Acoustics: How sounds are transmitted

'You voice is loud' my cousin said.

'Doesn't matter', I said, 'It isn't loud enough to reach the first floor. Even if the sound reaches it, our uncle can't make out that it is mine'.

My cousin looked at me strangely but  I knew exactly what I was saying because  I had my reasons to come to that conclusion. Let me explain. 

Regardless of their source, noises are transmitted in buildings by two methods.

Airborne Sound:

Sound waves traveling through air move between building areas - such as through open windows, doors, or stairwells.

Mechanically transmitted sound: 

When sounds move through solid building components such as floors, ceilings, walls, framing, carrying sound from one area to another the sound transmission is referred to more technically as impact insulation class transmission or IIC sound transmission.

In many cases  people  have difficulty tracing a sound to its source, or  are unsure if a sound that they hear at a known source (say a humming sound at an electrical component) is normal or means trouble or something else.

Because sound waves interact in fascinating ways with the environment around us. You must have noticed that an ambulance’s siren sounds different when it is in the distance compared with when the ambulance approaches and passes you?

This is because it takes time for sound to travel from one point to another, and the movement of the sound source interacts with the frequency of the waves as they reach the person hearing it. When the ambulance is far away, the frequency of the siren is low, but the frequency increases as the ambulance approaches you, which is a phenomenon known as the Doppler effect

 

How a sound is ''heard and perceived"depends on several things. 

Air movement:  The wind can slow down or accelerate the speed of sound, depending on whether it is blowing in the same direction or in the opposite direction to the sound signal.

Resound: When the place is an empty room the sound  bounces back just from the walls but when there are several objects around because of the physical properties of sound waves themselves, such as how they bounce or are otherwise disrupted by the objects.

Objects around: Sound is not only affected by distance, however, but also by other objects. Think back to a time when someone was calling for you from another room. You probably noticed that it was harder to hear them from another room than when he or she was right next to you. The distance between you is not the only reason a person is harder to hear when he or she is in another room. The person is also harder to hear because the sound waves are being absorbed by objects in the environment; the further away the person calling you is, the more objects there are in between you two, so less of the sound waves eventually reach your ears. As a result, the sounds may appear to be quiet and muffled, even when the person is yelling loudly.

Age of people hearing it and their hearing capacity: There are many causes of age-related hearing loss. Most commonly, it arises from changes in the inner ear as we age, but it can also result from changes in the middle ear, or from complex changes along the nerve pathways from the ear to the brain. Certain medical conditions and medications may also play a role.

Whether all doors are open or not: we all know that closed doors reduce the sound.

Whether other sounds around like AC and fans running, traffic, radio-TV - music system sounds, water flowing in the toilets in a house, neighbours voices are interfering with the sound in question. 

Shape of your head and the location of your ears.

Structure of the Ear

Our ears are complex anatomical structures that are separated into three main parts, called the outer ear, middle ear, and inner ear. The outer ear is the only visible part of the ear and is primarily used for funneling sound from the environment into the ear canal. From there, sound travels into the middle ear, where it vibrates the eardrum and three tiny bones, called the ossicles, that transmit sound energy to the inner ear. The energy continues to travel to the inner ear, where it is received by the cochlea - a mostly hollow tube in the inner ear that is usually coiled like a snail shell and which contains the sensory organs of hearing.  It contains the Organ of Corti, where sensory “hair cells” are present that can sense the sound energy. When the cochlea receives the sound, it amplifies the signal detected by these hair cells and transmits the signal through the auditory nerve to the brain.

Distance between the origin of the sound and the person who is hearing it: The more the distance, the less sound the person can hear.

Brain processes like whether the person hearing it can actually analyse and  recognize the sound as coming from a known person and source. How the mind of the person hearing it is distracted plays a large part in recognising a sound. Attention deficit? Yes! 

