Evening star. [volume] (Washington, D.C.) 1854-1972, 1930-10-26

and has worked out Q cabin wall de.
sign which, It Is claimed. meets the
need. The exterior covering Of the
cabin should he OF aluminium, q light,
sound-resisting metal Within this
there should be q lining Of some such
material rs balsam wood. at least iM
inches thick. A suitable interior finish
would be laid over this.
Experiments OF the bureau have
shown that all objects have the ca
pAeity Of absorbing and consuming
sound waves. Therefore, heavy seat
padding will decrease the noise. dis
posing of 1t as soon as 1t enters the
cabin. Heavy glass should be used In
the windows. three -sixteenths to one
fourth of an inch in thickness. As
glass Is very heavy, this means small
windows. 1t Is not practicable to de-
pend upon open windows for ventila-
ton. Other sources must be employed.
Cause Of Propeller Noise.
Airplane engineers admit that they
are not wholly informed as 10 exactly
what causes the deafening noise gem
Grated by q propeller. They believe
that such intense sound waves orig
mate In the pressure variations caused
by the turbulent flow of air past the
spinning blades. It Is not noise
caused by friction, by the clash Of
solid upon solid, but q noise produced
by the tremendous tumult OF the air
itself.
The loudest noise produced by ma
ture Is the thunder clap. This, science
has determined, is caused by two masses
Of air, which have been suddenly rent
by the passage of lightning, coming TO-
gether again. The meeting of these Inn
ponderable air masses creates tremen-
dous defamation.
So it is that lesser bodies of air.
sharply cut by swiftly moving propeller
blades, come together again behind the
propeller and Immediately adjacent to
the cabin of the plane, producing the
characteristic roar.
Experiments have been made with
different shaped blades and with diff
ferent speeds. These have included such
highly technical experiments as the
study Of relationship between the speed
Of the propeller blades and the speed
Of sound. Sound travels at the rate
blades sometimes revolve ct higher
rate than that.
At high speeds the closing air makes
continuous roar because each blade
stroke is 80 close behind that Of an
other that it Is impossible for the
ear to detect the interval between.
At slower speeds . more broken sound
is produced. It IS described by the
engineers as "burbling."
One thing has been very definitely
ascertained-that is, that the slightest
fluttering of the propeller due TO its
being the merest fraction out Of true will
greatly increase the volume of noise.
Nice mechanical adjustment and con-
slant inspection can remedy this un-
necessary source of noise. Study Of
blade materials Is going forward with
an idea Of finding some material which
will vibrate the least.
Airplane noise heard on the ground
varies with the wind. A person to
windWard of . plane In strong breeze
will scarcely hear o sound, although
the machine is fairly close, while Q per-
son to leeward will be annoyed by the
loud roar even though the plane may
be half o mile distant.
Work on the problem will be con-
tinued In the interest Of the comfort and
convenience Of passengers, and 1t q
reasonable to assume that the time will
come when smoking compartment stor-
les may be heard even when told in
whispers.