To begin a discussion on noise control, one must comprehend the basic terminology associated with this topic:
Sound Frequency (Hz) Noise Infra sound and ultrasound Decibel (dB) Noise level measurement Adding noise levels Octave band Sound transmission Resonance Sound reduction by distance Sound transmission loss (TL) Noise reduction (NR) Sound absorption
The Sound Factor!
Sound is produced when a source sets the air energy level nearest to it in a wave motion. The speed of sound is about 1116 feet per second or 340 meters per second in air. The speed of sound is however greater in liquids and solids; for example, 4,925 f/s in water and 16,417 f/s in steel.
Definition of Sound Frequency
The frequency of sound waves is a number measured in vibrations per second, and is given in a unit called hertz (Hz). Sound that we can hear ranges between the audible range of 20 to 20000 Hz.
The boundary between low to high frequency is normally considered to be at 1,000Hz. Sound may be thought of as pure single tone, however it is normally consisting of several tones at varying intensities.
What Effect Does Noise Have?
Noise is any sound that is other than desirable to our ears. The “noise effect” is linked both on the tone intensity as well as the frequency. For instance, high frequency is far more disturbing than low frequency sound. Pure single tones can also be more distraught to our ears than multiple tone emission.
Infra-sound Vs. Ultra-sound
Sound at frequency levels below 201-Hz is called infra sound. Sound with more than 20,000Hz is called ultrasound. It is thought that the sound emitted at over 20,000Hz in the ultrasonic region and not heard by humans is surely a health hazard, especially in workers at noisy or humming machine shops.
Decibel (dB)
The measurement for sound is the unit known as the decibel (dB). If sound level increases by 10 dB, our ears react as if it had increased by double the factor! Similarly a reduction of 10 dB gives us the impression that the sound intensity has reduced by half.
Measurement Of Noise Level
Instruments that are used for measuring the sound level are based on the human ear and its sensitivity to ‘noisy’ sound that is emitted at high and variable frequency. These special instruments give the “A-weighted sound level” unitized as dB(A).
Sound transmission
Sound transmission is in air and also in liquids or solids. Though the term “sound” normally is linked to waves transmitted through air, that makes our hearing possible.
What is Resonance?
All objects along with the air surrounding it ‘resonate’ or as it is termed as a re-sound. This resonance at particular frequencies reinforce the sound we hear. The frequency of resonance also is related to the construction as well as size of any object or the volume of air surrounding it.
Sound reduction by distance
Sound waves depend on the distance and medium conveying them. For example, every doubling of the distance results in reducing the level at least 6 dB or so, in measurement using air as the conveying medium. It is also true that other media or enclosed spaces have a different effect on sound transmission, where normally the reduction is significantly less than in air.
Transmission Loss And Sound (TL)
Let us take the instance of how sound relates to transmission losses and why!
Similar to study of Physics on the topic of “Sight”, and relating it to a solid wall, we can grasp this relationship by striking the wall. We find that only a part of sound transmits or is ‘absorbed’ through the wall and most of it bounces or ‘reflects’ back. The rating known as Transmission Loss or TL is quite stable for any wall construction and is again measured in decibels (dB).
Sound Absorption And Noise Control
Noise control uses the principle of sound absorption in many constructional materials to reduce the transmission losses. Generally, porous materials like Polystyrene Foam or Gypsum Expanded sheets prove very effective in controlling the transmission of high intensity sound waves.
Hemant Mehta is a professional copywriter who takes up freelance assignments on the Internet.
Tags: decibel, hearing, noise control, sound
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