Optimization Computer Tools for Room Acoustics
TAXI is the leading independent A&R company helping unsigned bands, artists and songwriters get record deals, publishing deals and placement in films and TV shows. WSDG is creating a series of articles on small listening / production room design and acoustics.
Article 6: "Optimization Computer Tools for Room Acoustics"
(this article has been primarily researched and prepared by Gabriel Hauser, of WSDG's European office in Basel, Switzerland)
1. The Problem
Everybody knows this effect: You buy a new pair of speakers that sounded so fantastic in the store (or you heard them in another room and liked them!). When you install them in your room, the result is not the same - in fact less than perfect. The difference is usually the room. Most of the difference is usually caused by differences in wavelength interaction at low frequencies. We have discussed this in previous articles.
There are two main issues to be dealt with:
a. Modal response of the room, also sometimes referred to as "standing wave effect". This has most to do with the relationship of wavelengths (remember each frequency has an associated wavelength) with the principal dimensions of a room. In small rooms this is a most curious effect, in that low frequencies, which of course have large wavelengths, often have wavelengths that are in the same order as or even larger than the room dimension themselves. This inability of a room to "resonate" fully at these frequencies will cause response anomalies that are not desirable.
b. Boundary effects - the effects of close reflective boundaries surrounding the speakers and the interference of these "reflections" at the critical listening position. This is referred to as speaker boundary interference or SBIR.
Optimizations for both issues can be performed manually (by ear) or with the help of a computer. This article demonstrates some relatively easy analysis that can be performed by two commercially available (and relatively inexpensive) computer programs for optimizing speaker and listener placement. Additionally, we explore some practical hints to be sure to get the right results.
2. Software for Speaker and Listener Placement
2.1 Room Optimizer by RPG, Inc.
RPG is a manufacturer and developer of acoustical materials in the USA. They have also developed a computer program that allows users to simply optimize their speaker setup automatically. It is relatively easy to use. All you need to enter are the room dimensions as well as possible speaker and listener locations with a maximal deviation range, and press start.
The biggest drawback is that it only analyzes rectangular rooms. For TAXI drivers, this is probably not that much of a liability. The analysis is performed between 20Hz up to 300 Hz - this is where most of the room boundary problems occur. Higher frequencies need other analysis tools. One defines the speakers in terms of low frequency transducers and their exact distance from walls and floors.
What you get as an output are the predicted frequency responses of the worst and the best tested speaker and listener positions. The program shows both the modal response and SBIR response plots in separate graphs.
The resulting locations are given as coordinates, which then enable you to locate your speakers. Remember what you are locating are the center points of the low frequency drivers in a speaker.
2.2 CARA by Elac
This Program was developed by Elac, a German Loudspeaker manufacturer. Nevertheless, the database that comes with the program features plenty more than only Elac Speakers, and customized loudspeakers are easily defined.
Unlike RPG's Room Optimizer, this program features arbitrary room shapes (CAD-like Interface) and allows calculations of the entire audio frequency band from 10Hz up to 40kHz.
- Optimization of speaker and listener position
- Predicted frequency response
- Pressure distribution in the room
- Temporal response
- Predicted reverb times over frequency
- Auralisation (you can listen to the predicted speaker-room-response)
3. Sample Room
The room chosen for simulation has the size of an average home listening environment. This could easily be a small workstation room or composing room. The room is rectangular and measures 6.8m x 4.5m x 2.2m. (22'-4" x 14'-9" x 7'-3"). The initial setup is a stereo situation; both programs will permit dealing with surround situations including dedicated subwoofer channels.
Before we start our calculations some "acoustical laws" have to be taken into consideration to find acceptable starting points for the optimization. The speakers should not be placed in room corners. Also, a symmetrical stereo triangle should be granted, including the fact that the distance of each speaker to room boundary is about the same. Good starting points for optimizations are shown in figures 1,2 and 3 (shown in both program displays). These distances more or less show the speakers a few feet from each of the corner boundaries and the listener is located on the centerline of the long axis of the room - about in the middle.
Room Optimizer allows for an exact definition of the initial positioning field, not just the available area that the speakers can be entered. It also will allow one to enter information about the dependency of the two speakers to each other or to the room center-axis or to the listener. Most of this type of data is probably not that important for TAXI type rooms.
In CARA, symmetry requirements can be defined partially: The distance of the speakers to the walls can be linked so that both speakers have the same distance to the rear and sidewalls. If this option is not activated, results can be very strange (like positioning the left speaker directly in front of the listener while the right speaker is located in a room corner, about 3m from the listening position). In the example used for this article (see drawings on
), the chosen loudspeaker type was a JBL TI2000, a bookshelf speaker that uses one low-mid frequency driver and a tweeter.
4.1 Room Optimizer (see figure 4)
Room Optimizer is very simple to use. It even offers "wizard" options where entering the room data and number of loudspeakers is all that's required. Results are shown as frequency responses of the modal response and the SBIR Effects. Drawbacks are that neither material properties can be entered nor can furniture be placed in the room, and that only rectangular room shapes are allowed.
In our example, the final calculated loudspeaker position turns out to be approximately:
1m (3'-3") from the front wall
.75m (2'-6") from the side wall
The height of the low frequency driver is 0.8m (2'-8") from the floor.
The listener is located in the middle of the room (along the room centerline - this will almost always be the case!), at 3.8m (12'-6") from the front wall. Notice that this is not exactly at the room center, but a bit back from center!
4.2 CARA (see figure 5)
CARA allows numerous different acoustical parameters to be calculated. The good news is that it offers a detailed simulation of the room. The bad news is that it requires more "intelligent" acoustical input to perform the calculations correctly. Perfectionists can even enter all furniture and material parameters of the room to get more accurate results.
In our example, the final calculated loudspeaker position turns out to be approximately:
.8m (2'-7") from the front wall
1m (3'-3") from the side wall
There was no change in the initial height of the speaker of 75cm off floor (bottom of cabinet). This was set in as a given, based on the speaker selected.
The listener is located 2.9m (9'-6") from the front wall and at equal distance from the sidewalls.
The frequency responses and calculated listening positions are slightly different in RoomOptimizer and CARA. First, CARA uses the "real" loudspeaker with its frequency response and RoomOptimizer works with an ideally flat response of the speaker. Secondly, RoomOptimizer shows resulting graphs with linear frequency axis while CARA uses logarithmic scaling. But if one compares the two graphs, the resulting responses are quite similar (Peaks at 50Hz and 80Hz, Dips around 25Hz and 70Hz).
In summary, for a quick optimization with fast results, or for the less experienced user, Room Optimizer is the product to use. For more detailed results, including reverb times, frequency responses up to 20kHz and even auralisations, CARA is the right choice.
One final note, it is interesting that both programs got us similar results but not the same. This is the nature of acoustics - it is an art and a science at the same time!
Both programs are in the below US $100 price range; Room Optimizer sells for $99.99 and CARA can be yours for US $64.95 - this includes a test-CD with low frequency tones and noises that helps aligning the speakers by ear or by measurement system.
Optimizing the speaker / listener positioning in your listening room with these types of "first pass" computer program outputs definitely helps to improve overall room performance. What they cannot do is completely set you free from the need for additional acoustical treatment against other types of reflections; room ratio analysis; and dealing with high room reverberation times at low frequencies.