Hi-fi speakers from piezoelectric tweeters to various kinds of mid range speakers and woofers which generally rely on circuit ant enclosures to produce quality sound,whether it dynamic , electrostatic or some other transducer based design engineers have struggled nearly for a century to produce a speaker design with the ideal 20Hz-20KHz capability of human hearing and also produce a narrow beam of audible sound.
Recent Technology
Audio spotlighting is a very recent technology that creates focused beam of sound similar to light beam coming out of a flash light. Specific listeners can be targeted with sound without others nearby hearing it i.e. to focus into a coherent and highly directional beam.it makes use of non-linearity of air.The audio spotlighting developed by American corporation uses ultrasonic energy to create extremely narrow beam of sound that behave like of light. Audio spotlighting exploits the property of no-linearity of air. A device know as parametric array employs the non linearity of the air to create audible by products from inaudible ultrasound, resulting in extremely directive and beam like sound.This source can projected about an area much like a spotlight and creates an actual specialized sound distant from a transducer. The ultrasound column act as a airborn speakers,and as the beam moves through the air gradual distortion takes place in a predictable way.This gives rise to audible components that can be accurately predicated and precisely controlled.
THEORY
The regular loudspeakers produce sound by directly moving the air molecules. The audible potions of sound tends to spread out in all directions from the point of origin.They do not travel as narrow beams.In fact the beam angle of audible sound is very wide just about 360 degree.This effectively means of sound you hear will be propagated through the air equally in all directions.Conventional loudspeakers suffer from amplitude distortion,harmonics distortion,inter-modulation distortion,phase distortion,crossover distortion etc..Some aspects of their mechanical aspects are mass,magnetic structure, enclosure design and cone construction.
In order to focus sound into a narrow beam,you need to maintain low beam angle and hence, more focused sound.The beam angle is also depeds on apeature size of speaker.A large loudspeaker will focus the sound over a smaller area.If the source loud speaker 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 pratical solution,thus the low beam angle can be achieved only by making the wavelength smaller and this can be achieved by making use of ultrasonic sound.
TECHNOLOGY OVERVIEW
History
The technology of using nonlinear interaction of high frequency waves to generate low frequency waves was originally pioneered by researchers developing underwater sonar tech. in1960.In 1975 an article cited on nonlinearity of air.Over the next two decades, several large companies including Panasonic and Ricoh attempted to develop a loudspeaker using this principle.They were successful in producing some sort of sound but with the higher level of distortion(>50%).In 1990 Woody Norris a Radar technician solved the parametric problems of this technology.
Difference Between Conventional And Audio Spotlighting
Audio spotlighting works by emitting harmless high frequency ultrasonic tones that human here cannot here. It uses ultrasonic energy to create extremely narrow beam of sound that behave like beam of light.Ultrasonic sound is that sound which have very small wavelength-in the millimeter range.These tones make useof non linearity property of air to produce new tones that are within the range of human hearing which results in audible sound.The sound is created indirectly in air by down converting the ultrasonic energy into the frequency spectrum we can here.
In an audio spotlighting sound system there are no voice coils,cones or enclosures.The result is Sound with a potential purity and fidelity we attined never before.Sound quality is no longer tied to speaker size.This sound system holds the promise of replacing conventional speaker in home,movie theaters and automobile-everywhere.
RANGE OF HEARING
The human ear is sensitive to frequency rangefrom 20 Hz to 20KHz.If the range of human hearing as a percentage of shift from the lowest audible frequency to the highest it spans a range of 100,000 percentage.No single loudspeaker element can operate efficiently over such a wide range of frequency.
Using this technology it is possible to design a perfect transducer which can be work over a with range of frequency which is audible to human hear.
WORKING
The original low frequency sound sound wave such a human speech or a music is applied into an audio spotlight emitter device.This low frequency signal is frequency modulated with ultrasonic ranging from 21kHz-28KHz.The output of the modulator will be the modulated from of original sound wave.Since ultrasonic frequency is used the wavelength of the combined signal will be in the order of few millimeter.Since the wavelength is smaller the beam angle will be around 3 degree,as a result the sound beam will be a narrow one with a small dispersion.
While the frequency modulated signal travels through the air,the nonlinearity property of air comes into action which slightly changes the sound wave.If there is a change in a sound wave,new sounds are formed with in wave.Therefore if we know how the air affects the sound waves,we can predict exactly what new frequency will be added into the sound wave by the air itself.The new sound signal generated within the ultrasonic sound wave will be corresponding to the original information signal with a frequency in the range of 20-20KHz will be produced within the ultrasonic sound wave.Since we can not hera the ultrasonic sound wave we only here the new sound s that are formed by non-linear action of the air.Thus in an audio spotlighting there are no actual speakers that produce the sound but the ultrasonic envelope acts as the airborne speaker.
The new sound produced virtually has no distortion of sound is freed from bulky enclosers.There are no woofers or crossovers.This technology is similar in that you can direct the ultrasonic emitter towarda a hard surface, a wall for instance and the listener perceives the sound as coming from the spot on the wall.The listener does not perceive the sound as emanating from face of the transducer,but only from the reflection from the wall.For the maximum volumn that trade show use demands,it is recommended that the audio spotlight speaker,more accurately called a transducer,is mounting no more than 3 meters from the avg. listeners ears,or 5 meter in the air.The mounting hardware is constructed with a ball joint so that the audio spotlighting are easly aimed wherever the sound is desired.
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.
Properties of ultrasound:
• The frequency ranges above 20 KHz
• The wavelength is less than 2crn
• Small beam angle hence highly coherent and directional.
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.
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)
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
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,
• 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
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 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
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.
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-
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.
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.
