- Email: [email protected]
Contracting acoustics
Journal of Sound and Vibration (1974) X(3),
444-449
CONTRACTING
ACOUSTICS-t
This letter covers the subject of the evaluation of tenders for both noise control products, and items with acoustic features which are evaluated at the tender stage of a contract. Examples taken from different fields show that, at the present time, there is a need for manufacturers of noise control products, etc.,, to understand normal tendering procedure. Such procedures are reviewed in the first part of this note.
1.
INTRODUCTION
: TENDERING
PROCEDURES
It would appear that few firms are in a position to tender for the supply of noise control equipment because most of the acoustic industry does not seem to recognize the purpose of a tender. At the present time many manufacturers have their own consultancy or advisory service to sell or back their products. Their customers may be satisfied with this arrangement but the fact remains some manufacturers are unable to provide an efficient service for the design engineer who needs tried and documented products rather than “specials”. The “package” contract (in which a promoter states his requirements only in broad terms and invites submissions of tenders for design, construction, installation, and sometimes commissioning of an item) is often used at present in the acoustic industry : indeed one preferred firm may be asked to offer a price for such a contract without reference to any other companies. We do not consider this to be a good practice in financial terms and because the best, or even an adequate solution, may not have been advanced. The “negotiated” contract (where only one company is involved) can be justified in special cases of urgency or where a particular company has exceptional experience. These forms of contract will not be discussed further since the object of this letter is consideration of more normal forms of tendering. 1.1. Why use a tender First of all let us define why a tender is sought and what part the engineer plays in the tender action. The engineer will know what he is looking for but he may not know where to place an order for that item if he is to get both good performance and low price. The primary reason for a tender action is to allow the engineer to find out. 1.2. The role of the engineer The engineer will have a list of possible contractors who he believes can offer the service he requires and he will constantly update his list as the “market place” changes. Representatives informing him of new products, technical information showing the performance, site visits to installed items to prove the various claims, are all ways in which the engineer is constantly reviewing the market. It must be noted that if the engineer has good reasons for assuming the “market place” not to have changed then he has no reason to resort to a tender action and
t In preparing this letter the author has drawn heavily from his experiences at Atkins Research and Development (W. S. Atkins Group) and it is a pleasure to acknowledge permission of the company to publish such material. 444
LETTERS
TO THE EDITOR
445
will recommend that an order is placed with the company he knows will give him the best service. Useless tendering does not help anyone since in the long run it must increase overheads and therefore prices. The open tender and inviting too many firms to tender are not recommended for the same reason. 1.3. Comparison of products This can only be achieved by comparing like with like; hence it is required that all tenders are given in the one form stipulated by the engineer. However, if a company wishes to submit a secondary tender, which carries modifications to the original form, it will not be penalized for this, but if this secondary tender is submitted it must be accompanied by sufficient information for the engineer to see all the implications of the proposed changes. Tenders will obviously be compared on the price and acoustic performance but often on many other things such as experience and proven design. Sometimes the acoustic performance of an item is only one part of a number of contract elements, e.g., structural requirements, airhandling performance or fire rating, and sometimes even the acoustic performance of an item might be described with as many as six parameters. It can be seen that correct choice is not a matter of black and white, and selection methods can become quite complex. i .4. Final selection The engineer will note all the pertinent facts, together with his reasons for his deciston and will present them to the promoter. He will also give reasons for rejecting tenders. Once an offer has been accepted, then the other applicants are quickly given notice that their quotations are unsuccessful in order not to prolong the work that they may be doing on the project since this abortive work only increases the overheads and hence the cost of hardware in general. The need for this rapid notice is perhaps highlighted by the difficulty of extracting from tenderers a full set of completed tender documents by the final selection date. not to mention the tender opening date.
2.
