INTRODUCTION

Imagine you could point the same way you can point light with, say, a flash light.

Suppose speakers existed that could send focused beams of sound where ever thy pointed. You

could speak in to a megaphone and send the sound to a single person. Or you could have five

types of music playing on the same dance floor. You wouldn’t have to worry about turning the

music down at night to keep the neighbors happy, so long as you didn’t point it in their

direction. Thanks to recent advancement in audio engineering, the kinds of products may soon

be a reality. Researchers have discovered a way to project acoustic waves as thin beam of sound:

step into the beam and projected sound fills your ears.

Directional audio is a technology that creates-focused beams of sound, similar to

the light beam coming out of a flashlight. The technology that powers this is known as audio

spotlighting. It uses a combination of non-linear acoustics and some complex mathematics in

order to focus sound into a coherent and highly directional beam. It is under development in

Holosonics Research Labs and the American Technology Corporation.

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DIRECTING THE SOUND

Properties of audible sound:

• The human hearing ranges from a frequency of 20Hz to 20 KHz.

• Wavelength varies between 2cm to 17m.

• Beam angle - 360 degrees.

The audible portion of sound tends to spread out in all directions from the point of

origin. The beam angle of audible sound is very wide, just about 360 degrees. This means the

sound that you hear will be propagated through air equally, in all directions, which is why you

don’t need to be right in front of a radio to hear the music.

In order to focus sound into a narrow beam the requirement is:

1. A low beam angle

-The smaller the wavelength, the lesser the beam angle and hence more focused the

sound. The human hearing ranges from a frequency of 20Hz to 20 KHz. Therefore the audible

sound is mixture of signals with varying wavelength between 2cm to 17m. Except for very low

wavelength, just about the entire audible spectrum tends to spread out at 360 degrees.

2. Large aperture size

A large loudspeaker will focus sound over a smaller area. If the source

loudspeaker can be made several times bigger than the wavelength of the sound transmitted,

then a finely focused beam can be created. But this is not a very practical solution.

This is where the ultrasound came to the rescue.

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PROPERTIES OF ULTRASOUND

The frequency ranges above 20 KHz

The wavelength is less than 2crn

Small beam angle hence highly coherent and directional.

SPECIAL FEATURES OF AUDIO SPOTLIGHT

A COMPARISON WITH CONVENTIONAL LOUD SPEAKER:-

Creates highly FOCUSED BEAM of sound

Sharper directivity than conventional loud speakers using Self demodulation of finite

amplitude ultrasound with very small wavelength as the carrier

Uses inherent non-linearity of air for demodulation

Components- A thin circular transducer array, a signal processor & an amplifier.

Two ways to use- Direct & projected audio

Wide range of applications

Highly cost effective

THEORY

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IN TO THE DEPTHS OF AUDIO SPOTLIGHTING

TECHNOLOGY

What ordinary audible sound & Conventional Loud Speakers lack?

What we need?

About a half-dozen commonly used speaker types are in general use today. They range

from piezoelectric tweeters that recreate the high end of the audio spectrum, to various kinds of

mid-range speakers and woofers that produce the lower frequencies. Even the most

sophisticated hi-fi speakers have a difficult time in reproducing clean bass, and generally rely on

a large woofer/enclosure combination to assist in the task. Whether they be dynamic,

electrostatic, or some other transducer-based design, all loudspeakers today have one thing in

common: they are direct radiating-- that is, they are fundamentally a piston-like device designed

to directly pump air molecules into motion to create the audible sound waves we hear. The

audible portions of sound tend to spread out in all directions from the point of origin. They do

not travel as narrow beams—which is why you don’t need to be right in front of a radio to hear

music. In fact, the beam

angle of audible sound is very wide, just about 360 degrees. This effectively means the sound

that you hear will be propagated through air equally in all directions.

