CHAPTER 01
INTRODUCTION
Hi-fi speakers range from piezoelectric tweeters of various kinds of mid-range speakers and woofers which generally rely on circuits at large enclosures to produce quality sound, whether it is dynamic, electrostatic or some other transducers- based design. Engineers have struggled for nearly a century to produce a speaker design with the ideal 20Hz -20,000Hz capability of human hearing and also produce narrow beam of audible sound.
The Audio spotlight developed by American Technology Corporation uses Ultrasonic energy to create extremely narrow beams of sound that behaves like beam of light. Audio spotlight exploits property of non-linearity of air. A device known as parametric array employs the non-linearity of air to create audible by products from inaudible ultrasound, resulting an 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 acts as airborne speaker, 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 predicted and precisely controlled.
This audio spotlight technology creates focused beams of soundsimilar to light beams coming out of a flashlight. Specific listeners can be targeted with sound without other hereby hearing it, i.e. to focus the sound into coherent and highly directional beam. It makes use of non- linearity property of air.
CHAPTER 02
2.1 THEORY
The regular loudspeakers produce audible sound by directly moving the air molecules. 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 meters (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 meters across is required, and to guarantee that all the audible wavelengths are focused, even bigger loudspeakers are needed.
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 2.1: 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.
Fig 2.2: F.Joseph Pompei At The Media Lab Of The Massachusetts Institute Of Technology Demonstrates How Invisible Ultrasonic Waves, As Illustrated Here, Could Help "STEER" Sound. (ABCNEWS.COM)
2.2 NON-LINEARITY OF AIR
Audio spotlighting exploits the property of non-linearity of air. When inaudible ultrasound pulses are fired into the air, it spontaneously converts the inaudible ultrasound into audible sound tones, hence proved that as with water, sound propagation in air is just as non-linear, and can be calculated mathematically.
A device known as a parametric array employs the non-linearity of the air to create audible by-products from inaudible ultrasound, resulting in an extremely directive, beamlike wide-band acoustical source. This source can be projected about an area much like a spotlight, and creates an actual sound distant from the transducer. The ultrasound column acts as an airborne speaker, 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 predicted and precisely controlled. However, the problem with firing off ultrasound pulses, and having them interfere to produce audible tones is that the audible components created are nowhere similar to the complex signals in speech and music.
Human speech, as well as music, contains multiple varying frequency signals, which interfere to produce sound and distortion. To generate such sound out of pure ultrasound tones is not easy.
Fig 2.3: Ultrasound waves
This is when teams of researchers from Ricoh and other Japanese companies got together to come up with the idea of using pure ultrasound signals as a carrier wave, and superimposing audible speech and music signals on it to create a hybrid wave. If the range of human hearing is expressed as a percentage of shift from the lowest audible frequency to the highest, it spans a range of 100,000%. No single loudspeaker element can operate efficiently or uniformly over this range of frequencies. In order to deal with this speaker manufacturers carve the audio spectrum into smaller sections. This requires multiple transducers and crossovers to create a 'higher fidelity' system with current technology.
Fig2.4:Parametric Loudspeaker- Amazing Audio Spotlight
(Airborne ultrasounds of 28 kHz are envelope-modulated with audio signals. Inherent non-linearity of the air works as a de-modulator. Thus de-modulated sounds impinge on our eardrums. We can hear those sounds! )
Using a technique of multiplying audible frequencies upwards and superimposing them on a "carrier" of say, 200,000 cycles the required frequency shift for a transducer would be only 10%. Building a transducer that only needs to produce waves uniformly over only a 10% frequency range. In this technology we can ‘put sound where we want’
Using sound with vision improves retention rates by up to 60%, but how do you get round the issue of noise pollution to the surrounding area? By using Audio spotlight - which concentrates the sound just as a spotlight does so only those in the "beam" can hear your message. Use it outside your shop window, or under your billboard. People can hear, but can't always know where the sound is coming from. Creative opportunities
AUDIO SPOTLIGHT TRANSDUCER
- 17.5”/445mm diameter, 1/2”/12.7mm thick
- Wall, overhead or flush mounting
- Black cloth cover standard, other colours available
- Audio output: 100dB max
- Usable range: 20m
- Audibility to 200m
- Optional integrated laser aimer 13”/ 330.2mm and 24”/ 609.6mm diameter also available
- Fully CE compliant
- Fully real-time sound reproduction - no processing lag
- Compatible with standard loudspeaker mounting accessories Due to continued development, specifications are subject to change.
