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Counterintuitive? Cardioid Subwoofers Can Be A Bit Claustrophobic

Investigating what happens to directivity when a cardioid sub is placed underneath a stage.
The subwoofer configuration at Chicago’s Auditorium Theatre referred to in the article.

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For a little while, this seemed to be the only focused experiments published in this area. Luckily, my colleague, Simon Lewis, discovered work by an Italian firm which replicated our experiments in late 2017. (That report can be found at illatooscurodellafase.wordpress.com.) In this case, the tests were conducted outdoors with a simplified stage (plywood was used). Results seem to indicate that the degradation of the cardioid response is even more pronounced than our experiments had shown!

Recent experiments by Kees Neervoort from Event Acoustics (Netherlands) once again replicated my original results and expanded the scope of the problem finding that stage “catwalks” further degrade sub directionality.

This isn’t quite the end of the story, though. There’s something else to keep in mind. Most festival stages have stage skirts to improve the “look” of the event. It’s important to check whether these are acoustically transparent or not. If they aren’t, you’ll find that sub output will decrease drastically.

This is due to the reflection coming off the stage skirt. The reflection, when it reaches the front of the subs, will be significantly out of phase from the direct sound (the precise relationship is strongly frequency dependent). This will cause partial cancellation of the forward-moving wave, thus reducing efficiency. This effect was recently confirmed experimentally at a mock live event as part of the above-mentioned research conducted at Event Acoustics.

The same goes if you’re indoors and place a cardioid subwoofer against a wall. I ran into this situation when on a gig with Adam Rosenthal a number of years ago at Chicago’s Auditorium Theater. The only place we could put the NEXO CD18 subs was on the offstage wings in front of the proscenium.

When we fired up the system, the low frequencies seemed to be extremely low level even though we were driving the system fairly hard. After thinking about it for a few minutes, we realized that the reflection off the proscenium was cancelling out our direct sound.

The solution? Turn off the rear drive unit, making the CD18s omnidirectional. After that, the low-frequency content came back to the expected levels and we had a good show. So, what are the takeaway messages here?

— Don’t place directional subs underneath a stage.
— Don’t place directional subs in front of a wall.
— Beware of non-acoustically transparent stage skirts.
— Do place directional subs slightly in front of a stage (about 3 feet of clearance is recommended).

As a final note, in case you think all of this stuff is new and I’m brilliant for highlighting these issues, I should note that Harry Olson and Richard Waterhouse figured out just about all of this before most of us (and even some of our parents and grandparents) were born. Read their papers and books! There’s nothing new here…

Academic papers this article is based on:

Hill, A.J.; J. Paul. The effect of performance stages on subwoofer polar and frequency responses. Proc. Institute of Acoustics – Conference on Reproduced Sound, Southampton, UK. November, 2016.

Hill, A.J.; M.O.J. Hawksford, A.P. Rosenthal; G. Gand. Subwoofer positioning, orientation and calibration for large-scale sound reinforcement. 128th AES Convention, London, UK. May, 2010.

And, selected “historical” reading:

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Olson, H.F. Elements of Acoustical Engineering, 2nd Edition. D. Van Nostrand Company, New York. 1957.

Waterhouse, R.V. Output of a sound source in a reverberation chamber and other reflecting environments. Journal of the Acoustical Society of America, vol. 30, no. 1, pp. 4-13. January, 1958.

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