The concept of high directional sound; or to grossly exaggerate 'SOUND BULLETS!' is one which has enjoyed a sporadic lifestyle, gaining interest and subsequent abandonment from various industries. It is the bell bottom jeans of technology; everyone thinks it’s a good look but have you seen anyone under 6ft wear them?
'Sound from ultrasound' would be the tagline that sums up my idea. The concept has been around since the 60's when hearing things from thin air was probably a regular occurrence. It gathered momentum from the defence industry to be used for transmission of covert messages disguised in a highly directional inaudible ultrasound wave but in recent years there has been an interest from niche entertainment venues to exploit this technology to a new level.
The audible sound is 'hidden' in a ultrasound wave whose frequencies cannot be heard by human ears and are revealed only when they hit surfaces or a listener is stood directly in its path. Imagine walking into a club and hearing sound burst from the walls and your own body. For those business types out there - projecting sound to individuals as they are walking through the mall, targeting them with information about opening times, discount brands and distances to the nearest McDonalds in a 'sonic bullet'.
To explain how this works lets go into a trance back to Yr 8 and that physics lesson you fell asleep in... the basics of sound waves are:
- A sound wave is basically like a punch to the face. The face of air that is. Unlike light it is a mechanical process which requires a medium to propagate through. Light and sound are both different types of energy and the energy carried by a sound wave is able to travel by doing a Mexican wave through air molecules.
- The shape of a sound wave is called sinusoidal. This basically means there are peaks and troughs that occur periodically, just like a tidal wave in the ocean.
- The frequency of a sound wave depends on how many peaks and troughs you can squeeze together, so a short wave will look bunched up and represents a 'high pitch' in sound. Ultrasound waves are extremely high frequency (classed as over 20kHz) and can't be heard because the middle ear acts as a low pass filter, basically like a bouncer who only lets in low frequencies.
The main features behind the effect are derived from properties
intrinsic to ultrasonic frequencies:
- Directivity: This is dependent on the ratio of the source size to the
frequency of the wave emitted by it. A normal loudspeaker will blast out
music in all directions because audible sound has a small frequency.
Ultrasound therefore is emitted in a narrow cone or beam
- Nonlinear properties: Nonlinear means the response of the wave, in this case, as it interacts with air molecules, does not follow a straight correlation i.e. doubling the strength equals double the effect. The nonlinear behaviour of ultrasound with air causes some parts of the sound wave to slow down, so that parts of the beam change
frequency. This is referred to as the Doppler effect where the change in velocity of a wave will also affect the frequency. Because parts of the ultrasound wave slow down, the peaks and troughs begin to spread out leading to a decrease in frequency (it goes from curly fries to a limp piece of wet spaghetti). You are left with bits of the wave at one frequency f1 and another frequency f2 which then undergo heterodyning (fancy word for mixing), to create sum and difference (f1+f2 and f1-f2) frequency components. Sum and difference literally does what it says on the tin; two slightly different high frequencies will crash together to produce both an even higher frequency wave (sum) and a tiny tiny small frequency wave (difference) but because the original waves were ultrasonic they still maintain the high directivity. The frequency difference component is important because this is where an audible sound
can be produced.
- The audible sound is only revealed after the ultrasound wave has propagated a certain distance and nonlinear-ed itself (great scientific lingo coming out today) or, an accelerated version of this effect is seen when the sound wave hits a surface. This is because the object will vibrate at the difference frequency like a loudspeaker.
And there you have it, the equivalent to a sound ninja. Unfortunately, despite how cool all of this appears to be, the sound ninja won't be kung foo chopping your ear drums anytime soon. The nonlinear property that allows ultrasound to be manipulated this way also creates a lot of energy dispersion, resulting in very short propagation distances and a sonic bullet that more resembles a sonic Frisbee - a bit feeble and lacking in force.