>Suspension Bridge Resonance
By Tom Irvine




Ni hao

Sound and vibration are significant effects in a
surprising number of contexts.

I recently came across a television program
about a horse breed called Paso Fino, or fine
step. The merit of a Paso Fino is judged
according to its staccato cadence as it tap
dances its way across a sounding board,
which amplifies the hoof beat sound.

On another topic, some years ago I was
standing in the Disneyland parking lot, in
Anaheim, California. The weather was damp
and foggy as is often the case during the
mornings in that area. A friend and I noticed
the sharp, crackling sounds emitting from
overhead power lines. The cause was corona
discharge, although we only vaguely
understood the principle at the time.

I have yet to hear the same effect in Arizona,
which has a very dry climate.

I hope you enjoy the articles dealing with
these sound and vibration topics.


Sincerely,

Tom Irvine
Email: tomirvine@aol.com



Feature Articles





Paso Fino Hoof Beat Sound

page 3




Power Line Noise
page 7
2




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3
Paso Fino Hoof Beat Sound


by Tom Irvine




Figure 1. Paso Fino (Photo courtesy of Jack Schatzberg)

The photograph shows Ms. K.K. DuBois riding her
performance mare over the sounding board at the Carousel
Horse Show in Scottsdale, Arizona. Note that the sounding
board has nearly the same color as the surrounding soil.



Introduction

The Paso Fino is a horse breed that has a
naturally smooth gait.

The Paso Fino breed originated in Spain.
The Andalusian, the Barb, and the Spanish
Jennet were crossed bred to produce the
Paso Fino.

Columbus brought Paso Fino horses to
Santo Domingo, in his second voyage to the
Americas. These horses became the
foundation stock for the Conquistadors.

Today, Paso Fino horses are used for trail
riding, cattle herding, and in competitions.



Furthermore, Paso Fino may have any
equine color.


Gait

Fino means fine, and paso means step.
The Paso Finos gait consists of rapid, small
steps. This gait produces very little bounce.
It is thus very comfortable for the rider.

The natural footfall of the Paso Fino is Right
Hind, Right Front, Left Hind, Left Front in a
perfect four beat gait.

Speeds

The Paso Fino has 3 speeds of square 4
beat gait:
4


FINO is a collected version of the gait,
performed with slower forward motion than
a regular walk. It is performed in
competitions.

The horse needs to be very fiery, giving the
impression he is about to explode, while
moving his feet with incredible speed and
accuracy with up to 10 beats per second,
while barely moving forward. The horse is
often ridden over a sounding board, so the
judges can hear that the cadence is even.
An image is shown in Figure 1. This is
strictly a show gait. It is unique to the Paso
Fino breed.

CORTO is the medium speed gait, like a
leisure trot. The Paso Fino prefers corto
walking or canter and will use it frequently in
the pasture, but some prefer trotting unless
carrying a rider. The horse can stay in corto
for many, many miles.

LARGO is the most extended version of the
gait, most horses perform it at the speed of
a fast trot, some much faster.

Sounding Board

Again, the cadence of the Paso Fino gait is
judged in competitions as the horse travels
over a sounding board. The exceedingly
rapid, staccato beat of the hooves is
magnified by the sounding board. The
amplified beat reverberates though the
arena.

The evenness of the footfall is the criterion.
This is determined by sound rather than by
sight. A judge may look away or down to
concentrate on the sound of the hoof beats'
rhythm.




In addition, the staccato beat is very
entertaining for the audience.

Sounding Board Construction


The sounding board is constructed of 4 ft x
8 ft sheets of plywood nailed to a base of 2
in x 4 in beams. The board is thus hollowed
out underneath.

The total board length is 48
to 64 feet. It is also called the Fino Board.

Paso Fino Sound Data

An example of an acoustic pressure time
history is shown in Figures 2 and 3.

This data originally came from a wav file
that is posted at a number of Paso Fino
websites. The author converted it to ASCII
text format using the CoolEdit software
program.

Furthermore, the author has posted the
audio file at:

http://www.vibrationdata.com/pasofino.wav

This energy in Figures 2 and 3 is the result
of the hoof impacts against the sounding
board.

A power spectral density of the time history
is given in Figure 4. The data shows that
most of the spectral energy is in the 80 Hz
to 300 Hz domain. This response is the
product of the boards structural response,
as well as the response of the air volume in
the hollow cavities underneath the board.

The energy distribution in the frequency
domain could perhaps be described as
narrowband random.









5
0
1
2
3
4
5
6
7
8
9
10
Approximately 10 beats per second.
TIME (SEC)
P
R
E
S
S
U
R
E

(
U
n
s
c
a
l
e
d
)
ACOUSTIC PRESSURE TIME HISTORY
PASO FINO TRAVELING OVER A SOUNDING BOARD

Figure 2.



6.45
6.50
6.55
6.60
6.65
6.70
6.75
6.80
6.85
TIME (SEC)
P
R
E
S
S
U
R
E

(
U
n
s
c
a
l
e
d
)
ACOUSTIC PRESSURE TIME HISTORY
PASO FINO TRAVELING OVER A SOUNDING BOARD


Figure 3.





6
0.01
0.1
1
10
10
100
1000
10000
FREQUENCY (Hz)
P
R
E
S
S
U
R
E

(
U
n
s
c
a
l
e
d

U
n
i
t
s
2

/

H
z
)
ACOUSTIC POWER SPECTRAL DENSITY
PASO FINO TRAVELING OVER A SOUNDING BOARD


Figure 4.




7
Power Line Noise

by Tom Irvine





Figure 1. Corona Discharge

The light-blue spots are evidence of Corona
Discharge. The relative humidity was
greater than 80%.


Corona Discharge

High tension power lines may produce
sizzling or crackling sounds during damp
weather. The crackling sound may also be
accompanied by a low frequency hum, with
a frequency of about 100 Hz.

This noise is a particular concern in costal
communities where fog is common.

The crackling sound is an effect of corona
discharge which forms around the
conductive wires. The corona discharge is
a type of electric spark. Sometimes, it is
accompanied by a blue light. The high
voltage breaks apart air molecules causing
the air to glow. This breakdown is a form of
ionization.

High-voltage power lines carry 230,000
volts or more. These transmission lines are
designed to operate below the threshold
voltage for discharge, but surface
irregularities such as rain drops or solid
debris can cause discharge to occur, with
the accompanying crackling sound.

"The presence of moisture in the air or on
the conductor promotes electrical
conduction and reduces the voltage
required for corona onset to occur," says
Richard Johnson, an electrical engineer at
the Public Service Company of New
Mexico. There is always a small amount of
corona discharge going on, he says. "The
presence of moisture simply magnifies the
effect."

Specifically, the electrical field around the
drops of water becomes strong enough to
tear electrons away from the surface or from
the negative ions that are always present in
the air.

The free electrons increase their speed in
the electronic field and release new
electrons from other gas molecules. An
avalanche of electrons is thus created.
Nevertheless, the electric field is quickly
weakened outside of the power line, and the
electrons lose speed and ability to ionize.
The free electrons are caught by gas
molecules that need electrons, such as
oxygen and halocarbons.

Effects

Corona discharge produces annoying sound
and electromagnetic interference. This
interference may disrupt TV, radio, and
telephone reception.

The power company must deal with these
complaints.

The Southwire Company of Georgia gives
the following data for sound measured 30
meters from power lines:




8
Level
Number of
Complaints
60 70 dB(A)
High
50 60 dB(A)
Moderate
40 - 50 dB(A)
Few

Additional Sources an