| |
For some perspective, a 22 inch parabolic dish will also give an approximately 12 dB (1kHz) frequency dependent free gain. It is also highly directional and rather expensive. The two-boundary rig gives 12 dB across the spectrum and has a wide hemispherical field of view. There are times when that is exactly what is wanted.
A web search turfs up virtually only commercial "boundary microphone" products with little in the way of technique or theory. Few resources discuss the use of boundaries to enhance sensitivity as a field technique. Basically the "Boundary Microphone" is seen as a recording method for studio or theater used to reduce or eliminate "comb filtering" which occurs when sound reaches a microphone from two separate paths, direct and reflected. Sensitivity gain is mentioned primarily as a "perk". The Crown website gives by far the most information including methods of mounting microphones on one or multiple boundaries and includes various polar and frequency response (cartoon) charts. They also provide a Frequency response formula that allows one to calculate the lowest frequencies attainable for a given surface side length. Everything in the context of studio or theater environment.
From now on I discuss "Boundary Microphone" in terms of sensitivity enhancement for field recording. As a two-boundary setup is almost trivial to construct I had to try this out. I own a small Evistr V508 portable dictating recorder. It records stereo at 1536 kbps CD quality, has an internal Li-ion battery and can record with battery and internal memory for up to 12 hours. I often use to record nighttime ambience and, hopefully, an occasional owl.
I have written to the manufacturer for data concerning the microphone (stereo) sensitivity and recorder self noise but have received no reply. I am on my own here! How do I know if this is working? Doing At-Home audio measurements is a fools errand. The needed equipment for minimum results is prohibitively expensive and with the equipment I do have access to most/all is woefully uncalibrated.
I Tested a homemade boundary jig measuring 23 x 11 inch lacquered 1/4 inch plywood panels.
F transition = 750/D = 750 / 1.9 = 391 Hz (two-thirds wavelength c = 1125, c * 2/3 = 750) F -6 dB = 188/D = 188 / 1.9 = 98 Hz (one-sixth wavelength c = 1125, c * 1/6 = 188)) where D is the side length of the panel in ft.
Measurements were conducted by 1) playing tones (250, 500, 1000, 2000, 4000, and 8000 Hz) with an online tone generator, 2) adjusting the % volume on the computer (attempting) to get a flat frequency vs volume dB recording from the V508 in the open (no boundaries). Given that, I then repeated the measurement with the recorder attached to the boundary rig. Tones were again played with the same volume % as gave the open field flat response. This is an iterative and time-consuming process and results are barely repeatable due to the uncalibrated/unspecified equipment used. The final results before I threw in the towel are plotted in Fig 2 below. It is clear I could not get a good "flat" open field response.
Fig 2
Open field measurement F Hz Vol Track L Track R Av LR % dB dB dB 250 32 -35.6 -32.9 -34.3 500 43 -38.3 -34.6 -36.5 1000 53 -38.4 -36.5 -37.5 2000 78 -36.7 -33.0 -34.9 4000 72 -41.8 -31.5 -36.7 8000 61 -34.3 -32.9 -33.6 Large Two-boundary measurement F Hz Vol Track L Track R Av LR Enhancement? % dB dB dB dB 250* 32 -33.1 -30.3 -31.7 2.6 500 43 -26.3 -23.5 -24.9 11.6 1000 53 -26.8 -24.3 -25.6 11.9 2000 78 -30.3 -30.4 -30.4 4.5 4000 72 -27.0 -24.6 -25.8 10.9 8000 61 -33.1 -28.3 -30.7 2.9 7.4 Av 11.9 Mx dB
*Note: The low enhancement at 250 Hz is explainable because the rig loses efficiency below 390 Hz. Something wonky was happening at 2000 and 8000 Hz and I was never able to get it right. I suspect it is my test environment.
The above testing and analysis confirms that a two-Boundary rig adds a 12 dB free gain across the audio spectrum above some lower limit defined by the size of the boundary employed. This is especially important at low frequencies say below 1000 Hz where a parabolic dish does not give significant gain. The following chart (Fig 3) gives curves of Gain versus Frequency for a 22 inch (20.5") dish (Telinga or Wildtronics) operating at 80 and 60% efficiency overlain by the expected response of the two-Boundary rig. The low frequency benefit is clear. Additionally I have plotted the power spectra of a Barred owl recorded (Fig 4) with my Evistr V508 while utilizing the two-boundary rig. The frequency signature of the owl is centered at 536 Hz where the two-boundary gain is higher than that of a dish.
Fig 3.
Fig 4.
Listen: Chouette Rayeé (file un-normalized)
The use of a dish or boundary is not a panacea for low frequency recording. Like a Telinga dish it requires that you provide a microphone of your choice. I have in my kit an unused Behringer B-5 microphone. This mic has an omni sensitivity of -40 dB. The boundary will improve this to -28 dB, very nice. Now, my Rode NTG5 short shotgun has a sensitivity of -23.5 dB, a better choice! Still, the Behringer B-5 for $75 plus a couple pieces of plywood (next time I will use plexiglass) gives you a descent result while the NTG5 sets you back over $400. If you have an unused Sennheiser MKH 8020 laying around (shame on you!) then its sensitivity of -30 dB will be improved to -18 dB. Now we are talking about a significant performance increase!
A boosted MKH8020 has 5.5 dB greater sensitivity than the NTG5, but costs $1100 more! At $200 per dB gained I leave it to you to decide which route to take. For me, the two-boundary rig is inexpensive and easy to construct and allows me to set out the little V508 more effectively. Some day when an anonymous doner gives me a MKH8020 then I will switch to that!
Conclusions:
The above analysis makes it clear that a two-boundary rig can add up to 12 dB of sensitivity enhancement to a field microphone. My data is rather scattered as expected from the lack of proper calibrated and/or specified measurement tools. Still, this does validate the boundary concept. A simple construct can seriously increase the sensitivity of a microphone. This has broad application in a field environment where soft ambient sounds are sought after. As a field recordist you should consider adding this to your kit.
KJS 02/2026