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CYBERLAB             2637 EUDORA   DENVER, CO  80207            (303) 320-6119
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                            A C O U S T I C A L C


                                    VER 1.0
                     (C) COPYRIGHT  1986  RICHARD E. SWEENY   


The science of acoustics is of vital  interest  to  anyone  who  is  seriously 
involved  with  music  or  sound  recording.  It  is  very  easy to spend many 
thousands of dollars on electronics without getting the  right  "sound"  in  a 
studio  or theater.  The ACOUSTICALC program is meant to assist the acoustical 
engineer, architect,  musician,  and hi-fi enthusiast in the search for better 
sound.  The program reveals problems inherent in the dimensions of a room.  

This  text  is  not  meant  to  be  a  complete  course on the many secrets of 
acoustics,  rather it is an introduction to some facts and fallacies,  and  an 
explanation of the printout provided by  ACOUSTICALC.  

Before  proceding,  the  user should return to the program and run it with two 
sets of room dimensions.  The first set is length = 50, width = 25, and height 
= 20.  The second set is length = 23.3,  width = 16,  height  =  10.  The  two 
printouts will be discussed in the following text.  

You  should  see "L W H" as the first three headings,  which stand for Length, 
Width, and Height.  The next three headings refer to modes of resonance in the 
room.  Axial refers to  two  parallel  walls  resonating  at  some  frequency, 
Tangential refers to the case where four walls are resonating, and Oblique has 
all  six  walls  involved.  Of the three modes of resonance,  the Axial is the 
strongest,  with Tangential being half as strong,  and Oblique half  again  as 
strong.  

Referring  to the first printout,  the first line of data contains 1,0,0 under 
the L,W,H headings,  and 11.3 under the Axial heading.  These data  mean  that 
the  room  will resonate in the Axial mode (only two walls involved) along the 
length of the room,  and the 1 under the L heading means that the  fundamental 
frequency  of  the  Length of the room is involved.  The 11.3 figure under the 
Axial heading shows that the frequency in this case is 11.3 Hertz.  

The next two figures under the Axial heading are identical (22.6).  A  pairing 
of  two  identical frequencies on the printout reveals a "collision".  In this 
case the room will resonate strongly at 22.6 Hertz, and the second harmonic of 
the Length of the room is reinforced by the fundamental frequency of the Width 
of the room.  

The next collision occurs in  the  tangential  mode  at  36.17765  Hertz,  and 
checking the first three columns show which sides of the room are involved and 
how  they  are  resonating.  Many  other  collisions  are  shown  by the first 
printout,  showing problems in all three modes,  and  frequencies  up  to  the 
fourth harmonic.  

The second printout,  with Length = 23.3, Width = 16, and Height = 10, shows a 
much better balance of room dimensions.  Although there are several  pairs  of 
frequencies which come very close to being a problem, there are no collisions.  
This  room  will have a more balanced sound with little of the bass "boom" the 
first room had.  

  




       
                         CORRECTING THE PROBLEM 
         

Correcting  acoustical  collisions,  such  as  those revealed by this program, 
involves constructing and installing tuned absorbers.  There are several books 
available on the subject,  so it will not be covered here.  If there is enough 
demand,  the  design  of  tuned  absorbers  will  be  in  a  future version of 
ACOUSTICALC.  Computer assisted design of speaker  enclosures  is  also  being 
considered for a future version.  







                          HOW THIS PROGRAM WORKS


The core of this program uses this rather awkward formula:

                        /---------------------------
               C       /   P*P        Q*Q       R*R
              ---  \  /   -----   +  -----  +  -----
               2    \/     L*L        W*W       H*H

Where:
       C = speed of sound
       L = length of the room in feet
       W = width of the room in feet
       H = height of room in feet
       P,Q,R = integers 0,1,2,3,4


The formula is used 125 times in three nested loops to calculate the resonant 
frequencies of the room.  While it is possible to perform this calculation by 
hand, the computer does it much faster, and with fewer errors.



