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Subject: Dancing

Dancing

From: Barry Knight <barry.knight>
Date: Tuesday, July 17, 2001
In Conservation DistList Instance: 15:6 Tuesday, June 26, 2001, I
wrote:

>Does anyone have any information on the likely impact of dancing on
>museum collections?

More thoughts about dancing

I had a number of replies to my query off-list, mostly personal
anecdotes about the problems of running functions in historic
buildings.  Thank you to those who responded.

I think the overall conclusions are clear: the risk of damage
occurring during an event, such as damage to floors by high heels,
spillages of food and drink, damage while setting up or dismantling
sound systems and staging, is far greater than the risk of damage
due to sound and vibration.

This is not to say that sound and vibration cannot cause damage.  We
should perhaps distinguish between sound and vibration: sound refers
to frequencies greater than about 30Hz, while vibration refers to
lower frequencies.  Also, sound is mostly transmitted through the
air, while vibration is mostly transmitted through the structure of
the building.  The vibrations caused by dancing are different from
those caused by earthquakes in that they are (at least in principle)
rhythmical, which means that there is a possibility that the
amplitude of the vibration may build up due to resonance.

It is quite difficult to work out how much a canvas might vibrate in
response to sound, but we can get some idea from the changes in air
pressure caused by sound.  The sound pressure level is measured in
decibels; an increase in sound pressure of 10dB is perceived as a
doubling in loudness.  The following figures give an idea of the
sort of sound levels we might expect (see eg
<URL:http://www.lhh.org/noise/decibel.htm>)

    90dB        heavy truck, shouted conversation
    100dB       boom box / ghetto blaster
    110dB       disco, symphony concert
    110-120dB   rock concert

To translate these figures into changes in air pressure, the sound
pressure level is defined as:

    20 log10 (effective sound pressure / reference pressure)

where the reference pressure is 2 x 10-5 Pascals. From this
equation, we can see that 90dB corresponds to an effective sound
pressure of 0.6Pa, 100dB to 2Pa, and 110dB to 6Pa.  These are very
small pressure changes, bearing in mind that atmospheric pressure is
about 100,000Pa, and will have a minimal effect on a canvas.

The only other concern is that the size of vibrations may be
amplified by resonance.  The resonant frequencies of paintings on
canvas are typically between 10 and 100Hz; tests by Peter Caldicott
(Art in Transit, 1991, page SE23) showed that the vibration of the
canvas could be amplified by 10 to 20 times if the vibration applied
happened to hit its resonant frequency.  However, Timothy Green (Art
in Transit, 1991, 65) found that these effects could be greatly
reduced by lining the canvas, increasing its tension, or fixing a
backboard.

The conclusion has to be that the pressure changes caused by sound
will have a minimal effect on a canvas.  These effects will be
reduced if the painting has a backboard, and will be effectively
eliminated if it is glazed as well.

Resonance in display cases may cause objects to "walk" off shelves,
but this can be reduced by damping the vibration of the shelves.

Barry Knight
English Heritage
+44 20 7973 3000
Fax: +44 20 7973 3001


                                  ***
                  Conservation DistList Instance 15:15
                 Distributed: Wednesday, August 8, 2001
                       Message Id: cdl-15-15-010
                                  ***
Received on Tuesday, 17 July, 2001

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