In order to focus sound into a narrow beam, you need to maintain a

low beam angle that is dictated by wavelength. The smaller the wavelength, the less the beam

angle, and hence, the more focused the sound. Unfortunately, most of the human-audible sound

is a mixture of signals with varying wavelengths—between 2 cms to 17 metres (the human

hearing ranges from a frequency of 20 Hz to 20,000 Hz). Hence, except for very low

wavelengths, just about the entire audible spectrum tends to spread out at 360 degrees. To create

a narrow sound beam, the aperture size of the source also matters—a large loudspeaker will

focus sound over a smaller area. If the source loudspeaker can be made several times bigger than

the wavelength of the sound transmitted, then a finely focused beam can be created. The

problem here is that this is not a very practical solution. To ensure that the shortest audible

wavelengths are focused into a beam, a loudspeaker about 10 metres across is required, and to

guarantee that all the audible wavelengths are focused, even bigger loudspeakers are needed.

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Here comes the acoustical device “AUDIO SPOTLIGHT” invented by Holosonics Labs

founder Dr. F. Joseph Pompei (while a graduate student at MIT), who is the master brain behind

the development of this technology.

FIG.1:-AUDIO SPOTLIGHT CREATES FOCUSED BEAM OF SOUND UNLIKE

CONVENTIONAL LOUD SPEAKERS

Audio spotlight looks like a disc-shaped loudspeaker, trailing a wire, with a small laser

guide-beam mounted in the middle. When one points the flat side of the disc in your direction,

you hear whatever sound he's chosen to play for you — perhaps jazz from a CD. But when he

turns the disc away, the sound fades almost to nothing. It's markedly different from a

conventional speaker, whose orientation makes much less difference.

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HOW IT EVOLVED

The technique of using a nonlinear interaction of high-frequency waves

to generate low-frequency waves was originally pioneered by researchers developing

underwater sonar techniques in the 1960s. In 1975, an article cited the nonlinear effects

occurring in air.

Over the next two decades, several large companies including

Panasonic, NC Denon and Rioch attempted to develop a loud speaker based on this principle.

They successful in producing some sort of sound, with extremely high levels of distortion

(>50% THD). This drawback caused the total abandonment of the technology by the end of

1980s.

In the 1990s, Woody Norris, a 65 year old west Coast maverick solved the

parametric problems of this technology with his breakthrough approach.

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HYPERSONIC SOUND EMITTER

ULTRASOUND IN AIR

Researchers discovered that if short pulses of ultrasound were fired into water, the

pulses were spontaneously converted into low frequency sound. Dr. Orhan Berktay established

that water distorts ultrasound signals in a nonlinear, but predictable mathematical way. It was

later found that similar phenomenon happens in air also. When inaudible ultrasonic sound pulses

are fired into the air, the air spontaneously converted the inaudible ultrasound into audible sound

tones, hence proving that as with water, sound propagation in air is just as non-linear, but can be

calculated mathematically. As the beam moves through the air gradual distortion takes place

giving rise to audible component that can be accurately predicted and precisely controlled.

The problem with firing off ultrasound pulses, and having them interfere to

produce audible tones is that the audible component created are nowhere similar to the complex

signals in speech and music which contains multiple varying frequency signals, which interfere

to produce sound and distortion.

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FIG.7:-CONVENTIONAL LOUDSPEAKER & ULTRASONIC EMITTER

BERKTAY’S EQUATION

In 1965, Dr. H.O. Berktay published the first accurate and more complete theory of

distortion of ultrasound signal in air. He uses the concept of modulation envelope. The air

demodulates the modulated signal and the demodulated signal depends on the envelope

function. Berktay assumes the primary wave has the form

P1 (t) = P1 E (t) sin (Wct)

Where we is the carrier frequency and E (t) is the envelope function which in this

case is the speech or music signal

The secondary wave or demodulated wave is given by

P2 (t) d/dt2E (t)

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This is called berktay’s far field solution. The berktay’s solution states that the

demodulated signal is proportional to the second time derivative of the envelope squared. This is

the fundamental expression for the output resulting from the distortion due to air.

