INTRODUCTION GENERAL SOUND POWER LEVEL Since 1991, Q-Tech, a Fantech company, has invested heavily in a continuous research and development program. ighlights of this program include the first published test data for a range of attenuators based on Australian sourced materials and the advent of the unique Q-Seal range of specialised attenuators. Q-Tech is not only the technical leader but also the marketing leader. Making acoustic technology more accessible and more easily understood is a priority. Q-Tech firsts include the computer selection program and the "Rapid System Analysis" which are both integrated into the Fantech CD. While Q-Tech has strong relationships with universities and CSIRO it is still independent. As you would expect of the market leader, Q-Tech has its own Insertion Loss Test rig, meeting the requirements of BS4718:1971. Such a facility shortens product development times dramatically. Q-Tech will continue to be involved in the latest acoustic technology and innovation and we will continue to provide our customers with products they can rely upon. The following pages, incorporating the original "Rapid System Analysis", will assist the user to determine whether a fan selection will require attenuation to satisfactorily meet the needs of the particular application. To proceed, it is necessary to understand the properties of, and differences between, Sound Power and Sound Pressure. 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 logarithmic scale is used for practicality; this provides us with a sound power range from 0 to 150 db, which is a lot more manageable. The sound power level is denoted as L w and is defined as:- ( sound power of source, W) L w =10log 10 ( reference power, 1pW) Where: W = Watts and pw = 10-12 Watts and is expressed in decibels, db SOUND PRESSURE LEVEL INFORMATION ON FAN NOISE TEST STANDARDS 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 1kz; 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: (s ound pressure, Pa) L p = 20 log 10-5 ( reference pressure, 2 10 Pa) Where noted in the product data pages within this catalogue fan noise levels are tested to BS848 Part 2: 1985 Fans for general purposes. Methods of noise testing. This test standard describes methods that may be applied to calculate the sound power level of fans. That is, the In-Duct method, the Reverberant Room method and the Free Field method. The sound pressure level of a product is measured using one of these test methods. A calculation is then used to convert the measured sound pressure levels to sound power levels. -22 NOISE CONTRORODUCTS FANTEC 2008
SILENCER INFORMATION STATIC INSERTION LOSSES DYNAMIC INSERTION LOSSES AIRWAY VELOCITY TYPICAL APPLICATIONS AND BENEFITS OF SILENCER TYPES 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. Fantech test attenuators 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 VAC system, the static insertion losses and dynamic insertion losses are virtually identical and can be assumed to be the same. For a given attenuator size a higher air flow results in a higher airway passage velocity. igher 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. Approx. NR25 Do not exceed 8 m/s In attenuator airway NR30 10 m/s NR35 13 m/s NR40 15 m/s NR45 18 m/s Critical noise level application should be checked by an acoustics engineer Model Application Benefits Small Circular Type Attenuators Bathroom and Toilet exhaust fans Lightweight CC Tenancy fit outs Low cost Apartment fans Semi-Flexible Circular & Rectangular Attenuators Car park exhaust fans Circular: Easy fitting C./C.P Return Air fans Circular Open: Low pressure drop & Swimming Pools Circular Pod: igh performance RT/RS Kitchen Exhausts Smoke Spill fans Rectangular: igh performance Cross-talk Attenuators Room to room air transfer ducts CS/T/U/Z Police stations Office areas Different designs to suit a wide range of wall/roof configurations Sound Bar Acoustic Louvres SBL1/2 Plant rooms Short lengths Weatherproof FANTEC 2008 NOISE CONTRORODUCTS -23
