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