Posted by: recordingsofnature | June 23, 2011

Optimized Pressure Equalizer for QTC40 for Semi/Far Distance Recordings

This post describes the microphone pressure equalizers I currently prefer to use for nature and location music recordings. They are easily constructed and shaped using a bundle of wooden sticks (match sticks, see the pictures below!)

Wooden pressure equalizer geometry

Wooden pressure equalizer geometry

The aim is to have a multipurpose-use for semi- and far distance recordings, to be used with the Earthworks QTC 40/ QTC 1 microphones. Thus the aim is to achieve a more forward facing focus and increased sensitivity, in terms of a directional HF rise. Applying an acoustic pressure equalizer to the QTC 4o microphone, -which originally is extremely omni directional, linear, fast and clean due to the small diaphragm, seems to be a nice combination for nature and location music recordings.

Shape and design

The pressure equalizer is made using the bundle of wooden sticks, (match sticks). In the center there is a small film roll fitting the microphone. This construction can be shaped easily in many geometries. The final geometry has been optimized mainly through trial and error.

Wooden pressure equalizer for HF rise

Wooden pressure equalizer for HF rise

Technical drawing:

Geometry of optimized pressure equalizer for semi/far distance

Geometry of optimized pressure equalizer for semi/far distance. Distances in millimeters.

As seen on the drawing, the geometry, which is cylinder symmetric, has a flat plateau in the center part,  Ø=21 mm. Further out, surface is smoothly curved backwards to a depth of 12 mm. In the center, the microphone head and the film roll is precisely aligned and offset by 0.5 mm!! This is a very delicate adjustment having some effect on the frequencies from 10kHz and up, as explained in the measurements below.

The shape is quite easy to reproduce, even in the field.  The geometry is a bit similar to DPA pressure balls, which are just round and made of some smooth plastic material.


Using the method described in the posts here and here, some measurements have been done to determine the frequency and directionality response of the pressure equalizer.

Frequency curves:

Frequency response of optimized pressure equalizer

Frequency response of optimized pressure equalizer

Directional frequency and phase response:

Directional frequency and phase response of optimized pressure equalizer

Directional frequency and phase response of optimized pressure equalizer

Some full datasheet can be found here:

As it can be seen, there is a clear and fairly smooth HF rise in the forward direction. It is starting already from 2.5 kHz to 25 kHz.  Above this, the pickup stays around 0 dB. There is a general +3 to + 4 db rise from 4 kHz going to 22 kHz, though increasing to about + 5 dB at 12 – 15 kHz. At 45 degrees, the HF rise follows the forward direction. For 90 and 180 degrees, the pickup is about 0 dB up to 10 kHz, where after it roll off fairly steeply.

The diameter of 48 mm of the pressure equalizers, makes them quite bulky, nevertheless, this allows them to affect frequencies starting from already 2 kHz. I think, in many cases this adds some good presence to the recordings.

In this way the sensitivity of the microphone in the interval is increased in the forward direction. The microphone now has a forward directionality starting from about 3 kHz instead of 8 kHz for the clean microphone. If needed, a softening EQ can be applied afterwards to compensate for the HF rise.

Fine adjustment of the microphone head

Ok, this last measurement is maybe going too much into completely irrelevant details! but still I think it is interesting! and it tells that in some cases, adjustments within 1 mm can actually affect the sound, especially if you are out for ultrasonic frequencies.

The fine alignment of the microphone head relative to the film roll, of only fractions of a millimeter actually has an effect on the frequency response for 12 kHz and up. Ok, the effect is maybe a little small but anyway the effect is clear.

effect of fine adjustments of microphone head

Effect of fine adjustments of microphone head

At 1 mm, the bump around 13 kHz is clearly increased to 7 dB, which start to be too high. For 0.2 and 0.5 both are fine. At 0.2 mm (and 0 mm), the very high frequencies, + 30 kHz maybe starts to roll off. Somehow, I like the 0.5 mm.



  1. A very interesting follow-up. I realise that you have optimised the microphone shroud geometry for a fairly flat response to suit your particular field recording use, but do you think the geometry could be altered to provide a similar 7dB or more gain as that shown at 13KHz, but at a lower frequency band?
    My thoughts are that this small microphone shroud optimised for directional gain, would make an excellent substitute for the large and unwieldy parabolic dish.


  2. Hi – I’m not sure if this pressure equalizer method can be as directional as e.g. cardioid type microphones. I think there are some limitations with this method, especially with the lower frequencies and the directionality. Ideally, doubling the dimensions of the pressure equalizer, will half the frequencies of the x-axis. So the horn might become quite large (and maybe the same size as the parabola) in order to reach deep enough frequencies. The directional pattern should remain the same, it is just the frequency axis that is scaled according to the scaling of the horn.

    It might be possible to try a more funnel-like shape, which will also reach to lower frequencies, and become more directional. But it will not be a very smooth frequency curve. Some different shapes are tried out here:


  3. […] Microphone configuration without the wind shield. The mikes are Earthworks QTC40s with specialized pressure equalizers attached. Those are described here […]


  4. My spouse and i likewise conceive therefore, perfectly composed article!.


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