Posted by: recordingsofnature | February 22, 2011

Pressure equalizer geometries and directionality

The purpose of the present investigations is to try to get an understanding of this, puzzling  topic of pressure equalizers. Applying them on an omni microphone it is possible to change the pickup in terms of directionality and frequency response. The understanding relates to the interaction sound waves interacting with a certain geometry. Since most of the audible wavelengths are comparable or larger than most microphone related geometries,  the result is a complex diffraction pattern, highly sensitive to the surface properties, sidewall curvatures and precise alignment of the microphone head.

5 mm concave, straight line sidewalls pressure equalizer

5 mm concave, straight line sidewalls pressure equalizer


It all comes down to the fundamentals of acoustic waves with a given wavelength (size) interacting on a geometry of given size and shape and how they reflect and especially diffract. This, however, also means, that by scaling the geometry up and down in size, it will be possible to correspondingly scale the frequency response.

Using the measurement method described in the previous post I have attempted to do a more systematic survey of the relation between the pressure equalizer geometry and the frequency response in different directions.

All the tests are made using a ø=30 mm film roll (constructed by 3 old 36 picture Afga films, see this post), which can be formed to in wide range of concave and convex shapes, which however always will be nicely cylinder-symmetrical. The images below show two examples of the film roll, one with round and one with straight convex profile (of 10 mm’s height).

Film roll pressure equalizers, Ø=30 mm

Film roll pressure equalizers, Ø=30 mm. Curved and straight line sidewalls


Film roll pressure equalizer fitted on Earthworks QTC40

Film roll pressure equalizer fitted on Earthworks QTC40


Below is a gallery of the directionality pattern of the frequency response for a set of different shapes of the pressure equalizer. The pickup pattern is visualized as described in the previous post. The geometries go from highly convex (straight or curved sidewalls) to highly concave. In a few of the tests, the microphone is adjusted 1.5 mm in the center position, to illustrate importance of fine adjustments.


A more comprehensive and complete set of  measurement data charts for all the tests can be found in this PDF REPORT

As is clear from the results, no simple relationship formula is obtained. The response is a complex diffraction pattern, which depends on the complete shape of the geometry. E.g. small adjustments of the position of the microphone in the film roll pressure equalizer gives quite large changes of the whole spectrum. Convex shapes have a gentle effect, with a quite smooth focusing of forward facing sounds. Straight sidewalls give a slightly more bumpy response compared to the rounded sidewalls. With the 30 mm film roll, a gain bump is present typically around 6-9 kHz.

In most cases moving the microphone back by 1.5 mm clearly creates more forward sensitivity, almost comparable to a cardioid pattern.

Concave geometries surely creates very  forward facing focus, which however also suffers from strong diffraction effects with also negative interference.

The present measurements cover very high frequencies, however, enlarging the geometric size will scale down the frequencies accordingly.

If we were able to see acoustic waves, like we see light, we might be more used to deal with strange diffraction patterns generated everywhere sound is present. Complex wave phenomenons, constructive and negative interference standing waves, room modes building up or changing, stationary and dynamic wave propagation. Maybe something like super high-speed sea waves! Maybe bats might have an opinion on this issue. The sensitivity to even small changes of the shape makes one understand the challenge of forming a shape for an acoustic function, and appreciate the workmanship of building musical instruments, trumpets, violins, organ pipes etc.

Next post will attempt to narrow down possible candidates in the process of finding the optimal pressure equalizer geometries for music, nature and high frequency recordings.


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