Comparison of Two Low Frequency Equalization Techniques

Download QuickTime 4 Channel Surround Comparison movie [ 6mb  .zip ]
Download QuickTime Stereo Comparison movie [ 4mb .zip ]

It is fairly common post production practice to use High Pass (HP) or "Roll-Off" equalization in addressing exaggerated low frequencies in field recordings. This movie compares this technique to one that does not use any HP filtering but rather a number of parametric "notches" at offending lower mid-range frequencies under 500 Hz. An EQ "plug-in" is inserted between two M-S matrix plugs in the mixing application so that separate EQ can be applied to the center and sides of the stereo field. (Here's a sample Reaper template using different, native plugs .zip]. Lower-mid range attenuation is most effective in the center of the stereo field as sound waves under 500 Hz pass freely to both sides of stereo mic arrays where they are summed, in phase. Boosting the level of the out of phase sound energy on the sides under 100 Hz can also improve the sense of spaciousness in some recordings. 

In the movie, two samples are repeated three times: (1) Flat, as recorded without EQ (2) Equalized only with Lower-Mid-Range Parametric Attenuation and  (3) Equalized only with 6 dB/Octave High Pass Filtering.  The samples come from an outdoor surround array comprised of two different stereo mic arrays: [Front S3]   [Rear PMBM1]. The arrays are separated 400 feet in a wooded valley. I chose material to demonstrate that the EQ technique works with different types of stereo arrays and that equalization is dependent upon the differing acoustics at the two positions. Overhead thunder provides ample, low frequency content that I found I could adjust more realistically than man-made low Hz "pollution."   I find lower-mid-range parametric filtering is effective at improving the clarity of most ambient field recordings. I've often wondered if sound wavelengths on the order of 4-24  feet (100-500Hz)  could be more prone to acoustic confusion in large spaces somehow. Mic technology is probably playing a role as well, but the wavelengths in this range seem to reinforce each other and form tight bundles of discordant frequencies that "mask" sounds with similar color very efficiently. There is more description of the steps I use when equalizing here. 

If you are already set-up for 4 or more channels of surround monitoring with QuickTime, download and play the 4 channel version on your computer.  If you play this version on a computer that is not set-up for surround playback, the rear stereo channels will automatically "fold" or mix into the front stereo pair. For those with stereo only, it will probably be easier to evaluate the results by downloading and playing the stereo version that has samples only from the rear array:  [ 4mb .zip ]

Both comparison movies have no equalization adjustments over 500 Hz and the attenuation of the lower mid-range content can make the overall tonal balance seem too "bright."  Here is a longer mix of the 4 channel surround recording made with with Lower-Mid-Range Parametric Attenuation and above 500 Hz equalization:

Download QuickTime 4 Channel Surround Comparison movie [ 25mb  .zip ]

Four Stereo Mic Arrays Compared - Take the blind test

View Array Comparison QuickTime Movie
AAC 320kbps Soundtrack [10 mb]
Adjust sound to comfortable playback level.

Alternate formats: sound-only .flac [download] ; sound-only [.mp3]

This test was conducted at the May 2009 "Recordist Campout," with the goal of comparing the localization (position) and depth (consistent distance) performance of stereo mic arrays. It features "pink noise" played with a boom box in 15 positions at a constant distance of 100 feet in an open, natural landscape. Although several arrays were tested, at the time of this writing, there are four to compare:

Rich Peet's "Cube Mic" with 4- Audio Technica AT-3202 Mics flush-mounted in four sides of high density foam cube shape. Designed for surround, two of the mics were used for this stereo test. They are angled 90 degrees and separated 14".

David Michael's Sennheiser MKH-40/30 M-S Pair. The MKH-30 is a figure 8 mic and the MKH-40 has a cardioid polar pattern. Care was taken to make sure the M-S decoding was optimized in post.

Gordon Hemptom's "Fritz" or Neumann KU81i binaural head mic with customized ears.

Rob Danielson's "Perp2Sphere" Mic with 2- Rode NT2000 mics in Omni mode with mic capsules facing-forward on opposite sides  8" diameter wooden sphere.

