Using Equalization to Highlight Bird Calls

Posted by Philip Tyler



My goal was to capture early morning birds at a Reed Fen in the Titchwell RSPB Nature Reserve. Upon listening to the recording at home, the "roar" of the surf and the "rumble" of traffic on the coastal road seemed to interfere with the clarity of the bird sounds. Oddly enough, these sounds on the right channel (to the North) were louder than sounds from a village about the same distance to the left.


Using a narrow "Q" setting in the Parametric Equalization plug within Audition, I swept the frequency spectrum listening for bandwidths that seemed most obtrusive. I settled on moderate curves centered on three frequencies: 291 Hz, 862 Hz and 1770 Hz. I reduced the volume of these bandwidths 16 dB, 15 dB and 20 dB respectively. At 291 Hz there was an objectionable rumble. At 862 Hz there was a pronounced roar from the road and surf and there was excessive airyness around 1770 Hz.

I sent the original recording to Rob Danielson to see how he might handle the task of making the bird calls more apparent. He equalized the recording to his tastes and produced a QuickTime video comparing my EQ settings with the original recording, his EQ settings with the original recording and our EQ settings against each other. Our results sound quite similar! I used only low frequency "cut" while Rob used a combination of attenuating low frequencies and boosting high ones. Rob also equalized the right channel differently to try to compensate for the heavier bass on the right side. The down side of the EQ that we used was lowered presence of the Collared Doves and Wood Pigeon whose calls were affected by the low frequency attenuation. I am pretty happy with the results and having Rob come-up with similar results is assuring. I'm convinced that if one is willing to spend time experimenting with EQ, you can get quite acceptable results.


My microphone rig was comprised of two "shotgun" Sennheiser ME66 mics in a crossed configuration angled about 25 degrees. The mics were powered and amplified by a Sound Devices MixPre and recorded on a Sony MZRH1 Hi-MD.

What mics are good for recording quiet subjects & spaces?

Budhaditya wrote on the Phonography list:
"I would like to hear from you about the critical choice of microphone in field recording, specifically for capturing quieter sounds..."

I can get you started on researching this question. The rule of thumb for recording subtle sounds and ambience in quiet locations is to use mics with no more than 16dB(A) self-noise. Here's a list [htm] of suitable mics that I compiled and is reasonably complete. (Other lists)

Your Soundman OKM II K binaural mics "spec" with 32 dB(A) "self-noise" which is better suited for recording in loud enviroments and louder effects. Used In quiet places where the gain has to be increased considerably, the mics' self-noise will contribute audible hiss regardless of the recorder used. Should you be happy with the Soundman mics, you can consider buying a less expensive recorder because the high self-noise will always "mask" or cover-up the very low-noise performance of the mic preamps in the Fostex FR2-LE and Sound Devices 700 recorders. You Sennheiser MKH-416 mic requires phantom powering though which these two recorders provide. Your MKH-416, with 13dB(A) self noise, should be a lot quieter than the Soundman mics.

Your Soundman OKM II mics may require what is called "Power In Plug" or "PIP" in which the sound recorder supplies the DC voltage to run the mics through the 3.5mm stereo mic input jack. The Fostex or SD recorders don't have this input. Their XLR type connectors supply 24/48 volts phantom for condenser mics. You can buy or build an power adapter to run the Soundman mics on a SD or Fostex recorder.

Nature recordists tend to do ambient or "diffuse field" recording pretty regularly and the largest group/Listserv of such recordists has an archive with a lot of relevant discussion you can search & read.
http://bioacoustics.cse.unsw.edu.au/archives/html/naturerecordists/

To save you some reading time, below are some searches, by model number, of omni-directional mics that are frequently discussed on the list:

Phantom-powered, omni-directional mics for recording ambience in quiet places:
Audio Technica "AT-3032" Inexpensive. Surprisingly, low self-noise.
Senheiser "MKH-20" popular, expensive mic. Newer model is MKH-8020
Senheiser "MKH-800" Popular more expensive, multi-pattern mic.
Rode "NT2000" mid-priced, multi-pattern mic. Heavy.
Senheiser "MKH-30" Expensive. Used primarily as the "side" mic in "MS Pairs"

For small mics with a litle less self-noise, the Shure "WL183" [23dB(A)] and the Danish Pro Audio DPA 4060 [23dB(A)] are popular. The DPA mics are small enough to fit inside of the ears. For small mics with sunstantially less self-noise,~14dB(A), the Telinga "EM-23" or "EMKS-23" mics can be made in a fairly small package for PIP. Klas Strandberg ,who is currently the only source for this EM-23 configuration, might have to be coaxed into making them and they are not cheap. As the Primo EM-23 capsules can be purchasd (sans FET) for $60 each, folks on the MicBuilders list are currently trying FET options to achieve similar results. Here's a comparison test with some of the popluar small, electret mics people are using. If you are curious, the self-noise of your Soundman mics "spec" similar to that of the WM-61A's in the test.

