Studying Boundary Mics

Clippings from Crown’s PZM Application Guide [pdf] with photos of rigs made by Curt Olson,Walter Knapp and others. Compilation by Rob Danielson 10.2006

The Pressure Zone Microphone (PZM)







A boundary microphone is a microphone designed to be used on a surface for these benefits:


When a microphone is placed near a reflective surface, sound travels to the microphone from two paths: (1) directly from the sound source to the microphone, and (2) reflected off the surface. The PZM design allows the direct and reflected sound waves to be added in phase to increase the output of the mic capsule by 6dB.



The reflected sound waves travel a slightly longer path than the direct Sound waves so the reflected sound is delayed a bit in time. This difference could introduce high frequency phase cancellation, but as long as the distance between the mic capsule and the collector plate is kept small, the amount of phase cancellation created is minimal.


High-end response at 20 kHz is -3 dB when spacing is .085" (1ŕ8 wavelength at 20 kHz)


High-end response at 20 kHz is -1 dB when spacing is .052" (1ŕ13 wavelength at 20 kHz).


High-end response at 52 kHz is -1 dB when spacing is .020" –- the spacing used by the Crown PZM microphone.





You can greatly broaden your range of applications by mounting the PZM’s on one or more boundaries. A boundary is a stiff, nonabsorbent surface such as a floor, table, or plexiglass panel-- any stiff, sound-reflective material can be used.


Single, large boundary. Now suppose the PZM capsule is placed very near (within .020" of) a single large boundary, such as a wall. Incoming sound reflects off the wall. The reflected sound wave adds to the incoming sound wave in the “pressure zone” next to the boundary. This coherent addition of sound waves doubles the sound pressure at the microphone, effectively increasing the microphone sensitivity 6 dB.


Here’s one called the PZM single boundary rig called the, “Wedge:”






Two boundaries at right angles to each other. Now suppose the PZM capsule is placed at the junction of two boundaries at right angles to each other, such as the floor and a wall. The wall increases sensitivity 6 dB, and the floor increases sensitivity another 6 dB. Thus, adding two boundaries at right angles increases sensitivity 12 dB.


A boundary for each stereo channel using the floor as the second boundary.



At the junction of three boundaries at right angles. Now let’s place the PZM element at the junction of three boundaries at right angles, such as in the corner of the floor and two walls. Microphone sensitivity will be 18 dB higher than what it was in open space.


L2 Floor Array

Here’s another stereo PZM array (Fig. 47) designed by recording engineers Mike Lamm and John Lehmann. It simulates the O.R.T.F. stereo mic technique. According to one user, “You can take this array, set it down, and just roll. You get a very close approximation of the real event.” Suspending the inverted array results in less bass and more highs, while placing it on the floor reverses the balance. When this array is used on a stage floor, the construction shown in Fig. 48 is useful. It has decreased side pickup and increased pattern overlap. The axes of the left and right polar patterns may be at any desired angle, just so the 120° boundary angle and 6.7-inch capsule spacing are maintained.



Frequency-Response Effects of PZM Mounts


The size of the boundary on which the PZM is mounted affects the PZM’s low-frequency response. The bigger the boundary, the better the bass. Specifically,

the response begins to shelve down 6 dB at the transition frequency FT, where

FT = 750/D D is the boundary dimension in feet. The response is down 6 dB at the frequency F-6 where F-6 = 188/D.


For example, if the boundary is 2 feet square, FT = 750/D = 750/2 = 375 Hz and F-6 = 188/D = 188/2 = 94 Hz. If a PZM is mounted on a 4' square boundary,

FT = 750/4 = 178 Hz and F-6 = 188/4 = 47 Hz. This is called the “4' – 40 Hz” rule.


What are the acoustic causes of these frequency-response effects?

When sound waves strike a boundary, pressure doubling occurs at the boundary surface, but does not occur outside the boundary. Thus there is a pressure difference at the edge of the boundary. This pressure difference creates sound waves.