CHAPTER 03
3.1 TECHNOLOGY OVERVIEW
The technique of using non-linear interaction of high-frequency waves to generate low-frequency waves was originally pioneered by researches developing underwater sonic techniques in1960’s. In 1975, an article cited the non-linear effects occurring in air. Over the next two-decades, several companies develop a loudspeaker using this principle. They were successful in producing some sort of sound but with higher level of distortion (>50%). In 1990’s Woody Norris a Radar technician solved the parametric problems of this technology.
Audio Spotlighting works by emitting harmless high frequency ultrasonic tones that human ear cannot hear. It uses ultrasonic energy to create extremely narrow beams of sound that behave like beams of light. Ultrasonic sound is that sound which has very small wavelength-in millimeter range. These tones make use of 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 frequency spectrum we can hear.
In an Audio Spotlighting sound system there are no voice coils. The result is ‘sound with a potential purity and fidelity which we never attained before.’ Sound quality is no longer tied to speaker size. The sound system holds the promise of replacing conventional speakers in home, movie theaters and automobile-everywhere.
Fig3.1: Conventional speakers
Fig 3.2: Audio spotlighting
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
3.2 RANGE OF HEARING
The human ear is sensitive to frequency ranging from 20Hz to 20,000Hz. If range of human hearing is expressed as a percentage of shift from the lowest audible frequency to the highest it spans a range of 100,000%. No single loudspeaker element can operate efficiently over such a wide range of frequencies.
Using this technology it is possible to design a perfect transducer which can work over a wide range of frequency which is audible to human ear
Fig 3.3: Range of Hearing
The idea of amplitude modulation (AM), a technique used to broadcast commercial radio stations signals over a wide area. The speech and music signals are mixed with the pure ultrasound carrier wave, and 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, one slightly higher and one slightly lower than the hybrid wave. Berktay’s equation holds strong here, and these two sidebands interfere with the hybrid wave and produce two signal components, as the equation says. One is identical to the original sound wave, and the other is a badly distorted component. This is where the problem lies the volume of the original sound wave is proportional to that of the ultrasounds, 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. It was at this point that all research on ultrasound as a carrier wave for an audio spotlight got bogged down in the 1980s.
CHAPTER 04
WORKING PRINCIPLE
The original low frequency sound wave such as human speech or music is applied into an audio spotlight emitter device. This low frequency signal is frequency modulated with ultrasonic frequencies ranging from 21KHZ to 28KHZ. The output modulator will be the modulated form of original sound wave. Since the ultrasonic frequency is used the wavelength of the combined signal will be in the order of few millimeters. Since the wavelength is smaller than the beam angle will be around 3 degree, as a result the sound beam will be a narrow beam with small dispersion.
Fig 4.1: Audio spotlighting Emitter
While the frequency modulated signal travels through the air, the non-linearity property of air comes into action which slightly changes the sound wave. If there is a change in sound wave new sounds are formed within the wave. Therefore, if we know how the air affects the sound waves, we can predict exactly what new frequencies (sound) 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 original information signal with a frequency in the range of 20HZ to 20KHZ will be produced within ultrasonic sound wave. Since we cannot hear the ultrasonic sound wave we can hear the new sounds that are formed by non-linear action of air. Thus in an audio spotlighting there are no actual speakers that produce the sound but the ultrasonic envelope acts as the airborne speaker.
