Understanding Noise

Sound & Sound frequency

Sound loudness

What is noise?

How can noise levels be measured?

The unit used in acoustics to define and measure sound intensity is Decibel (dB). Decibel uses a logarithmic scale which works very differently from a linear scale. Human perception of loudness is not linear, so a logarithmic scale is well suited to express our experience of sound. In fact, when the human ear receives a sound stimulus it tries to amplify very weak sounds and to attenuate very strong sounds.

So calculating the Decibel is based on the comparison between the input signal and the output signal. A linear scale as used on a ruler, where a distance of 30 inches is three times as long as a distance of 10 inches, couldn't adequately express a decrease or increase in sound intensity.

When using a logarithmic scale to measure noise levels, adding 3 dB means multiplying the loudness by two. If we divide the intensity of a sound by 2, the sound level only drops by 3 dB.

If we measure noise on a scale from 0 to 130 Decibels: 0 dB represents the threshold of audibility, 130 dB represents the threshold of pain. Most of the sounds of everyday life are between 30 and 90 dB. 

Decibel levels for everyday noises:

What is the increase in sound intensity between two different Decibel levels ? 

Sound intensity is the energy needed to produce a given level of sound and not to be confused with sound volume which is the level at which we perceive the resulting sound.The mathematical relationship between Decibel (dB) and sound intensity works as follows: each 10 dB increase results in a 10-fold increase in sound intensity which we perceive as a 2-fold increase in sound volume. Thus, from 0 dB to 10 dB there is a 10-fold increase in sound intensity, just as there is from 10 dB to 20 dB or from 34 dB to 44 dB. 

Decibel levels for everyday noises VS. increase in sound intensity and perceived volume:

Actual sound levels VS. noise perception

The intensity of a sound can be measured by using a microphone to convert the sound into power and then interpreting the power obtained in terms of Decibels (dB). However, this physical sound level does not really correspond to the perception of the sound that we are having.

Measuring human noise perception

The human ear does not perceive all the sound frequenciesin the same way: we are deaf to low-frequency sounds (below 20 Hz) and high-frequency sounds (above 20,000 Hz), and are most sensitive to sounds around 2000 Hz.This is why there are different versions of the "dB" scale which are decibel readings that have been adjusted to take into account the varying sensitivity of the human ear to different frequencies of sound:

• dBA (A-weighted sound levels) - measurement of common sound intensities, using a filter to account for the loudness as perceived by a human ear (everyday noises).

• dBC (C-weighted sound levels) - measurement of higher sound intensities, specifying the effect of peak and high impact sounds on the human ear (extreme loudness such as a live concert).

It is not possible to reproduce the behavior of the "average" human ear with a single filter (and even less that of the ears of each particular individual), but in most situations our hearing can be measued with the the dBA scale. This scale is a good starting point from which to measure sound perception, but gives us only an idea of the perceived noise levels. 

Factors impacting noise perception

In reality, the overall noise is composed by many different sound levels all across the frequency spectrum, which is why measuring the perception of sound is very complex and depends on a large number of factors:

• Individual sensitivity (depending on age, culture, time of day...)

• Spatial configuration (inside/outside, construction of the building, furniture in the room...)

• Distance to the noise source (sensor position)

• Sound frequency (low, medium, high)

• Sound repeatability (continuous, intermittent, impulsive)

• Temporal exposure (long-term, short-term)

These are the most common reasons that explain a difference between the actual sound and the perceived noise. The Decibel measurements can only give a limited representation of the perceived "reality", especially as the sensitivity to noise varies greatly from one individual to another. Some populations can be more vulnerable to noise exposure than others, such as students during an exam or people affected by hearing loss.

Types of noise

There are many different categories of noise, from natural to man-made. In big cities we are confronted with man-made sounds, often quite aggressive.

We try to find soundproof solutions to create a quieter environment and get rid of diffrent types of noise:

• continuous noise (like machinery that runs without interruption, car traffic): is noise which remains constant and stable over a given time period

• intermittent noise (trains, aircrafts, sirens): is noise which does not remain constant and stable over a given time period

• impulsive / impact noise (gun shot): is a very short burst of loud noise which lasts for less than one second.

The noise we find the most disturbing and that we are the most exposed to (unless you live in the countryside) is traffic noise. Unfortunately, this is also the hardest type of noise to reduce, let alone cut out.

The best bet to dampen traffic noise is to combine different solutions: noise-reducing curtains, rugs, window frame insulation foam, etc. 

The difference between sound proofing and sound absorption

To understand the difference between sound absorbtion and sound proofing let's remember how the noise travels: when a sound wave is emitted, it is modified by parameters such as distance or obstacles. When a sound wave meets an obstacle like the wall of a room, a certain amount of energy is reflected back into the room. These successive reflections are called reverberation.

Sound proofing

Sound proofing (or sound insulation) attempts to stop noise by blocking the acoustic wave from traveling to adjacent spaces. 

Insulation can only be achieved with mass and/or multiple layers ot material in the walls or windows. It means, that the construction of the building is essential in this matter, and that additional sound proofing is not easy once the structure is already built.

Sound absorption

Sound absorption aims to reduce noise by limiting the reverberation of the sound wave within a space.

Absorption can be obtained with light materials (such as specific fabrics) that are applied to walls or windows, meaning it is easier to achieve once the structure is already built.

How can "soundproof" curtains help?

The impact of any sound insulation curtain depends on the kind of noise you are experiencing and the construction of your building.

As mentionned before, there are different types of noises with either high, medium or low frequencies. Different applications and treatments are needed to confront this spectrum of noises and there is no one-for-all solution.

Each sound environment is different and the intricacies of one's home have to be taken into account, because the noise does not come in simply from your windows but also seeps in through the walls and various openings.

Moondream sound insulation curtains aim to adress both - sound proofing and sound absorbtion - as best as possible with a solution that is equally functional and aesthetic. Our patented curtains have three strategically assembled layers of fabric to reduce the transmission of the sound wave and absorb the reverberation of the sound as well:

• The incoming sound energy is dampended and divided by 4, reaching a drop of up to 7 decibels. 
• The transmission of the sound wave is reduced and the reverberation is reduced: for a surface area of approximately 215 square feet, the reverberation time is reduced by 0.4 seconds.

The result of both actions is a quieter acoustic environment that will contribute significantly to your wellbeing. To improve the curtain's results you can add other absorbent materials to your floors such as carpet or a rug.

You have to be aware that if you wish to cut out certain low frequency sounds such as traffic noise completely, a curtain might not be enough (soundproof windows and thick walls would be necessary). Moondream curtains can however dampen the noise which can already be an important improvement and is a much more cost effective solution, especially if you are just renting.