We are immersed in noise, and sorting out meaningful signals from ambiance is an amazing capability of our brain. Although we do not have the advanced sonic capabilities of bats, owls, wax moths, elephants, dolphins, and other animals, human hearing is quite sensitive. How we process and interpret what we hear plays a central role in shaping our interpretation and experience of our surroundings. From helping us recognize danger, to assisting in relaxation--sounds trigger physiological and psychological responses that affect our behavior and interactions with the world around us.

Automatic defensive response is a central feature of all sensory systems. With vision, understanding that what one sees is dangerous, is learned from painful personal experience or preventive social guidance: Until we are thought that tigers can devour us, we have no reason to be afraid of the beautiful, furry animal. 

The particular characteristic of a sound, such as volume, pitch, and tone, produce automatic physiological responses that reinforce or contradict the experience intended by those producing the sound. If you have ever sat through a wind-instruments concert performed by second-graders, you understand how auditory processing can work against the creator's intentions. On the other hand, if you ever jumped out of your chair to run downstairs in response to an ear-piercing fire alarm, the qualities of that sound have facilitated the intention of its designer very well.

Many products have an auditory component. The design of user experience for such products requires consideration of the physiological, emotional, and behavioral responses elicited by various characteristics of sound. Some examples are straightforward:

• Sudden sounds trigger an instant pause in whatever a person is doing. The sound maybe loud and high pitched like in an alarm system, or presented in short beeping bursts like when the car alerts the sleepy driver that it's drifting out of its lane.
• Continuous soft sounds of slightly lower pitch, such as ocean waves, are used in sleep-aid noise machines for their known calming effect.

The physiological responses to auditory stimuli are almost universal, and experience designers can usually count on eliciting reactions like the ones described previously because they trigger autonomic nervous system responses.

The image above illustrates two categories of sound qualities:

• Sounds that alert the body to danger (whether real or imagined). The physiological stress that follows might include sweating, pounding heart, freezing in place, or a desire to escape. We can put sudden loud bangs, explosions, screams, and sometimes an eerie quiet in this category.
• Sounds with qualities of rhythm, tempo, harmony, and melody that elicit pleasant and grounding response, and cause the body to relax and move in calming rhythmical ways.

But there are also significant individual differences in response to sounds. An individual's response to sound is affected by thresholds, habituation, and the personal-emotional and cultural context within which the auditory processing occurs.

While imagery is passive--we have to focus our attention on an object--sound waves spread in the air, and hit the eardrums of anyone within reach. Thresholds differ among individuals. For some, loud music is fun, while for others it is torturous. But loud sounds can be dangerous, and cause irreversible permanent hearing loss, which is why the design of volume-controls often includes a visual representation of the loud to quiet range, and portable music players in smartphones and other devices include an option to limit the volume level of headphones.

Kindergarten teachers need high thresholds for noise; otherwise they could not function in a classroom full of loud five-year olds. And if that teacher feels that sometimes even their high thresholds does not offer sufficient protection in the classroom, they can use products like the Deluxe Yacker Tracker (shown in the preceding image)--a noise level monitor, which clearly indicates to the classroom when they have gone too far-too red.

It is interesting to note that thresholds for sounds are also contextual. A loud music performance may feel okay at a rock concert, but cause discomfort and agitation when it blasts through a neighbor's window.

Appliance designers use sound to alert users. For example, when the water in the kettle is boiling, when the temperature level of an oven has reached a desired temperature, or when the baking time set by the user has ended. In these cases, the designers must balance the need to sound an alert that will spring the user to action, without causing unnecessary stress: piercing beeps may cause frequent and needless disruption, while soft beeps might be too low to register if no one is in the kitchen. Sounding a progressively louder alarm is a good solution.

Habituation is the term for decreasing response to a repeated stimulus. Without habituation, we would all go mad. We are blissfully saved from constant awareness of all the sounds around us because the brain "gets used" to sounds and ignores them. We can sleep in a building near the train tracks, read in a coffee shop, and pay attention to our boss in a long meeting thanks to habituation.

But just as with thresholds, there are individual differences in this ability. Extreme cases of poor auditory filtering of environmental sounds can be a seriously disabling condition. Most people, however, habituate to sounds enough to carry on with their daily activities. Experience designers consider habituation when the goal is to overcome it or to use it to reinforce a specific experience--emergency vehicle sirens use rising and falling sounds to ensure continued alertness on the part of drivers, whose default is to ignore the sounds surrounding them on the road.

More than anything else, however, individual differences in how auditory information impacts experience, is in the emotional associations elicited by sounds. Sounds can generate a wide range of emotional responses.

Sounds have a large emotive potential. Sounds from nature, sounds from man-made instruments, as well as the sound of the human voice, can be pleasant, irritating, stimulating, or relaxing. Sounds can change our mood and influence our activities.

Individual differences in emotional responses to sounds may be a matter of what we can call "taste"--preferences that do not have specific or exact explanations, but are weaved into the fabric of personal histories and characteristics, as well as cultural and aesthetic norms. From the experience design point of view, individuals' likes and dislikes of certain sounds invite serious consideration, and not only in connection to products or situations where sound is the main feature of the experience.

For products in which sound is a central feature, offering selection is a good approach:

• Doorbells come in various shapes and more importantly, with various sounds from the classic buzz to a birdsong, there are options for the customer's ears.
• Personal GPS systems offer a selection of voices, including the option to record a favorite voice to guide one's road adventures.

In other situations, sound is a component, but not a central feature of the experience. Background music is a good example. Background music is developed through research on how various sounds/music impact people in different situations. The goal is to identify sound patterns that promote situational success.

• The proverbial elevator music is intended to reduce stress for people enclosed in a crowded metal box hanging on wires, while supermarket music is indented to slow us down, uplift our mood, and hopefully purchase many more items than those listed in our shopping list.
• Auditory input that includes recorded sounds from nature (birds, sea waves, and so on), music or spoken words, is used to help children and adults go through scary and/or painful medical procedures, like the MRI machine or dental work.

The Mozart effect was a widely-embraced marketing gimmick that claimed that listening to Mozart and other classical music enhances the development of cognitive skills in young children. For a while, it was almost unthinkable to raise a child without a CD, video cassette, or DVD of the Mozart effect--much to the delight of the company that produced them. The Mozart effect failed to withstand the scrutiny of research, and a direct link between Mozart's music and cognitive enhancement, has not been established.

Nonetheless, the idea that certain music or other auditory stimulation can help people learn and improve skills such as memory, attention, and spatial orientation, has substantial anecdotal evidence and continues to interest researchers as well as experience designers in various areas of practice.

At the opposite end of the response to sound spectrum, experience designers tackle the challenge of eliminating unwanted, unpleasant, and annoying sounds with product in categories such as:

• Sound proofing technologies and designs for home and office
• Personal and environmental noise-cancelling and/or masking devices such as noise-cancellation headphones, or products like Sono, noise-cancelling gadgets that selectively convert an annoying background noise into a more pleasant sound