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7/28/2019 Corona Photo Sample

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Acoustics Shock Vibration Signal Processing March 2003 Newsletter

Introduction

The damping in suspension bridges isfairly small. A small oscillating are some

times used as vibration exciters duringstructural tests of bridges, as reportedby Bishop in Reference 1.

Suspension Bridge ResonanceBy Tom Irvine

Ni hao

Sound and vibration are significant effects in asurprising number of contexts.

I recently came across a television programabout a horse breed called Paso Fino, or finestep. The merit of a Paso Fino is judgedaccording to its staccato cadence as it tapdances its way across a sounding board,which amplifies the hoof beat sound.

On another topic, some years ago I wasstanding in the Disneyland parking lot, in

Anaheim, California. The weather was dampand foggy as is often the case during themornings in that area. A friend and I noticedthe sharp, crackling sounds emitting from

overhead power lines. The cause was coronadischarge, although we only vaguelyunderstood 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 withthese sound and vibration topics.

Sincerely,

Tom IrvineEmail: [email protected]

Feature Articles

Paso Fino Hoof Beat Sound page 3

Power Line Noise page 7


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The Vibrationdata CD contains dozens of software programs, over 100 tutorialpapers, as well as NASA and military handbooks.

Topics include

Acoustics Pyrotechnic Shock

Shock Response Spectrum Random Vibration

Signal Processing Dynamic Data Acquisition and Analysis

Modal Testing Finite Element Methods

Structural Dynamics Earthquake Engineering

The CD also contains: Arthur W. Leissa, NASA SP-160, Vibration of Plates

The price for the CD is $75.00 (US).

To order, please send a check or money order to:

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Payment via PayPal is also accepted.


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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 herperformance mare over the sounding board at the CarouselHorse Show in Scottsdale, Arizona. Note that the soundingboard has nearly the same color as the surrounding soil.

Introduction

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

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

Columbus brought Paso Fino horses toSanto Domingo, in his second voyage to theAmericas. These horses became thefoundation stock for the Conquistadors.

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

Furthermore, Paso Fino may have anyequine color.

Gait

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

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

Speeds

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


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FINO is a collected version of the gait,performed with slower forward motion thana regular walk. It is performed incompetitions.

The horse needs to be very fiery, giving theimpression he is about to explode, whilemoving his feet with incredible speed andaccuracy with up to 10 beats per second,while barely moving forward. The horse isoften ridden over a sounding board, so thejudges can hear that the cadence is even.An image is shown in Figure 1. This isstrictly a show gait. It is unique to the PasoFino breed.

CORTO is the medium speed gait, like aleisure trot. The Paso Fino prefers cortowalking or canter and will use it frequently inthe pasture, but some prefer trotting unlesscarrying a rider. The horse can stay in cortofor many, many miles.

LARGOis the most extended version of thegait, most horses perform it at the speed ofa fast trot, some much faster.

Sounding Board

Again, the cadence of the Paso Fino gait is

judged in competitions as the horse travelsover a sounding board. The exceedinglyrapid, staccato beat of the hooves ismagnified by the sounding board. Theamplified beat reverberates though thearena.

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

In addition, the staccato beat is veryentertaining for the audience.

Sounding Board Construction

The sounding board is constructed of 4 ft x8 ft sheets of plywood nailed to a base of 2in x 4 in beams. The board is thus hollowedout underneath. The total board length is 48to 64 feet. It is also called the Fino Board.

Paso Fino Sound Data

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

This data originally came from a wav file

that is posted at a number of Paso Finowebsites. The author converted it to ASCIItext format using the CoolEdit softwareprogram.

Furthermore, the author has posted theaudio file at:

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

This energy in Figures 2 and 3 is the resultof the hoof impacts against the soundingboard.

A power spectral density of the time historyis given in Figure 4. The data shows thatmost of the spectral energy is in the 80 Hzto 300 Hz domain. This response is theproduct of the boards structural response,as well as the response of the air volume inthe hollow cavities underneath the board.

