Do You Know Your dB (Spl) From Your dB (A)?

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How loud is too loud? Is it when your ears start hurting or when you suddenly find you’re deaf? How do we know what is a safe level? That’s where dB (SPL) and dB (A) come in.

Some of us will face some hearing loss in the long term due to exposure to loud sounds right now so measuring sound levels is an important part of making certain what is a safe sound level. There is another problem though. Our ears are more sensitive to a specific range of sounds. For example, if you had a high pitched sound and a medium pitched sound both of which had the same sound energy, the high pitched sound would appear quieter than the medium pitched sound. Think of a piano. The sound of the notes around the middle of the keyboard seem louder than the sounds of the notes at the high end of the keyboard. The same happens with low pitched sounds. So we need a way of measuring sound levels so we can protect our hearing from damage.

The units used to measure sound are decibels and written as dB. It’s not an absolute unit like kilograms or kilometers, but a ratio of two measurements based on a logarithmic scale. Sound pressure levels are measured using dB (SPL), where SPL stands for Sound Pressure Level. A level of 0 dB (SPL) is at the threshold of hearing, that is, the minimum that can be heard by a young, healthy ear. A level of 140 dB (SPL) is at the threshold of pain. In between these levels we have our normal range of hearing. For example, 30 dB (SPL) is whispering and 60 dB (SPL) is normal conversation between people who are about one meter apart.

But SPL is not how our ears hear sound. A more usual way to measure sound levels is if an ear was listening to them. To measure sound levels we use an electronic instrument, which is set to measure dB (SPL), but it is possible to switch on a filter that mimics, as close as possible, how our ears hear sound. The instrument will then measure the sound as dB (A). The ‘A’ is the name of the filter. The measurements taken as dB (A) determine the measurement of what is too loud for our ears.

The European Union Noise at Work regulations say that 80 dB (A) for 40 hours per week is the minimum point at which action must be taken to protect hearing. This is equivalent to standing near a busy road. But the limit works on the equal energy principle. That is if you double the sound level then you must halve the exposure to 20 hours per week and so on. To keep your hearing safe look for these measurements because they give you information about the risks you may be taking when exposed to high sound energy levels.

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