Stereo Frog Recording

Most of my frog recordings are made in mono. There is only a single channel of the recorded call and it plays exactly the same over the left and right speakers. The reason for this is that I generally use a shotgun microphone to help isolate individual frog calls and most shotgun microphones do not capture stereo. Furthermore, when trying to focus on an individual frog you are recording sound coming from what is in all practicality, a point source. Because the sound is coming from a single point, there is no difference in what it sounds like coming from the left and the right. 

But we normally hear in stereo because they have two ears on the opposite sides of their head. Most sounds coming towards your head are picked up by both ears, but sounds coming from the left side reach the left ear slightly sooner than they do the right ear. Yes, the difference in arrival time is very small at the speed of sound, but there is a difference.

Furthermore, your head blocks some of the sound coming from the left side as it moves over to the right ear. So the right ear receives the sound slightly later and slightly less loud. Your brain is able to interpret these differences in arrival times and volume to give you a stereoscopic auditory image.

To some degree, the relative asymmetry of the pinnae and auditory canals also allows you to differentiate sounds coming from above and below as well. So by comparing the signals between your right and left ears, you can localize where a sound is coming from in space. (It is more complicated that this as studies have shown that it isn't simply the position of the ears that determines individual's ability to discriminate the source of a sound. - for example, Claes, et al., 2015).

So your perception of space in the auditory landscape is dependent on the position of the sound source and the difference in arrival time and intensity at each ear and the ability of your brain to assimilate and interpret that information correctly.

Localizing Space in Recorded Sounds

But now stop and think about listening to a recording.  For simplicity, let's imagine you are wearing headphones and listening to a stereo recording.  The relative position of the speakers does not change so to your ears, the sound source for each ear is the same.  Furthermore, your left ear and right ear are hearing different recordings (tracks).  Therefore, the only mechanism you have for localizing the source of a sound is to compare the sound arriving in each ear and extrapolate distance information from those differences.

Here's a simple demonstration of how this works.

Here is a recording of a Cajun Chorus Frog




It is playing exactly the same recording in both of your ears (both channels) therefore you can't localize where the sound is coming from or your brain will tell you it is coming from right in front of you.

Now listen to this version of exactly the same recording.  In the first call, the frog is clearly in the center.  In the second it sounds like it is coming from the right and in the third it is coming from the left.  




So why does it sound different? It isn't just a matter of me turning down the signal from the left then right speakers.  The sound is coming into both speakers.  You can prove this to yourself by removing the headphone from your left ear and just listening through the right ear and vice versa.

So why does exactly the same recording sound like the frog has moved from the center to the right and to the left?  Simple.  I took the two channels of the mono recording and added an extra 1/1000th of a second of silence in front of the left channel in the middle call and the right channel in the last call.  So at first the two channels are playing simultaneously, then during the second call the left channel is playing 1/1000th of a second later than the right, and in the third call the right channel is playing 1/1000th of a second later than the left.  So when the sound is getting to your right ear a fraction of a second later your brain tells you the sound is "coming from" your left.

Last little bit of brain trickery....now listen to this frog (again same original recording).

 



It starts off in front of you but gets further and further away to the left with each call. Of course the frog and your speakers aren't moving.  The first call is arriving at both ears simultaneously and at equal volumes.  The last three calls are arriving 1/1000th of a second later to the right ear (putting the frog on your left) and then each subsequent call is reduced in volume on the right channel 1db.  Your brain interprets that reduction in sound in the right ear relative to the left as a measure of distance.

So your brain is localizing the position of a sound source by the difference in arrival time and volume between your ears.  Of course, your ears (pinnae) point forward on your head, so you can actually differentiate whether sounds are coming from behind your or in front of you (sounds in front of you will sound louder).  Therefore sophisticated sound systems often rely on 5 or even 7 speakers to create that sense of sound localization called surround sound.

Recording in Stereo

Recording sounds in stereo is really a bit of trickery.  You have to record the sounds in such a way that the human ears will feel like they are there in the actual environment.  So you have to control the timing and volume of the sound coming into each microphone in such a way to approximate the experience the listener would have had in the field.

This is achieved by using multiple microphones and positioning the microphones so you can control the timing of the arrival of sound to each microphone.  Some of the different mechanisms have involved positioning two microphones close to each other but facing the opposite directions (at various angles), facing each other at various angles, spaced apart different distances, etc., etc., etc.  Then there are the various methods of baffling the sound between the two microphones in order to approximate the effect of the human head.  Other approaches have included things like making fake heads and putting mics in the ears.  There are lots of good discussions of the pros and cons of different methods online.

One popular approach among nature recordists is to place the microphones in an arrangement called a SASS (Stereo Ambient Sampling System) array.  This type of arrangement is known for producing a realistic stereo effect and also has the added advantage of amplifying the signal at the same time.  Vicky Powsys has a great discussion of SASS and how it works for field recording on her wonderful blog, the Capertee Birder.