NOISE RATINGS db(a) LEVELS The ear responds not only to the absolute sound pressure level of a sound, but also to its 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. 140 jet aircraft taking off (25 metres) Deafening 130 120 threshold of pain rock concert (front row) 110 sheet metal shop (hand grinding) Very Noisy 100 90 jack hammer (1 metre) lawn mower, heavy trucks (6 metres) 80 electric drill (1½ metres), busy street Noisy 70 60 loud radio (in average domestic room) busy general office, restaurant 50 normal speech, general office Quiet 40 quiet office 30 quiet bedroom, whisper Very Quiet 20 10 still day in the country away from traffic, tap dripping 0 threshold of hearing DECIBELS db(a) Figure 1. owever the ear is not as sensitive to lower frequency sound pressure levels as it is to higher frequency sound pressure levels. In the 1930 s, experiments were carried out on 11 people by arvey Fletcher at the Bell Telephone Laboratories in New York to determine how loud tones of different frequencies sounded subjectively. 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. The A frequency weighting corrections are shown below. Octave Band Centre Frequency, z 63 125 250 500 1000 2000 4000 8000 A frequency weighting corrections -26-16 -9-3 0 +1 +1-1 The A frequency weighting suggests that if a tone of 40 db is played at 1000 z, a 40 db tone played at 63 z would sound 26 db quieter, or be 14 db(a). Due to its simplicity and convenience, the A frequency weighting has become popular and is now used for many different noise sources at different levels. In fact, most legislation regarding noise is written using db(a)s, in addition nearly all manufacturers of fans and other noise generating machines quote their noise levels in db(a)s at 1, 1.5, or 3 metres assuming spherical distribution. It is therefore important that we understand the A frequency weighting and how db(a)s are calculated. -24 NOISE CONTRORODUCTS FANTEC 2008
CALCULATING db(a) LEVELS Published db(a), or A frequency weighted, sound pressure levels are theoretical values. These are, in fact, calculated from the sound power level data and are quoted at a specified distance i.e. 1, 1.5, or 3 metres. For example, using the Fantech model AP0804AP10/23 (duty 7000 L/s @ 80 Pa, inlet side), by applying an A frequency weighting correction to the fan sound power levels for each frequency and then logarithmically adding the values from left to right the resultant overall sound power level for this unit will be 98 db(a). A further calculation is required to convert this value from the A weighted sound power level to an A weighted sound pressure level at a prescribed distance from the noise source i.e. 77 db(a) @ 3m. See below for a detailed example of this calculation. db(a) CALCULATION EXAMPLE 1. A weighting corrections In-duct Sound Power Levels, L w db re 1pW Frequency (z) 63 125 250 500 1k 2k 4k 8k AP0804AP10/23 Inlet 94 88 95 94 94 91 88 80 'A' frequency weighting correction -26-16 -9-3 0 +1 +1-1 db(a) Sound Power level 68 72 86 91 94 92 89 79 2. Calculating an overall sound level For each sound power level: a. Calculate the difference between the sound power level and the sub total. b. Use Figure 17 on page -35, to determine the value to add. c. Add the value to add to the highest of the sub total and the sound power level. Octave Sound Sub Total Freq. z Power Level Difference Add Sum - 63 68 - - 68 68 125 72 4 1.5 73.5 73.5 250 86 12.5 0.2 86.2 86.2 500 91 4.8 1.2 92.2 92.2 1k 94 1.8 2.2 96.2 96.2 2k 92 4.2 1.4 97.6 97.6 4k 89 8.6 0.6 98.2 98.2 8k 79 19.2 0 98.2 98.2 db(a) is rounded to 98 db(a). 3. Converting Sound Power to Sound Pressure To convert this 'A' weighted sound "power" level to an 'A' weighted sound "pressure" level (which is calculated for a specified distance from the source) the following equation is used: Where: =L -20log d -11 L w = Sound Power Level re 10-12 W (db) L p = Sound Pressure Level re 20µPa (db) d = Distance from fan in metres (m) W Therefore, to determine the db(a) sound pressure level at a distance of 3m: Note that the above calculation assumes that the fan behaves as a point source of noise, that the noise radiates in all directions equally, and that no reflected sound is present. 10 = 98-20 log 3-11 10 =98-10.5-11 =98-21 = 77 db(a) @ 3m FANTEC 2008 NOISE CONTRORODUCTS -25
