Supergirl’s Silver Banshee: Shattering With Sound

In the episode of CBS’ Supergirl entitled “World’s Finest” that recently aired, a couple of super cool things happened: 1) The Flash guest –starred and 2) the Silver Banshee made her official, screamy debut as one of Supergirl’s villains. Having first appeared in Action Comics #595 in 1987, the Silver Banshee jumped to the small screen with her powers largely intact.

(The episode can still be viewed at CBS.com as of posting time)

In the episode, the Banshee lets out a scream that breaks things – glass, mostly and maybe an occasional eardrum. The scream has so much power it shows up as a series of widening circles coming from the Banshee’s mouth as she screams. It’s loud. And has a lot of energy and Supergirl and the Flash found out.

Sound-based powers often show up in comics with the likes of Green Arrow cast member Black Canary and Tyroc of the Legion of Super-Heroes at DC, and Banshee (and his daughter Siryn) as part of the X-Men family at Marvel. Along with Silver Banshee, these characters all have similar abilities, and today, let’s focus on the more destructive: breaking things.

Silver Banshee showed this ability from the very start – screaming to shatter windows, pushing people and things away from her, and injure people who were too close to her when she screamed. So how do you shatter things with sound? Let’s dig into the science of screaming at glasses.

First off, a quick sound primer: all the sound you hear is the result of a vibration, or oscillation. The sound you hear from the person next to you, your speakers, and your earbuds happens because something (vocal cords in an animal, the membrane of a speaker) vibrates the air between it and you. The vibrating air hits your eardrum, vibrates it, and activates specific neurons of your auditory nerve.

The frequency (or roughly, the pitch – how high or low a sound is) depends on the number of sound waves that your ear experiences per second – high frequency, high pitch, low frequency, low pitch. And finally, the volume of the sound you experience depends on the energy that’s producing the sound. The louder the sound, the more energy is being used to produce it. The softer the sound, the less energy.

Just a couple of other things about sound, and then we’re off to shattering stuff:

Normally (although it can be focused), sound energy travels from the source as a sphere or bubble. The energy dissipates according to the inverse-square law, which means as your distance from the sound source doubles (increased by a factor of 2), the sound intensity decreases by a factor of four (the square of the distance). Triple your distance from the source, and the intensity drops by a factor of nine. In other words, the farther away from a source of sound, the quieter it gets. Sound intensity is measured in decibels.

Sound waves must have a medium (or stuff) in which to travel. No medium, no sound. Explosions in space – pretty, but absolutely silent, unless something hits your spacecraft and then it will ring with a sound related to the impact’s energy. Related to this, sound travels fastest when the particles (molecules) in a medium are closer together, so as a result, sound travels the fastest through solids, second fastest through liquids, and slowest (about 340 meters/second) through air.

Humans can hear from about 20 waves per second (a unit called Hertz, or Hz) to 20,000 Hz. Dogs can hear higher frequencies, animals like elephants and whales hear – and communicate – in lower frequencies.

Got it? Good – let’s move to shattering glasses with sound.

Matching Pitches and Shattering Wine Glasses

It’s a common trick or joke in stories – an opera singer sings so loud (or so badly) that nearby wine glasses (and eyeglasses and windows…) shatter.

How?

Let’s just worry about wine glasses.

If you tap the rim of a wineglass, it will ring with a specific tone. The frequency of that tone is the glass’ natural resonance frequency. Simply, it’s the frequency at which it will vibrate due to its molecular structure and amount of impurities.

Wine glasses are most often used for these examples because these high quality wine glasses have little damping – a phenomenon which causes vibrating objects to lose energy over time. Impurities in a wine glass made out of normal glass would act to damp it, just as impurities in window glass would do the same thing – the vibrations would interact with the impurities and reduce the total energy of the vibration.

Wine glasses are also very thin and on stems – which help keep the vibrations in the bowl of the glass itself, rather than being damped by sitting on a surface. Lead crystal wine glasses (which contain at least 24% lead oxide) are most often used in these demonstrations.

To shatter the glass, the resonant frequency of the glass has to be directed at the glass, and in response, the glass will start to vibrate. But not shatter. To make the glass shatter, you need more energy. That is, you need to pump the resonance frequency at the glass with a lot of energy – or volume. The more energy, the stronger the vibrations of the glass will be. Energy – or sound intensity – is measured in decibels, remember? The intensity needed to shatter a crystal wineglass is somewhere above 100 db. At that point, the glass structure will not allow it to continue to vibrate, and it will shatter as it tries to vibrate as strongly as the sound wave that’s hitting it.

And just for reference, a human voice in conversation is between 50-55 db.

The inverse-square law can be a problem as well – since you don’t want to waste any of the sound energy, the glass should be very close to the sound’s source, and the sound should be “focused,” usually a wooden panel with a hole in it is place between the glass and the sound.

Finally – the glass needs a small flaw, such as a scratch, to be a start point for the shattering. No flaws – no shatter.

Can wine glasses be shattered by nothing more than the power of a human voice?

Thank you, YouTube.

Notice – the straws in some glasses are there to show that the glass is indeed vibrating. The more violently the straw moves, the stronger the vibrations in the glass.

Notice also – the singers are extremely close to the glasses, which goes to say, if you’re going to try this at home (which we do not recommend), do it through a microphone-amplifier setup, or wear goggles/glasses to protect your eyes – you’re trying to make a glass shatter in your face.

One final thing about shattering and breaking things with sound – in the episode of Supergirl, Kara stood full in the blast of Silver Banshee’s scream and afterwards, realized that her ear was bleeding. This was mostly likely due to her eardrum being ruptured – or perforated – by the sheer intensity of the Banshee’s scream (sound intensity is made up of pressure waves in the air – stand in front of a subwoofer or bass drum sometime to feel them), not “shattering” due to the scream matching her eardrum’s resonant frequency. Like most of the materials in our everyday world, there’s too much damping going on with an eardrum for it to maintain a resonant frequency up to the scale at which it would break.