There is a lot of misinformation around about what the terms ‘audible’, ‘inaudible’ and ‘ultrasound’ mean. This post aims to give readers a better understanding of each, the reasons behind some of the confusion, and why the differences here matter.
Simply put, the term ‘audible’ means ‘able to be heard’. This may seem blindingly obvious, but as we’ll see — it sets the scene for more subtle distinctions later in this discussion.
When we say that we are ‘able to hear’ something, we are making a statement about several different characteristics of a particular sound and its acoustic context. Each of these qualities must be within certain ranges in order for the human auditory system to detect a sound.
A couple of the most significant qualities affecting human hearing of particular sounds :
- The type and amount of any other sounds playing which are competing to be heard
- How loud it is
- The frequencies it contains (is it very high or very low pitch?)
Let’s assume here that the first one is controlled by us — that we have quiet conditions for listening, so that the main remaining 2 factors become a sound’s loudness and its frequency composition.
In terms of frequencies, the common definition for the human audible frequency range is 20Hz and 20kHz  — however our ear’s sensitivity to sound varies dramatically over this range.
Our ear is most sensitive between the ranges of ~2kHz and ~5kHz (see the dip in the graph above). This range of particular sensitivity corresponds directly to the main frequency components of human speech.
In our most sensitive frequency regions we are able to perceive sound pressure levels of under 20μPa — this corresponds to the vibrating air molecules which together constitute audible sounds over incredibly small distances, on the order of 1/100 of a millionth of a centimetre, or 1/10th the diameter of a hydrogen molecule .
This sensitivity changes not only with frequency of the sound, but with the age of the listener — with younger folk having a much greater sensitivity to higher frequency sounds. There are many different ways a person can lose hearing sensitivity, most commonly this happens as a natural process of ageing as we (unlike frogs & fish) gradually loose the tiny hair cells in the inner ear (presbycusis) which translate vibration to electrical impulses finally interpreted by the brain.
Inaudible then — ‘unable to be heard’.
There may be multiple potential reasons for this — a sound could be inaudible over the background noise level (i.e. ‘masked’), could be outside the human audible frequency range, or be too quiet to be heard even in ideal listening conditions.
Again we must be careful to differentiate between academic, general definitions of whether a sound is audible or not, and the ability of single individuals to detect these sounds- the definition of frequencies inside the 20Hz-20kHz range being ‘audible’ does not mean that all frequencies within this range are accessible to everyone.
When we say that a sound is ultrasonic, or lies in the ultrasonic range, we generally mean that it contains only frequencies which are above the upper limit of the human audible frequency range - defined generally as frequencies above 20kHz.
‘Ultrasonic’ then, is a sound rendered inaudible specifically due to its frequency content.
Although the definition of ultrasound covers the majority of the population, there are those (especially younger people) who will be able to hear frequencies at or above the textbook definition of 20kHz.
For some individuals then ‘ultrasonic’ (as most standardly defined) does not necessarily mean ‘inaudible’.
Equally, for many (especially older) individuals, higher frequency sounds will be ‘inaudible’ though they lie well below the 20kHz limit.
Audibility and data-over-sound
Chirp and most other companies offering ‘inaudible’ data over sound technologies use a frequency range at the very upper limit of human hearing, but below the standard 20kHz definition of ‘ultrasound’, typically in the 18–19kHz range.
Whilst this lies above the audible range for most of us individual — especially if you are older — it is not, with respect to standardised definitions, ultrasonic.
In the next post of this series, we will look at the various affordances and appropriate use cases for both audible and inaudible sound as a means of transmitting data.
- Rosen, Stuart (2011). Signals and Systems for Speech and Hearing (2nd ed.). BRILL. p. 163
- Everest, F. Alton. The Master Handbook Of Acoustics. 1st ed. New York: McGraw-Hill, 2001. Print.
- Horowitz, Seth S. The Universal Sense. 1st ed. New York: Bloomsbury, 2012. Print. p.91
For more information on Chirp’s data-over-sound technology solutions, please visit Chirp.io