SOUND ENGINEERING
A sample grid of bricks
that can focus sound
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Marco Scisetti /stock.adobe.com
operate is
a constraint
that you put into
the ‘machinery’ – into the
algorithm that we invented. The lters
that we had in 2017 were very good at a
single frequency – all of the sound passed
through. If you pay a bit in terms of
decibels, by say 2-3dB, then you can have
wider bandwidth.”
Still, extending the bandwidth remains
a crucial question of metamaterial
research. But he also inquires: how much
bandwidth do we really need? “With our
ears, we can hear 11 octaves. Even our
pianos don’t go so high; they manage
eight, or 10 if you have a grand piano.
With our voice, we cover three octaves if
we are very good. But in our phone, in the
one I’m using now, we use two and a bit.
That is for me is an exquisite problem.
Whenever we think about sound, we think
of all of the spectrum, but we only really
use little bits of it. So maybe there is a
future where metamaterials don’t need to
cover 11 octaves, but maybe just the one
needed for the speci c application.”
Regardless, current technology can
mimic existing directional speaker
technology – called audio spotlights – at
a fraction of their cost, says Memoli.
These high-end speakers use ultrasound
to transmit a narrow beam of sound,
into which they add in the audible
frequencies (music). Because of the
electronics involved, those speakers cost
some £2,000 according to Memoli; but
3D-printing a metamaterial grid to mount
in front of that would cost a tenth of the
price – and you can do it yourself.
But the uses of the technology go
beyond beaming sound in straight lines.
Last year, Memoli published a paper
describing a way to bend the sound
around an obstacle between it and a
focus
point.
“Maybe one day,
if we get better on the
frequency, you’ll be able to have
this in a disco and have an area in the
middle of the dance oor with less sound.
Imagine that you can place it at the bar,
so you can nally order without shouting
all of the time.”
MORE APPLICATIONS
Another use is noise cancellation, about
which Memoli and his team were writing a
patent application in late July. This draws
on the same physical principle that is used
in noise-cancelling headphones: inverting
the waveform to cancel it out. Although
reluctant to explain the idea of his brick
arrangement in full, he hints: “If one of
the bricks lets sound go through, and the
other one inverts the sound going through,
and if you nd the proper distribution of
bricks, maybe you can cancel what’s on
the other side.”
This is just scratching the surface of
the technology’s potential, Memoli argues.
“Imagine that in a building you can design
different parts of a door to keep out some
of the sounds and let others through.
Or you can bend sound to go around a
corner or around a person, or you can send
different content to a person in front of
you and to a person to your side. This is
all fascinating, and it’s completely different
from what we had before.”
He continues: “The most exciting part
for me is that it’s not the material itself;
it’s the design that you put in it which
does the trick. That boundary between
art and engineering is what gives the
result. That’s why I work so much with
artists.” He is working with musicians
to write compositions for speakers with
metamaterials; with digital artists to nd a
way to use metamaterials to track a user
navigating a space with sound but without
headphones; with theatre performers
on a different way of interacting with the
audience by sending sound messages to
speci c parts of the auditorium.
As to their strategy to commercialise
this technology, researchers at Sussex are
currently trying to nd the most pro table
product opportunity from a wide eld.
“It’s so broad that making a company is
dif cult, because there are such a large
number of possibilities. So we are still
exploring which one has the most market
potential. That’s because we have been
contacted by so many people; it’s dif cult
not to be excited by all of them.”
Each brick contains an internal labyrinth of varying length. The right arrangement can bend sound
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