A CAUTIOUS WORD ON TE USE OF db(a) LEVELS The db(a) sound pressure level is used almost universally to describe the noise level of many items of noise emitting machinery. owever, published db(a) sound pressure levels should be used for comparative purposes only, they are not designed to reflect actual installed noise levels. The assumptions that are used to calculate the db(a) are rarely replicated in real life situations and, therefore, published db(a) values will not necessarily represent the actual noise levels that may be experienced on site. In order to determine the actual db(a) sound pressure level that may be expected from an installation, an acoustic analysis of the system, using sound power levels and taking into account the surrounding acoustic environment, should be performed. NR LEVELS 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 -37) 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 -37) As an example, using the following sound power levels, a graph can be drawn on an excerpt of the Noise Rating Cures shown on page -37. Frequency, z 63 125 250 500 1000 2000 4000 8000 db(a) @ 3m Sound Pressure Level 52 47 49 40 35 30 32 30 44 60 Noise Rating Curves 60 Octave band sound pressure level, db 50 55 50 40 45 40 30 35 63 125 250 500 1k 2k 4k 8k Octave band centre frequency, z For this example, NR40 clears all of the sound power levels, and is therefore the equivilant NR level. 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 -27 may be used. -26 NOISE CONTRORODUCTS FANTEC 2008
Recommended design sound level, db(a) Environment NR Curve Satisfactory Maximum General open Offices, reception areas 40 40 45 Conference rooms 30 30 40 Executive Offices 35 35 40 Office Buildings Foyers 45 Public Areas 45 40 50 Computer rooms 45 45 50 Undercover Carparks 50-60* 55 65 ospital wards 35 35 40 Intensive care wards, operating theatres 30 40 45 Laboratories 40 45 50 ospital Casualty areas 40 40 45 Kitchens, sterilising and service areas 45 50 55 Surgery, dental clinics and consulting rooms 40 40 45 Waiting rooms and reception areas 45 40 50 Classrooms 35 35 45 Lecture theatres without speech reinforcement 30 30 35 Lecture theatres with speech reinforcement - 35 45 Conference rooms 30 35 40 Assembly halls up to 250 seats 30 30 40 Schools Assembly halls over 250 seats 30 30 35 Recreation halls 40 - - Gymnasiums 40 45 55 Laboratories (Working) 45 40 50 Engineering workshops 45 50 60 Music practice rooms / office areas 40 40 45 Toilets, changing rooms and showers 45 45 55 Radio and T.V. studios T.V. recording studios 20-25 Note 1* Note 1* Audience studios 30 - - Concert and opera halls 25 Note 1 Note 1 Music practice rooms 30 - - Auditoriums and Music alls Cabarets and theatre restaurants - 35 40 Lecture alls 30 - - Lobbies 40 - - Dining rooms 40 40 45 Restaurants 40 45 50 otels / Motels Sleeping areas near major roads 35 35 40 Sleeping areas near minor roads 35 30 35 Kitchens and laundries 45 45 55 Bars and lounges 45 45 50 Shop Buildings Supermarkets 50 50 55 Shopping malls 45 45 55 Public buildings Municipal building administrative offices 35 35 40 Library reading areas 35 40 45 Billiards and snooker rooms 45 40 45 All other indoor sports with coaching - 45 50 Indoor sports buildings All other indoor sports without coaching - 50 55 Gymnasiums, squash courts and bowling alleys 50 - - Swimming pools 55 - - Toilets 45 50 55 General Service areas for all buildings Corridors 45 45 50 Plant rooms 70 - - Reproduced with permission from the Australian Institute of Refrigeration, Air Conditioning and eating (Inc.). The NR data is extracted from the andbook which is available from the AIRA Office at Level 7, 1 Elizabeth Street, Melbourne, Victoria, 3000. More complete db(a) data is available from AS/NZS 2107:2000 "Acoustics - Recommended design sound levels and reverberation times for building interiors." Note 1: Specialist advice should be sought for these spaces. * Added by Fantech FANTEC 2008 NOISE CONTRORODUCTS -27