The Oxford dictionary defines the word 'ultrasonic' as meaning .... "of sound waves with pitch above upper limit of human hearing". Sound on the other hand is defined as ... "Sensation caused in the ear by vibration of surrounding air; what is or may be heard; vibrations causing sensation of sound; similar vibrations whether audible or not .....". Just to make things clear ... when I talk about 'sound' .... I am referring to something we 'think' we can hear. I will take that one step further and say that it is something, that we think we can hear well enough to receive and understand intelligence contained therein. For one reason or another .... Ultrasonic sound allows just that !.
The Superheterodyne Principle.
Patented about 1931. It is based on the fact that if two frequencies F1 and F2 are mixed together in a non-linear device, the end products will be F1, F2, F1- F2, F1 + F2 etc, etc. The key to this application is "a non-linear device". So if we beam a 40 kHz ultrasonic signal at someone's head .... and at the same time beam a 40.1 kHz ultrasonic signal at the same head ... if a non-linear device was present, then the head would hear a 40.1 - 40 kHz difference audio signal .... ie 100 Hz. If one of those ultrasonic signals was varying in frequency at an audio rate ..... ie the latest top of the pops ..... that is what the head would 'hear' !.
Non-linearity in human hearing.
Everything that follows is hearsay and has not been confirmed by me. The first practical approach to projecting sound involved two separate ultrasonic projectors and the system was reported to work well up to distances of about 100 feet. The explanation for the non-linear element was that where the two ultrasonic beams crossed (and only there !) demodulation took place in air. It was simply explained that air was a non-linear medium .... and that the system worked ... so there !.
As far as I could establish there were two main contenders for the discovery of the equipment to make this practical. The main advance at this stage was that a way was found to send both F1 and F2 using a single Ultrasonic projector. Now, one simply pointed the single device at the target and the target heard sound. This continued while the target was within the projectors beam. From the targets point of view they could be walking down the middle of the street and suddenly hear music or speech. Since ultrasonic waves can be very directional it was easily possible to control the area over which sound was heard. From a commercial point of view, it was ideal for people wandering around exhibitions, art galleries etc, since a different narration could be presented at each exhibit. No-one else outside of the intersection of the two beams could hear anything !.
I am profoundly deaf. In technical terms I am 70 db down at 1 kHz and even less at higher frequencies, or in other words I have less than 5% of the hearing range that I ought to have at my age. If I want to listen to the TV the volume has to be turned up to a level, that is painful to 'normal' people. With the ultrasonic system one does not need induction loops or headsets, because the target can listen at any level they want and no-one else in the room can hear a thing !.
The human ear.
The human ear is a very complex, mechanical, hydraulic, electronic assembly. I may have simplified that description !. At the end of this system is the cochlear. Basically it contains thousands of very fine hairs sensitive to audio levels and frequencies. Each hair cell is electrically connected to the brain. When we 'hear' something it is the result of the brain interpreting the electrical signals from the hair cells. Most of my cochlear hairs were sheared off in an explosion. Unfortunately the damage to the hairs and underlying cells means that, they have become permanently traumatized. The explosion of 40 years ago, I still hear today, every second of the day, and this is called Tinnitus. Obviously for me to hear anything it has to be of a higher level than the tinnitus noise. This is the problem that deaf people have ... they need to listen to sound at a much higher level than 'normal' people. The ultrasonic spotlight device is ideal for that application because the deaf person can listen to the sound at any level and no one else in the room can hear a thing. It also follows that everyone in a room or car can listen to sound of their own choice without disturbing anyone else and without having to use ear phones.
Non-linearity inside the head.
It has long been thought that a non linear element existed inside the human head or brain, that could directly demodulate ultrasonic signals.
Ultrasonic control of anti-social behavior.
Fifty years ago there was a lot of interest in ultrasonic 'ray' guns. In the USA these eventually became the subject of legal restriction and were classed as "Infernal Machines". The use of them against humans amounted to assault and was treated as such in the courts. So I was a little surprised to find that the British police actually recommend an ultra sonic device, called a Mosquito, which sole purpose is to be used against youths involved in anti-social behavior ..... for example hanging around in intimidating groups. I was even more surprised to find that the theory of operation, was something I had completely overlooked.
The 'Mosquito' device works on the principle that as you get older the high frequency response of the ear becomes lower, so that ultrasonic noise above about 19kHz can be heard by the young, but not by the average adult. So if a group of children form intimidating groups, for example out side of a shop, a Mosquito device can be fitted to beam irritating noise down on that area and cause them to move on.
What makes the device acceptable from a regulation point of view is it's output level. The Mark I Mosquito had it's output level set at 85db(A) which was the work place standard for 8 hours exposure. At 3 metres, a level of 76db(A) would require 16 hours exposure at this level to become unacceptable. The mark II Mosquito has a VOX facility which automatically sets the output level to just 5db(A) above ambient noise level. To be 'legally acceptable', any design for an ultrasonic repeller has to take these levels into consideration. It will be interesting to see whether the purpose of such devices could be classified as assault and any subsequent ruling by The Court of Human Rights.
I was asked by some 'victims' to have a look at the problem, which I did and then produced some proto-types. I was never terribly impressed with the idea and later found that some councils had removed Mosquito's , from their buildings. Sometime later I began to wonder if we had short changed the children and began the children's project.
Proto-type Electronic 'Pest' Eliminator Equipment (EPEE).
Top is a 75 watt tweeter, with a good response around 20 kHz. These are available from Maplins for about a fiver and from Bardwell at two for a fiver. The top PCB is a four transistor driver board with a 300 volts working voltage at 500 mA, way over the top for this application but it is a standard building block I use in lots of projects. The bottom PCB contains an 18F1320 PIC to produce a modulated waveform for the driver board. I did not incorporate a +5db(A) VOX auto output level facility on this board but it would be easy enough to add. My Mark II version uses a MAX1771 switched mode power supply, that can produce a stabilised output voltage in the range 0 - 300v.
The bottom right trimmer pot sets the maximum voltage and the upper blue trimmer pot sets the actual output voltage between 0 - 300 volts.
Ultrasonic riot control.
The theory behind riot control is that if you can get rid of the front rank of the rioters .... the riot ends. Water cannon and rubber bullets work quite well, but the latter can cause fatal injuries. Another approach was the use of ultrasonics to disorientate and disperse the rioters. Ultrasonics have advantages over the earlier audio range disorientation devices such as the 'curdler'. Much research went into finding the most unbearable sound and one popular idea was that it was the sound of a baby crying .... played backwards !. I can think of a lot of pop stars that would top my list of the unbearable !. The advent of ultrasonic beam sound projectors meant that very small sections of a riot crowd could be targeted, without any adverse effects against nearby riot control personnel.
Ultrasonic weapons of war.
The Inverse Square law.
Sound waves spread out as they leave their source. The intensity of the waves is inversely proportional to the square of the distance of the wave from it's source. So if we require 1 watt of energy to achieve a range of one meter, we will need 4 watts to get the same signal level at two meters. Nine watts at three meters, sixteen watts at four meters, 25 watts at five meters and 36 watts at six meters etc.. If we require long ranges then we either need very high ultrasonic power levels .... or we need to stop the ultrasonic beam dispersing in the first instance. One means of reducing beam dispersion is to use a parabolic reflector to produce a very narrow beam. Most advanced military and civilian devices use the parabolic reflector solution. See Parabolics.
John Kent 2007