Page 4 - Science Focus (issue20)
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Helium and Why

                      It Makes You Sound Like Mickey Mouse

                                         氦氣         -  為何它使您的聲音像米奇老鼠

        By Henry Lau 劉以軒

            Have you ever thought that it was cool to have    seen people do this in parties or on TV, or even during
        a really deep and husky voice like Darth Vader or     classes, for its great entertainment value. Next time you
        Batman? What if we could temporarily change our       want to go incognito, you could try this out. Now, what
        voice for fun? Listen closely – I am going to let you in   exactly is it with helium that produces this property?
        on a little not-so-secret secret...the power of helium.   Before we delve into the mechanism of helium
        Being the first in a group of elements dubbed the     speech, here’s some background information about
        noble gases, you’d think that helium would be a       helium. Helium was first discovered in 1868, when
        dignified sort of element. Indeed, helium is well known   scientists were studying the Sun [1]. When light from
        for its inertness, a property unique to noble gases,   the Sun was resolved into a spectrum, a previously
        which is essential for its use in industry. However, helium   undocumented line in the spectrum was observed.
                          can also be used for comic effect:   Scientists then pursued this further and proved the
                            upon inhaling helium, one’s       existence of helium in 1895. Denoted by the chemical
                              voice becomes squeaky and       symbol “He”, helium is the second lightest element
                               seemingly higher pitched.      known to humans. Subsequently, helium was found
                               This phenomenon is called      to be very inert, which means it rarely undergoes
                               “helium speech”. So, I guess   chemical reactions with other substances.
                               I lied; you won’t sound
                               like Batman, but you will          If you’ve heard someone speak after inhaling
                               sound like a Mickey Mouse      helium, you may describe their voice as “squeaky”.
                               impersonator! You may have     But in reality, helium doesn’t change the pitch of your
                                                              voice; rather it messes with the timbre (or quality) of
                                                              your voice [2]. In order to speak, your vocal cords
                                                              have to vibrate, which causes the air nearby to vibrate
                                                              at the same frequency, termed the fundamental
                                                              frequency. This also creates a set of weaker harmonics
                                                              (or overtones), at frequencies which are the integral
                                                              multiples of the fundamental frequency with lower
                                                              amplitudes. As those frequencies pass through
                                                              the vocal tract, they are all amplified (in terms
                                                              of amplitude, or loudness) due to the vocal tract
                                                              resonances, but to different extents. Some frequencies
                                                              tend to be amplified more, which make up spectral
                                                              peaks called formants, if shown in a power-frequency
                                                              graph. It is the profile of the set of sound waves
                                                              produced at multiple frequencies with different
                                                              amplitudes (and hence the resultant waveform) that
                                                              make up the quality of one’s voice, known as timbre.
                                                                  Since sound travels much faster in helium than it
                                                              does in normal air, when we inhale helium and speak,
                                                              the resonances and formants shift towards higher
                                                              frequencies [2]. In other words, the frequencies of the
                                                              fundamental and harmonics remain unchanged
                                                              because the vocal cords still vibrate at the same
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