“Radio” Chihuahua Explained?
Frankie the Chihuahua is a hot news item. The Ohio Chihuahua is being reported on as having ears that can “pick up radio signals”. Why? Because people can hear a high pitched noise that sounds a bit like radio static being emitted from his ears.
But here’s the thing. Frankie is a real, live Chihuahua – he’s not an electronic toy. There aren’t any diodes or transistors in his head to collect and modulate radio waves. And I doubt that there are any coils or crystals in there to create a crystal set that could simply amplify the radio waves present in the air around him.
What I believe we have here is a case of a lot of people jumping to an inaccurate conclusion based on a single piece of information: what the noise coming from Frankie’s ears sounds like. From video available here it is apparent that while the noise from Frankie’s sounds somewhat similar to the high-pitched static or interference one sometimes gets from a radio or television set, there appears to be a more atonal quality to it than most interference exhibits.
Folks may think it sounds like radio interference but I suspect that what’s really going on is that Frankie’s ears are acting as Helmholtz resonators.
Hermann von Helmholtz first introduced his resonators in the 1863 classic, On the Sensations of Tone as a Physiological Basis for the Theory of Music. Helmholtz resonators act as sound filters and amplifiers and each Helmholtz resonator is designed to pick up and magnify a specific frequency.
The classic example of a Helmholtz resonator is a soda bottle. The air in the neck of the bottle oscillates as a unit of mass and compresses the air inside the bottle, which then acts kindof like a spring, continueing the oscillation and creating sound waves.
Another common example of a Helmholtz resonator is the seashell. The shell acts as a resonator amplifying sound at specific frequencies, which are controlled by the dimension and the shape of the shell (or ear?).
Helmholtz resonance is the phenomenon of air resonance in a cavity. The name comes from a device created in the 1850s by Hermann von Helmholtz to show the height of the various tones. An example of Helmholtz resonance is the sound created when one blows across the top of an empty bottle.
When air is forced into a cavity, the pressure inside increases. Once the external force that forces the air into the cavity disappears, the higher-pressure air inside will flow out. However, this surge of air flowing out will tend to over-compensate, due to the inertia of the air in the neck, and the cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreasing each time.
This effect is akin to that of a bungee-jumper bouncing on the end of a bungee rope, or a mass attached to a spring. Air trapped in the chamber acts as a spring. Changes in the dimensions of the chamber adjust the properties of the spring: a larger chamber would make for a weaker spring, and vice-versa.
The air in the port (the neck of the chamber) is the mass. Since it is in motion, it possesses some momentum. A longer port would make for a larger mass, and vice-versa. The diameter of the port is related to the mass of air and the volume of the chamber. A port that is too small in area for the chamber volume will “choke” the flow while one that is too large in area for the chamber volume tends to reduce the momentum of the air in the port.
The length of the neck appears in the denominator because the inertia of the air in the neck is proportional to the length. The volume of the cavity appears in the denominator because the spring constant of the air in the cavity is inversely proportional to its volume. The area of the neck matters for two reasons. Increasing the area of the neck increases the inertia of the air proportionately, but also decreases the velocity at which the air rushes in and out.
An ocarina is essentially a Helmholtz resonator where the area of the neck can be easily varied to produce different tones. While several variations exist, an ocarina is typified by an oval-shaped enclosed space with four to twelve finger holes and a mouth tube projecting out from the body.
So – an ocarina is a Helmholtz resonator; seashells and soda bottles also act as Helmholtz resonators. Are there any other examples of this fascinating device in nature? Well – in a lovely bit of serendipity, researchers at Goethe-University, Frankfurt have proposed that the bulla, or middle ear cavity, may act as a Helmholtz resonator. And if one looks at the photo of a collection of Helmholtz resonators below:
And compares them to the shape of a dog’s bulla shown in this cross-section of a dog’s ear (the bulla is the large, round cavity with a narrow neck at the top shown at the bottom of the diagram):
You’ll note that there is a remarkably strong resemblance in the appearance of the two structures.
Does being a living Helmholtz resonator make Frankie any less special or less interesting than being a living radio receiver? I don’t think so. And — Frankie’s story is just one more reminder that jumping to conclusions all too often leads one into the wrong place.