SPL & Perceived Loudness

Sound Pressure Levels (SPL)

Since this is a basic website geared towards audio, we are going to focus on sound pressure levels in air (rather than, say water).

SPL is a measurement of acoustic pressure caused by a sound wave moving through the air.

It is represented in decibels (dB) which is a logarithmic unit of measurement. Decibels are measurements that are used to compare a measurement to a standard, accepted reference. In the case of sound pressure levels, the reference point (0dB) is the threshold of hearing.

This is the smallest sound that can be heard by the human ear with no background noise or other sounds present.

You may also see dBA and dBC. These are weighting curves for measuring audio frequencies. A weighting greatly reduces low frequencies and relatively high frequencies in an effort to more closely represent the way we hear. This is helpful when we want know more accurately how the measured sound pressure levels impact our hearing. 

C weighting reduces low frequencies and high frequencies but to a much less extent both in terms of when it starts to roll off as well as how much. This is a measurement that is better used for peak levels and usually is used for sound pressure levels over 100dB as our hearing curves at loud levels tend to flatten out more rather than heavily favoring certain frequencies.

Included, courtesy of OSHA, is a graph including examples of sound pressure levels in A weighting. This will give you a good idea of what these numbers mean when someone is discussing them. 

I also recommend reading both the CDC and OSHA pages about hearing loss prevention. It is easier than you might think to damage your hearing permanently. Simple precautions like ear protection can make all the difference when going to concerts, working with your power tools, or going to the range.

I personally use Decibullz, and if at the range, I also wear ear muffs in addition (make sure to be careful when molding the Decibullz earplugs - they get hot and need some time to cool!).

Perceived Loudness

Let's say you have two different frequencies, at the same sound pressure levels, being heard by the same person. In all likelihood, these sounds will be perceived as having different loudness, even though they have the same sound pressure levels. This is because our ears are more sensitive to certain frequencies than others. There are graphs that pictorial represent our peceived hearing curves; you'll usually hear these referred to as the Fletcher Monson curves, or equal loudness curves. Although this is the case, this data is slightly outdated and if you are looking for something more up to date, I would recommend the ISO 226:2003 curves

Units for loudness are typically referenced as phons. For example 85 Phons is the loudness of 1kHz at 85dB SPL. 
Units for loudness, in a digital system, will be listed as LUFS or LKFS.