Dorte Hammershoi is a Danish professor in electronic systems. She works in the field of human-sound perception, which includes electro-acoustics, hearing damage and the measurement of noise sources close to the ear. Through the story of Dorte Hammershoi, we will explore the physics of sound.
Hammershøi received her Master of Science in Electrical Engineering with specialization in biomedical engineering from Aalborg University in 1989. In 1995, she was awarded a PhD in acoustics for her work on binaural techniques. She is currently working as a professor at the Department of Electronic Systems at Aalborg University and is investigating how hearing aids can be improved in order to help people with hearing problems. The problem of customizing hearing aids has been known for many years, and there are many alternative ways to conduct hearing tests. Among other things, there is a test where one must recognize words that are said in different kinds of background noise. By its very nature, this provides a clearer picture of how well hearing is working. In the words of Hammershoi herself, “The reason we are so focused on audiograms is that right now we can’t put our finger on a specific measurement and say: that’s the test that could enable us to adjust hearing aids better. There are a whole battery of various tests and examinations, but they take time and may require special equipment or training. That is why we need to find out which methods work best” . Today, researchers like Dorte Hammershoi are using new physics to solve problems that affect all of us.
What is sound?
Sound is a form of energy (just like light, heat, electricity). Sound is caused by vibrations travelling through a medium (i.e. air) and can be detected by our ears. Sound travels as a wave and has different speeds, depending on the medium. In air, sound has a speed of 340 m/s while in liquid it travels at 1400 m/s and in solid materials it travels at 5000 m/s. Think back to the last lightning storm you were in, notice that you always see the lightning before you hear the thunder. But they both actually happen at the same time. We get a time difference because light travels much faster than sound (300,000,000 m/s versus 340 m/s)
Reflection of sound
In the same way that a light wave can bounce off an object to form a reflection, sound waves can also bounce off objects. We call this an echo. Reflection of sound has been put to use in loads of different ways – from the sonar used by ships and submarines, to ultrasound imaging used to view children in the womb. Bats use echolocation to find their way around dark caves and hunt their prey. By sending out a high pitched sound, the bats then use the reflections to know what the shape of the cave is.
How our ears detect sounds
The human ear is designed to pick up sounds and send signals to our brain. The ear is made up of three difference sections:
- The outer ear: The part we can see! The ear and ear canal
- The middle ear: Contains 3 bones called the hammer, anvil and stirrup
- The inner ear: Containing the cochlea
When a sound reaches our ears, the sound vibrates our eardrum. The vibrations are passed along the hammer, anvil and stirrup and sent through nerves to the brain as an electrical signal.
The loudness of sound is measured in decibels (dB) and if the decibel level of a sound is too high, we can do real damage to our ears. Anything over 120 dB will more than likely cause pain in your ears. This is roughly the noise level of a plane taking off right beside you. Fireworks and gun shots clock in at around 140 dB. High noise levels can cause severe damage, which is why researchers like Dorte Hammershoi are looking at how out bodies interact with sound and how we can better protect our ears from the dangers of sound energy.