largely depending on who is using them.  You can check this by
doing Google searches and looking at how the words are used.  In addition,
most of these meanings relate to psychoacoustic perception, which is much
more than just harmonicity.

In the first place, "tonality" means something else entirely in music
theory.  In addition, it is often also used to mean the same as
"tonalness", which is a purely perceptual concept, referring to the
sensation of pitch of a sound complex.  Pitch perception is much more
complex than the question of the degree of harmonicity.  For example, pitch
can be heard in sounds that are far from being harmonic, as explained in
the articles by E. Terhardt on pitch that are available on
http://www.mmk.ei.tum.de/persons/ter.html or by R. Meddis and M.J. Hewitt
(JASA, 89 (6), June 1991).  There have been attempts to predict tonalness
based on various theories, such as Terhardt's virtual pitch concept or the
Meddis and Hewitt temporal approach (q.v.).  I haven't seen them, but I
believe that there are even standards about this: ASA 118-1995 and DIN
45681 (the latter still in draft form a few years ago, but may be final by

Another similar concept with a similar name is "tonality measure", which is
used when deriving auditory masking thresholds (also a perceptual concept)
for use in audio coding algorithms such as MPEG.  This refers to the degree
to which the individual sine wave components stand out above the noise
floor.  Masking models often treat "tonal components" differently from
others when calculating the auditory masking threshold.

Also, in some speech coding work these or similar terms may also refer to
the degree to which individual partials of a tonal complex can be "heard
out" individually (usually only the lower partials), still another
perceptual concept, and similar to (but different from) the masking concept
above.  If partials can be heard out, they are sometimes coded differently
from those that can't be heard individually.

The fact that these terms (tonality etc.) are often used to mean different
things by different authors is alone a good reason to avoid them unless
they are clearly defined when used.

I therefore think that degree or measure of harmonicity (or similar) is a
better term when referring to the degree to which a signal is
harmonic.  This is a relatively straightforward physical concept (though
with many possible ways of defining or estimating it), and is much simpler
than the concepts underlying the other terms (tonality, tonalness, virtual
pitch etc.), which are mostly perceptual in nature and have shifting
meanings depending on who uses them.

******************** While still on this topic, another variation of my first harmonicity measure H1 is obtained with a different definition of ACF:

     RB(k) = SUM (x(n) * x(n+k)),

where the sum is taken over all n in the range [0, N-1], with values of
x(n) outside this range being set to zero.  (This definition is the one
used by the Matlab function xcorr.m.  It applies a window to the signal,
and, apart from a constant scale factor, gives a biassed estimate if
certain statistical assumptions are made, unlike the previous
definition.)  The resulting harmonicity measure is then

     H11 = MAX (RB(k)) / RB(0),

where the maximum is taken over k in the range [1, MAX], where MAX should
be larger than any likely period of the harmonic component.

(Incidentally, I should also have written [1, MAX] instead of [1, N-1] for
this range earlier, since both R(k) and RB(k) are unreliable for large lags
k, though for different reasons.)

     Harvey Holmes

At 09:45 22/01/2005, Jim Beauchamp wrote:

That said, it turns out that this is not really what Chris
was interested in. He is interested in something called "tonality",
which is something that has been mentioned in the audio
literature many times, but I have not seen a simple definition.
But basically if a signal is composed of harmonic or quasi-harmonic
sinusoids, it is "tonal". The other extreme is a noisy, random
signal. And, of course, signals can be combinations of both.

Jim Beauchamp


End of AUDITORY Digest - 21 Jan 2005 to 22 Jan 2005 (#2005-14)