EXAMPLES
We present below some of the methods and experiences of Atkins Research and Development when dealing with products with acoustic features..They are written for the specialist in the particular fields although some anecdotes are included for the general reader. It will be seen from the examples that we cannot claim to be a model of tendering orthodoxy. The conditions existing in the noise control field preclude our acting in a “normal” manner. However, we would state that (a) we are not lessening the responsibility ofthe sub-contractors by our actions but are fulfilling our obligations as consultants, and (b) when deviating from normal practice we use the guideline of “what is best for our clients, both present and future”. 2.1. Constant colume boxes Five companies were listed as being able to supply the required number and range of C.V. boxes but incomplete “in-house” knowledge existed on the acoustic performance of their particular boxes. The manufacturers were asked for the relevant data, in-duct noise, casing radiated noise, insertion loss of the box, attenuation of a final attenuator and final regenerated noise after the attenuator, for the range of volumes and pressures that the boxes were to work at. It should be borne in mind that there is also a manufacturer’s choice of box size to be reckoned with. For some parameters we asked for the manufacturer’s figures whereas for others we specified limits not to be exceeded. Some firms were unable to supply data; others made no attempt to comply with our limits. Since some of the variables had to be fixed in
446
LETTERSTOTHEEDITOR
order to make a comparison of the boxes we were obliged to ask for a second tender from such firms. This is not normal practice but we believed it was in the best interests of our client to do so. The non-provision of data weighs heavily against the manufacturer in our scheme of assessing the merit of a particular tender, since an evaluation system is used where marks are awarded (e.g., high marks for low noise level) but non-submission of data scores zero. Two tables are set out below. Table 1 is a copy of a sheet for a particular volume flow, one of ten used in the full analysis. Table 2 gives the final scores. The full evaluation method is not given here; however the lesson of non-provision of data is evident.
Example
TABLE 1 of a tender analysis form
Selection parameters
Manufacturers
Selected flow (cfm 4500)
’ V Induct
W Casing radiated
X Insertion loss of
noise
noise
box
A B
0 5
0 5
2 5
0 3
C D E
4 3 2
0 0 4
0 4 3
4 0 5
zt , Final regenerated
Y Attenuator
noise
2:
2 18 8 7 14
t Note: Column Z is evaluated by a different scheme.
TABLET
Summation V
Manufacturers
sheet of tender analysis forms W
X
Y
A
23
0
18
0
B C D E
41 35 26 19
45 28 20 29
49 0 37 34
30 40 0 50
Z
CT; 41 165 103 83 130
Comment: Take out A and re-evaluate.
The comparison of different manufacturers’ figures for various specified box duties made an interesting study. Table 3 shows one column of our summary table, it is for box 56/13 with a required volume flow of 3180 cfm. It was from this summary table that we were able to inform one manufacturer that either his laboratory figures were wrong or he had a bad design of box. Here again the best interests of the client were served by advising the manufacturer of our findings and allowing time for him to make his own check. 2.2. Fans Since the noise of fans has been well researched and adequately documented and since the BS 848 deals with the measurement of noise from fans one would expect that fan manufacturers would be able to supply data on their products in a standard form. This is true for only a very few companies.
LETTERSTO THE EDITOR
447
TABLE 3 One coLw~m of the C. V. box summary table: V = 3 180 cf.1, CVB 56/l 3 Frequency (Hz) Manufacturer
Box size
AP
(in wg) -
h
r
\
500
I k
2k
60 57 67 71 70 88 78 73 Casing radiated noise No data 51 44 65 58 69 69 86 78 68 Insertion loss of the box 3 7 12 14 20 No data 9 16 4 6 14 Attenuator No data 28 34 3 12 No data 6 9 16 Final regenerated noise 54 44 38 29 Extra cost for a silencer Limit guaranteed 82 62 58
52 55 62 67 62
45 49 54 63 54
41 48 45 62 46
34 52 63 60
36 47 54 54
34 42 48 42
33 38 46 39
15 31
24 33
35 30
25 36
31 20
35 30
36 31
28
42 17
46 20
48 21
45 21
19
22
22
20
34 19
21 11
23 10
17
42
32
25
28
68
41
39
36
35
63
125
250
4k
Induct noise A B C D E
1.5 0.8 1.1 0.77 0.7
A B C D E A B C D E
0.8 1.o
0.5
1.0
A B C D E A B C D E Required limit
1.5 1.0
1.0 1.5 0.7 0.5
1.5 0.9
0.7
61
62
59
51
48 41 49
Note: 0.5 Box size means that two boxes will be used to achieve required duty.
