Date: Thu, 25 Mar 2004 05:52:32 -0600

Author: "Marc Kossover"

Subject: Re: Resonance and flame bending

Post:

"Dick Heckathorn" writes:
>Have you put the candle near the speaker and see how strong the flame is
>deflected from it?

Putting a candle next to speaker is an experiment that we do in our high
school physics class. Even with the volume turned way up (40 Watt speaker,
just below clipping, varying from 40 to 80 Hz), the candle flame is not
deflected by the motion of the speaker. On close inspection, the flame is
seen to be vibrating back and forth to what appears to be the same
frequency as the motion of the speaker cone. This outcome seems reasonable
since, to a first approximation, the medium does not have any net change
in position, but just moves back and forth.

An aside that might be relevant to the resonance situation in a tube: Our
speakers are ported, meaning that they have a horizontal tube that
connects the inside of the speaker cabinet with the outside. If you put
your hand a couple of cm away from the port, it feels like the port is
puffing air towards your hand, and it will deflect a candle away from the
hole. Certainly, the speaker isn't continuously shooting air out the port,
after all, the cabinet is sealed (we checked) everywhere else, and I am
not ready to repeal conservation of matter. Air must be going into the
port as well.

So, to check the situation, we placed a piece of paper somewhat larger
than the diameter of the port over the hole. The paper stayed upright by
the hole, alternately moving towards and away from the hole, vibrating and
making a noisy racket. So, air is going in as well. Why can't we feel it?
I suspect that it is like the front and rear of a fan, where it is easy to
feel the air in front of the fan, but hard to feel it from behind. I
figure that the the blades push the air largely forward, away from the
blades (in a vortex?), keeping the air together. While on the rear side of
the fan, only air pressure brings new air to region where the air has been
pushed away by the blades. Air pressure works in all directions equally,
so although the same number of molecules must enter the rear side of the
fan, they come in from all directions, not making a distinctive stream of
air like the front of the fan does.

Marc "Zeke" Kossover