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Re: [OM] The Concert

Subject: Re: [OM] The Concert
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Tue, 7 Dec 2010 12:13:28 -0600
>  OK Ken I have to ask. What is a "slap echo" and how is it different from
a
> regular echo?


"Regular echo", which is generally considered by most audio engineers as
"reverb" is where sound bounces off of various surfaces which absorb,
reflect and disperse sound in a way that provides a room tone as well as a
general decay of sound. In a room where all surfaces are 100% absorbant, the
sound originates from a point source and immediately dies upon encountering
the walls.

You can have a room which is highly reflective, but if the sound is
adequately dispersed (diffused) upon striking the walls, then the room will
be "live" but will have a sloping decay time in sound-pressure-level (SPL).
When engineering rooms, we calculate the RT60 time--which is the lengh of
time it takes for the reflected sound to drop 60 dB below the direct sound.

Calculating RT60 is easy IF all surfaces are relatively uniform. You just
need the average absorbtion coefficent of the surfaces, total surface area
and cubic space (volume) of the room in question. Where things get trickier
is when the three axis of a room have different absorbtion coefficients. I
built an extensive calculation tool that calculates three-axis as well as
determines a slew of other nifty equations like %ALCONS which give a good
indication of the acoustic health of a room. My program determines %ALCONS
for user-defined distances (and lateral positions taking into account
absorbtion of surfaces of all six) and I even developed a tool for
simulating sound at various listening positions. My three-axis equation has
proven to be EXTREMELY accurate and I've proven it in a couple tricky rooms
to within 5%--which is unheard of accuracy. To get it that accurate, I did
have to modify the equations to correct for a couple of faults relating to
the diffusion characteristics of most surfaces as well as the null-zones
along edges of surfaces and corners. Just to be pendentic, I'm going to
suggest that if any audio contractor gives you the standard RT60 calculation
based on the simple Coefficient/Surface Area/Volume equation, RUN!!! To do
it right, you really do need to use a 3-axis equation.

My modified 3-axis equation is as accurate as probably is possible without
resorting to ray-tracing programs. I probably have 500-750 man-hours into
developing it and testing it and has proven to be extremely accurate in most
circumstances. Every time I measured a room, I digitally recorded the room
and did an analysis on it to fine-tune my equations. After "correcting" a
room I compared calculated fixes with actual and further fine-tuned the
equations. Odd-shaped rooms were a further development of the program and I
can do fan-shaped as well as egg-shaped rooms.

But, all that assumes standard-fare echo.  That doesn't assume reflective
walls or even curved walls which even when highly absorbant will be an
acoustic disaster. These can only be identified with the eyes, not the ears.

Slap echo happens when you have a large, flat surface which sound can bounce
off of. Typical room design has been LEDE (Live End, Dead End), which is
fine for RT60, but will provide a delayed sound which can provide
comb-filtering as well as just irration to an audience or performer. Things
will get really nasty when you have two opposing surfaces which are not
diffusing the sound adequately. In this case you'll have a sound that
bounces back and forth between the two walls until evenually enough
absorption in the air has occured. Kinda like an infinity-mirror.

Fan-shaped auditoriums generally have no slap echo if designed correctly.
Unfortunately, most aren't. The balcony bulkhead is almost always a problem
because the archetect has this surface vertically positioned and the entire
face of it is curved to match the fan-shape of the room. This gives a
reflective surface which focuses a sound originating from the stage directly
back to the stage. Just sloping the bulkhead (bullnose) upward or curving
the shape will help dramatically. Another trick is to shape the fan shape of
the back walls, bulkheads and seating arrangement in such a way that no
refocusing occurs. When you do have refocusing caused by the seating area,
an ancient solution for absorption is the Helmholtz Resonator which dates
back at least 2500 years and was commonly used in greek amphitheatres.

AG (too much information) Ears
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