waves

You
find
an
old
abandoned
mining
tunnel
in
the
side
of
a
hill.
You
want
to
know
how
long
the
tunnel
is
without
going
inside.
You
set
up
a
very
low
frequency
loudspeaker
and
find
that
there
are
standing-足-wave
resonances
at
6.5
Hz
and
9.1
Hz,
but
at
no
frequencies
between
these.
The
air
is
quite
cold
inside,
so
the
speed
of
sound
is
about
335
m/s.
How
far
is
it
to
th…