The energy distribution in the frequencydomain could perhaps be described asnarrowband random.


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0 1 2 3 4 5 6 7 8 9 10

Approximately 10 beats per second.

TIME (SEC)

PRESSURE(Unscaled)

ACOUSTIC PRESSURE TIME HISTORYPASO 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)

PRESSURE

(Unscaled)

ACOUSTIC PRESSURE TIME HISTORYPASO FINO TRAVELING OVER A SOUNDING BOARD

Figure 3.


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0.01

0.1

1

10

10 100 1000 10000

FREQUENCY (Hz)

PRESSURE(UnscaledUnits

2/

Hz)

ACOUSTIC POWER SPECTRAL DENSITYPASO FINO TRAVELING OVER A SOUNDING BOARD

Figure 4.


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Power Line Noise by Tom Irvine

Figure 1. Corona Discharge

The light-blue spots are evidence of CoronaDischarge. The relative humidity wasgreater than 80%.

Corona Discharge

High tension power lines may producesizzling or crackling sounds during dampweather. The crackling sound may also beaccompanied by a low frequency hum, witha frequency of about 100 Hz.

This noise is a particular concern in costalcommunities where fog is common.

The crackling sound is an effect of coronadischarge which forms around theconductive wires. The corona discharge isa type of electric spark. Sometimes, it isaccompanied by a blue light. The highvoltage breaks apart air molecules causingthe air to glow. This breakdown is a form ofionization.

High-voltage power lines carry 230,000volts or more. These transmission lines aredesigned to operate below the thresholdvoltage for discharge, but surfaceirregularities such as rain drops or soliddebris can cause discharge to occur, withthe accompanying crackling sound.

"The presence of moisture in the air or onthe conductor promotes electricalconduction and reduces the voltagerequired for corona onset to occur," saysRichard Johnson, an electrical engineer atthe Public Service Company of NewMexico. There is always a small amount ofcorona discharge going on, he says. "Thepresence of moisture simply magnifies theeffect."

Specifically, the electrical field around thedrops of water becomes strong enough totear electrons away from the surface or fromthe negative ions that are always present in

the air.

The free electrons increase their speed inthe electronic field and release newelectrons from other gas molecules. Anavalanche of electrons is thus created.Nevertheless, the electric field is quicklyweakened outside of the power line, and theelectrons lose speed and ability to ionize.

The free electrons are caught by gasmolecules that need electrons, such as

oxygen and halocarbons.

Effects

Corona discharge produces annoying soundand electromagnetic interference. Thisinterference may disrupt TV, radio, andtelephone reception.

The power company must deal with thesecomplaints.

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


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Level Number ofComplaints

60 70 dB(A) High

50 60 dB(A) Moderate

40 - 50 dB(A) Few

Additional Sources and Concerns

Corona activity may also occur along sharpedges on energized hardware, brokenconductor strands, or defective insulators.

The discharge from a broken insulator canfind a path to ground in certain cases,

resulting in a highly destructive ground fault.

Detection of corona discharge is thus criticalin the power industry.

Prevention

Even well-designed transmission lines maysuffer corona discharge during wet weather.The threat can be reduced, however,through proper design and maintenance.

A design mitigation method is to transmitthe power over a bundle of lines, with eachline spaced 18 inches apart. This reducesthe electrical power than each line mustcarry.

Furthermore, the lines must be properlymaintained. Smooth conductor surfacesminimize electrical stress, thus reducing thepotential for corona discharge.

On the other hand, older conductors canbecome pitted or scratched and thus moresusceptible to water penetration. Coronadischarge is thus more likely to occur.

St. Elmos Fire

Corona discharge is called St. Elmos Fire,when it occurs naturally.

St. Elmos Fire may develop around pinetrees, masts, and lightning rods duringthunderstorms. It also occurs on the wingsand surfaces of aircraft flying throughheavily charged air masses.

A buzzing or hissing sound may accompanySt. Elmos Fire.

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