HYPERSONIC SOUND TECHNOLOGY

Hypersonic sound technology works by emitting harmless high frequency ultrasonic tones that

we cannot hear. These tones use the property of air to create the new tones that are within the

range of human hearing. The result is an audible sound wave is created directly in the air

molecules by down converting ultrasonic energy into the frequency spectrum we can hear.

In a hypersonic sound system, there are no voice coils, cones cross-over networks or

enclosures. The result is sound with a potential purity and fidelity which we attained never

before. Sound quality is no longer tied to speaker size. The hyper sonic sound system holds the

promise of replacing conventional speakers in homes, movie theaters, automobiles- everywhere.

The ultrasound signal is used as a carrier wave and the audible speech and music

signal are superimposed on it to create a hybrid wave similar to the amplitude modulation. The

resultant hybrid wave is then broadcast. As this wave moves through the air, it creates complex

distortions that give rise to two new frequency sets,

(i) One slightly higher than the hybrid wave. This sideband is identical the original sound

wave

(ii) Slightly lower, than the hybrid wave. This sideband component is a badly distorted

component.

These two sidebands interfere with the hybrid wave and produce the two signal

components - the normal and the distorted components. But the problem that arises is that the

volume of the original sound wave is proportional to that of the ultrasound, while the volume

of the signal’s distorted component is exponential. So, a slight increase in the volume drowns

out the original sound wave as the distorted signal becomes predominant.

An MIT Media labs researcher, Joseph Pompei, managed to crack the problem

by studying current technique and he realized that the focused should have been on the signal’s

distorted component. The technique to create the audio beam is simple,

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SourceSignal

ProcessorUltrasoundamplifier Emitter

Modulate the amplitude to get the hybrid wave

Calculate what the berktay’s equation does to this signal

And do the exact opposite

In other words distort it before the distortion by air takes place. When this wave

is passed through air and what you get is the original sound wave component. But this time

(a) The volume of the original sound wave component is exponentially related to the

volume of the ultrasound beam

(b) The distorted component volume now varies directly as the ultrasound

THE HSS SYSTEM

Source: the audio program source is the source of signal such as a CD player or microphone.

Signal processor: the music or voice from audio source is converted to a highly complex

ultrasonic signal by the signal processor. That is audio source signal modulates ultrasonic signal.

Ultrasonic amplifier: the processed signal is the amplified by the ultrasonic amplifier.

Ultrasonic emitter or transducer: the ultrasonic signal is then emitted into air by

the transducer.

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Since the sound that is contained in ultrasonic energy is highly directional, it forms

a virtual column of sound directly in front of the emitter, much like the light from a flashlight.

All along that column of ultrasonic sound, the air is creating new sounds i.e. the sound that we

already converted to an ultrasonic wave. Since the sound that we hear is created right in the

column of ultrasonic energy, it does not spread in all directions like the sound from a

conventional loudspeaker; instead it stays locked lightly inside the column of ultrasonic energy.

Since you can control and focus the ultrasound beam just like a flashlight, you can

direct it such that you would hear the sound only if you were in the path of the beam. This is

called directed sound

You could also bounce the beam off a reflecting surface, so that people in the path

of the audio reflection can hear the sound. This is known as projected audio. In short, unlike

ordinary speakers, you will hear the sound only if you disrupt the sound beam, whether you

stand in “its path or in the path of a reflection from an acoustic mirroring surface. If you step

away from the path of the sound, you will hear nothing. The sound’s source is not the physical

device you see, but the invisible ultrasound beam that generates it.

Alternative technology:

There is another alternative approach to creating targeted audio, other than the

ultrasound modulation technique. One is the parabolic dish approach that essentially uses

antennae .to focus and direct sound. Here a relatively omni-directional loudspeaker is placed at

the focal point of a parabolic dish pointing towards it. When the loudspeaker generates the

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sound signal, it acts as a point source, emitting waves that reflect off the parabolic dish that is

pointed towards a particular direction. This is very much in use, but the size of the parabolic

dish required to accommodate the longer wavelengths of lower frequencies is too large.