All of the recorders were "rolling" at once so sound stimuli in all of the tests are identical. The landscape is sloped at angle of about 10 degrees from left to right so other changes in apparent elevation can be attributed to differences in the mics/arrays. Here is another, slower-paced version of the test made with longer durations of the same clips.

At first, the pans can seem quite similar, but by playing the test a number of times at a comfortable sound level, significant differences can emerge. Using processes of elimination and what they already know about mic array traits, the field recordists I have shared the test with have been able to identify arrays used with a high degree of accuracy. They have pointed-out differences in horizontal spacing as well as depth (apparent distance from the mic across the stereo field) and overall consistency or smoothness. Several have commented that it helps to close your eyes and visualize a boom box playing short excerpts of pink noise across a 200' wide landscape. The different mics used in the arrays account for much of the frequency response differences heard. After you have noted the performance differences and allocated them to appropriate array types, here's a chart identifying the mic arrays to see how you fared.

Visitors are welcome to leave observations and questions about the test below. Rob D.

Comparing apEQ to Eqium 2.0

Ever since Paul Jacobson pointed Nature Recordist list readers to apEQ, I've been wanting to compare it to Eqium 2.0 . The later plug is currently sold as UNIQUEL-IZER for considerably more than ApEQ. A question posed by Justin on the Nature Recordist group prompted me into action.

To conduct an A/B test, I made a recording of the dusk interactions in a rural setting including car traffic and hubbub from a village a mile away in order to create a sample of a recording that would likely benefit from EQ.

As I've mentioned on the topic of field recording equalization before, "I never met a lower octave I did not like." I rarely use "roll-off" filtering of the type Justin is experimenting with because I feel that I can better address the most exaggerated frequencies, individually, with narrow-ish parametric EQ "curves." Its time-consuming, but I feel this technique preserves more of the fundamentals of the sound waves that are helpful in recreating useful overtones in the lower mid-range.

Here's the comparison as a 7mb QuickTime movie. The soundtrack is full resolution (16 bit/48K). Here's screen shot of the curves used.

A few observations:

(1) I'm really surprised at how differently the two EQ plugins affect the recording-- especially within the range of 80 Hz to 500 Hz. The result of attenuation made with a single apEQ "peak" curve has more impact. Even with very careful "Q" or width settings, apEQ tended to remove a little more of the "body" and adjacent tones than I'd prefer at times. However, the difference is subtle and only shows up after a more complex curve is in effect. With a wider boost/cut range of 40 dB, its easier to use a + dB "peak" in apEQ to audibly locate an offensive tone or bandwidth than with Eqium. Recordists who like to attempt to "remove" man-made drone sounds as might like the greater expediency of apEQ. I found that I could get impressive improvement in the field recordings I experimented with as few as 5 to 8 curves. apEQ might be preferable when needing to quickly but effectively equalize a recording. I'll probably stick with Eqium when I'm trying to coax "space" out of an ambience recording, but the sound quality differences are curious and worth more experimentation for sure.

(2) The controls of apEQ are fantastic. After you create a Peak EQ curve by clicking anywhere on the master curve, you can click on one control variable in the floating box to drag-change that setting without affecting the others. This is very handy for fine tuning Q and the Gain after you find the frequency. I also like the "bypass" button for A/B comparing just one setting. I didn't find a "B" buffer option for comparing two sets of EQ's; maybe I missed this.

(3) The superimposed FFT display of apEQ is also fantastic-- I found that I referred to it a lot. (I have to open a separate FFT window when I used Eqium and this clutters the screen and is not as visually efficient.)

(4) I could not find a master gain knob in apEQ. Also, I couldn't find a global balance setting. Stereo mic pairs usly need some tweaking. Of course, both of these corrections can be achieved with other plugs and time line settings, but I missed having these basic adjustments saved with the plug setting document. Or, maybe I missing them too?

(5) I'm not sure how many bands of EQ one can create with apEQ but it seems like plenty. (One can create 99 bands with Eqium.) apEQ probably taxes the CPU more than Eqium but the preview audition mode seemed very responsive on my 2004 model 2GHz Dual G5 PowerPC Mac.

