PRODUCTS & SERVICES
surface microphones for
noise and so much more
The world’s first surface measurement
microphone is still the choice for acquiring
flight test data – fast and accurate the first time
AEROSPACETESTINGINTERNATIONAL.COM // SEPTEMBER 2019 103
Brüel & Kjær aerospace surface
microphones (Types 4948 and 4948-B)
are specially designed for aerospace
applications and have a broad range of
applications beyond wind-tunnel testing and
acoustic fatigue measurements, both in high
intensity test chambers and during flight. The
microphones can acquire substantial
quantities of data in a wide variety of problem
areas, such as aerodynamic noise
measurements, ground pressure
measurement during fly over tests and skin
pressure measurements. They feature a
dynamic range from 55 to 160dB (up to 180dB
peak for the high-pressure version) and are
also suitable for placement in confined
spaces where traditional microphone setups
would be impossible or provide invalid data.
To ensure that the microphones are able to
handle the practical challenges for use in the
potentially extreme aerospace testing
environments, some special considerations
were taken during the design phase. Some of
these features may be eye-catching but
result from form following function. The
microphone’s all-titanium construction
provides more than a high-quality finished
look – the material was chosen because it
provides a uniquely high resistance to
corrosion and other environmental
influences. The microphone’s flush diaphragm
and housing design gives it a sleek
appearance, but the purpose is to minimize
the microphone’s influence on the
measurement by reducing its own windgenerated
noise.
The effort to provide quality data is
reflected in many unseen design
considerations as well. For example, the
microphone’s pressure equalisation vent is
placed just next to the diaphragm on the front
of the microphone. The design and placement
of the vent provides multiple benefits. The
placement, for example, ensures that the
static pressure can vary rapidly according to
Surface microphones, as indicated by the
name, offer a high degree of mounting
flexibility. Different mounting accessories are
available to enable a secure mounting setup.
The all-titanium (UA-1640) mounting flange is
the most secure and maximizes corrosion
resistance, but there are other options
available with their own pros and cons. The
tape-over method is by far the easiest way to
mount a surface microphone, but it’s not
well-suited for flight testing due to the high
flow speed and wide temperature range. In
many cases, the glue-on method is better
but less convenient, and the mounting pad
may be damaged when it is removed from
the surface.
A compromise between the two methods
is double-sided very-high bond (VHB)
adhesive tape square pads that bond to the
microphone’s flexible mounting pad. \\
1 // Brüel & Kjær
Aerospace Surface
Microphone Type 4948
2 // Surface microphone
mounted on an aircraft
wing using the all-titanium
mounting ring (UA-1640)
3 // The glue-on method
for mounting a surface
microphone is more
secure than using tape
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the microphone’s measurement position in
the applications, so the measurement is
referenced to true static pressure at the
measurement position. Additionally, the
measurement refers to average local static
pressure because of the design of this
circular slot. Also, to make sure that
contamination and partial blockages, such as
fluid, smashed bugs and ice have no
influence, the lower limiting frequency is wellcontrolled
because it is determined solely by
an internal channel.
To ensure ease of use, the surface
microphone has a built-in preamplifier, so it
can be plugged into a data acquisition
system. Also, because of its transducer
electronic data sheet (TEDS) functionality, it is
automatically detected by LAN-XI systems,
and the identity and loaded sensitivity of the
microphone are readily available for
application and documentation purposes.
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