When we invited five companies to tender for some axial fans and asked that they give noise data, some provided the data as PWL1, and others as SPL at 3 fan diameters. As a basis for comparison either measurement will suffice, but for a calculation of the silencing needed for a fan in a ventilation duct “in-duct PWL” is required or a method of converting SPL at 3 diameters to “in-duct PWL”. However, we then asked that all manufacturers supplied data in either or both forms if possible so that we might obtain five sets of comparable information irrespective of whether it were SPL or PWL. After much telephoning and cajoling the PWL table was completed first and our assessment was based on these figures. The parameters this time were PWL for eight octave bands for sixteen fans working at given duty points. The analysis showed that there was little difference in the noise of those fans produced by the better manufacturers. In fact we resorted to statistics and showed an apparent difference was not significant. 2.3. Silencers The important physical measures of the performance of a silencer, for a given flow rate through it, are its size, the pressure drop through it, the regenerated noise and the dynamic
448
LETTERS
TO THE EDITOR
insertion loss. Since all of these factors are inter-related it is best if some of them are fixed for the purpose of a tender action. Insertion loss is stated as a minimum number in the various octave bands. Pressure drop is stated (based on previously obtained manufacturers data) at a realistic level which should not be exceeded. Size is sometimes critical on a project and is fixed to within small limits. Regenerated noise data is not always called for in a tender but the companies that are tendering will have provided information on the regenerated noise of their series of silencers before being invited to tender. We find that some companies pay little attention to the desired insertion losses; in fact it is not unknown to ask a company to resubmit its tender and to pay particular attention to the insertion loss values. In one case over 50 % of the silencers selected by a manufacturer were not to specification. We usually find that pressure drop, being the resultant after fixing all the other parameters, is only likely to be of the right order rather than below a given limit. All that can be done here is to observe that no unexpected pressure drops occur that might show up suspect data, design, or manufacturing method and to see if one manufacturer has consistently higher or lower pressure drops than his competitors for much the same reason. Some manufacturers wish to submit their own variations on the desired sizes, etc., so that they might achieve the requisite amount of insertion loss at a lower price. We find that this is helpful but it is not often rewarded by a contract for the alternative quotation because of implications that only the designer can see. It should be pointed out that in the above case of re-submission of a tender because of insufficient insertion loss on 50 % of his silencers, the manufacturer thought that he knew best and went below the specification because he felt that the odd few dB would not be noticed in practice and this enabled him to offer keener prices. He may be correct, but we were intent on evaluating silencers with equal amounts of insertion loss and so had to ask for a re-quote or disqualify his original tender. Here again we were resorting to the argument of “what is best for the client” and gave the manufacturer the chance of re-submitting. 2.4. Acoustic tiles (supply andfix) An old and respected firm submitted a tender, but two months later wrote to explain that they had subsequently read the “invitation to tender” and could they now please change their tender specification and price. A specification for acoustic tiles is not complicated and the engineer rightly expects the manufacturers or sub-contractors to show that his installed product will meet the required values of absorption coefficient. When we sent out invitations to tender to some acoustic tile manufacturers, the manufacturers either sent the invitations on to their choice of sub-contractors or sent us a list of selected sub-contractors to whom we had to issue extra invitations to tender. It may seem improbable but the sub-contractors, although not understanding the specification sent to them, still submitted tenders. Since no one had met the specified requirements (this was judged not by claims to fulfil the requirements of the tender but by analysing the materials and airspace, where given, in the tenders and trade literature) much time was spent in meeting with a tile manufacturer (who was most probably to be selected) and his subcontractor to achieve a tile conformation that would meet our specification. The company finally accepted had their design independently checked to BS 3638 and this surely underlines the problem of so little expertise in the industry. The ratio of lowest to highest tender prices on this tiling contract was 1: 5 and probably due to the same reason. 2.5. Doors and windows (supply andfix) Doors and windows are perhaps the most difficult items to assess by tender action since low prices might be either because of inexperience or, conversely, a sure knowledge of how the job can be done.
