The Sound of Science - "Hemholtz Resonators"

Jun 22, 2018

With me today I have a Hemholtz resonator. This is an incredibly complex instrument, so I’ll do my best to describe it. It’s a glass bottle with a long neck, and I got 6 for about 10 bucks. Okay, it’s a beer bottle, but it is in fact a Hemholtz resonator.”

A Hemholtz resonator consists of a relatively large chamber of air and a small neck. The amount of air, the shape, and size of the neck are the main factors in this instrument. When we talk about this resonator, we need to keep in mind: Air can be compressed and stretched. In fact, this compression and stretching acts like a spring, bouncing back and forth until it returns to its original pressure.

Now imagine a Jack-in-the-Box and how it bounces around after it pops out. In that toy, there is the spring and the head, which acts as a mass. Without that mass on the end, it wouldn’t have that characteristic bounce, it would just be a spring standing straight up. If you add too much mass, the spring wouldn’t pop out. Essentially, the mass affects how fast and how much the spring compresses and stretches.

Jeremy Benson and Sam Watt (R) of NIU STEAM.

Back to our Hemholtz resonator. Remember how I said air can compress and stretch? The air inside the body of the bottle is our spring in the system. What about the mass? Without it, the spring wouldn’t wiggle, right? The air in the bottle’s neck doesn’t compress as much as the air in the body, so it gets pushed and pulled like the head of a jack-in-the-box.

As we learned in our episode on resonance, vibrating air causes interference. Some frequencies are damped while others are enhanced. The result is the sound we all can recognize.

We’re almost out of time, so think about this question: Why do we blow across the opening instead of into it?

Learn the answer, and more, on The Sound of Science.  I’m Sam Watt, and you’re listening to WNIJ, where you learn something new every day.