SIGNAL PROCESSING

In order to convert the source program material to ultrasonic signals, a modulation

scheme is required. In addition error correction is needed if distortion is to be reduced without

loss of efficiency. The goal is to produce the audio in the most efficient manner while

maintaining acceptably low distortion levels. The type of modulation adopted also has

importance the requirement is for a method for modulation and distortion reduction mat

Is able to minimize distortion by creating output that matched the ideal modulation

envelope while simultaneously

Does not increase bandwidth requirements i.e. reduction of bandwidth

Allows high modulation index for good efficiency

Allows the lowest possible ultrasound operating frequency for greater output

Preprocessing:

There should be necessary preprocessing for reducing the distortion due to air.

Referring back the Berktay’s equation it can be seen that the demodulation due to the medium

gives an output that is the two-time derivative of the envelope square. Therefore the necessary

preprocessing required are

1. Double integration and

2. Square rooting

The two time derivative operations Berktay’s solution translates to a 12db/octave

high pass slope in the output which can be corrected independent of the modulation scheme,

with an equalization factor.

The Berktay’s solution says that the audio signal will be proportional to the

envelope. Not the spectrum. Therefore there is considerable freedom in choosing the modulation

scheme. The two modulation schemes used are

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1. Double sideband amplitude modulation (DSB) with square root

preprocessing - which results in many sidebands

2. Single sideband amplitude modulation (SSB) - so that the interaction

between the sidebands are eliminated.

DSB SYSTEM

Advantages:

Modulation index can be reduced to decrease distortion, but

Square rooting gives the proper envelope.

Disadvantages:

Reduced efficiency

It requires large bandwidth

The other scheme for modulation is single sideband SSB modulation

SSB SYSTEM

SSB modulation either chooses upper sideband (USB) or lower sideband (LSB)

modulation has a number of advantages

a. USB

Disadvantage:

The frequency response is somewhat erratic

Above resonance or carrier frequency the ultrasonic attenuation and saturation

levels both increase with frequency.

b. LSB

Band limited LSB system should provide the best of both worlds with a potential

for greater output and much more effective distortion reduction.

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Advantages:

Lower side bands are better because there may be efficiency available in emitter design

without sacrificing linearity.

High efficiency resonant devices remain well behaved below resonance.

The low audio frequencies are at higher ultrasonic ranges and therefore have greater

directivity associated with them and vice versa for high audio frequencies further

helping to maintain high directivity at low audio frequencies.

Narrower the bandwidth the greater the system efficiency

Besides being effective from distortion reduction standpoint

Attenuation levels are minimal with decreasing ultrasonic” frequencies.’

Narrow bandwidth provides much greater output by interacting more effectively with

the associated transducer.

Utilizing the above mentioned information, it can be seen that the system that

provides significant advancement is a Single Sideband Processor utilizing a square rooted

envelope reference to calibrate a recursive, zero bandwidth distortion canceller operating as a

lower side band modulator. This is the basis for the proprietary processor currently being

implemented for audio spotlighting.

Square rooting the audio before the modulation gives the proper envelope for a DSB system.

Comparing the envelopes of DSB with square rooting:

The envelope of DSB with square rooting-

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The envelope of SSB-

It can be seen that both the schemes result in a waveform that has the same envelope.

The following is the waveform both put together for comparison.

The blue is the DSB line. The red gives the SSB waveform. It can be seen that though they are

of different values they result in the same envelope.

Hence SSB gives a distortion free signal with no preprocessing or additional signal

conditioning so in case of no preprocessing; SSB is vastly superior to DSB.

SSB also gives a controlled measure of self equalization to the demodulated

audio thus eliminating the effect of the 12db/octave roll off.

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TRANSDUCER TECHNOLOGY

1. To cover a certain frequency range.

2. To have a certain dispersion pattern which In order to make this technology work,

ultrasonic energy must be emitted into the air. Electrical signals are converted into

these acoustic signals by means of an ultrasonic transducer. Acoustic transducers or

emitters can be designed Is sharp.

