Meyer Sound research and development personnel used the companys MAPP Online program to model the frequency response of an M2D Compact Curvilinear Array loudspeaker system as measured from a specified physical position in a known concert hall. The researchers then created a corresponding physical loudspeaker system in the specified concert hall and measured the systems frequency response at the specified position. Predicted and measured responses are plotted herein for comparison. The data show very good correlation, indicating a high degree of confidence in the programs acoustical predictions.
Meyer Sounds MAPP Online acoustical prediction program is a high-resolution visualization tool for professional sound system designers.
Using an Internet-connected personal computer, a sound designer specifies Meyer Sound loudspeaker models, their locations, how they are aimed and, optionally, the locations and composition of walls in a physical space, as well as environmental factors such as atmospheric pressure, temperature and relative humidity.
This information travels over the Internet to a powerful, remote server computer. Running a sophisticated acoustical prediction algorithm using highly accurate measured data that describe each loudspeakers directional characteristics, the server predicts the sound field that the loudspeakers will produce, as well as average and peak SPL and frequency and impulse responses at specified microphone positions. (The complex loudspeaker polar data magnitude and phase used in MAPP predictions are measured with 1/24th octave frequency resolution and 1° angular resolution in a calibrated anechoic environment.)
Because MAPP Online is intended to be a tool for audio industry professionals to use in specifying real-world loudspeaker systems that must meet stringent performance guarantees, it was deemed vital that the programs accuracy be proven through equally stringent empirical testing. Accordingly, research and development personnel at Meyer Sound obtained the use of a representative concert hall with the intention of hanging an array of loudspeakers and comparing that systems actual measured performance with predictions made with MAPP Online.
Installation and Measurements
Zellerbach Hall is a 2014-seat concert hall on the campus of the University of California at Berkeley. It features first-floor orchestra seating, a second-floor mezzanine and a third-floor balcony. Customarily used for a variety of performances ranging from classical chamber ensembles to fully amplified popular music and world music acts, the Hall was selected for the present purposes as a typical venue representative of those that sound system designers are likely to encounter.
As shown in Figure 1, multiple loudspeaker arrays were rigged for a variety of testing purposes. This paper concerns itself with an array of six M2D Compact Curvilinear Array loudspeakers and one M2D-Sub Compact subwoofer suspended from trusses at the approximate center of the stage (the array is shown in the upper right corner of Figure 1).
Fig. 1. Loudspeaker arrays installed in the test venue
For the measurements published here, the research team located a calibrated microphone in the hall approximately forty feet from the M2D array. The exact microphone location is shown in this MAPP Online configuration file.
MAPP Online version 1.1.11 and higher can load and save configurations of loudspeakers, microphones, and room information. Readers with access to MAPP Online may right-click (Ctrl-click for Macintosh users) on the link above, choose "save link as " and save the configuration file to a local hard drive. In MAPP Online, choose the "Open" menu item in the "File" menu, and select the file "zms.csv". MAPP Online should display the image shown in Figure 2, which is a section view of Zellerbach Hall with the loudspeaker array and measurement microphone installed.
(Note: Readers who do not have MAPP Online can request it here.)
Fig. 2. M2D Array in Zellerbach Hall as Modeled in MAPP Online
The six M2D's and the M2D-Sub are shown here in side view. The M2D Sub is "off" in this configuration file, as it was in testing. You can see the exact splay angles of the array by dragging the mouse to select the loudspeaker array and then using the menu item "Loudspeaker Array" in the "Configure" menu.
If you click the "Predict" button in the upper left-hand corner of the MAPP Online window, the Java client will connect to the computational server at Meyer Sound R&D in Berkeley, CA and transmit the array data to it. You must have an active connection to the Internet to make a prediction. (If you do not have an active connection, however, you can still set up loudspeaker arrays and microphone positions.)
After a few seconds, you should see a color image similar to Figure 3. The colors indicate the predicted coverage pattern in the octave band centered at 500 Hz (the default octave). If you want to change the octave band, use the "Prediction Parameters" menu item in the "Configure" menu.
Fig. 3. MAPP Online Sound Field Prediction
By clicking on the "Virtual SIM" tab, you can bring up the Virtual SIM view, which includes an estimate of the 1/3rd octave spectrum response, the frequency response (smoothed to 1/6th or 1/3rd octave, or unsmoothed), and the band-limited impulse response. Each view is available in the "Measurement" menu, and the smoothing and zoom options are found in the "Util" menu.
Figure 4 shows the 1/6th octave smoothed frequency response estimate for the designated microphone position. Note that MAPP Online accurately models the high-frequency attenuation caused by air, which varies with temperature, humidity, pressure and distance. You can change these environmental parameters in the "Natural Environment" dialog under the "Configure" menu.
Fig. 4. 1/6th Octave Smoothed Frequency Response as Estimated by MAPP Online
Comparison Between the MAPP Prediction and the Measured Response
Using a SIM System II FFT Analyzer, Meyer Sound engineers measured the frequency response of the six M2Ds from the designated microphone position in Zellerbach Hall. Figure 5 shows this measurement in progress.
Fig. 5. Measuring the Array's Response in Zellerbach Hall
Data from the SIM measurement were exported to the MatLab application for analysis, as was the Virtual SIM frequency response prediction (export was achieved using the MAPP Online menu item "File -> Export Frequency Response Data as "). Figure 6 shows the two data sets overlayed in a single plot.
Fig. 6. Predicted and Measured Frequency Response at 1/24th Octave Frequency Resolution
In Figure 6, the blue trace is the actual measurement data, and the red trace is the MAPP prediction. Overall, the match is excellent, especially from 300 Hz to 10KHz.
Below 300 Hz, the effects of the room (not yet modeled in MAPP Online) add to the low-frequency buildup. Similarly, anomalies above 10 kHz (where the sound wavelength is very small) are undoubtedly caused by immediate boundary conditions, and will vary radically with small changes in the measurement position. Note, however, that MAPP Online very accurately predicts the general trend of the system response in these regions.
The comparison between predicted and measured responses for a physically realized curvilinear array of loudspeakers in a practical performance space demonstrates that MAPP Onlines predictions are highly accurate, and may be relied upon in designing real systems. It is worth noting that MAPP Online is the only acoustical prediction program that yields frequency response data, and that the programs predictions are remarkably accurate even when viewed at the finest frequency resolution. Among other benefits, this capability affords the opportunity to predetermine equalization and delay settings with great accuracy, assuring that installed systems will perform to specifications with minimal onsite adjustments.