AES Section Meeting Reports

Los Angeles - February 25, 2014

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The February 2014 meeting of the AES-LA featured 3 speakers on the topic "Taming the Bass Monster." With us for the meeting were Todd Welti from Harman International, Steven Klein of Sound Control Room, Inc. and Nick Antonio of ARUP Acoustics.

First up was Todd Welti, who summarized the research done over the years by Harman on room modes, bass response and subwoofer placement. One of the really thorny problems with bass is that seat to seat variation in bass response, especially in small rooms, can be quite large, very audible, yet difficult to fix. In particular, the seat to seat variation cannot be corrected simply with changes to EQ or level, as what fixes the bass response for one position may make it worse at another.

One possible solution is to use multiple subwoofers, to take advantage of mode room mode cancellation — using the output of one subwoofer to cancel out the output of another at the problematic areas in the room. The trick then is to determine the optimum number and placement of subwoofers to create a nice, even response across listening positions.

Todd and the team at Harman generated a series of Matlab simulations to research this, which required the creation of a couple of metrics to use for the simulations. The first is Mean Spatial Variance (MSV), a number (expressed in dB) meant to capture the variation in frequency response across a randomized set of seating positions in the 20Hz — 80Hz range. The second is Mean Output Level (normalized to the number of subwoofers) or MOLn. This is a measurement of average low frequency energy averaged over all of the seats in the 20Hz -- 40Hz range.

Having defined these metrics, the team ran a series of simulations over rooms of varying size, from a 4X4 meter square up to a 9X9 meter square at 0.2m increments in each dimension, for a total of 625 different rooms. For each set of 625 rooms, they could then generate the calculated MSV and MOLn figures for different examples of subwoofer configurations — for example, one sub in each corner, or 2 subs placed at the midpoints of each side wall. They then charted the calculations, looking for patterns that led to desirable MSV and MOLn figures for different configurations of room shape and size and subwoofer placement.

Many of the results track conventional wisdom — square rooms are not good for even bass response, and as expected 4 corner-based subs lead to strong (albeit not terribly even) bass. Some results were a little unexpected, such as the strong showing of a 4 subwoofer configuration, on opposing walls (either front/back or left/right) at the quarter points of the walls.

The full white papers and graphs of the results are available for download from Harman.

Next up was Steven Klein of Sound Control Room, Inc. Steven is a designer of recording studios and control rooms, which is an environment with bass management challenges of its own. In particular the question of whether to use subwoofers in a control room environment has its partisans; Mr. Klein comes down in the affirmative. The logic here is that if you lack subwoofers, you wind up turning up the volume to hear the bass, possibly to the detriment of what you are trying to accomplish with all the other frequencies.

One must be cautious with both room design and equipment spec, though, as the effects of high-pass filters in your control room monitors can affect tracking. Mr. Klein also advocates careful acoustic room design so as to avoid the necessity of electronic equalizers to provide flat response.

Our final speaker was Nick Antonio of ARUP Acoustics. Whereas Mr. Welti and Mr. Klein focused on smaller rooms, ARUP specializes in very large room acoustics, which have challenges all their own. He opened with the fair question: what is a large room? For Arup's purposes, a large room for music (in which you need flat response down to 30Hz) is 250,000 cubic feet or greater. For speech (with a required flat response down to 80Hz) anything 35,000 cubic feet or greater is considered a large room.

The "bass monster" in these large rooms lies in achieving an idealized bass ratio, which leads to a perception of a pleasing "warmth" to the sound. This sounds fuzzy, but there is some math lying behind it — the bass ratio is calculated by dividing the mid-frequency reverb time by the low frequency reverb time.

Measurements of 45 concert halls, all regarded as having very good or ideal sound, show that they all have a bass ratio falling in the 1.1 — 1.4 range, making that the target range for Arup projects. This can be achieved by, naturally, providing for enough low frequency power to begin with, but then attempting to tame the reverb time and room modes with acoustic absorption -- either full on absorption (as with curtains) or utilizing Helmholtz panels, which absorb low frequencies but diffuse the higher frequencies.

It was a real treat to hear about approaches to "taming the bass monster" from our 3 guests, and the AES-LA would like to thank them for helping to educate us.

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