MAPP Online Pro Acoustical Information

Surfaces

Currently surfaces that have been activated, and for which a material was chosen, model only the first reflection from each surface by generating a single image source for each loudspeaker on the other side of that surface. Surface materials are modeled by multiplying the frequency response of each image loudspeaker by a magnitude only frequency response representing the material. The frequency response of each material is interpolated up from octave band absorption values taken from standard acoustical texts. The initials at the end of the material name indicate which book they are from:

CH = Cyril Harris, "Handbook of Acoustical Measurements and Noise Control", Third Edition, Acoustical Society of America, 1998.
LB = Leo Beranek, "Acoustics", Acoustical Society of America, 1996.
KF = Kinsler, L, Frey, A, et al., "Fundamentals of Acoustics", Fourth Edition, John Wiley and Sons, 2000.

Sound Field Scaling

Sound field predictions are normalized such that the largest value in the sound field is set to 0 dB. The dynamic range of the sound field plot is 42 dB. Positions in the sound field that are more than 42 dB down from the maximum are displayed with the darkest color blue from the colorbar.

Frequency Response Scaling

Frequency Response data is normalized to 0 dB at 1000 Hz. This is important to keep in mind when comparing two or more frequency responses. Because the third-octave band spectrum is displayed in absolute dB SPL, it is often more intuitive to compare Band Spectra.

Meyer Sound Loudspeaker Data

The apparatus used to measure the polar response of each loudspeaker model consists of a Meyer Sound SIM System II analyzer, a Bruel & Kjaer 4133 omnidirectional measurement microphone, a B&K 2639 preamplifier, and a B&K 2807 power supply.

Each loudspeaker is placed in an anechoic chamber on an automated turntable positioner (accurate to within 0.1 degrees rotation), with the microphone placed 4 meters away from the geometric center of the loudspeaker.

The turntable is rotated through a full 360-degree arc with 1-degree increments. This measurement is performed once along the horizontal on-axis plane, and again (separately) along the vertical on-axis plane.

A piecewise approximation to a constant-Q transform is utilized in the measurement so that the frequency resolution is consistent across the full frequency range. This transform affords greater than 1/36th-octave resolution from 20 Hz to 20 kHz.

The microphone to preamp sensitivity is 11.5 mV / pa (or -38.8 dBV @ 94 dB SPL, when expressed logarithmically). This sensitivity becomes part of the measured polar response transfer function data that MAPP Online Pro uses for each loudspeaker model.

SPL Calibration

The Meyer Sound loudspeakers available in MAPP Online Pro have had their maximum linear-frequency-weighted, slow-time-weighted, average SPL measured at two meters while being driven with pink noise at the onset of limiting. This is done by driving the loudspeaker with a pink noise signal, increasing the amplitude of the noise, and watching the level of the frequency response in the SIM analyzer.

Because a frequency response measures the ratio of output to input, if the loudspeaker is functioning linearly, increasing the input will cause a proportional increase in the output and the frequency response will not change. As the input level is increased you eventually reach a point where the speaker can no longer reproduce the highest peaks. Because the input increased, but the output doesn't increase due to limiting, the frequency response decreases.

When a loudspeaker's maximum average SPL is measured, the level of the pink noise is increased until the decrease in frequency response just begins to happen. The linear-frequency-weighted, slow-time-weighted, average SPL of that speaker is measured for that drive level. The sound level meter is then switched to peak-reading mode to ensure that the peak to average ratio (crest factor) of the output sound is the same as the crest factor of the input pink noise. Because the crest factor of SIM II pink noise is 12.5 dB, the peak SPL reported by MAPP Online Pro is always 12.5 dB higher than the average. However, this peak SPL computation is invalid when the prediction contains only subwoofers, since subwoofers are band-limited to the lower octaves of the audio spectrum. The peak SPL computation for a loudspeaker plus subwoofers is correct. We are working on an improved peak SPL computation algorithm that could recognize band-limited systems (e.g. subwoofer, subwoofer arrays, low-passed loudspeakers, etc.) and correctly predict the peak SPL based on the operating bandwidth.