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 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
BEAM DISPERTION
COMPONENT OF AUDIO SPOTLIGHTING SYSTEM
1.Power supply
2.Frequncy oscillator
3.Modulator
4. Audio signal processor
5. Microcontroller
6. Ultrasonic amplifier
7. Transducer
1. Power Supply: Like all electronic systems, the audio spotlighting system works off DC voltage. Ultrasonic amplifier requires 48V DC supply for its working and low voltage for microcontroller unit and other process management.
2. Frequency oscillator: The frequency oscillator generates ultrasonic frequency signals in the range of (21,000 Hz to 28,000 Hz) which is required for the modulation of information signals.
3. Modulator: In order to convert the source signal material into ultrasonic signal a modulation scheme is required which is achieved through a modulator. In addition, error correction is needed to reduce distortion without loss of efficiency. By using a DSB modulator the modulation index can be reduced to decrease distortion.
4. Audio signal processor: The audio signal is sent to electronic signal processor circuit where equalization and distortion control are performed in order to produce a good quality sound signal.
5. Microcontroller: A dedicated microcontroller circuit takes care of the functional management of the system. In the future version, it is expected that the whole process like functional management, signal processing, double side band modulation and even switch mode power supply would be effectively taken care of by a single embedded IC.
6. Transducer: It is 1.27 cm thick and 17” in diameter. It is capable of producing audibility up to 200 meters with better clarity of sound. It has the ability of real time sound reproduction with zero lag. It can be wall, overhead or flush mounted. These transducers are arranged in form of an array called parametric array in order to propagate the ultrasonic signals from the emitter and thereby to exploit the nonlinearity property of air.
MODES OF LISTENING
There are two modes of listening
Direct mode
Projected mode
Direct Mode:
Direct mode requires a clear line of approach from the sound system unit to the point where the listener can hear the audio.To restrict the audio in a specific area this method is appropriate.
Projected or Virtual Mode:
This mode requires an unbroken line of approach from the emitter of audio spotlighting system,so the emitter is pointed at the spot where the is to be heard.For this mode of operation the sound beam from emitter is made to reflect from a reflecting surface such a wall surface or a diffuser. A virtual sound source creates an illution of sound source that emanates from a surface or direction where no physical loudspeaker is present.
ADVANTAGES
1. Can focus sound only at the place you want.
2. Ultrasonic emitter device are thin and flat and do not require a mounting cabinet.
3. The focused or directed sound travels much faster in a straight line than conventional loudspeaker.
4. Dispersion can be controlled very narrow or wider to cover more listening area.
5. Can reduce or eliminate the feedback from microphone.
6. Highly cost effective as the maintenance required is less as compared to conventional loud speakers and have longer life span.
7. Requires only same power as required for regular speakers.
8. There is no lag in reproducing the sound.
HYPERSONIC SOUND SYSTEM: FACTS AND LIMITS
• 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.
• Greater modulation index increases gain
• Single sideband envelope is equal to square rooted envelope for a single ton
APPLICATIONS:
1.Automobiles: Beam alert signal can be directly propagated from an announcement device in the dashboard to the driver .Presently Mercedes Benz buses are fitted with audio spotlighting speaker so that individual travelers can enjoy the music.
2.Retail sales: Provide targeted advertising directly at the point of purchase.
3.Safety officials: Portable audio spotlighting device for communication with a specific person in a crowd of people.
4.Public announcement: Highly focused announcement in noisy environment such as subways,airport,traffic intersections etc..
5.Emergency rescue: Rescue can communicate with endangerd people far from reach.
6.Entertainment system: in home theatre system tear speaker can be eliminated by the implementation of audio spotlighting and the properties of sound can be improved.
7.Museums:In museums audio spotlighting can be used to describe about a particular object to a person standing in front it ,so that the order person standing in front of another object will not be able to here the description
8.Military applications:Ship to ship communication and shipboard announcements.
9.Audio/video conferencing:
Project the audio from a conference in four different language,forma single central device without the need for headphone.
10.Sound bullets:Jack the level 50 times the human threshold of pain and an offshoot of audio spotlighting sound technology become a nonlethal weapon.
FUTURE OF AUDIO SPOTLIGHTING:
Even the best loudspeaker are subject to distortion and their omni directional sound is annoying to the people in the vicinity who do not wish to listen.Audio spotlighting system holds the promise of replacing conventional speakers.It allows the user to control the direction of propagation with sound. Audio spotlighting really “put sound where you want it”.
CONCLUSION:
Audio spotlighting is really going to make a revolution in sound transmission and the user can decide the path in which audio signal propagate. Due to the unidirectional propagation it finds application in large number of fields. Audio spotlighting system is going to shape the future of sound and will serve our ears with magical experience.
REFERENCE
1. F. Joseph Pompei. The use of airborne ultrasonics for generating audible Journal of the Audio Engineering Society, P. J. Westervelt. Parametric acoustic array. Journal of the Acoustical Society of America.
2. Thomas D. Kite, John T. Post, and Mark F. Hamilton. Parametric array in air: Distortion reduction by preprocessing. Journal of the Acoustical Society of America.
3. Jacqueline Naze Tjotta and Sigve Tjotta. Nonlinear interaction of two collinear, spherically spreading sound beams.
Fig 1 : F.Joseph Pompei at the Mit Lab. Propagation of Sound Beam from Audio Spotlighting Device._
Fig:6: Directivity
Fig.8: The envelope of DSB with square rooting
Fig.9: The envelope of SSB Fig.10 waveform both put together for comparison11 An Array Of PVDF Diaphragm
Fig 12: Dispersion of Sound Beam
Fig 14: Parametric Loudspeaker
Fig 15: Directed Audio And Projected Audio
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