LETTERS
449
TO THE EDITOR
One large company sent four men to meet an engineer with the suggestion of a “negotiated contract” for tens of thousands of pounds without submitting any details of how the work was to be done and only undertaking that “certainly some small changes to our design can be accommodated if this is really required by the engineer”. Needless to say that firm was not entertained for more than that day as a prospective sub-contractor. It is important for the contract to be one of “supply and fix” because of the division of responsibilities that would otherwise occur on site, with disputes over bad fixing of the doors, etc., being much more likely to occur ifthe main contractor is allowed to build-in the window or door. Nevertheless, the door or window contractor has all too often displayed an unfamiliarity with site conditions and work trades associated with their product: e.g., the quality and tolerances of modern brickwork, or acceptable standards of welding. Although there are a few notable exceptions we have found that it is difficult to obtain sufficiently detailed information about installation at the tender stage from most manufacturers and often they are apparently unaware of the finer points of acoustic design. The acoustic specification for a door or a window is simplicity itself (it will state that the door etc., must have an Average Sound Reduction Index (SRI) of XdB or wording similar to this) but behind this simplicity lies a most difficult design problem; a good acoustic door can be likened to precision engineering. We have found nineteen companies who say they will manufacture acoustic doors and six who will make acoustic windows, yet it seems that there is a distinct lack of proven products and ability to fix the doors and windows in their structural openings. We have had many novel designs or features submitted to us during our searches for doors and windows, but all were offered as possible solutions, none as a tried and tested remedy. No engineer is going to go out on to a limb with some novel idea if he can easily obtain a satisfactory standard solution at an equal or less cost. 3.
THE FUTURE
Many other items of plant are now receiving the attention of a “noise man” at the tender stage. Since it is often a requirement of the tender that noise figures are provided it should be stated that PWLlz in eight octave bands is the preferred method since the dB(A) cannot be used for design purposes and is therefore almost worthless for tender purposes. With future government legislation it will probably be mandatory for suppliers to certain industries to provide such figures. It can be foreseen that noisy plant will suffer a cost penalty (that of silencing the equipment) and that failure to provide the required information may well mean disqualification from a tender action. In the process industries it is now common practice to give a maximum level of noise not to be exceeded by a single item, or an ensemble, being provided by one contractor. That contractor is also required to show calculations to justify his statement that his equipment does not exceed his noise quota. The methods adopted by the construction engineers in the process industries show the way ahead, and the outlook for the acoustics contractor who is unprepared is very bleak. Contracts will only be let to those who can guarantee their noise levels or other acoustic parameters beforehand. Increased prices to cover the cost penalties incurred by the guarantee are valid but they are accepted as part of the price we pay for the quiet we desire and must be compared with the high cost of remedying an unsatisfactory situation. It always costs much more to put something right afterwards than to get it right in the first instance. 193 Barnetwood Lane, Ashtead, Surrey, England ( Received 14 January 1974, and in revised form 18 April 1974)
M. D. G.
RAVDALL
444-449
CONTRACTING
ACOUSTICS-t
This letter covers the subject of the evaluation of tenders for both noise control products, and items with acoustic features which are evaluated at the tender stage of a contract. Examples taken from different fields show that, at the present time, there is a need for manufacturers of noise control products, etc.,, to understand normal tendering procedure. Such procedures are reviewed in the first part of this note.
1.
INTRODUCTION
: TENDERING
PROCEDURES
It would appear that few firms are in a position to tender for the supply of noise control equipment because most of the acoustic industry does not seem to recognize the purpose of a tender. At the present time many manufacturers have their own consultancy or advisory service to sell or back their products. Their customers may be satisfied with this arrangement but the fact remains some manufacturers are unable to provide an efficient service for the design engineer who needs tried and documented products rather than “specials”. The “package” contract (in which a promoter states his requirements only in broad terms and invites submissions of tenders for design, construction, installation, and sometimes commissioning of an item) is often used at present in the acoustic industry : indeed one preferred firm may be asked to offer a price for such a contract without reference to any other companies. We do not consider this to be a good practice in financial terms and because the best, or even an adequate solution, may not have been advanced. The “negotiated” contract (where only one company is involved) can be justified in special cases of urgency or where a particular company has exceptional experience. These forms of contract will not be discussed further since the object of this letter is consideration of more normal forms of tendering. 1.1. Why use a tender First of all let us define why a tender is sought and what part the engineer plays in the tender action. The engineer will know what he is looking for but he may not know where to place an order for that item if he is to get both good performance and low price. The primary reason for a tender action is to allow the engineer to find out. 1.2. The role of the engineer The engineer will have a list of possible contractors who he believes can offer the service he requires and he will constantly update his list as the “market place” changes. Representatives informing him of new products, technical information showing the performance, site visits to installed items to prove the various claims, are all ways in which the engineer is constantly reviewing the market. It must be noted that if the engineer has good reasons for assuming the “market place” not to have changed then he has no reason to resort to a tender action and
