Noise Control  General Information 

Introduction 

SOUND POWER LEVEL  
The sound power is defined as the rate at which a sound source emits energy. Since sound energy in everyday situations ranges from 10^{12 }Watts to 1000 Watts, a logarthmic scale is used for practicality; this provides us with a sound power range form 0 to 150 dB, which is a lot more manageable.  
The sound power level is denoted as L_{w }and is defined as:  
L_{w} = 10log_{10} (Sound power of source, W) (reference power, 1pW) and is expressed in decibels, dB 

Where: W=Watts and pW = 10_{12} Watts 

SOUND PRESSURE LEVEL  
The sound pressure is what you actually hear and is the effect of the sound power in the hearing environment. It will be a function of the volume of the space, its acoustic absorption qualities and the distance of the listener from the sound source.  
Sound pressure level is also expressed in dB and is relative to the quietest sound which a healthy young person can hear at 1kHz; 2 x 10^{5} N/m^{2} (or Pa)  
The sound pressure level, like sound power is expressed on a logarithmic scale and denoted as L_{p}. It is defined as:  
L_{p} = 20log_{10} (Sound pressure, Pa) ference pressure, 2 x 10^{5} Pa) 

STATIC INSERTION LOSSES  
BS 4718 : 1971 "Methods of Test for Silencers for Air Distribution Systems" requires manufacturers to test and publish static insertion loss figures.  
An insertion loss is defined as "the reduction in noise level at a given location due to the placement of a silencer in the sound path between the sound source and that location". A static insertion loss is the insertion loss with no airflow passing through the silencer.  
Therefore placing a silencer in between a fan and the measuring position, will reduce the noise level at the measuring position by the insertion loss.  
DYNAMIC INSERTION LOSSES  
Attenuators are tested to BS4718: 1971 "Methods of Test for Silencers for Air Distribution Systems". This test standard sets out a procedure for the testing of static insertion losses; i.e. the measuring of insertion losses without airflow.  
Some overseas companies publish dynamic insertion losses; that is the testing of insertion losses with airflow involved. At higher passage velocities the static insertion loss can vary from the dynamic insertion loss by a small margin, depending on the direction of the airflow compared to the noise propagation direction.  
For typical velocities associated with a HVAC system, the static insertion losses and dynamic insertion losses are virtually identical and can be assumed to be the same.  
AIRWAY VELOCITY  
For a given attenuator size a higher air flow results in a higher airway passage velocity. Higher passage velocities will increase the regenerated noise level of the attenuator. This is particularly critical when the attenuator is serving a low noise level zone; i.e. film studio. A number of suggested maximum passage velocities with the appropriate room NR level are tabulated. Critical noise applications should be checked by an Acoustics Engineer.  


Critical noise level application should be checked by an acoustics engineer  
TYPICAL APPLICATIONS AND BENEFITS OF SILENCER TYPES  


NRCURVES AND dB(A) LEVELS  
The ear responds not only to the absolute sound pressure level of a sound, but also to it's frequency content. It actually gives a weighting to the level of sound according to its frequency content, and ascribes a certain loudness. This means that if we want to know how a person will judge the sound, we must somehow translate our objective measured units of sound pressure level and frequency content into subjective units of loudness.  
A sound level meter accepts all of the frequency components of a sound, and adds all their absolute levels together to give an overall sound pressure level, dB (Linear).  
Figure 1 shows typical overall sound pressure levels produced by some everyday sources.  
However the ear is not as sensitive to lower frequency sound pressure levels as it is to higher frequency sound pressure levels. Therefore the "A" weighting (or the "A" in dB(A) was devised so that the sound meter would filter each frequency of sound by a certain amount before adding them together to give a loudness that more closely follows the sensitivity of the human ear.  
While measuring with the "A" weighting is a convenient method of estimating loudness, at certain times we need more information than this single figure can give us.  
The dB(A) tells us virtually nothing about the sound's frequency content. Is the noise too high over the whole frequency spectrum, or are there just one or two frequency components which are excessive? Is the noise problem due to a tonal component which stands out above the general noise level?  
Therefore, to try and help with these deficiencies, a NR curve is used in Australia (while in New Zealand PNC curves are often used). The NR curve is a series of Octave Band frequency curves (as shown on Figure 18, page G33) on which the octave band spectrum of the noise in question is plotted on the same grid. The NR level of the noise is the highest NR curve touched. This system lets the engineer know which frequencies need to be attenuated to achieve a certain NR curve. (PNC curves are shown on Figure 19, page G33)  
Therefore both the dB(A) and NR curves are subjective units which give a representation of how the ear actually assesses noise, although work is currently being done to develop more accurate representations.  
For some suggested limiting values for both dB(A) and NR levels, the table on page G32 may be used. 