I'm a Project Manager now, dontcha know? And after my granola powershake this morning, I decided to brain surge some new ideas using my
humble physics background. And KABOOM. It happened. I thought-mapped a
brilliant idea that parallels the awesomeness of THX, which by the way,
harnesses an important PM ethos 'if you can't trademark a product,
trademark the quality assurance instead'.
I was probably not the first to conjure up this interestingly useless
idea; nor will I be the last. The concept of high directional sound; or,
to grossly pimp it up 'SOUND BULLETS!' is one which has enjoyed a
sporadic lifestyle, gaining interest and subsequent abandonment from
various industries. It is the Lindsay Lohan of technology; it has the
foundations for a prosperous career, earning big bucks in the
traditional market of defence and obtaining IP, but it lacks advancement
because it is just too difficult to work with. All the substance but
with no class.
'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. I
liken it to a Jason Pollock painting - with spots of sound coming from
all directions. Imagine literally hearing Kanye West erupting from the
face of your boyfriend. Or, 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 if you imagine the wave to consist
of Mexican wrestlers pulling a can of whoop ass on each other in a neat
sequential formation.
- 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 attractive low frequencies but knocks back yer 2Hz girlfriend.
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. double 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 moreso resembles a sonic Frisbee - a bit feeble and
lacking in force.