t In preparing this letter the author has drawn heavily from his experiences at Atkins Research and Development (W. S. Atkins Group) and it is a pleasure to acknowledge permission of the company to publish such material. 444
LETTERS
TO THE EDITOR
445
will recommend that an order is placed with the company he knows will give him the best service. Useless tendering does not help anyone since in the long run it must increase overheads and therefore prices. The open tender and inviting too many firms to tender are not recommended for the same reason. 1.3. Comparison of products This can only be achieved by comparing like with like; hence it is required that all tenders are given in the one form stipulated by the engineer. However, if a company wishes to submit a secondary tender, which carries modifications to the original form, it will not be penalized for this, but if this secondary tender is submitted it must be accompanied by sufficient information for the engineer to see all the implications of the proposed changes. Tenders will obviously be compared on the price and acoustic performance but often on many other things such as experience and proven design. Sometimes the acoustic performance of an item is only one part of a number of contract elements, e.g., structural requirements, airhandling performance or fire rating, and sometimes even the acoustic performance of an item might be described with as many as six parameters. It can be seen that correct choice is not a matter of black and white, and selection methods can become quite complex. i .4. Final selection The engineer will note all the pertinent facts, together with his reasons for his deciston and will present them to the promoter. He will also give reasons for rejecting tenders. Once an offer has been accepted, then the other applicants are quickly given notice that their quotations are unsuccessful in order not to prolong the work that they may be doing on the project since this abortive work only increases the overheads and hence the cost of hardware in general. The need for this rapid notice is perhaps highlighted by the difficulty of extracting from tenderers a full set of completed tender documents by the final selection date. not to mention the tender opening date.
2.
EXAMPLES
We present below some of the methods and experiences of Atkins Research and Development when dealing with products with acoustic features..They are written for the specialist in the particular fields although some anecdotes are included for the general reader. It will be seen from the examples that we cannot claim to be a model of tendering orthodoxy. The conditions existing in the noise control field preclude our acting in a “normal” manner. However, we would state that (a) we are not lessening the responsibility ofthe sub-contractors by our actions but are fulfilling our obligations as consultants, and (b) when deviating from normal practice we use the guideline of “what is best for our clients, both present and future”. 2.1. Constant colume boxes Five companies were listed as being able to supply the required number and range of C.V. boxes but incomplete “in-house” knowledge existed on the acoustic performance of their particular boxes. The manufacturers were asked for the relevant data, in-duct noise, casing radiated noise, insertion loss of the box, attenuation of a final attenuator and final regenerated noise after the attenuator, for the range of volumes and pressures that the boxes were to work at. It should be borne in mind that there is also a manufacturer’s choice of box size to be reckoned with. For some parameters we asked for the manufacturer’s figures whereas for others we specified limits not to be exceeded. Some firms were unable to supply data; others made no attempt to comply with our limits. Since some of the variables had to be fixed in
446
LETTERSTOTHEEDITOR
order to make a comparison of the boxes we were obliged to ask for a second tender from such firms. This is not normal practice but we believed it was in the best interests of our client to do so. The non-provision of data weighs heavily against the manufacturer in our scheme of assessing the merit of a particular tender, since an evaluation system is used where marks are awarded (e.g., high marks for low noise level) but non-submission of data scores zero. Two tables are set out below. Table 1 is a copy of a sheet for a particular volume flow, one of ten used in the full analysis. Table 2 gives the final scores. The full evaluation method is not given here; however the lesson of non-provision of data is evident.
Example
TABLE 1 of a tender analysis form
Selection parameters
Manufacturers
Selected flow (cfm 4500)
’ V Induct
W Casing radiated
X Insertion loss of
noise
noise
box
A B
0 5
0 5
2 5
0 3
C D E
4 3 2
0 0 4
0 4 3
4 0 5
zt , Final regenerated
Y Attenuator
noise
2:
2 18 8 7 14
t Note: Column Z is evaluated by a different scheme.
TABLET
Summation V
Manufacturers
sheet of tender analysis forms W
X
Y
A
23
0
18
0
B C D E
41 35 26 19
45 28 20 29
49 0 37 34
30 40 0 50
Z
CT; 41 165 103 83 130
Comment: Take out A and re-evaluate.