Sound and the Brain

While the ears are responsible for receiving sound from the environment, it is the brain that perceives and makes sense of these sounds. The auditory cortex - the area of the brain located in the temporal lobe that processes information received through hearing -  of the brain is located within a region called the temporal lobe and is specialized for processing and interpreting sounds. The auditory cortex allows humans to process and understand speech, as well as other sounds in the environment. What would happen if signals from the auditory nerve never reached the auditory cortex? When a person’s auditory cortex is damaged due to a brain injury, the person sometimes becomes unable to understand noises; for instance, they may not understand the meaning of words being spoken, or they may be unable to tell two different musical instruments apart. Since many other areas of the brain are also active during the perception of sound, individuals with damage to the auditory cortex can often still react to sound. In these cases, even though the brain processes the sound, it is unable to make meaning from these signals.

 

• Diagram of a sound source traveling through the ear canal and turning into neural signals that reach the auditory cortex.
• The sound is directed into the ear canal by the outer ear, and is later turned into neural signals by the cochlea. This signal is then transmitted to the auditory cortex, where meaning is assigned to the sound.

Hearing Sound from Over Here, or Over There?

One important function of human ears, as well as the ears of other animals, is their ability to funnel sounds from the environment into the ear canal. Though the outer ear funnels sound into the ear, this is most efficient only when sound comes from the side of the head (rather than directly in front or behind it). When hearing a sound from an unknown source, humans typically turn their heads to point their ear toward where the sound might be located. People often do this without even realizing it, like when you are in a car and hear an ambulance, then move your head around to try to locate where the siren is coming from. Some animals, like dogs, are more efficient at locating sound than humans are. Sometimes animals (such as some dogs and many cats) can even physically move their ears in the direction of the sound!

Humans use two important cues to help determine where a sound is coming from. These cues are:

(1) which ear the sound hits first (known as interaural time differences-

The difference in the arrival time of sound received by the two ears ),
 and

(2) how loud the sound is when it reaches each ear (known as interaural intensity differences-

  If a dog were to bark on the right side of your body, you would have no problem turning and looking in that direction. This is because the sound waves produced by the barking hit your right ear before hitting your left ear, resulting in the sound being louder in your right ear. Why is it that the sound is louder in your right ear when the sound comes from the right? Because, like objects in your house that block or absorb the sound of someone calling you, your own head is a solid object that blocks sound waves traveling toward you. When sound comes from the right side, your head will block some of the sound waves before they hit your left ear. This results in the sound being perceived as louder from the right, thereby signaling that that is where the sound came from.

You can explore this through a fun activity. Close your eyes and ask a parent or friend to jingle a set of keys somewhere around your head. Do this several times, and each time, try to point to the location of keys, then open your eyes and see how accurate you were. Chances are, this is easy for you. Now cover up one ear and try it again. With only one ear available, you may find that the task is harder, or that you are less precise in pointing to the right location. This is because you have muffled one of your ears, and therefore weakened your ability to use signals about the timing or intensity of the sounds reaching each ear.

Also, Sound travels faster in warmer air, so the sound waves are refracted upward, away from the ground. Various atmospheric conditions can cause a temperature inversion, with air temperature increasing with height. This causes some of the sound energy from a source near the ground to be refracted back toward the ground. So at noon, the sound waves reaching upward are less.

Not all people can hear sound in the same way:  What is loud sound to you may not be loud to others around you! High frequency sounds are very directional. If one person’s ear was pointed directly at the sound source, while the other person was positioned with the back of their head to the sound source, the sound might sound very differently, particularly in volume, but the person with ears “pointing” at the sound will pick up more of the high frequencies. Also, sound has to bounce around to get down the ear tube to the ear drums. People have different shaped ears, their head shapes also differ so different people may hear this reflected sound differently. Different people have different hearing abilities. Some may hear certain frequencies better than others. Even though two people may be standing next to each other, the position of a doorway or window through which a sound is coming from might be closer to one than the other, and one person may in fact be blocking much of the sound from the other, especially the high frequency directional sounds. All of us have varying ear characteristics and the frequency response of ear, its sensitivity, shape of ears (capability of capturing sound waves and subsequent passive amplification), ear-brain conduit efficiency, fast sound/voice processing ability of brain etc. Our echo-cancelling ability is different. At older age, people are downed by the ‘muffling sound factor’ more than others. Language processing ability of the brain may also cause some differences. They may hear the sound with almost equal discernibility but the INTELLIGIBILITY aspect causes them to have different feelings.