These sound waves generated at the edge of the boundary travel to the microphone in the center of the boundary. At low frequencies, these edge waves are opposite

in polarity to the incoming sound waves. Consequently, the edge waves cancel the pressure doubling effect. Thus, at low frequencies, pressure doubling does not occur;

but at mid-to-high frequencies, pressure doubling does occur. The net effect is a mid-to-high frequency boost, or – looked at another way  - a low-frequency attenuator.





Crown PZM 180 Polar Hz Response


Crown PZM 180-- Polar pattern: Hemispherical when mounted on a surface boundary. On a stand the mic is omni-directional at lower frequencies and unidirectional at higher frequencies.



The Phase Coherent Cardioid ( PCC)


The Phase Coherent Cardioid (PCC) is a surface-mounted super-cardioid microphone which provides the same benefits previously mentioned for the PZM. Unlike the PZM, however, the PCC uses a subminiature super-cardioid mic capsule. Its directional polar pattern improves gain-before-feedback, reduces unwanted room noise and acoustics, and rejects sound from the rear. The below figure shows the difference in construction and polar patterns of the PZM and PCC.



The diaphragm of the PZM is parallel to the boundary and facing down whereas the diaphragm of the PCC is perpendicular to the boundary and the main axis is parallel with the plane. If a unidirectional polar pattern is used, the PCC should have a 6 dB higher direct-to-reverberation ratio than the PZM; consequently, distant sources will sound closer and clearer.


Note that the PCC rig is designed to be place on a large boundary surface like a floor.


On a boundary, the direct and reflected sounds arrive at the diaphragm in-phase. This coherent addition of direct and reflected waves increases sensitivity 6 dB and prevents phase cancellations. If the mic capsule is small, it ensures phase coherency up to the highest frequencies in the audible spectrum, resulting in a wide, smooth frequency response free of phase interference. A small electret condenser mic like the EM-158, Rapid 35=0190 or Panasonic WM-61A seems to meet the criteria:



Based on sound traveling through air @ 300 meters/second, the direct sound from a source at 10 meters will reach the mic capsule in 33.33333 ms (milliseconds). Assuming that the sound reflected from the adjacent boundary could travel as much as an additional .01 msec and arrive in 33.3433333 ms, the difference of .0100033 ms or 1/100,000 sec is capable of producing phase cancellation only for with sounds higher in pitch than 50 K Hz, which is more than 1 octave higher than humans can hear.


Crown states about the PCC,”clarity and reach are also enhanced by this design… thus eliminating comb filtering in the audible spectrum.  Note there is a lift, or a rise in response at high frequencies in their diagram.








Curt Olson’s Experiments


Curt Olson is an Audio Producer/Editor/Mixer from Minneapolis Minnesota who has been experimenting with boundary mic rigs for the last few years. His instructive posts can be found on the Nature Recordist List  and he present a number of sound recordings with corresponding mic fixtures here.


His most most recent stereo boundary rigs using omni-directional mic capsules are similar to the PCC design in that the capsules are mounted facing “forward,” and perpendicular to the boundary with the mic bodies strapped directly onto the boundaries. (Curt feels there is little or no resonance created by this direct contact.)




Curt calls this a, “head-spaced parallel barrier array.” It uses standard 1”X 4” wood stock for two, parallel boundaries (3.5” x 9”) with the mic capules recessed about 2.5” from the front edge of the boundaries. The capsules are about 6” apart to approximate the spacing of human ears. Here are some field recording samples from this rig:


Frog Chorus (1:22)

Spring Thunder w/Siren (1:31)

Belching Bubbles (1:25)


These recording were made on a Sony MZ-NHF 700 Hi-MD Stereo Recorder (~$200), using the above pictured condenser Audio-Technica AT-3032 omni-directional microphones (~$170 each) powered by an Art Phantom II portable phantom power supply ($50).



Prior to this, he built a rig featuring slightly larger boundaries with the omni capsules set back about 6” from the front edge of the boundaries.



(I could not find any soundfiles on Curt’s site that were identified as made with this rig.)



