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Q What is Noise? and Soundproofing all about i am new to this?

In all cases the logarithmic measure is used to compare the quantity of interest with a reference value, often the smallest likely value of the quantity. Sometimes that reference can be an approximate or average value.

Most often in common acoustics, the decibel is used to compare the sound pressure level (SPL) in air with a reference pressure. The reference level for sound intensity (I), sound power level (PWL) and sound pressure in water are amongst others that are in common use:

Reference sound pressure (in air) = 0.00002 = 2E-5 Pa (rms) " " intensity = 0.000000000001 = 1E-12 W/m^2 " " power = 0.000000000001 = 1E-12 W " " pressure (water) = 0.000001 = 1E-6 PaAcousticians use the dB scale for the following reasons:
1) Quantities of interest often exhibit such huge ranges of variation that a dB scale is more convenient than a linear scale. For example, sound pressure radiated by a submarine may vary by eight orders of magnitude depending on direction; expression in linear units carries with it the confusion of the location of the decimal point. Decibels values are characteristically between only -999 to +999.
2) The human ear interprets loudness more easily represented with a logarithmic scale than with a linear scale.

Sound Pressure Level = 20 x lg (p/0.00002) dB

Often, the "maximum" level and sometimes the "peak" level of the sound being measured is quoted. During any given time interval the peak level will be numerically greater than the maximum level and the maximum level will be numerically greater than the (rms) sound pressure level;

peak>max>rms.

Unfortunately, human perception of loudness vis-a-vis frequency changes with loudness. When sound is very loud - 100 dB or more, the perception of loudness is more consistent across the audible frequency band. "B" and "C" Weightings reflect this trend. "B" Weighting is now little-used, but C-Weighting has achieved prominence in evaluating annoying community noises such as low frequency sound emitted by artillery fire and outdoor rock concerts. C-Weighting is also tabulated in 8.2.

The first electrical sound meter was reported by George W Pierce in Proceedings of the American Academy of Arts and Sciences, v 43 (1907-8) A couple of decades later the switch from horse-drawn vehicles to automobiles in cities led to large changes in the background noise climate. The advent of "talkies" - film sound - was a big stimulus to sound meter patents of the time, but there was still no standard method of sound measurement. "Noise" (unwanted sound) became a public issue.

The first tentative standard for sound level meters (Z24.3) was published by the American Standards Association in 1936, sponsored by the Acoustical Society of America. The tentative standard shows two frequency weighting curves "A" and "B" which were modeled on the response of the human ear to low and high levels of sound respectively.

With the coming of the Walsh-Healy act in 1969, the A-Weighting of sound was defacto presumed to be the "appropriate" weighting to represent sound level as a single number (rather than as a spectrum). With the advent of US FAA and US EPA interests in the '70's, the dBA metric was also adapted by them. (Along with the dBA metric has come an associated shortfall in precision in accurately presenting the capacity of a given sound to produce hearing loss and the capacity to create annoyance.)

[Editor's Note: A single number metric such as dBA is more easily understood by legal and administrative officials, so that promulgation, enforcement and administrative criteria and actions are understandable by more parties, often at the expense of a more precise comprehension and engineering action capability. For instance, enforcement may be on a dBA basis, but noise control design demands the octave-band or even third-octave band spectral data metric.]

The most commonly referenced weighting is "A-Weighting" dB(A), which is similar to that originally defined as Curve "A" in the 1936 standard. "C-Weighting" dB(C), which is used occasionally, has a relatively flat response. ""U-Weighting"" is a recent weighting which is used for measuring audible sound in the presence of ultrasound, and can be combined with A-Weighting to give AU-Weighting. The A-Weighting formula is given in section 8 of this FAQ file.

In addition to frequency weighting, sound pressure level measurement can be time-weighted as the "Fast", "Slow" or "Impulse" response. Measurements of sound pressure level with A-Weighting and fast response are also known as the "sound level".

Many modern sound level meters can measure the average sound energy over a given time. this metric is called the "equivalent continuous sound level" (L sub eq). More recently, it has become customary in some circles to presume that this sound measurement was A-Weighted if no weighting descriptor is listed.

Combined sound level = 10 x lg ( 10^(62/10) + 10^(73/10) )

= 73.3 dB

Note: for two different sounds, the combined level cannot be more than 3 dB above the higher of the two sound levels. However, if the sounds are phase related ("correlated") there can be up to a 6dB increase in SPL.

The basilar membrane is wider at its apex than at its base near the oval window; the cochlea tapers towards its apex. Groups of the delicate hair sensors on the membrane, which membrane varies 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. Refer to the Tinnitus FAQ for more information on associated hearing disorders.

The safety aspects of ultrasound scans are the subject of ongoing investigation. One metric that has been expressed is that exposure to ultrasound should not exceed 85dB in the 16kHz octave band.

Health hazards also result from extended exposure to vibration. An example is "white finger" disease, which is found amongst workers who frequently use hand-held machinery such as chain saws.

