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Helmotz Resonator Design - Perforated and Slat Type
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Studio Design Categories:  Modal Analysis Acoustic Materials Quadratic Diffusor Acoustic Equations Reports Database Maintenance |
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Studio Design 2010 allows the user to enter in room dimensions of length, width, height
and the reverb response for the room. It calculates all the modal frequencies for axial, tangial, oblique modes
and graphs the results for easy to see modal spacing and mode buildups. In graph mode, the user can tweak the room dimensions to optimize the modal spacing and build-ups. In material mode, the user can optimize acoustic materials for desired room reverb response. Low frequency modal build-ups are corrected by building helmotz resonators tuned to the troublesome mode(s). | ||
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The screen allows entry of parameters and calculates the resonant frequency. The program will estimate the coefficients of absorption for 125hz through 4000hz. Apply adjustments to the coefficient estimates for insulation, percentage of perforation, and percentage of slat-gap. Save the design by clicking the Add to Acoustic Materials button. |
Low Frequency Resonator options
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Perforated Type Helmotz resonators
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The example demonstrated here requires low frequency absorption of 43hz, 70hz, and 111hz. The screen above shows a resonant frequency of 43hz. It uses a 1/2" plywood top with 1/8" holes drilled into it every 5" in a triangular pattern. The sides of the absorber are 2.125" wide plus 1/2" for the top and 1/2" for the back. Also allow some width for an inset such as fabric cover, trim, and lip. A 1"x5" pine board would be ideal... 3.125" for air gap, top, and bottom... 1/8" for fabric cover and 3/4" for wood trim... and 1/2" for lip. That's 4 1/2" wide. |
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The coefficients of absorption are recalculated when insulation is not being used. Without insulation, the band- width around the resonant frequency is reduced which lowers the absorption coefficients. |
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The graph above is used to provide the estimates. Yellow is for adjustments to perforation percentage, Blue is to estimate adding insulation, and pink is without insulation. This is a normalized graph. |
Saving a resonator design
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Click the Add to Acoustic Materials button. A default name is provided along with the coefficients of absorption. |
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Click the Add button and make selections for supplier, category, and freq band. Click Save to add this into the acoustic materials database. The absorber will be available to use in the design! Close the screen to go back to the absorber design. |
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Another troublesome modal build-up in this example is 70hz. Changing the hole spacing from 5" to 3" provides the desired result while keeping the construction the same as for the 43hz absorber. Add this absorber to the database also. The remaining troublesome mode was 111hz. Just change the perforation to rectangular and hole spacing to 1.75" for an 112hz absorber and add it into the database too. Slat type absorbers are a nice variation and look great. They are most practical over 150hz depending on air gap and thickness of lumber used. |
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