Examination of Witnesses (Questions 19580
- 19599)
19580. Have you ever been asked, or have you
ever designed a studio to meet criterion expressed in that way?
(Mr Bell) I have been asked
to design a studio that meets the NC25 and NC20 criterion, which
are the ones we are bothered about in those two areas, and it
is always expected of me that external noise is inaudible within
those studios.
19581. That is not an answer to my question,
with the greatest respect, Mr Bell.
(Mr Bell) Yes, it is.
19582. Have you ever designed or have you ever
been asked to design a studio applying a criterion which is expressed
as an NC curve, minus a particular level of dB expressed in the
third octave band? Yes or no.
(Mr Bell) It depends whether
we are trying to go through the semantics of your question or
whether we are trying to get into the spirit of what it is that
we are trying to achieve. The background noise in the studio is
NC25, in the control room. It is expected that extraneous noise
is not audible. In order that extraneous, pulsed and non-continuous
internal noises are not heard in the control room then they must
be below the NC25 broadband threshold. That, as I was suggesting
earlier on, applies to external noise generated by a train, and
in our ability to measure it at Grand Central, that means it needs
to be 8dB below NC25 or, as you express it, NC20-3.
19583. I have asked the question twice. I have
a simple rule which is I ask three times and if the witness does
not answer I move on and make submissions about it. I put the
question again: have you ever designed or been asked to design
studios that would accord with an NC rating minus a particular
decibel level expressed as third octave bands? Yes or no.
(Mr Bell) I have been asked
to design control rooms with NC25 and in which external noises
like trains are inaudible. That is an implication of the standards
to which you refer. So the correct answer could probably be "yes".
19584. NC20-3dB expressed in third octave bands
was a level identified through measurements within the studio,
was it not?
(Mr Bell) It was indeed.
We had a meeting on October 13 when the two sides met and it was
agreed that we should quantify what level would render the trains
inaudible. We duly met and measured the background noise and that
was the standard that was derived following that measurement process.
19585. That measurement was achieved by switching
off all the equipment in the studio and everybody leaving the
room. Is that right?
(Mr Bell) Absolutely, because
what was required was a mathematically correct representation
of the external noise of the train so that it could be measured
later and compared with any subsequent noise made by the railway
being constructed. Thus all the background noise of the studio
and anybody there and any other extraneous noise was removed from
the equation for that measurement process to be accurate and correct.
It was an attempt to quantify the level of tube noise alone that
needed to be met in order for it not to be discernible once the
rest of the studio was switched on and working. This methodology
was agreed between us before we did it by both Crossrail's acoustic
specialist and ourselves as the best way to quantify the tube
noise so that it could be compared to the forecast for the rail
noise, because the forecasts for the rail noise are for the railway
alone and not for the rail and anything else.
19586. I want to ask you a couple of questions
about tonality. As I understand it, tonality occurs where there
is a pronounced spike in a particular frequency in a sound.
(Mr Bell) You introduced
the term "tonality". Tonality is where there is a frequency
or frequencies which are predominant in the background. There
are various types of noise which are declared as neutral. For
this case, NC25 would be declared as neutral.
19587. Can we have, please, page 11 of the exhibit
9304, which should be a graph.[16]
Here we have the forecast of the temporary construction railway
with no rail joint. Where in that graph is the tonal spike?
(Mr Bell) Where it is expressed
in third octaves it may be foreseeable but it is expressed in
octave bands, which tend to mask it.
19588. Just to confirm: you have produced no
evidence, have you, of the third octave band spikes in your evidence-in-chief?
(Mr Bell) Third octave band
spikes of what?
19589. That support your contention that there
would be tonal noise with Crossrail.
(Mr Bell) Crossrail have
produced enough graphs of their own which show potential tonal
spikes.
19590. Chairman: That was not actually
the question.
(Mr Bell) I have produced
none. Unfortunately, we are in the position of being studio designers
and not railway forecasters. Therefore, we have to rely on the—
19591. I understand that, but it was the question
put.
(Mr Bell) We have not produced
any evidence on tonality.
19592. Mr Taylor: I have one last point.
We heard before from Mr Taylor that you had a discussion with
him informally when he was still in the building as to whether
steps could be taken in the design of the studios to take into
account the possibility that Crossrail might come along—in
other words, that you could make some sort of tacit provision.
(Mr Bell) I am sorry?
19593. You will remember when I asked Mr Taylor
about what steps—
(Mr Bell) I am sorry, wrong
Taylor!
19594. Mr Ivor Taylor explained that you had
a discussion about what steps might be taken. What technical work
did you do in advising the studios to examine what might be done
to make tacit provision for additional mitigation in the event
that Crossrail were to happen?
(Mr Bell) There is a reasonably
long answer. Yes, we had that conversation and my answer is that
we took the best we could do at the noise that was available there
and then on the basis that, as I said at an earlier point, the
Central Line is reasonably close and reasonably loud. If we could
meet the current noise generation by the Central Line my judgment
was we would stand as good a chance of keeping the new, modern,
well-constructed isolated railway noise out. The fact of further
mitigation is that we did consider digging down a floor to try
and get us enough headroom, but it was not possible to do so;
the foundations of Grand Central, which I think is a good thing
for everyone concerned, are massive and did not prove excavatable.
19595. So what actual technical work did you
do to examine the possibility of making tacit provision for further
mitigation in the event that Crossrail came along?
(Mr Bell) I am sorry; I
am not certain what technical work you are asking about.
19596. Did you attempt to identify what the
forecasts were for groundborne noise in this area that Crossrail
published at the time? Did you take those forecasts into account
in any design?
(Mr Bell) We were unaware
of any forecasts being made for that building at that time. I
believe subsequently we looked but we found that no forecasts
had been made for that building and were not made until quite
recently.
19597. Did you ask Crossrail for any information
about forecasts of ground-borne noise?
(Mr Bell) No, I did not.
We took the advice from the clients that Crossrail did not appear
to be forthcoming with such information.
19598. Last point: you explained in your evidence-in-chief
that one of the things you are asked to do when you are advising
clients on design is to state whether the building is suitable.
Did you advise your clients whether this building would be suitable
if Crossrail was built?
(Mr Bell) No, because there
was no criterion available for what noise Crossrail might make
for such advice to be offered.
19599. So the position is this, Mr Bell, is
it not: you built the studios not making any provision whatsoever
for changes in the studio should Crossrail be built?
(Mr Bell) No provisions
were possible in the footprint of the studios and given the height
of the building as available.
16 Committee Ref: A221, Grand Central Studios-basement
studio 9-temporary construction railway, no rail joint (WESTCC-9304-011). Back
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