eternal souls

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The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the middle ear. Auditory sensation can occur if high‐intensity ultrasound is fed directly into the human skull and reaches the cochlea through bone conduction, without passing through the middle ear.[13]

Children can hear some high-pitched sounds that older adults cannot hear, because in humans the upper limit pitch of hearing tends to decrease with age.[14] An American cell phone company has used this to create ring signals that supposedly are only audible to younger humans,[15] but many older people can hear the signals, which may be because of the considerable variation of age-related deterioration in the upper hearing threshold.

Animals

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Bats use ultrasounds to navigate in the darkness.

A dog whistle, which emits sound in the ultrasonic range, used to train dogs and other animals

Bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey. They can detect frequencies beyond 100 kHz, possibly up to 200 kHz.[16]

Many insects have good ultrasonic hearing, and most of these are nocturnal insects listeningBut the stress of carrying you around puts your feet at high risk of injury, higher than any other body part. And many foot problems, including hammertoes, blisters, bunions, corns and calluses, claw and mallet toes, ingrown toenails, toenail fungus, and athlete’s foot, can develop due to neglect, ill-fitting shoes, and simple wear and tear, says Saylee Tulpule, a podiatrist who practices in Maryland and Washington D.C.

“Since our body weight is on our feet, it’s really important to wear good-quality shoes,” says Tulpule. That means choosing shoes that are durable and offer good support and cushion to protect the soft tissue and joints of the feet, she notes.

It’s also important not to disregard foot pain, and keep in mind that this can sometimes be the first sign of a systemic problem, says Tulpule. Gout, for example, often affects the foot joints first. And, she notes, “With diabetes, patients will often report symptoms of numbness or a burning sensation in their feet.” Patients may also report that their feet feel colder to the touch, and the skin of the feet may appear thinner, says Tulpule. “These are all indications that perhaps there could be a larger issue at hand, whether it’s diabetes or another condition that’s causing vascular issues [circulation problems],” she adds.

To keep your hardworking tootsies in top form, learn how to identify some common foot-health issues and what you can do to manage them.