*For a plane wave, the sound power that passes through a surface of A square meters is defined as the ratio of the pressure squared to the air impedance

I = p^2/(rho*c)

When combined with the propagation unit vector, this defines the rate of sound energy transmitted in a specified direction per unit area normal to the direction. When measured in practical units, we can compute intensity after the relation that

Numerically, the sound intensity is related to the sound power as follows: In free air space, a source emitting Lw dB re 1 picowatt produces the sound pressure level Lp at a distance R feet as

Lp=Lw-20logR-0.6

At a one foot radius, that sound power is distributed over a surface of 4*pi = 12.57 square feet or (*.3048^2=.0920*) 1.17 square meters. 10log1.17=0.7dB. So within 0.1 dB, the coincidence exists that the sound intensity in picowatts per square meter is numerically equal to the sound pressure level in dB!

NOTE: This identity holds true only when the impedance, rho*c is exactly 400 mks rayls. This occurs for sea-level at 39 degrees C. For 22 C, rho*c = 412; a 0.13 dB difference arises. But at higher elevations, air density decreases for a given temperature. At an elevation of 840 feet above sea level, rho*c reduces to 400 at 22 C. (fortunate for much of Midwestern US!). The 0.13 dB difference at sea level is not usually significant for acoustical measurements.

Sound intensity meters are popular for determining the quantity and location of sound energy emission.

I = Io/D^2

This is relatively simple to reliably calculate, provided the source is small and outdoors where no echoes occur. (But indoor calculations in a reverberant field are rather more complex. )

If the noise source is outdoors and its dimensions are small compared with the distance to the monitoring position (ideally a point source), then as the sound energy is radiated it will spread over an area which is proportional to the square of the distance. This is an 'inverse square law' where the sound level will decline by 6dB for each doubling of distance.

Line noise sources such as a long line of moving traffic will radiate noise in cylindrical pattern, so that the area covered by the sound energy spread is directly proportional to the distance and the sound will decline by 3dB per doubling of distance.

Close to a source (the near field) the change in SPL will not follow the above laws because the spread of energy is less, and smaller changes of sound level with distance should be expected.

If the observation position very close to the source, at a distance that is small compared to the size of the source, the sound level changes very little with location in that source area. One may be able to determine the "virtual center" of the whole sound field, whence inverse square law calculations can proceed in reference to that distance, for locations outside the source area.

The surrounding environment, especially close to the ground, and in the presence of wind & vertical temperature gradients, has a great effect on the sound received at a distant location. Ground reflection affects sound levels more than a few feet away (distances greater than the height of the sound source or the receiver above the ground). Wind and air temperature gradients affect all sound propagation beyond 100 meters over the surface of the earth. Sound propages well downwind (traveling with the wind), and very lirrle upwind. When the ground surface is cooler than the air just above it ("inversion"), typically late at night and just before dawn, sound will travel great ditances across the landscape even without any wind.

In addition it is always necessary to take into account attenuation due to the absorption of sound by the air, which may be substantial at higher frequencies. For ultrasound, air absorption may well be the dominant factor in the reduction.

3] Vibration

It is sometimes more useful to know the vibrational velocity or displacement. Often, moving coil transducers are used to directly measure vibrational velocity. A single integration of that signal provides a measure of displacement.

If only an accelerometer is available, it is necessary to integrate the acceleration signal once for velocity and twice for displacement. If the vibration is sinusoidal at a known frequency, f, then an integration is calculated by dividing the original by 2 x pi x f (noting that there is also an associated phase change).

Example: A machine is vibrating sinusoidally at 79.6 Hz with an rms acceleration of 10 m/s^2.

Its rms velocity is therefore 10/(2 x pi x 79.6) = 20 mm/s
Its rms displacement is 10/(4 x pi^2 x 79.6^2) = 0.04 mm

The final result may also be expressed in terms of zero-to-peak, which is found as the square root of two [sqrt(2)] times the rms value. The peak-to-peak value is twice again that.

Thus, one has three measures (acceleration, velocity, displacement) and three scales (rms, 0-p, p-p) totalling nine possible explicit measures of one and the same vibration. Couple that with three possible directions (E-W, N-S, up-down) one faces 27 separate possible values... and then there are inches, mils, microns and millimeters... Needless to say, one must be eternally vigilant and explicit in their vibration measurement and reporting nomenclature!

If the vibration is produced by a motor inside a machine, it is necessary that the natural frequency of the supporting system is well below frequency of motor oscillations (the forcing frequency). This is achieved by increasing the mass or decreasing the stiffness of the system as appropriate.

The method of vibration isolation is demonstrated with a weight held from a rubber band. If the band is moved up and down very slowly the suspended weight will move by the same amount. At resonance the weight will move much more and possibly in the opposite direction. But as the frequency of vertical movement is further increased, the weight will become almost stationary. Springs are more often used in compression than intension.

Important:-

Intuitive attempts to reduce vibration from machinery can sometimes instead aggravate the problem. This is especially true when care was originally taken to minimize vibration at the time of design, manufacture and installation.

Another method of vibration control is to cancel the forces involved using a Dynamic Vibration Absorber. Here an additional "tuned" mass-spring combination is added so that it exerts a force equal and opposite to the unwanted vibration. They are only appropriate when the vibration is of a fixed frequency.