3. A bandwidth from around 20 KHz to infinity.

4. A sharp dispersion pattern that gives a collimated beam of ultrasound

5. Unlimited output capabilities.

What is practically possible is a usable bandwidth of 20 KHz for use with SSB

modulation giving 20 KHz of audio bandwidth, a resonant peak where the carrier will be placed,

and a falling output level with frequency to provide a measure of self-equalization in the system.

The frequency response of a transducer designed for 500Hz to 20 KHz flat audio response is

much more realistic, because the overall performance will be much better. These will be output

below 500Hz just not at the same level as the rest of the bandwidth.

Collimated beam is a must. In a point source the wave fronts are expanding

spherically around the source, so the intensity falls as the surface area of the sphere grows. With

a plane wave source where the radiating surface area of the diameter is much greater than the

wavelength being emitted, the wave front do not spread appreciably and a collimated beam

results. The only losses in intensity occur due to molecular friction. The attenuation is gradual

over distance. The attenuation grows with increasing frequency so lower operating frequencies

are desirable for minimizing losses.

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SOME OF THE EMITTERS USED ARE

1. Monolithic dim ultrasonic transducers

2. Electrostatic

3. Piezoelectric film

4. Planar magnetic emitters

5. Pressure based PVDF

In the thin film transducers the piezo film generates the greatest ultrasonic output

per unit area while providing easily scalable singular structures of any diameter desired for a

given application. Piezoelectric Film Transducer

The most active piezo film is Polyvinyl dine diflouride or PVDF for short. In order

to be useful for ultrasonic transduction, the film must be polarized or activated. The film needs

to have a conductive electrode material applied to both sides in order to achieve a uniform

electric field through it.

The piezoelectric films operate as transducers through the expansion and

contraction of the x or y axes of the film surface. For use as an emitter, the film will not create

effective motion in the z direction unless it is curved or distended so that the expansion and

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contractions can be converted into z axes movement and create displacement generating

acoustic output.

In one of the simplest implementations of the concept, a sheet of PVDF is taken

and it is laid over a metal late witn an array of holes in it. Pressure or vacuum can be applied to

one side of that plate to create an array of PVDF diaphragms, each with the diameter of the hole

under it. A schematic cross-section of such a device is shown below

The size of the hole is related to the resonant frequency of the carrier signal.

Therefore there is flexibility in calibrating the resonant frequency. Through the use of a new

type of proprietary PVDF film, which is the first purpose built transducer, the current emitter is

stable, repeatable and very practical device to manufacture. It has the following advantages:

Very high efficiency

Attenuated, self equalization slopes at the sideband frequency

Adjustable resonant frequency

Correct bandwidth needed to reproduce the widest band audio.

Repeatable, simplified construction.

Greater than 140db ultrasonic output capability.

Inherently low distortion

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COMPONENTS AND SPECIFICATIONS

Audio Spotlight consists of three major components: a thin, circular transducer array, a

signal processor and an amplifier. The lightweight, nonmagnetic transducer is about .5 inches

(1.27 centimeters) thick, and it typically has an active area 1 foot (30.48 cm) in diameter. It can

project a three-degree wide beam of sound that is audible even at distances over 100 meters (328

feet). The signal processor and amplifier are integrated into a system about the size of a

traditional audio amplifier, and they use about the same amount of power.

SOUND BEAM PROCESSOR/AMPLIFIER

Worldwide power input standard

Standard chassis 6.76”/171mm (w) x 2.26”/57mm (h)x 11”/280mm (d), optional rack

mount kit

Audio input: balanced XLR, 1/4” and RCA (with BTW adapter) Custom

configurations available eg. Multichannel

AUDIO SPOTLIGHT TRANSDUCER

17.5”/445mm diameter, 1/2”/12.7mm thick, 4lbs/1.82kg

Wall, overhead or flush mounting

Black cloth cover standard, other colours available

Audio output: 100dB max

~1% THD typical @ 1kHz

Usable range: 20m

Audibility to 200m

Optional integrated laser aimer 13”/ 330.2mm and 24”/ 609.6mm diameter also

available

Fully CE compliant

Fully realtime sound reproduction - no processing lag

Compatible with standard loudspeaker mounting accessories Due to continued

development, specifications are subject to change.

