Measuring sound

Michael, can’t you practice quieter? I can’t hear myself think over your trumpet! It’s so loud! I bet my trumpet playing isn’t half as loud as your band practicing! Both Michael and Jenny make very loud sounds when playing their instruments.

Many factors influence how we perceive sound — from how long a sound lasts, to its pitch. And we can measure these different properties of sound! Sound is vibrations of particles, travelling through a medium, such as air, in the form of a wave. As the sound wave travels from the source, it creates areas of higher pressure - compressions - and lower pressure — rarefactions — in the medium. Let’s draw a sound wave on a graph, to see how we can measure some properties of sound!

The highest points of the wave — peaks, represent compressions, while the lowest points — troughs, represent rarefactions. We can draw a horizontal line through the centre of the wave. This shows the so-called rest position. Now, let’s take a look at the axes of the graph. The horizontal axis represents time in seconds.

We can count how many times a wave repeats every second. This is the frequency of a sound wave, which we measure in hertz. Frequency tells us how high- or low-pitched a sound is. Now, let’s look at the position of the peaks and troughs. They represent the maximum amount of displacement of the vibrating particles from their rest position.

This is the amplitude of the wave. Amplitude also tells us how loud a sound is — the greater the amplitude, the louder the sound! Amplitude is represented on the vertical axis of the graph. So, can Jenny and Michael use amplitude to determine whose instrument is louder? Yes and no.

They could definitely look at the sound waves their instruments produce, and measure the amplitude. But amplitude doesn’t tell us much about how we sense sound, which is important to consider, when talking about how loud a sound is. For this purpose, we use amplitude to determine the sound volume. Volume is measured in units called decibels. A normal conversation is around 60 decibels.

The sound of rustling leaves is 20 decibels, but doesn’t feel that much quieter. Meanwhile, the sound of a large truck is around 100 decibels and that feels a lot louder. In each case, the difference is 40 decibels, but the perceived difference in sound is far from equal! This is because the decibel scale expresses a value in relation to another value and, it represents changes as a ratio, rather than a simple difference between values. The decibel scale is a logarithmic scale.

If a person is exposed to sounds above 70 decibels for long periods of time, their hearing could be damaged. That’s one reason why manufacturers of home appliances and sound systems often include information about the volume of sound. Volume is usually expressed in a different unit — A-weighted decibels or dBA. A-weighted decibels take into account how the human ear responds to sound. However, it is unlikely that Jenny and Michael’s musical instruments come with A-weighted decibels written on them.

So how can they solve the dispute of which instrument is the loudest? Look Jenny, I found this phone app that we can use to measure the volume of our instruments. Good, now we can finally settle this!