Posted in physics, science

How do headphones work?

With a market value set to reach $15.8 billion by 2025, chances are you own a pair of headphones. However, the odds of the average person knowing how they work are significantly worse.

At the most basic level, all speakers are made up of three main constituents: a cone (sometimes referred to as a diaphragm), a coil of wire and a permanent magnet. As shown in the main image, the cone is attached to the coil. The permanent magnet is situated within the coil.

Electricity is fundamental to the function of headphones – when it flows through the coil, it becomes an electromagnet. If the flow of the electricity is altered, the electromagnet either attracts or repels the permanent magnet. This combination of both attraction and repulsion induces motion in the coil, causing the cone to move. These movements generate vibrations (a.k.a. soundwaves) in the air – much like those produced by banging on the skin of a drum. As discussed in my previous post ‘How do we hear?’, the ear detects these vibrations and converts them into signals that can be processed by the brain.

Once the soundwaves are being produced, factors such as volume and pitch are controlled by the strength and frequency of vibration. In order to generate a louder sound, larger and more powerful vibrations are required. Pitches are determined by the frequency of the soundwave. The frequency is defined as the rate of the wave, illustrated by Figure 1. Whilst pitch is altered simply by increasing the power of vibration, frequency is governed more by the size of the speaker. Hence, huge subwoofers produce much better bass than a tiny in-ear headphone.

Figure 1

Although humans are capable of hearing many different types of sound, headphones and other sound-systems can typically only produce two: monoaural and stereo. Monoaural is when the sound only comes from one source, such as someone speaking. This is the type of sound encountered during a phone conversation or from an old radio. Stereo sound is encountered when two speakers generate slightly different vibrations in order to emulate a 2D soundscape. Have you ever had a song begin in one headphone and move across to the other? That’s an example of stereo. Whilst it is possible to generate 3D sound artificially, it is a lot more time consuming. To be effective, speakers must be placed around the audience and each must play subtly different things. Currently, the only place that this is practical is for ‘surround-sound’ in cinemas. Though, given the passage of time, it may well become a future household staple.

Tabitha Watson

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Posted in biology, science

How do we hear?

In order to hear the world around us, humans and other mammals rely on vibrations travelling in the air. To interpret these vibrations, the ear has evolved a complex system of canals and tiny hair-like protrusions, all working in harmony to generate sound as we know it.

The ear is a complex structure composed of three distinct parts, each with separate functions. The outer ear (or pinna) is the external section that protrudes from the side of the head. Responsible for the placement of sounds in relation to our bodies, the complex arches and valleys funnel vibrations into the ear canal and create a three dimensional soundscape.

Once the vibrations begin to move down the ear canal, they enter the middle ear. This section is composed of the auditory canal, which terminates at the eardrum (also known as the tympanic membrane). The eardrum is then attached to three tiny bones called ossicles, surrounded by a small pocket of air. Individually, these bones are the malleus, incus and stapes (or alternatively, the hammer, anvil and stirrup). The ossicles are then attached to a fluid-filled structure called the cochlea. It is here that the inner ear begins.

The primary function of the cochlea is to convert vibrations into electrical impulses to be sent down the auditory nerve and interpreted by the brain. In order to change the vibrations to impulses, a rather ingenious method is employed. Once the vibration reaches the cochlea from the ossicles, it travels down the basilar membrane whereupon it is detected by approximately 16,000 to 20,000 hair-like cells called cilia. These cilia are attached to a specialised part of the ear canal called the Organ of Corti, and it is here that the raw vibrations are converted to nerve impulses and passed along the auditory nerve to the brain. This is achieved by the deformation of the cilia – as they are moved by the vibrations, specialised ion channels are pulled open and the resultant influx of potassium and calcium ions depolarises the cells and produces an action potential.

So how are humans able to hear such a spectrum of different sounds and pitches? Well, it’s all down to the tapered shape of the cochlea. Due to their individual amplitudes, different frequencies of sound wave peak at different times as they travel down the ear canal. As higher frequency waves have larger amplitudes, they are not able to travel as far as lower frequency waves. Due to this, each section of cilia is sensitive to a particular frequency of wave – this is what enables the detection of such a vast spectrum of sound.

Tabitha Watson

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