The peak level reported by a loudspeaker in MAPP Online Pro may be lower than the peak level reported in the datasheet for that loudspeaker. The peak level reported in the datasheet is measured using a short burst of music. Meyer Sound loudspeakers can reproduce short bursts of high power before the limiters engage. For this reason the peak level for music reported in the datasheet may be higher than the peak level for pink noise which is reported by MAPP Online Pro.

MAPP Online Pro has been calibrated such that a virtual microphone placed on axis to a virtual speaker set to a relative level of 0dB will report the same average and peak SPL as the actual loudspeaker when driven with pink noise at the onset of limiting.

Low Frequency Polar Data Acquisition

The high-resolution polar data used by MAPP Online Pro is acquired at one-degree spatial resolution using an automated positioning device in our anechoic chamber. The measurement computer uses a SIM-type algorithm to acquire 1/24th octave complex (magnitude and phase) data from 0 Hz to 20 kHz.

However, the anechoic wedges used in our chamber are only rated to 100 Hz. Below 100 Hz, the data in our anechoic chamber, while "pretty good," is not accurate enough to give good results in MAPP Online Pro.

Building an anechoic chamber that would give a good measurement of the free-field (anechoic) response of a subwoofer would be very difficult and expensive. The wavelength of sound at 30 Hz is 11.4 meters (38 feet), and therefore the anechoic wedges would need to be on the order of 10 to 20 feet long in order to absorb, and not reflect, low-frequency energy.

However, measuring outdoors has its problems as well. Cars and trucks create low-frequency rumble that can contaminate measurements. Wind noise can also be a big problem. But if done carefully, outdoor measurements correlate well with indoor anechoic chamber measurements.

The figure below shows an on-axis frequency response measurement of a single Meyer Sound M2D loudspeaker at four meters, measured on a flat surface with the microphone placed directly on the ground. The ground is known as a "half plane," and on axis to a single loudspeaker it creates a very accurate measurement of the frequency response compared with a single loudspeaker measured in an anechoic chamber. The half-plane causes the magnitude of the response to increase by 6 dB compared with the anechoic chamber measurement.

The blue trace in the figure below shows the outdoor SIM half-plane measurement of a single M2D, minus 6 dB. The red trace shows the anechoic chamber measurement of a single M2D. Note the excellent correlation between the two measurements in the range from 100 Hz to 10 kHz. Above 10 kHz, ground plane measurements are inaccurate due to the size of the microphone compared with the small wavelengths of the sound. Above 10 kHz, we have found through extensive testing that our anechoic chamber is very accurate.

Below 100 Hz, as expected, the two traces differ. Especially visible in the red trace is a 10 dB hole centered at around 80 Hz. Since our chamber wedges are only rated to 100 Hz, we have found that ground plane measurements are more accurate.

Boundary Element Modeling

In order to "correct" our anechoic chamber measurements below 100 Hz, we could measure outdoors from a distance of four meters, at one degree rotational increments. However, this is tedious and prone to error.

Instead, we have chosen to simulate the polar response of our loudspeakers below 100 Hz using a mathematical technique called Boundary Elements.

Boundary Element Methods in computational acoustics are similar to Finite Element methods in mechanics. A mesh is created of the object to model, and the physics of wave phenomena are solved for each element in order to provide a solution of the wave equation. The figure below shows a model mesh of an M2D loudspeaker. Even though this mesh might look "boxy," it is more than sufficient for accuracy at low frequencies. A general rule of thumb used in Boundary Element Modeling is to use a mesh grid that corresponds to 1/10th of a wavelength of the highest frequency predicted. At 100 Hz, the wavelength of sound is approximately three meters (about 12 feet), and so 1/10th of a wavelength is about one foot, which is larger than the model mesh used for the M2D. This mesh would not yield accurate results for mid or high frequencies, but for these frequency ranges we use the very accurate data from our anechoic chamber.

By specifying the velocity boundary conditions of the model loudspeaker mesh, we can inform the model of where the vibrating structure corresponds to the dual 10-inch woofers of an M2D. The other mesh elements are then assumed to be rigid (reflecting).