Searching the models terms I put in quotes will get you started. Another very important factor is the "stereo"array" used and also shock-mounting and wind-protection. Curt Olson's simple to make wooden Stereo Mic Rigs are certainly worth a look and listen. Walt Knapp has some good photos of popular MS pairs using Sennheiser mics. Rob D.

Attenuating Louder Tree Frogs

Debbi would like to bring-out the subtler, quieter calls of Reg Legged Frogs in a recording where Pacific Tree Frogs are singing much louder. If equalization proves to help, she may decide to work on learning some EQ skills. To get the ball rolling, I've made a QuickTime Movie with four sound clips with my first attempt at EQ and EQ with an effects plug designed to reduce sibbilance. Here's description of the four clips in the movie:

(1) Debbi's original mono field recording.

(2) The recording equalized with 28 bands of parametric EQ using Eqium. I primariy addressed the raspiness of the tree frog calls in the upper mid-range, exaggerated high frequencies and I tried to address a mysterious very low frequency throbbing sound (idling truck?).

(3) The above equalization AND the "De-Essing" plug in Logic Audio (mild high hat setting). This plug was set to apply compression around 2K Hz.

(4) Debbi's equalized sound file she made in Audition.

I tried a number of ways to reduce the very low frequency throbbing in the recording without much success. I elected to leave it as is rather than eliminate it with a high-pass filter.

What do you think Debbie? Which of the four clips best serves your purposes? Rob D.

Equalization To Improve Spatial Transparency

I recorded some impulses from a frozen water main repair a half block away with Rode NT2000's in a parallel boundary rig with 2.5" inset using a Sound Devices 744T recorder.

Here's a link to the recording as a 256 kps mp3 after I equalized it:

The equalization I used is based on a +10dB "lift" 1000-4000 Hz with lower-mid "notching" of the resonant tones. The concrete crusher (which sounds more like a pile driver on the frozen clay) is on the other side of a brick, three story, apartment building. All of the sound waves from the equipment are indirect reaching the rig after reflecting from local surfaces. The difference in EQ used between the two channels above 4K stems partially from noise performance in the two mics-, but also from trying to balance the pronounced hiss from a gas meter that was mostly on one side. Blue = left channel and red = right (I flipped the output after EQ).

Here's a link to the original recording for comparison, roughly volume matched also as a 256 kbs mp3.

I believe it is easier to "visualize" the location of the concrete crusher in the equalized file. The complex "stuttering" echoes created by nearby building surfaces are also more evident. Rob D.

Rising Low Frequency Response

Hi Rob -- On the WL183 and Low Frequency Response
All the mics I'm now using: NT4, NT1A, WL183 have extended bass response, which is good in comparison to the ECM909 I began with. But I notice that what is recorded with all three mics at lower frequencies is much more pronounced than I hear it myself: distant jet aircraft are more noticeable (a big problem where I live). ECM 959 and 999 don't seem so pronounced at LF.

When I look at the Wavelab FFT analysis screen when editing, I see that response goes down to almost dc levels, with NT1A having a hump from about 9Hz to just under 200Hz with a peak at about 40Hz. WL183 not so obvious but still enhanced.

[Spectrum image at top] is from the WL183s in a SASS setup, recording a few seconds of quiet woodland ambience. Does this show a rising LF repsonse to you, or is it an artefact of the analysis method perhaps?

I recorded a couple of music tracks indoors with theWL183 whilst on holiday - and these don't seem particularly bass heavy. Whereas outdoors, the extended bass is more noticeable. I realise this is all a bit hand-wavingly subjective, but I just thought I'd ask. As it is I tend to EQ out at the bottom end to return things to what I remember in the field.

Regards
Allan H

QuickTime Movie on Spatial Clarity 125-700Hz

Here's a QuickTime movie concerning posting techniques for field recordings added here for reference. It is the best example I've prepared to show some of the ways current recording technology are significantly challenged in reproducing sounds in this spectrum in "quiet," low-reverberation conditions and high mic preamplification is used.

If you encounter difficulties playing QuickTime movies with your browser or OS, you can right-click-download the movie in this directory (titled: MicPreClarity125-700HzTest.mov) and use the free Video-Lan movie player to play it. Rob D.