I would definitely give the free demo of apEQ a try if you are contemplating spending some money on EQ. Rob D.

Equalization Technique for Diffuse Field Recordings

At 2:27 PM +0300 4/2/09, Mike emailed me:
Hi Rob, Do you employ any structures around the mic rigs to equalize / compensate for the high frequency roll off associated with the diffuse field?

Hi Mike--

I'm not sure I get additional HF loss with the mics capsule oriented Perpendicular to Sphere in my rigs. However, I do find that all far-field recordings usually benefit from EQ if clarity or improved "realism" (based on naked ear experience) is the goal. I've never used, "perfect mics/arrays;" please let me know if you find or make one!

As I've posted a few times on the naturerecordist list (with some dissatisfaction, I'm sure), I feel that each field recording or significantly time-separated event usually requires different EQ. Below is a step-by-step description of the way I tend to EQ field recordings:

[In performing the below steps, I am fond of the Eqium plug developed by Elemental Audio because its quite transparent and supports unlimited bands. Its now sold by Roger Nichols under a different name, Freqium. The EQ app/plugin must also have very quick response in "preview mode." You can use a simpified version of the below steps (fewer bands) if the paratmetric bands of the EQ plug you use aren't very narrow. The resulting tonal balance will probably turn out more natural than one without a procedure to address the biggest problems, first.]

(1) Set playback level at desired final playback level. Mark that point. Make sure the playback volume between the two tracks is balanced.

(2) Fade-up the volume to that point listening to frequencies that "stand-out" or have a drone quality. I calls this "Fade to Find." Make a mental note of the tone of the loudest one (the one you hear first as your fade-up the volume. Most of the time, they will occur between 125-700 Hz. If they are in the HF or VLF, ignore them for now and focus on the 125-700 Hz range.

(3) Use a sweeping narrow parametric band of EQ to locate that frequency between 125-700 Hz that is most exaggerated. After you find it, fine adjust "Q" width and and "tame" or gently attenuate it until the tone blends more naturally with adjacent frequencies. I never try to eliminate anything- this leads to overall imbalance. Basic rule, if it sounds good, cut the effect in half.

(4) Repeat steps (2-3) until the lower mid-range sounds more transparent-- meaning you can hear details that were previously "masked" by the sustained tones. Sometimes, there are as many as 6-8 bandwidths. I never try to eliminate anything- this leads to overall imbalance. Don't forget to use the "fade in to find trick" so you address the problem bandwidths in correct priority. [Note, sometimes the exaggeration is more pronounced on one channel, Eqium allows you address the channels separately].

(5) Explore boosting the upper mid-range (500-3.5K Hz) with 1-3 bands. This can add spatial clarity and depth.

(6) Perform narrow parametric on 16Hz-125Hz with same techniques as in step (2-3). Takes excellent monitoring. I use both phones and the best speakers I have to compare impacts. Don't forget to use the "fade in to find trick" so you address the problem bandwidths in correct priority.

(7) Address HF exaggerations-- most often with quite a few, very narrow bands of parametric EQ. The process is like "drilling out" very thin "zizz." Often its the "edgy" noise that comes from mic self-noise (which will show-up almost every time) and also HF distortion of content with lots of HF (like "sizzling" of leaves, grass in the wind). Done carefully, this method can preserve considerable HF "brilliance" while reducing the excessive HF content that make the HF in the space feel unnaturally close. Note that I've waited to do this step last because HF impression is very dependent on the space established by the mid range and LF.

(8) Adjust output level of the "mix" to get decent saturation-- no less than -15 dB peaks. Up to -.3 dB is no problem. For some reason, the effects of the EQ will be closer to what you heard in preview mode if the levels in the output file are robust. Determine overall dynamics on the EQ'd file in the mastering stage.

(9) When exporting or mixing-out the file, "bump-up" to 24 bits the results if the recording is not already in 24 bits.

So, now you know why others dread this discussion. Rob D.

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.