MICHAEL GREEN AUDIO / Different Methods of Acoustical Control -- A Historical Perspective


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Username: Mark_mclaughlin

Post Number: 1
Registered: 02-2004

Different Methods of Acoustical Control -- A Historical Perspective

In the modern day of acoustics, especially since the age of amplification and loudspeakers, there have been several new approaches to controlling acoustics. These different methods have been designed to work together with the amplification of music. In the past, it was up to the acoustician to develop architecturally sound environments. It was at best hit and miss and very timely. All of the old venues of acoustics were designed with a cooperative effort between the musician, the audience, the instrument builders and the architects. Modern acoustical design has become somewhat more complicated.

With the introduction of the age of electricity, a new world was born. It was not merely sticks and stones anymore, but engineering and circuitry. It was a change from producing and projecting the music to producing, storing, amplifying, mixing, restoring, reproducing, amplifying and projecting the music. With the introduction of the microphone, the recording equipment and the playback equipment, the art of the music performance has changed forever. Several new acoustic venues had to be created. The live performance was suddenly affected not only by natural acoustics, but by amplified loudspeakers. Instead of one horn or main acoustical source, you now had the performance area picked up by microphones, sent through amplifying equipment into loudspeakers to acoustically fill the auditorium.

There were also other types of rooms and functions which needed to be created in this new world. One was the recording room. This was a room specifically designed to play an instrument (or instruments) and have them picked up by a microphone. These rooms opened the door for different acoustical theories. One well known theory was called the "Dead" theory. The "Dead" theory was built on the premise that the walls of the room should not participate in the sound at all. Absorbent materials, commonly foam or fiber glass, were placed on all the surfaces in hopes that the sound waves coming from the source, whether it be instrument or voice, would go directly into the microphone and would not return from the walls, thus giving us the word anechoic -- meaning no echo or no audible return. This was a room commonly used to also test loudspeaker performance. By placing a loudspeaker three feet away from a microphone, it was believed that the microphone would be able to get a true reading of how the loudspeaker performed. Many loudspeakers were then tailored to give a reading of a standard accurate response which the engineers called "flat". This "flat" response was based on frequencies from 20 Hz to 20k Hz. The idea was that if the speaker did not vary over a predetermined decibel level, whether it be a peak or a minus at any particular Hz, it was to be determined "flat" and therefore was a speaker of merit.

In the recording application, this method became somewhat troublesome for the engineer, as the recordings were sounding too dead. Therefore, a new theory was adapted. This theory was called "Live End, Dead End". It was too simplistic in its approach -- one end of the room was built much like the anechoic chamber and the other end of the room was built "live" out of mostly wood. The engineer's job was to then determine in what proportion, from "dead" to "live", in which he wished to do his testing or recording. This method, of course, began to evolve into a mix of "live" and dead". Test facilities and recording studios began experimenting with different shapes of reflection versus absorption. The reflective surfaces were then commonly made out of several different materials -- from brick, stone, concrete to various types of wood. The absorbent materials were commonly made out of different density fiberglass or foam. Some of this foam was sculptured in hopes to do a little of both -- diffusion and absorption.

The next era of acoustical treatment become even more interesting. As the development of loudspeakers and microphones were advancing and developing, as well as instruments, ranging from the lightest of materials for resonant principles to very heavy materials designed for anti resonance, so did the acoustical products change. Stand alone and retrofit acoustical products were now developed. Products were designed in different shapes including round, curved, flat and angled to achieve desired results. There were products developed to "trap" acoustical space, to diffuse acoustical energy, to resonate acoustical energy and to absorb acoustical energy. These products were commonly placed all over the room, regardless of whether the room was built "live" or "dead".

Soundproofing was also a big part of the acoustical problem. The "trapping" theory believed that if you trap the basic fundamental nodes of any given room, depending upon the size of the room, you could equalize the room and maintain "flat". This was accomplished through the use of cylinder traps made of varied sizes to capture particular ranges of frequencies. The opposing side was a diffusion system. This diffusion system was designed to allow frequencies to find their particular tonal spot or resonance on the diffusion panel and therefore diffusing their counterpart nodes (peaks) in the room. By placing either one of these tools in particular problem areas of the room, you could choose between the two results. At this same time, many people would use all three -- dampening, trapping and diffusion.

In 1988, a new method was designed. This was the method of "acoustical barricading". The idea was to build a room with as much energy as possible using materials that were commonly the same types of materials as what would make the natural sound of a musical instrument and then placing barricade products in strategic places to control the room. This method was called "Pressure Zone Controlling". This method was based on the idea that inside a room, sounds were created more by pressure (the pressure of the entire room) than it was by sound traveling in a straight line. In the mid 50's text books on acoustics, you will find that sound is commonly referred to as traveling in a straight line, creating different reflection points, depending on the direction that the source or creator of the sound is facing. The pressure zone control theory was based on the idea that sound, as all energy, travels in spherical patterns. Therefore, sound should be controlled from more of an aerodynamic approach as opposed to a "beam" approach.

The next stage evolving from "Pressure Zoning" is "Variable Pressure Zone Control". By placing materials, built like musical instruments, to variably tune the entire audible frequency range and placing them in the strategic pressure zone areas, you can now variably control the sound of the pressure zone. The next step is building the wall (again, like a musical instrument) with variably controllable devices built into the wall to not only gain control of the acoustics (i.e., laminar flow and horn loading of the room), but also now the mechanics of the room. By adding more or less tension to the now variably tunable walls, you can change the mechanics which ultimately not only change the node or nodes of a room, but you can linearly align the patterns of the laminar flow. The key here is not to absorb or remove any frequencies, but to control them and therefore preserve, retain or augment any signal in a mechanical or acoustical manner.

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Silver Member
Username: Mixneffect

Orangevale, Ca. USA

Post Number: 759
Registered: Apr-05
My friends anechoic chamber cost him $60,000. It is 30 X 50 x 25. The walls are 24" thick and padded with insulation. The outer walls are covered with sound absorbtion materials. The floor is raised by 24" as well, and it got the same treatment as did the ceiling. When you walk in it for the first time, your ears plug up (like in a plane at high altitude).
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