Acoustical Engineering

Most room environments are not designed with the acoustical properties in mind, but rather other aesthetic or practical objectives are of primary importance - until it becomes necessary to either work with sound or enjoy the listening experience without the room's signature detracting from the ability to distinguish detail, proper spectral balance or spatial cues. The answer is to either design the room from the beginning taking into consideration the acoustical properties, or to correct the problems later using any of several techniques.

RR Audio provides acoustical analysis and design services for
recording and project studios as well as for home environments.

If you are planning a new room design or want to correct the one you have, give us a call.

Acoustics

    Acoustical problems in critical listening environments can be a real nightmare. In the case of a project studio in the home, what normally would make for a comfortable den or bedroom can be a disaster when you are trying to evaluate your latest recording. In the case of a working recording studio, anomalies in the room acoustics can be inadvertently compensated for when mixing the recording, causing the mix to be incorrectly balanced when taken out of the original environment. In the home listening environment, although acoustical problems are not threatening your livelihood, they can be extremely annoying in that they interfere with the ability to "get inside of the music", creating the illusion that you are "there".

    In each of the above cases, what is needed is to evaluate the environment to determine where the problems lie in the frequency and time domains, to extract from this information a model of what modifications need to be done to the room to alleviate the problems, and to decide which types of devices and materials should be used to both achieve the desired results and stay within the budget. The same thing holds true when creating a room from scratch. If the room is designed from the ground up to be acoustically balanced, not only is the end result much more satisfactory, but the cost to build the room is generally less than fixing mistakes after the room is done.

    The first step in correcting the acoustics of a room is the analysis :

Analysis

    Acoustical analysis is accomplished by generating a series of impulses (similar to handclaps) which contain all frequencies up to the sampling rate used and feeding the signal into the room via the playback system. Several different signals with different sampling rates are used to allow for the maximum detail in the data to be acquired (tests are done with data to 23kHz, then another test is done with data to 4kHz, then again with data to 500Hz). The reason for this is it allows for more data points to be available at the lower frequencies, so that when zooming in on the lower frequencies there is enough data (without large gaps between data points) to evaluate the problems without having to interpolate data. Microphone placement in the listening position is tested first, then additional tests are done with the microphone in specific problem areas to confirm that the problem data is correct.

    Once data is acquired, the information is displayed in the time domain. This allows the viewing of the reaction of the system to the impulse, and shows when in time any reflections occurred (which allows the calculation of their origination based upon the distance from the microphone) as well as the amplitude of those reflections and any ringing, etc. This data can then be transformed into the frequency domain, where frequency spectrum response curves (amplitude vs. frequency), phase response (phase shift vs. frequency), group delay (time delay, both positive and negative, vs. frequency), spectral decay (amplitude vs. frequency vs. time "waterfall" plots) and other data presentations can be generated. These plots allow for very detailed analysis of the problems, and provide the information required to find the problem areas with certainty and design the correct modification program the first time.

Evaluation

    Once the data has been acquired and transformed into the various plot types, the information is evaluated and design work is begun to correct the problems. Many different solutions are available to correct the various kinds of problems which may exist some methods being more costly than others. After determining which are the primary causes of the problems, several methods of correction are outlined and the required materials and devices for correcting the problems are listed in ways which allow choice as to method vs. budget. The client is then consulted as to which method is preferable, at which point design of the final system is begun.

Design

    Computer-aided design of devices, layout of the room, and plans for building the various devices are completed and final approval is gained from the client before construction is begun. Some of the devices used in the correction of typical acoustical problems include:

    Quadratic Diffusors generally n=17 or n=29 types, these are devices which reradiate sound, allowing the frequency response of the reradiated sound to be much smoother than when reflected off of a flat surface. They consist of carefully calculated wells of specific depth and width at staggered frequencies. A complex diffusor would be a composite of an n=29 unit on the left which operates from 240Hz to 1700Hz, a center n=17 unit which operates from 966Hz to 7728Hz, and a duplicate of the left n=29 unit on the right. An n=29 type has 28 wells, and diffuses 14 frequencies and the corresponding harmonics - n=17 types have 16 wells and diffuse 8 frequencies + harmonics.

N=17 Diffusor

To the right are a hinged dual and a free-standing  composite panel acting as a reflective trap zone (left), an n=29 diffusor (center), three parabolic reflectors on the ceiling, and the walls were treated with carefully-placed absorptive and reflective zones and covered with fabric. This small control room has a neutral sounding acoustic that makes mixing in the room for long periods of time possible, and the original problems in the room were well-controlled.

N=29 Composite diffusor
(two fundamentals for very wide bandwidth)

    Parabolic Reflectors these are curved devices which are generally placed on walls (a different type is used on ceilings) to spray the soundwave on paths which tend to break up standing wave modes. When used on ceilings (generally over a larger area than the wall-types) these devices not only break up the standing wave between parallel ceiling and floor, but reflect the soundwave towards the walls, where absorption by the wall treatment and any acoustical panels can occur.

    Wall Treatment many times the most attractive and least expensive method (depending upon chosen materials) of creating absorption and reflection zones is to treat the wall surfaces permanently using absorptive materials chosen for the proper absorption coefficient for the specific problem area and reflective materials (like sheets of damped wall paneling) interspersed strategically. Absorptive materials (and non-finish-grade reflective materials) are covered with fabric and trimmed with moulding, piping, etc. to maintain the decor of the room. In many cases, the finished result does not look like any acoustical treatment has been done at all (these are affectionately called "camouflage jobs" by our silly designer).

    Acoustical Panels these are permanent or portable devices which either absorb energy, reflect energy, or do a combination of both jobs. Panels when strategically placed can damp problem areas, reflect energy into trap zones, or form the trap zones themselves. These devices can be constructed in such a way as to be very attractive or extremely inexpensive, depending upon requirements and the materials which are used. We have made panels with everything from simple cloth-covered luan exterior frames with inexpensive cloth over the standard inner frame and damping materials to solid-hardwood framed exteriors with raw silk over the interior materials.

    Bass Traps and Resonators many times a single frequency and its harmonics are causing a serious problem. In these cases, it is generally preferable to trap the problem frequency with a tuned device which cancels or reinforces the fundamental frequency rather than use an equalizer, as any signal processor adds noise and other anomalies. These devices work on the same principle as blowing across the mouth of a Coke bottle. If there is liquid in the bottle, depending upon the volume of the bottle and the size of the mouth, a specific note is emitted. Drink some of the liquid and the note changes. Helmholz resonators of this type can greatly reduce problems associated with specific frequencies, and in most cases eliminate them entirely. Other types of Trapping devices can be created by designing a damped hidden chamber which is tuned to one or more frequencies, or using structures (like highly absorptive panels) to create a trap zone in a corner or other strategic area. When properly used, a minimum of devices can correct for a major low frequency problem.

If you would like to ask questions regarding the products and services we offer, or would like us to send you more information, please let us know. We would be happy to provide you with whatever information you require.

RR Audio Laboratory
636 E. Harvard Rd. unit B
Burbank, CA  91501  USA
Telephone : (818) 843-8212
Facsimile : (818) 563-9372
E-mail : 
rr@trapagon.com