Recently, "Active Vibration Control", using techniques akin to Active Noise Control has evolved. This senses the unwanted vibration of a structural member to produce a reversed phase signal to drive a transducer attached to the same member to counter the motion. In that way, for instance, the vibration of rolling wheels of a vehicle is prevented from being transmitted into the body of that vehicle through the chassis
4] Architectural & Building Acoustics

What is reverberation time?

The time for sound in a room to decay 60 decibels. Scientific work on room acoustics was pioneered by Wallace Clement Sabine 1868-1919 (see his Collected Papers on Acoustics, 1922). The reverberation time, T, is defined as the time taken for sound energy to decay in a room by a factor of one million in energy (60 dB). It is dependent on the room volume and the total amount of sound absorption contained in the room. In metric units

0.161 x room Volume T = ---------------------------------------------- sum of Surface areas x absorption coefficients

In US English units, dimensions are in feet and the constant is 0.049.

It can be determined on small material samples with an "impedance tube" or on large samples in a laboratory "reverberation room". The impedance tube evaluates sound absorption at normal incidence only, and produces absorption values that are sightly lower than those found in the reverberation room where the "Sabine coefficient" is measured over a wide range of incidence angles.

For the purposes of architectural design, the Sabine coefficient is preferred, though the normal incidence absorption may be used in the absence of any other information. Interestingly some absorbent materials are found to have a Sabine coefficient in excess of unity at higher frequencies. This is due to diffraction effects. Where this occurs the value can be taken at face value for small material patches and as 1.0 for very large absorbers (entire walls). The Odeon computer program includes a file of absorption coefficients.

What is the difference between sound absorption & sound insulation ?

There is often confusion between sound insulation and sound absorption.

Sound is absorbed when it encounters a material which will convert some or all of it into heat, or which allows it to pass through not to return. For this reason good sound absorbers do not of themselves make good sound insulators. Sound insulators rarely absorb sound. Sound absorbers contribute little to sound insulation. They are treated separately in sound control design.

Sound insulation prevents sound from traveling from one place to another, such as between apartments in a building, or to reduce unwanted external noise inside a concert hall. Heavy materials like concrete are the most effective materials for sound insulation - doubling the mass per unit area of a wall will improve its insulation by about 6dB. It is possible to achieve good insulation over most of the audio frequency range with less mass by instead using a double leaf partition (two independent walls separated by an airgap filed with a sound absorber).

Test procedures (e.g. ASTM E-90 in the lab and E336 in the field) generate a loud and consistent broadband spectrum of steady noise on one side of a partition or specimen of the material under test, 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 time 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.

A single-number rating of that range of noise reductions or transmission losses van be had by fitting them to a segmented curve.

In North America, this procedure is ASTM E413. The fitted range is from 125-4000 Hz. The value of that curve at 500 Hz is called the Noise Isolation Class (NIC) or Sound Transmission Class (STC) respectively. Internationally, ISO140-3 produces the noise reduction and transmission loss data in the same way. But the single number rating is according to ISO 717 which uses data in the 100-3150 Hz range. This single number rating is called "R'" and "R" respectively.

Similar methods are applied to impact ("footfall") noise (a problem in multifamily residential buildings). A standard tapping machine is used to hammer on the floor, lightly and steadily at the rate of 10 taps per second. The sound pressure level in the room below are measured. ASTM E492 and ISO 140-4 and 717 apply. (See ASTM e-33 Web Site .)

The method of noise insulation will depend on the exact situation; generalities are extremely difficult to devise. Situations are more often than not unique, depending on the nature of the building infrastructure that the architect or his informal successors have devised. More often than not, successful noise isolation improvement requires the advice of a competent and experiences person and at an early stage of the renovation. The following ideas may serve as initial guidelines.

When the noise is from an external source such as a main road it may be possible, if planning authorities permit, to screen with a noise barrier. These can be effective providing that the direct line of sight between traffic and house is concealed by the barrier.

The weak point for sound transmission to and from a building is most often via the windows. Double glazing will usually afford noticeably better protection than single glazing, but in areas of high external noise it might be preferable to have double windows with a large air gap (25 to 100 mm) and acoustic absorbent material on the perimeter reveal around that gap. For a few people, the resultant lower room background noise level can make noise transmitted through party walls more apparent. The fitting of new windows may reduce the level of air ventilation, and it will be vital to compensate for this, if necessary with by improving the noise insulation of certain party walls.

Noise through party walls can be reduced by the addition of a false wall. This is constructed from a layer of sound insulating material, commonly plasterboard, separated from the party wall by a large void containing acoustic quilting. The false wall must not be connected to the party wall because that would allow sound transmission paths. The quality of construction is an important consideration if optimal levels of attenuation are desired. It is advisable to contact an independent noise consultant before allowing any building works to commence.

Noise through party walls can be reduced by the addition of a false wall. This is constructed from a layer of sound insulating material, commonly plasterboard, separated from the party wall by a large void containing acoustic quilting. The false wall must not be connected to the party wall because that would allow sound transmission paths. The quality of construction is an important consideration if optimal levels of attenuation are desired. It is advisable to contact an independent noise consultant before allowing any building works to commence.