On Wed, 9 Jan 2008 00:30:01 +0000, Jim Crowther
<Don't_bother@blackhole.do-not-spam.me.uk> wrote:

Jim,

>>Trouble is it depends on just how far the microphone is from it. The slow
>>speed of sound makes a large difference when you're talking about
>>fractions of a frame.
>>
>
>IIRC, we used to assume 7.5m for sound for a field delay.  Sounds a bit 
>low , but not by much.  Directors used to complain in the edits about 
>the difference between close mics and booms on long shots.  The close 
>mics of course looked 'wrong'.  So why were they specified...

I use a very rough and ready reckoner of 40ft distance equals a
(25fps) frame,  which isn't too far away from your 7.5m per field.

Close mics on the actors would be used to ensure that you got good,
intelligible sound with comparatively low background noise.  You can
never guarantee that when working with a (hyper) cardiod mic at
distance.  

You can make the sound of close mics look right by slipping the sync
to match what you expect.  As humans we are used to seeing/hearing
late sync in real life from distant objects,  so seeing a distant
object in perfect sync will always look wrong.   You can also make the
close sound effect sound better by use of a controlled level of
additional atmos and creative use of the facilities on a good sound
desk.

Jim.

On Wed, 09 Jan 2008 10:31:55 +0000, Jim Guthrie <> wrote:

>>IIRC, we used to assume 7.5m for sound for a field delay.  Sounds a bit 
>>low , but not by much.  Directors used to complain in the edits about 
>>the difference between close mics and booms on long shots.  The close 
>>mics of course looked 'wrong'.  So why were they specified...
> 
> I use a very rough and ready reckoner of 40ft distance equals a
> (25fps) frame,  which isn't too far away from your 7.5m per field.

Speed of sound in air (at standard temperature and pressure) is 331.45m/s so
that gives 13.26m per frame (43.5 feet).