Sennheiser 2011 Annual Report Download - page 22

42
NIGHT CONCERT

20 Hz
and uses it to emit low frequencies of 20 Hz in the 36 m2
(387 sq. ft.) room; what acousticians call a “standing wave”
quickly forms. All of a sudden, the indicator shoots up to
more than 20 dB. Indoors, the pressure level produced by
infrasonic waves is much more intense than outdoors.
Acoustics expert Rüdiger Borgmann, author of a
book on infrasound, confirms: “With their geometry, cities
are especially vulnerable to infrasound because the high re-
verberant building facades of urban canyons create stand-
ing waves.” So, too, the bedroom.
There are countless sources of artificial infrasound
in cities: almost every tall office building, residential com-
plex, hotel and hospital are equipped with air-conditioning,
ventilation systems and pumps. Heap on other factors such
as planes, railroads, subways, power stations and all the
electrical equipment companies’ use: “As technology in-
creases, so will infrasound,” says Borgmann. Infrasound in-
teracts with the solid-state vibrations of cooling equipment
and machinery that, at levels below 20 Hz, emit airborne
sound. Some researchers believe it even amplifies it.
The Oldenburg acoustician Mellert once ran a test in
which he took repeated measurements of air conditioners
in open-space offices. He found that the long ventilation
ducts and shafts used in tall buildings create resonances
similar to those of low-register organ pipes, forming a
standing wave. “Using the Sennheiser high-frequency con-
denser microphone MKH 110, we were able to measure
sounds waves below 10 Hz,” says Mellert. For him, the ver-
dict is still out on whether urban canyons can also produce
the same type of standing waves: “That needs to be stud-
ied further.”
Back in the Ruhr Valley, Prof. Krahé is trying to crack
his latest infrasound case. Fitting a tiny Sennheiser MKE-1
onto Friedrich’s ear, he hopes to better identify the low-fre-
quency sounds that are causing her so much grief. Krahé al-
ways comes up with new ideas to catch the culprit. Maybe
the key lies in the brain. That’s why he is running a series of
EKG tests in his anechoic chamber. He wants to find out
whether infrasound alters brain waves. If it does, then he
would be able to show how infrasound also impacts people
who aren’t able to hear the hum – and finally prove that
the vibrating city is not a figment of the imagination. ■
1. After dark, for some people, a mysterious night concert
begins in the megacities 2. Professor Detlef Krahé researches
with a Sennheiser KH 870 subwoofer
KH 870
The newest electronics and eight analog
channels make for dry bass tones to 18 Hz
Free field frequency
response
18–300 Hz, ± 3 dB
Bass 7.1 bass manager with
80 Hz crossover
Impedance electrically
balanced
XLR, 8 x 13 kiloohm
Weight 47.1 kg
Neumann KH 870 Active studio subwoofer