Posted by: recordingsofnature | November 14, 2015

Windy November forest – going semi-binaural

Forest recording of a warm windy, and sunny Sunday morning, November 8th 2015 around 9:30 am. Appx 10C. The recording is from almost the same spot as the June all-night-recording.
Leaves have just fallen these days. A quick storm passed through the country during the night, and it has really helped thin out in the treetops. In the morning, waves of windy gusts were still blowing through the forest, though at the forest bed was barely more than a light breeze.
November forest and microphones on tree trunk
The sound of rustling leaves has changed into a more light and thin nature, with the addition of a deeper howling from the bare branches.

Sunday mornings are the time where the nearby motorway noise is at a minimum, still it is a throughout present sound.

Brønbyskoven november 2015-3

brønbyskoven november 2015

Going semi-binaural

It has become a never ending (but highly interesting) endeavour finding the just-right recording configuration.

With this recording I have tried a different approach with my tree-ears platform. Using the same microphone capsules and wind shields, this time I have not used the human ear forms but rather a nearly flat surface. Only feature is a round (adjustable) bar about 1 cm behind the capsules.

tree ears simple setup

This new setup is an attempt to overcome some of the fundamental limitations I have experienced with binaural recordings (with human-shaped ear models). Those limitations include the very non-linear spectral response of the ear cavity with a broad resonance peak around 2 to 7 kHz, and a strong high frequency roll-off already from 8-9 kHz.

As a result, this means the recordings need to be post processed and equalized in order to sound natural, i.e. attain a flat average frequency response (diffuse field EQ). This prevents the resonance effects to pass through the ears twice. This is a real problem for long mp3 recordings, which I cannot post process without decoding/re-encoding, causing significant quality loss.

Ideally, the binaural microphone should only add the directional spectral cues centred around a flat average frequency response i.e. leaving out the non-directional gains.

Semi-binaural recordings are free of these constraints, and are typically of much higher quality in terms of linearity and smoothness. But the trade-off is the lack of control of the up/down elevation nor forward-backward directions.  For example, with my old semi-binaural setup I often feel that sound comes from above, when it is from directly in front.

So, this is the background of this new semi-binaural microphone setup. The stating point is a clean configuration with microphones just sticking out on the surface on each side. It is then a matter of adding specific directional reflectors/shadows to mimic certain directional cues. I actually believe only a few directional cues are needed in order create a convincing spatial effect.

tree ears simple setup front

With this recording I have a placed the round bar about 13 mm behind the center of the capsules to mimic the cues for forward and forward-up directions. This bar produces a reflection for sounds coming from in front, resulting in a quarter-wave notch around 343m/s  / (0.013m * 4)  ~ 6600 Hz. This effect has some resemblance with  typical HRTF cues for sound coming from in front. This reflection gradually diminishes for larger angles and is nearly absent for directions 90 degrees off, i.e. from the side or from above.  So it appears placing a bar behind the capsules is a way to control the forward direction cue and help to distinguish between directly forward and forward-up.
I will definitely do some more investigations on this in a future post.

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