at its base near the oval window; the cochlea tapers towards its apex. Groups of the delicate hair sensors on the membrane, which vary in stiffness along its length, respond to different frequencies transmitted down the spiral. The hair sensors are one of the few cell types in the body which do not regenerate. They can therefore be irreparably damaged by large noise doses.

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-  What makes sound?

Sound is produced when the air is disturbed in some way, for example by a vibrating object. A speaker cone from a high fidelity system serves as a good illustration. It may be possible to see the movement of a bass speaker cone, providing it is producing very low frequency sound. As the cone moves forward the air immediately in front is compressed causing a slight increase in air pressure, it then moves back past its rest position and causes a reduction in the air pressure (rarefaction). The process continues so that a wave of alternating high and low pressure is radiated away from the speaker cone at the speed of sound

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- Can you describe how changes in air pressure make sound?

When the rapid variations in pressure occur between about 20 and 20,000 times per second (i.e. at a frequency between 20Hz and 20kHz) sound is potentially audible even though the pressure variation can sometimes be as low as only a few tens of millionths of a Pascal. Louder sounds are caused by greater variations in pressure. A sound wave of one Pascal amplitude, for example, will sound quite loud, provided that most of the acoustic energy is in the mid-frequencies (1kHz – 4kHz) where the human ear is most sensitive. It is commonly accepted that the threshold of human hearing for a 1 kHz sound wave is about 20 micro-Pascals.

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- What is the difference between acoustics and soundproofing?

Acoustics is the general field that describes how waves propagate in a medium, such as air. Soundproofing is a process of blocking, absorbing and diffusing these waves.

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- Does sound become unsafe at certain levels?

OSHA has defined limits of exposure to noise at 85 dBA for a period of 8 hours. It is strongly recommended that unprotected exposure to sound pressure levels above 100 dBA be avoided. Damage to hearing from loud noise is cumulative and is irreversible. (See Question 1 above) Exposure to high noise levels is also one of the main causes of tinnitus. Health hazards also result from extended exposure to vibration.

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- Is it possible to create a truly soundproof barrier? 

Just like watch makers state that the watches are “water resistant”, instead of “waterproof”. There are several companies that claim that there is no such thing as “soundproof”. We can reduce sound significantly to the point where it is not audible, but it is probably not possible to reduce the sound so much that there will not be any pressure fluctuations in the air.

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- What are some different types of noise?

Airborne Noise – Sounds transmitted between rooms via flanking elements instead of directly through separating elements or along any path other than the direct path. To reduce the amount of sound heard by the other person you would need to install a barrier or sound insulator between the sound source and you, the thicker and denser the barrier then the better the sound reduction will be. (Sonic-Shield Soundproofing Made Simple FAQs)

 

Impact Noise – This is the transmission of sound via the connection of different materials to each other i.e. footsteps on a floor of a house. (Sonic-Shield Soundproofing Made Simple FAQs)

 

Flanking noise - noise reaching a room by an indirect path. Often, people will apply sound proofing to a wall, but the noise might actually enter the room through the ceiling. It is necessary to consider all of the potential noise transmission paths to successfully reduce the noise. (Sonic-Shield Soundproofing Made Simple FAQs)

 

Vibration Noise - Noise from vibration will generate both flanking noise and airborne noise depending on its location. To reduce vibration transmission, you need to either stop the sound getting into the floor, out of the ceiling or a combination of both, by adding a “floating floor” or an acoustic ceiling system. (Sonic-Shield Soundproofing Made Simple FAQs)

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- What is a sound barrier?

A sound barrier is another way of describing a sound blocker and normally comprises insulation with a high mass which then reduces the amount of noise that can pass through it. A simple door can be described as a noise barrier when it is closed to reduce the sound of noisy kids playing just outside. Sound waves flow like water and air so it is impossible to use a sound barrier such as a fence or screen to stop noise but they are effective when used to reduce noise immediately on the other side of them. From distances farther away the noise reduction will be less efficient.

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- What is sound damping or sound dampening?

Noise or vibration that is transmitted to panels can excite vibration modes of these panels. The panel can behave like a loudspeaker, radiating noise from its surface. Typically, structure-radiated sound can be eliminated by “tuning” the panel resonances out of the audible frequency range. This is done by adding mass (to reduce the natural frequency) or stiffness (to increase the natural frequency). It is also possible to add a damping material (such as a visco-elastic constrained damping layer) to spread out the panel resonance over a wider frequency range, thereby reducing the peak energy at a single frequency.

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- Can you describe sound insulation?

Sound insulation is a measure of the sound stopped by a barrier such as a partition wall. We can measure the sound reduction index in a laboratory. This consists of two adjacent reverberant rooms, the difference between the level of the sound in the source room and the receiver room is measured, and the properties of the receiver room are taken into account by calculation. The measurement method depends on the particular situation. There are standards for the measurement of the insulation of materials in the laboratory, and for a number of different field circumstances. Usually test procedures generate a loud and consistent broadband spectrum of steady noise on one side of a partition or specimen of the material under test, and then measure the amount of this sound that passes through that material. The ratio of the incident sound to the transmitted sound is the “noise reduction”, usually expressed as 10 times the logarithm of this ratio. If the noise reduction is also corrected for the amount of sound absorption to be found in the receiving room, 10 times the logarithm of the corrected ratio is called the “transmission loss. This is performed for 1/3 octave bands of noise from 100 to 4000 Hz.

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- What are sound transmission class ratings?

The American Society for Testing and Materials established sound Transmission Class (STC) ratings to help compare noise reduction products such as windows, doors, walls and building materials. STC ratings are measurements of sound isolation for various products. Measured in decibels, the STC rating is the average amount of noise resistance at 18 different frequencies. Each number in the rating system represents a significantly higher measurement than the one before, similar to the Richter scale used to measure earthquakes. The higher the STC class rating is, the greater the noise reduction.

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- What does IIC stand for?

Impact Isolation Class. The higher the IIC number the better. Assemblies having IIC ratings greater than 50 usually are adequate to muffle normal walking noise

Finally while Sonic-Shield Soundproofing Made Simple FAQs are listed above, and the chart below is not a question, the chart may answer some questions for you that you did not even know that you had about common indoor and outdoor noise levels.

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- What noise levels do we experience daily?

The chart below shows common indoor and outdoor noise levels. Although sound is perceived differently from person to person, this is a good baseline to understanding the common types of noise we experience daily and be more aware of our surroundings and how they are affecting our health.