                                                    

                              SYSTEM REQUIREMENTS

                        IBM PC or equivalent with 64K.
                        One floppy drive                    
                        Printer
                        Monochrome or Color display
                        No math co-processor is needed


                        

  

                     THE MOST COMMON ACOUSTICAL FALLACIES 
                     ==================================== 


(1)  The  recording studio has acoustical padding on every surface,  just like 
     inside a good speaker, so it must be acoustically correct.  
     
                            ---- THE TRUTH ---- 

     There is a major difference between the inside  of  a  speaker,  and  the 
     interior of a recording studio.  Too much sound absorbing material on the 
     inside  walls  will  kill  any  high  frequency  reverberation,  and  the 
     resulting recordings will sound dead and unnatural.  

     While  speaker  enclosures  are designed to boost weak bass sounds ,  the 
     studio environment will overemphasize the lower frequencies,  leading  to 
     the  "boom" heard so often.  The proper way to correct this problem is to 
     design, build,  and install a tuned bass absorber.  The usual approach of 
     using  a graphic or parametric equalizer is much less effective,  and the 
     "boom" is still not corrected in the room.  

(2)  The control room is not as important to good sound as the recording room, 
     so it does not need to be treated acoustically.  

                            ---- THE TRUTH ----

     The control room is at least as important as the recording room.  In fact 
     the control  room  must  have  a  shorter  reverberation  time  than  the 
     recording  room in order to accurately hear the sound produced.  The only 
     practical alternative is to wear  headphones  during  recording,  if  the 
     control room is not corrected.  

(3)  The  weakest  link  in the studio is the speakers,  since their frequency 
     response is the least flat of all the equipment used.  

                            ---- THE TRUTH ----

     The weakest link in the studio is the acoustics,  which can vary as  much 
     as  20db  in the band between 15HZ and 500HZ.  The speakers are a distant 
     second, and the microphones are third.  The tape equipment is fourth, and 
     the remaining electronics are the fifth weakest link.  

(4)  A spectrum analyzer and a graphic equalizer were  used to  "balance"  the 
     room, so nothing further can be done.  

                            ---- THE TRUTH ----

     The preceding technique is the proper  one  to  use  AFTER  the  room  is 
     analyzed  and corrected using acoustical techniques.  Phenomenal sound is 
     possible if the correction  is  done  in  the  proper  order.  Without  a 
     properly corrected environment,  the graphic equalizer is just a band aid 
     approach where surgery is needed.  

(5)  Reverberation is the mortal enemy of the recording  studio,  and  must  be 
     reduced as much as possible.  Zero reverberation is the ideal.  

                            ---- THE TRUTH ----

     This  old myth dies hard.  A slight reverberation is needed for a natural 
     sound.  The proper amount varies with the room volume,  with  .4  seconds 
     being about right for speech recording,  and .7 seconds correct for music 
     recording.  Having adjustable acoustics will be an advantage when dealing 
     with both music and speech.  

(6)  The  best  way  to isolate the studio from outside noise,  and keep sound 
     inside the studio, is to add padding to the walls.  

                            ---- THE TRUTH ----

     The ONLY methods that work are heavy walls,  and multiple  walls  set  to 
     resonate  at  different  frequencies from each other.  The padding on the 
     walls will control reverberation times,  but is powerless to isolate  the 
     studio.  An open door is just as good as padding for this purpose.

(7)  When recording speech, any well padded room the size of a large closet is 
     adequate, since only one or two people will be in the recording room.  

                            ---- THE TRUTH ----

     A room of less than 1500 cubic feet will have several built in resonances 
     which are strong enough to defy a cost effective solution.  Unfortunatly, 
     the  remedy  is  to enlarge the volume by moving walls,  which is neither 
     cheap nor easy.  Additional padding is ineffective in reducing the "boom"
     which is so common in small rooms.




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     CYBERLAB          2637 EUDORA    DENVER, CO.  80207        (303) 320-6119
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