There is a lot of misinformation around about what the terms ‘audible’,
‘inaudible’ and ‘ultrasound’ mean. This post aims to give readers a…

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 [1] — 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 [2].

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[3]) 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

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.

  1. Rosen, Stuart (2011). Signals and Systems for Speech and Hearing (2nd ed.). BRILL. p. 163
  2. Everest, F. Alton. The Master Handbook Of Acoustics. 1st ed. New York: McGraw-Hill, 2001. Print.
  3. 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