The comparison of different manufacturers’ figures for various specified box duties made an interesting study. Table 3 shows one column of our summary table, it is for box 56/13 with a required volume flow of 3180 cfm. It was from this summary table that we were able to inform one manufacturer that either his laboratory figures were wrong or he had a bad design of box. Here again the best interests of the client were served by advising the manufacturer of our findings and allowing time for him to make his own check. 2.2. Fans Since the noise of fans has been well researched and adequately documented and since the BS 848 deals with the measurement of noise from fans one would expect that fan manufacturers would be able to supply data on their products in a standard form. This is true for only a very few companies.
LETTERSTO THE EDITOR
447
TABLE 3 One coLw~m of the C. V. box summary table: V = 3 180 cf.1, CVB 56/l 3 Frequency (Hz) Manufacturer
Box size
AP
(in wg) -
h
r
\
500
I k
2k
60 57 67 71 70 88 78 73 Casing radiated noise No data 51 44 65 58 69 69 86 78 68 Insertion loss of the box 3 7 12 14 20 No data 9 16 4 6 14 Attenuator No data 28 34 3 12 No data 6 9 16 Final regenerated noise 54 44 38 29 Extra cost for a silencer Limit guaranteed 82 62 58
52 55 62 67 62
45 49 54 63 54
41 48 45 62 46
34 52 63 60
36 47 54 54
34 42 48 42
33 38 46 39
15 31
24 33
35 30
25 36
31 20
35 30
36 31
28
42 17
46 20
48 21
45 21
19
22
22
20
34 19
21 11
23 10
17
42
32
25
28
68
41
39
36
35
63
125
250
4k
Induct noise A B C D E
1.5 0.8 1.1 0.77 0.7
A B C D E A B C D E
0.8 1.o
0.5
1.0
A B C D E A B C D E Required limit
1.5 1.0
1.0 1.5 0.7 0.5
1.5 0.9
0.7
61
62
59
51
48 41 49
Note: 0.5 Box size means that two boxes will be used to achieve required duty.
When we invited five companies to tender for some axial fans and asked that they give noise data, some provided the data as PWL1, and others as SPL at 3 fan diameters. As a basis for comparison either measurement will suffice, but for a calculation of the silencing needed for a fan in a ventilation duct “in-duct PWL” is required or a method of converting SPL at 3 diameters to “in-duct PWL”. However, we then asked that all manufacturers supplied data in either or both forms if possible so that we might obtain five sets of comparable information irrespective of whether it were SPL or PWL. After much telephoning and cajoling the PWL table was completed first and our assessment was based on these figures. The parameters this time were PWL for eight octave bands for sixteen fans working at given duty points. The analysis showed that there was little difference in the noise of those fans produced by the better manufacturers. In fact we resorted to statistics and showed an apparent difference was not significant. 2.3. Silencers The important physical measures of the performance of a silencer, for a given flow rate through it, are its size, the pressure drop through it, the regenerated noise and the dynamic
448
LETTERS
TO THE EDITOR
insertion loss. Since all of these factors are inter-related it is best if some of them are fixed for the purpose of a tender action. Insertion loss is stated as a minimum number in the various octave bands. Pressure drop is stated (based on previously obtained manufacturers data) at a realistic level which should not be exceeded. Size is sometimes critical on a project and is fixed to within small limits. Regenerated noise data is not always called for in a tender but the companies that are tendering will have provided information on the regenerated noise of their series of silencers before being invited to tender. We find that some companies pay little attention to the desired insertion losses; in fact it is not unknown to ask a company to resubmit its tender and to pay particular attention to the insertion loss values. In one case over 50 % of the silencers selected by a manufacturer were not to specification. We usually find that pressure drop, being the resultant after fixing all the other parameters, is only likely to be of the right order rather than below a given limit. All that can be done here is to observe that no unexpected pressure drops occur that might show up suspect data, design, or manufacturing method and to see if one manufacturer has consistently higher or lower pressure drops than his competitors for much the same reason. Some manufacturers wish to submit their own variations on the desired sizes, etc., so that they might achieve the requisite amount of insertion loss at a lower price. We find that this is helpful but it is not often rewarded by a contract for the alternative quotation because of implications that only the designer can see. It should be pointed out that in the above case of re-submission of a tender because of insufficient insertion loss on 50 % of his silencers, the manufacturer thought that he knew best and went below the specification because he felt that the odd few dB would not be noticed in practice and this enabled him to offer keener prices. He may be correct, but we were intent on evaluating silencers with equal amounts of insertion loss and so had to ask for a re-quote or disqualify his original tender. Here again we were resorting to the argument of “what is best for the client” and gave the manufacturer the chance of re-submitting. 