Once we specify the velocity boundary conditions, the software solves the Kirchhoff-Helmholtz Integral Equation in order to find the surface pressure on the loudspeaker mesh. The figure below shows a colormap of the surface pressure (at 90 Hz).

Once we know both the velocity boundary conditions and the surface pressure on the loudspeaker mesh, the software can solve the integral equation to find the pressure at any point in free space. In our case, we have asked the software to solve for the pressure on a four-meter circular radius, which corresponds to the four-meter radius in our anechoic chamber.

The following two figures show how we calculate the complex pressure at four meters every one degree. In our software package (Sysnoise), this is called Directivity.

By combining the on-axis, four-meter, outdoor half plane measurement and the Boundary Element one-degree directivity calculations, we can supply the accurate complex polar data that MAPP Online Pro requires below 100 Hz. The next three figures show a MAPP Online Pro prediction of a single M2D with a microphone at 1 meter. Notice how the Frequency Response and the Band Spectrum calculations now extend (accurately) below 100 Hz.




Not all of the loudspeakers in MAPP Online have been corrected at this time. MAPP Online will automatically display the spectrum and frequency response below 100 Hz if the polar data file has been corrected. If information below 100 Hz does not display, the data have not yet been corrected.

More information about Boundary Element Methods in acoustics can be found in the book, Inverse Acoustic and Electromagnetic Scattering Theory: Second Edition, David Colton and Rainer Kress, Springer-Verlag, Berlin, 1998.

Acoustical FAQ

When I "corner load" a subwoofer, I only get a 10dB increase in SPL. I thought a corner increased a subwoofer output by 12dB?

As mentioned in the "Acoustical Information," MAPP Online Pro currently models only the first reflection from boundary surfaces. This accurately models their "comb filtering" effects. However, it underestimates the effect of "corner loading" subwoofers. For one boundary, it is accurate. i.e. a subwoofer placed on the ground gives 6dB more SPL then a subwoofer in free space. However, for two boundaries (a wall-floor intersection, for example) this first-reflection model shows an increase of 10dB, which is less than what is expected from a two-dimensional " theoretical corner loading", i.e. 12dB. In three dimensions, (which MAPP Online Pro does not currently model), the SPL of a corner-loaded subwoofer increases 18dB. Note that this ONLY works for low-frequency sources placed very close to corners. It does NOT work for higher frequencies, when sound starts to become directional. (source: Acoustical Engineering, H.F. Olson, 1954).

Why does a UPA seem louder than a CQ in MAPP Online Pro?

When the SPL of a loudspeaker is measured for MAPP Online Pro it is driven with full-spectrum pink noise at the onset of limiting. Because a CQ covers a wider frequency range (more low frequencies) than a UPA it limits earlier for full-spectrum pink noise than a UPA. If you were to drive both speakers with A-weighted pink noise, a CQ would be louder than a UPA. Meyer Sound is in the process of developing a method of measuring the maximum SPL of each loudspeaker on a per-octave basis.

What are the initials at the end of surface material names (i.e. the "CH" in "concreteblockpaintedCH")?

The initials at the end of the material name indicate which text book they are from:
CH = Cyril Harris, "Handbook of Acoustical Measurements and Noise Control", Third Edition, Acoustical Society of America, 1998.
LB = Leo Beranek, "Acoustics", Acoustical Society of America, 1996.
KF = Kinsler, L, Frey, A, et al., "Fundamentals of Acoustics", Fourth Edition, John Wiley and Sons, 2000.

Why are there no ceilings or floors in MAPP Online Pro?

Every effort has been made to avoid labeling any part of the sound field up/down/left/right/top/bottom so that you can feel free to think of it as either a floor plan (four walls) or a section plan (two walls a floor and ceiling).

Why wasn't (material name here) included as a surface material?

There are certain materials that we have not included because we want to measure their absorption values ourselves. Lightweight materials over an air space are intended to achieve some of their absorption by resonating (especially at low frequencies). We expect that this will effect the phase to a large enough extent that it cannot be ignored.