HYPERSONIC SOUND SYSTEM: FACTS AND LIMITS

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The output is proportional to the area of the ultrasonic column.

Ultrasonic design is based directly on emitter diameter,

Directivity directly depended on the length of the ultrasonic column.

Lower modulation index decreases distortion.

Greater modulation index increases gain

Single sideband envelope is equal to square rooted envelope for a single tone.

APPLICATIONS

The audio spotlight is made of a sound processor, an amplifier and the transducer.

Aim the transducer anywhere, and direct and project a three degree wide sound beam that is

audible even at 100 metre.

The following are a brief list of applications made possible by

directed audio

1. Personalized messaging:

Using targeted sound, you could message people in high activity areas, without using

headphones.

2. Discreet announcements:

In museums and exhibitions, using audio spotlighting technique to discreetly inform people,

without raising ambient sound levels, describing each exhibit only to the person standing in

front of it. It can be used in theme parks.

3. Automobiles:

Daimler Chrysler MAXXcab prototype truck has four individual audio spotlights in the

truck to let all the passengers enjoy their own choice of music.

4. Audio/video conferencing:

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Project the audio from a conference in four different languages, from a single central device,

without the need for headphones.

5. Targeted advertising:

With targeted announcements at crowd shows, audio spotlighting technology may

possibly be the best way to exhibitors to draw people to their stalls and demonstrate their

products to potential customers. It can be used in supermarket and retail stores. It can be built

into vending machine.

6. Home theatre:

You could be wired up for spotlighting with your home theatre audio system, and

could experience sound as it was originally heard, possessing direction and movement and this

will be tuned to your favorite position for viewing. If you have to watch a show everybody else

detests. Then personalized spotlight can turn of the audio for everyone else.

7.Realistic movies:

With targeted sound, movies could become more realistic, with the sound moving,

along with its source on the screen. Movies could be truer to life and enormously

entertaining.

8. Paging system:

Direct the announcement to the specific area of interest.

9. Ventriloquist systems:

By using tiny sound-focusing devices to beam out voices and having them scatter

against rocks and natural obstacles to the path, they can give the impression of the presence of

people in uninhabited places. This of sort of tricks is known as Psy-ops short form

Psychological operations that are used to fight war of wits against troops.

10. Military applications:

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Ship to ship communication

Shipboard announcement

It’s a substitute to the radio operator’s headphones. To keep track of what is going on

around them, as well as the radio chatter, uses generally keep one ear off the radio

handset. The information can be piped into the operator’s ears, without them having to

wear bulky handsets.

Non lethal acoustic rifles that fire sound pulses. Pump up the normal sound being

transmitted to about 150dB or greater, and you could fire out pulses that could

disorient human targets even causing them severe physical pain. The weapon could be

fine tuned to bring on further discomfort.

Jeep mounted units that can be used or deployed as a mob deterrent.

There are even more interesting application in the pipeline - car based safety audio

systems, discrete speaker phones and many more such interesting application

CONCLUSION

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Directional audio or the hypersonic sound Technology is simply the most revolutionary

sound reproduction system of this century. The opp rtunities for applying this characteristic to

the reproduction of sound-are limitless. We will be able to reproduce sound just the way we

experience it in the real world. Over the next few years, the way we experience sound is going

to change dramatically. It is a true technological paradigm shift. These are just a few of the

virtually limitless number of potential applications. Within the next 3to 5 years sound beam

technology should begin to find its way many deferent areas of our everyday lives. We should

also begin those new many applications in number of deferent areas. So we can conclude- Audio

Spotlighting really “put sound where you want it” and will be “A REAL BOON TO THE

FUTURE.”

REFERENCE

www.ieee.org

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www.google.com

www.answers.com

www.efymag.com

www.digitmag.com

www.illumin.usu.edu

www.thinkdigit.com

www.holosonics.com

www.spie.org

www.howstuffworks.com

www.abcNEWS.com

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