Thanks Ken. It never ceases to amaze me what I learn on this List. Too bad
I can't get CME credits here.
Charlie
On Tue, Dec 7, 2010 at 1:13 PM, Ken Norton <ken@xxxxxxxxxxx> wrote:
> > 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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