Sennheiser 2011 Annual Report Download - page 21

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NIGHT CONCERT

20 Hz
At night, when the roaring city traffic comes to a halt
and the construction noise dies down. When the
world’s metropolitan areas finally turn down the dial
on the throbbing street life, most people are sound asleep.
Others, however, are being tortured, tossing and turning in
bed.
Sleepless in the city. Around the world, from San
Francisco to Auckland, from Sydney to Calgary, and in con-
gested urban centers like the five-million-strong Ruhr Val-
ley, there’s a deep hum that only 2 to 5% of the population
can hear. Like a power drill boring through their heads, an
uncomfortable sensation of pressure also builds up in their
ears. Some feel a vibration course through their bodies, ex-
perience tremendous anxiety and even shortness of
breath. Its audience has long known the title of this night
music: infrasound. At levels of 20 Hz and below, why are
they able to register frequencies so low that, in the field of
acoustics, it would be considered absolutely impossible?
Does infrasound really cause such harmful side effects?
In the outskirts of a city somewhere between Dort-
mund and Bochum, a teacher named Anne Friedrich returns
to a home she was forced to flee during the night of Febru-
ary 23 last year. She couldn’t stand the hum any longer.
Prof. Detlef Krahé of the University of Wuppertal has at-
tached a microphone to the headboard of her bed and con-
nected it to a sound-level meter. The mic is so sensitive, it
can record sounds down to 5 Hz. Every time she hears a low
tone that bothers her, she jots it down and then protocols
to what extent. Fuelled by his
scientific curiosity, the profes-
sor of communications engi-
neering is using her bedroom
to conduct research. Is what
the teacher hears measurable?
And if so, when do the sounds
appear and at what levels?
In 2005, the 59-year-
old teacher with dark curly hair
bought a townhouse in the
Ruhr Valley. It wasn’t until mid-
September 2007 that she was
torn from her sleep for the first
time. “Wummwummwumm,” she
growls in a low voice. “It was like
there was a truck parked right in
front of the house, and the driver
was gunning the engine.” What’s
more, she could feel vibrations go-
ing straight through her. The mad-
dening bass tone is sometimes
weaker during the day, but she only
completely escapes it when she
leaves the house. Odd that none of
her neighbors seem to notice.
Friedrich had all the build-
ing equipment and appliances thor-
oughly inspected. Nothing. The first
hunch came from a building biolo-
gist: infrasound. The teacher points
through her front window: “There
it is. The bane of my life.” During
the summer of 2007, a new cooling
system had been installed in the warehouse across the
street and that’s exactly when the torturous sounds and
physical ailments first appeared. By February 2010, she
couldn’t deal with the insomnia anymore and fled her
home. Today, she only returns to water the plants – or let
Krahé in to take measurements.
In Germany, Denmark and Canada, hum hearers are
banding together, forming associations, community groups
and Internet forums. They are getting environmental au-
thorities and scientists on board, and keeping up with the
latest scientific research. It’s not just about bringing an end
to their suffering. They want people who don’t hear the
noise to know they aren’t just some crackpots, imagining
sounds that aren’t there. In the 1990s, the long-suffering
residents of Taos, New Mexico were even able to get the
U.S. Congress involved in what has become officially known
as the “Taos Hum”. But its genesis was just as elusive in
Taos as it was in Munich and Bristol: “It’s like trying to solve
a crime,” says Krahé of his difficult investigation to find the
culprits.
Krahé has finished studying the mea-
surements he took at Friedrich’s house: “I regis-
tered distinct peaks of 33 dB at vibrations of 20
Hz and 40 Hz. But there were also noise compo-
nents below 20 Hz.” Noises? Frequencies below 20 Hz are
generally considered inaudible. Are official reference levels
misleading? Krahé adds with a sigh: “According to DIN
45680, these infrasound values are well below the thresh-
old of perception.”
Acousticians at a university located in the Danish
city of Aarlsborg have come up with the same type of re-
sults. They have been conducting measurements in an in-
frasonic pressure chamber for years, looking to find out
how infrasound affects people. Their surprising result,
which has been confirmed by many other international
studies and experiments, is that people are able to per-
ceive sounds well below the previously defined threshold
of 20 Hz. If they are especially sensitive, they even react to
frequencies of just a few hertz. It’s a bit like the fluttering
heard when rolling down the window of a moving vehicle.
With sounds below 20 Hz, there is no sensation of pitch,
but there is a sensation of sound. At least, for some people.
Despite all the studies, there are still many open questions
and there will have to be more research. Foremost of all:
Are there any negative health effects from infrasound?
What is the difference between infrasound and the
noise caused by high frequencies? Acoustician Volker Mel-
lert of Oldenburg University has been researching infra-
sound since the 1980s. As he explains: “Contrary to high
frequencies, the threshold of perception between barely
hearing something and hearing it loudly can be minuscule.
Especially at very low frequencies. That’s why frequencies
in the infrasonic range can quickly morph into a major an-
noyance.” Mellert discovered how low frequencies can af-
fect the body the hard way. He and his team ran an experi-
ment on themselves. As the 5 Hz emit ting from
low-frequency loudspeakers in their anechoic chamber be-
gan to generate infrasound, they started to feel an un-
pleasant sensation of pressure and ran out of the room.
They all felt terrible – some were even about to pass out.
Mellert is convinced that artificial infrasound is
more disturbing than the infrasound produced by nature.
Machines generate “isolated single-frequency sinusoidal
low pressure fluctuations,” which can be very unpleasant.
Natural infrasound on the other hand, such as the low-fre-
quency sounds of the ocean or wind (or, as Mellert de-
scribes them, broadband) are simply more pleasurable.
Infrasonic waves up to 300-meters long can travel
almost unimpeded over large spaces. However, unlike high-
frequency noise, they are not absorbed by air or insulation.
As Prof. Krahé describes it, “It cuts through glass like a hot
knife cuts through butter.” In an experimental chamber,
Krahé demonstrates how infrasonic waves intensify sound
pressure. He hooks up his Sennheiser KH 870 subwoofer
NO SOUND IS MORE EX-
TREME THAN INFRASOUND.
IT PASSES THROUGH GLASS UNMUFFLED.
1. Air conditioners often generate resonances
that resemble those of deeply tuned organ pipes
2. Night in Hong Kong: cities in particular are
susceptible to infrasound, due to their geometry
LOW FREQUENCY MICROPHONES
0.1
hertz is the lower
threshold frequency
that a high-frequency
microphone can mea-
sure. Professional Sennheiser micro-
phones of the MKH series work according
to this process, which is today used only
by Sennheiser. It enables the construction
of measuring devices for extremely low
frequencies. Thus, in the 1960s, the low
frequency microphone MKH 110 was
created. The acoustician Volker Mellert
used this microphone to detect infrasound
sources. Its lower threshold frequency was
1 hertz. The sister model, MKH 110-1, can
even go as low as 0.1 hertz.