2.4. Acoustic tiles (supply andfix) An old and respected firm submitted a tender, but two months later wrote to explain that they had subsequently read the “invitation to tender” and could they now please change their tender specification and price. A specification for acoustic tiles is not complicated and the engineer rightly expects the manufacturers or sub-contractors to show that his installed product will meet the required values of absorption coefficient. When we sent out invitations to tender to some acoustic tile manufacturers, the manufacturers either sent the invitations on to their choice of sub-contractors or sent us a list of selected sub-contractors to whom we had to issue extra invitations to tender. It may seem improbable but the sub-contractors, although not understanding the specification sent to them, still submitted tenders. Since no one had met the specified requirements (this was judged not by claims to fulfil the requirements of the tender but by analysing the materials and airspace, where given, in the tenders and trade literature) much time was spent in meeting with a tile manufacturer (who was most probably to be selected) and his subcontractor to achieve a tile conformation that would meet our specification. The company finally accepted had their design independently checked to BS 3638 and this surely underlines the problem of so little expertise in the industry. The ratio of lowest to highest tender prices on this tiling contract was 1: 5 and probably due to the same reason. 2.5. Doors and windows (supply andfix) Doors and windows are perhaps the most difficult items to assess by tender action since low prices might be either because of inexperience or, conversely, a sure knowledge of how the job can be done.
LETTERS
449
TO THE EDITOR
One large company sent four men to meet an engineer with the suggestion of a “negotiated contract” for tens of thousands of pounds without submitting any details of how the work was to be done and only undertaking that “certainly some small changes to our design can be accommodated if this is really required by the engineer”. Needless to say that firm was not entertained for more than that day as a prospective sub-contractor. It is important for the contract to be one of “supply and fix” because of the division of responsibilities that would otherwise occur on site, with disputes over bad fixing of the doors, etc., being much more likely to occur ifthe main contractor is allowed to build-in the window or door. Nevertheless, the door or window contractor has all too often displayed an unfamiliarity with site conditions and work trades associated with their product: e.g., the quality and tolerances of modern brickwork, or acceptable standards of welding. Although there are a few notable exceptions we have found that it is difficult to obtain sufficiently detailed information about installation at the tender stage from most manufacturers and often they are apparently unaware of the finer points of acoustic design. The acoustic specification for a door or a window is simplicity itself (it will state that the door etc., must have an Average Sound Reduction Index (SRI) of XdB or wording similar to this) but behind this simplicity lies a most difficult design problem; a good acoustic door can be likened to precision engineering. We have found nineteen companies who say they will manufacture acoustic doors and six who will make acoustic windows, yet it seems that there is a distinct lack of proven products and ability to fix the doors and windows in their structural openings. We have had many novel designs or features submitted to us during our searches for doors and windows, but all were offered as possible solutions, none as a tried and tested remedy. No engineer is going to go out on to a limb with some novel idea if he can easily obtain a satisfactory standard solution at an equal or less cost. 3.
THE FUTURE
Many other items of plant are now receiving the attention of a “noise man” at the tender stage. Since it is often a requirement of the tender that noise figures are provided it should be stated that PWLlz in eight octave bands is the preferred method since the dB(A) cannot be used for design purposes and is therefore almost worthless for tender purposes. With future government legislation it will probably be mandatory for suppliers to certain industries to provide such figures. It can be foreseen that noisy plant will suffer a cost penalty (that of silencing the equipment) and that failure to provide the required information may well mean disqualification from a tender action. In the process industries it is now common practice to give a maximum level of noise not to be exceeded by a single item, or an ensemble, being provided by one contractor. That contractor is also required to show calculations to justify his statement that his equipment does not exceed his noise quota. The methods adopted by the construction engineers in the process industries show the way ahead, and the outlook for the acoustics contractor who is unprepared is very bleak. Contracts will only be let to those who can guarantee their noise levels or other acoustic parameters beforehand. Increased prices to cover the cost penalties incurred by the guarantee are valid but they are accepted as part of the price we pay for the quiet we desire and must be compared with the high cost of remedying an unsatisfactory situation. It always costs much more to put something right afterwards than to get it right in the first instance. 193 Barnetwood Lane, Ashtead, Surrey, England ( Received 14 January 1974, and in revised form 18 April 1974)
M. D. G.
RAVDALL