My Stereo Setup

My first forays into stereo recording were accomplished by unplugging my shotgun microphone from my Olympus LS-10 or LS-11 recorders and recording using the XY stereo microphones which came on the recorder itself. 
Here is a recording of a chorus of Hurter's Spadefeet calling from a flooded area in DeWitt County, Texas. 





This gave a satisfactory stereo image and you can clearly hear that the sounds are coming from two sides.  But the image is rather "narrow".  The frogs don't seem to occupy much space in the auditory landscape.  I found myself wanting to get a better sense of space and more amplification.  Inspired by examples built by Vicky Powsys, I decided to make my own "field hardy" SASS unit.  Of course, I am not any kind of handyman and the idea of cutting wood at precise angles, etc., was very unappealing.  So I decided to try to make a "sort of" SASS unit out of a dense foam Yoga Block.  I downloaded the SASS dimensions from Vicky's blog and shrunk them down to fit on the largest yoga block I could find and I made my unit from there.  After I made it, I was happy to see that Curt Olson had already established that smaller units will provide good stereo images.  

My SASS array isn't pretty (I'm wondering is I shouldn't call it the HALF-sASS 😉).  But it does give me a much better stereo image as well as a boost in gain (volume).  And I invested all of about 20 minutes making it and $4 for the Yoga Block.   Furthermore, it is one unbreakable piece and only weighs 5oz (142 grams) total so it is great for travel.
The green foam in the "nosepiece" is intended to allow a bit more sound to pass between the left and right sides than the dense foam of the yoga block.  The nosepiece is essentially hollow but blocked with that more open foam (I just hollowed out the center with a knife).  The pink things sticking out under the microphone are sections of rubber bands that hold the microphones flush in place at the edge of the yoga block (without these, the mics are a bit loose and too easily pulled out while positioning the unit).  The blue rubber band is just to hold the excess microphone cable out of the way while in use.  I thought about attaching a tripod screw or quick release plate, but I generally just put it on the ground, a stump or even hold it in my hands for shorter recordings.
  
For microphones I use a stereo pair of small EM-172 based microphones I bought from Micbooster.com.  These EM-172 mics are very popular among nature recordists for being very quiet.  You could buy the small mic capsules yourself and make your own microphones for less money but as I said, I am not handy and the Micbooster set comes pre-assembled - no soldering required!

The results from the SASS array are much more pleasing to my ear.  This is a chorus of several species of frog (Gray Treefrogs, Spring Peepers, Cajun Chorus Frogs and Southern Leopard Frogs) in Davy Crockett National Forest in East Texas in March,  2016 with version I of my foam SASS unit.  You can hear how much more "width" there is in the recording and I think an improved sense of space.  I am not sure there is enough center in the stereo image so I think some modifications might be in order once I do some research.



To try and give me a better "center" image to my stereo recording, I have also been experimenting with the the ORTF technique (named for the Office de Radiodiffusion-Télévision Française who developed it).  For this technique two cardioid microphones are place 17cm apart (approximating the distance between the human ears) and at 110° angles to each other.   To my ear, this produces a more natural stereo sound that better approximates the experience you hear in the field.  Here's an ORTF recording of a chorus from Guadalupe County, Texas in February 2017 (Cope's Gray Treefrogs, Blanchard's Cricket Frogs, and Southern Leopard Frogs) -

Frogs in Stereo - Is it worth it?

Of course, this begs the question "Is it useful/valuable to record frog choruses in stereo?".   

There are two issues with this:
1.  A frog chorus is generally fairly localized to a small area such as a small pond or drainage ditch.  If you are any distance from this source you really don't get much stereo image since all the calls are effectively coming from one spot.
2.  If you do get a large area or get close to the chorus, the sounds coming from each side/area are roughly the same.  So even though there is a difference in timing and volume in the stereo image you don't get as much sense of space due to the uniformity across the space.   

Here's an example of a situation where recording in stereo adds very little to the ambience of the recording.  This was a very loud chorus of Mexican Spadefeet, Texas Toads and Spotted Chorus Frogs in Schleicher County, Texas.

Here's the stereo version



And here's the mono version of exactly the same recording.



You can hear that there really isn't a huge difference in sound between the two.   So recording this in stereo probably wasn't necessary.

In contrast, when there are fewer frogs more widely spaced, such as these Canyon Treefrogs in the Davis Mountains of West Texas, a stereo recording does add more "information" to the recording.
Here is a stereo recording of these Canyon Treefrogs. You can tell where they were in space and which ones were closer to the recorder (Olympus LS11 by itself in this case). 




And here is the same recording recorded to mono. I think you can agree there is a loss of auditory information in this recording.




In the right situations, stereo recordings of frogs can add a lot to your sense of "being there" which is really the point of any amibence recording.  It is a bit more work to get a good stereo recording, but when it works it is worth the effort to capture the essence of the "frogscape" you experienced.


© Chris Harrison 2017