What is the quietest sound we can hear? What is the slightest pressure we can feel? What is the weakest light we can see? Such questions fall in the domain of psychophysics, an interdisciplinary science that deals with physical stimuli and human sensations. The concept of an absolute threshold provides the answer to these questions.
We all know that a certain minimum stimulation of the sense organs is required before any sensory experience will occur. This is what defines the “absolute threshold” of a sensory system.
The minimum amount of energy required to produce a sensation is called the “absolute threshold” of a sensory system. (There are at least five sensory systems in humans that relate to sight, sound, smell, taste, and touch.)
Absolute Threshold Examples for Different Sensory Systems
When we measure the absolute threshold for various sensory systems, it gives us an idea about how sensitive we are. For example, only three photons are required to strike our eyes to produce a sensation.
We have different absolute thresholds for different sensations. When it comes to vision, our eyes can catch a candle flame as far away as 30 miles on a clear night. As far as the hearing is concerned, we’re able to hear the tick of a watch from 20 feet under quiet conditions. Similarly, for taste, we can appreciate sweetness caused by a teaspoon of sugar in 2 gallons of water. We can smell one drop of perfume diffused into a three-room apartment. And last but not least, we can detect a bee’s wing falling on our cheek from one centimeter above.
Absolute Threshold and Upper Threshold
Most sensory systems have an upper limit as well as a lower limit or the absolute threshold. The most suitable example in this regard is the pitch sensitivity of sound. We humans can’t hear voices that are below 20 hertz (vibrations per second) as well as those that are above 20,000 hertz. It makes sense to have an absolute threshold for sensory systems. This has to do with practical reasons. For example, if we could hear voices below 20 hertz, we would hear the movements of our muscles. It would be an unnecessary load to our nerves. Nobody would want to listen to the music of their bodies, creaking and groaning like an ancient ship as their muscles moved, would you?
The 20,000-hertz upper threshold, on the other hand, is a blessing in disguise. If we could hear above this limit, we would be surrounded by an unnerving noise of bats that use this frequency to navigate their way and detect their prey.
Dogs, bats, cats, and some other members of the animal kingdom can hear sounds above a 20,000-hertz threshold. Ever heard of a “silent dog whistle?” It produces a sound above the upper threshold for us, humans, to hear, sometimes reaching as high as 50,000 hertz. While we can’t hear a sound of a frequency so high, dogs hear it. This is the reason why this whistle is called “silent.” Note that it’s silent only for us and not for dogs.
How the Absolute Threshold is Measured
The method used to determine an absolute threshold for a sensory system is very familiar. An observer is subject to different intensities of a certain stimulus until they are unable to detect the stimulus. However, the observer usually undergoes thousands of trials before a threshold is determined for a particular stimulus.
In practice, instead of declaring a single value below which people detect a stimulus and above which they always do, a range of values is determined and the absolute threshold is taken to be the value at which people detect a stimulus 50% of the time.
While the concept of an absolute threshold is useful and pragmatic, it is not an absolute reality. Even a single person might not be able to detect a weak stimulus multiple times. Moreover, scientists are unable to calculate the exact amount of energy present at or below an absolute threshold for a particular stimulus, sometimes because of limits to the physics of measurement.
Many other factors can affect an observer’s ability to recognize an absolute threshold, including personal expectations, motivations, and cognitive processes. If a person is adapted to a particular stimulus, it’s more likely for them to not recognize a stimulus at its absolute threshold. Long exposure to environmental stimuli can decrease our ability to apprehend those stimuli, a deliberate muting of our senses by our brain. It’s just how our brain works and functions.