A conventional speaker radiates sound in a very wide pattern, often with side lobes (also depicted in Fig 4.2) Side lobes do not exist in Audio Spotlight systems; the beam is much like that from a flashlight. The Audio Spotlight uses the interaction of ultrasonic sound waves with the air to reproduce audible sounds in a highly directional pattern. By contrast, a conventional speaker radiates sound in a wide pattern, often with side lobes that the Audio Spotlight avoids.
Fig 4.2: Directivity pattern
Focusing on the signal’s distorted component, since the signal component’s behavior is mathematically predictable, the technique to create the audio beam is simple; modulate the amplitude to get the hybrid wave, then calculate what the Becktay’s Equation does to this signal, and do the exact opposite. In other words, distort it, before Mother Nature does it.
Finally, pass this wave through air, and what you get is the original sound wave component whose volume, this time, is exponentially related to the volume of the ultrasound beam, and a distorted component, whose volume now varies directly as the ultrasound wave.
By creating a complex ultrasound waveform (using a parametric array of ultrasound sources), many different sources of sound can be created. If their phases are carefully controlled, then these interfere destructively laterally and constructively in the forward direction, resulting in a collimated sound beam or audio spotlight. Today, the transducers required to produce these beams are just half an inch thick and lightweight, and the system required to drive it has similar power requirements to conventional amplifier technology.
Fig 4.3:Computer Simulation Of Sound Propagation: Complex Set Of High-Intensity Ultrasound Signals Intermodulates Air. Among The Products Is A Collimated Audio "Spotlight".
CHAPTER 05
ARCHITECTURE
COMPONENTS OF AUDIO SPOTLIGHTING
- Power supply
- Voltage Regulator
- Frequency Oscillator
- Modulator
- Audio Signal Processing
- Microcontroller
- Ultrasonic Amplifier
- Transducer
Fig5.1: Block Diagram of Audio Spotlighting System
- Power Supply:
Like all electronics system, the audio spotlighting system works off DC Voltage. Ultrasonic amplifier requires 48 V DC supply for its working and low voltage for microcontroller and other process management.
- Voltage regulator:
As the name itself implies, it regulates the input applied to it. A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. In this diagram, power supply of 5V is required to the microcontroller. In order to obtain this voltage level, 7805 voltage regulator are to be used. The first number 78 represents positive supply and the numbers 05 represent the required output voltage levels.
Fig 5.2: Pin diagram of 7805 Fig 5.3 : Block diagram of 7805
3. Frequency Oscillator:
The Frequency Oscillator generates ultrasonic frequency signals in the range (21,000Hz to 28,000Hz) which is required for the modulation of information signals.
Fig 5.4: Frequency Oscillator
4.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.
Fig5.5: DSB modulator waveforms
5.Audio Signal Processing:
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.
Fig5.6: Audio Signal Processor
6.Microcontroller:
A dedicated microcontroller circuit takes care of the functional management of the system. Microcontroller controls overall operation of the system. In this A/D convertor is in built in microcontroller. 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.
7. Ultrasonic amplifier:
High efficiency ultrasonic power amplifier amplifies the frequency modulated wave in order to match its impedance of integrated transducers. So that the output of emitter will be more powerful and can cover more distance.
Fig 5.7: Ultrasonic Amplifier
8.Transducer:
It is 1.27cm thick and 17” diameter. It is capable of producing audibility upto 200meters with a better clarity of sound. It has ability of real time sound reproduction with zero lag. These transducers are arranged in the form of array called parametric array in order to propagate the ultrasonic signals from the emitter and thereby to exploit the non-linearity property.
Fig 5.8:Parametric Transducer
CHAPTER 06
MODES OF LISTENING
DIRECT AUDIO AND PROJECTED AUDIO:
There are two ways to use Audio Spotlight. First, it can direct sound at a specific target, creating a contained area of listening space which is called“Direct Audio”. Second, it can bounce off of a second object, creating an audio image. This audio image gives the illusion of a loudspeaker, which the listener perceives as the source of sound, which is called “projected Audio”. This is similar to the way light bounces off of objects. In either case, the sound’s source is not the physical device you see, but the invisible ultrasound beam that generates it.