Sound Pressure & Volume

Sound Pressure and Volume

The human ear hears pressure – not volume. The difference between the two is that the closer to the source of the sound you are, the more pressure you will hear, and conversely the further away you get from the source of sound, the lower the pressure.

As you increase the pressure, generally you are increasing the volume. Volume does affect sound, even though it does not directly affect how we perceive how loud something is. For example, a good quality (efficient) speaker can produce more pressure with less volume.

In audio we have a chain of factors involved in how loud we perceive something to be. These are:

  1. How ‘loud’ is the recording we are playing back – how big is the input signal into our chain.
  2. What volume level do we select on our amplifier (and in-fact how efficient is our amplifier at converting the input signal into a ‘louder’ output)
  3. How efficient is our speaker driver at converting the electrical signal it is fed into sound pressure
  4. How far away from the speaker are we sitting when we listen.

It is important to have a good balanced match between all the factors to avoid having to stress any one part of the chain described above which can result in distortion, which can actually be more harmful to hearing and equipment than listening at a higher sound pressure level without distortion.

So how do we measure how loud something is?

The unit of measure for ‘loudness’ is Sound Pressure measured in decibels. A decibel (or dB) is a ratio between two different things, and is a logarithmic measurement.

In sound pressure, we measure how loud something is relative to a sound pressure of 0dB which is the absolute lower limit of audibility. This is a theoretical limit however as there exist various laboratory test environments designed for measuring sound, that have a lower background noise level than 0dB – these test rooms (referred to as anechoic chambers) can be very unnerving to go into as there is absolutely no sound to be heard, and no reflections of sound from the environment around you allowing you to hear your breathing or voice as you talk.

To give context to this, the following list of sound pressure levels can be used*:

176dB – .30-06 rifle being fired, measured 1m to the side of the shooter.

150dB – Jet engine at 1m

130-140dB – threshold of human pain

135dB – human shouting, measured 2cm from the mouth

120dB – risk of noise induced hearing loss (measured at the ear)

100dB – jack hammer / kangol hammer measured at 1m

80-90dB – Traffic on a busy road measured at 10m

85dB – Hearing damage over long term exposure – measured at the ear

60dB – TV set (at normal listening level) measured at 1m

40-60dB – normal conversation at 1m

20-30dB – very calm room

0dB – Auditory threshold (point at which it is not possible for the ear to hear anything quieter) – measured at the ear

-12.4dB – Anechoic chamber, University of Salford

-20.35dB – Anechoic chamber – Microsoft

*referenced from https://en.wikipedia.org/wiki/Sound_pressure

 

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