Before trying the above, very simple designs using front facing mics strapped onto the boundaries, Curt experimented with omni-directional capsules there were mounted to the boundaries and angled at about 110 degrees loosely imitating Crown’s SASS rig.





One Curts’ earlier rigs (which he feels may be less successful) was based on some principles one can observe in Crown’s “SASS” design (below). This features including mounting the mic capsule so the diaphgram is flush with the surface of the wood barrier.  Note that Curt’s design does not include the SASS’s foam baffle but rather an opening.  It looks as though the front edge of the opening is about 2.5” from the capsules– close where the foam baffle would be.


An overhead view of Crown’s SASS enclosure for comparison. Olson’s opening between the wood barriers seems to correspond with two simplified boundaries meeting the location of Crown’s foam baffle. Original photo by Walt Knapp.



Here are some recordings Curt made with his wooden version of the SASS sans foam baffle stereo mic rig:


Feather Dance (1:10) Saturday morning in mid-September 2005, I dropped in on a Native American festival at Harriet Island Park directly across the Mississippi River from downtown St. Paul, Minnesota. One of the activities was an exhibition of Native American dances. Here is a short excerpt from the Feather Dance. Recording equipment: Sony MZ-NHF 800 Hi-MD recorder, Sony ECM-155 (omni-directional) microphones.


Thunder Overhead (1:00) I managed to capture two clean thunder claps almost directly overhead, with some slap echoes off the back of my home. The two discharges actually occurred about 2 minutes apart, but I pulled them together into this single clip. Sony MZ-NHF 800 Hi-MD recorder, Shure WL-183 microphones.


Grouse Drumming and more... (1:00) A close-up recording of a male ruffed grouse drumming (vigorous fluffing of the wings intended to declare the bird's territory and attract females). These drumming episodes occur at intervals of about four to six minutes and can go on for many hours at a time -- often all night. This clip was captured shortly after sunrise. Notice also winnowing snipe in the background and a low duck fly-over at the end. Sony MZ-NHF 800 Hi-MD recorder, Shure WL-183  mics.


Dawn Woodpeckers, Loon and more... (3:08) The Chippewa National Forest in north central Minnesota is alive with birds in early spring -- especially woodpeckers. In this fairly long clip, I enjoy the spaciousness, the busyness, and the sense of being surrounded by all kinds of interesting critters, near and far. Sony MZ-NHF 800 Hi-MD recorder, Shure WL-183 microphones.











With this design, Curt again uses flush-mounted omni-directional capsules but the barriers joint in a “V” the “Wedge” Rig suggested above in Crown’s Literature. I believe the pictured windscreens are made from wire baskets that have been covered with the thick “Terrycloth” weave fabric.  Here are some recordings Curt made with this rig:


Beavers At Work (2:25) A remote lake in Minnesota. I positioned the rig at the shoreline, and walked away for about an hour as the sun was going down. Sony TCD-D7 DAT recorder, Shure WL-183s omni-directional mics.


Feathered Flight (1:20) Wings beating the air as an unknown large bird flies low and slow past my microphone, on the left. A few seconds later, two ducks fly by from right to left. Sony MZ-NHF 800 Hi-MD recorder, Shure WL-183 microphones in a tree-mounted wedge array.







This rig seems to be very similar to the one above except the “wedge” angle formed seems to be very close to 90 degrees. Here’s one recording Curt posted with this rig:


Pheasant Fluff (0:16) On April 1, 2005, I accompanied friend Rich Peet to Crex Meadows, pre-dawn this time, to capture wildlife sounds at sunrise. We set up my wedge microphone and digital recorder in one location, then moved to another with Rich's gear. Back at home later that day, I discovered this nice pheasant call followed by a vigorous feather fluff, among many other quieter sounds. Sony TCD-D7 DAT recorder, Shure WL-183 microphones.



Walter Knapp’s SASS Modifications


Another sound recordist who spends many hours per year in the field recording amphibians and stereo ambience in and around Georgia (USA), Walt Knapp has been modifying the Crown SASS fixture which is also is based on boundary principles. He has be replacing the fairly noisy Crown mic capsules with much low noise omni directional mics like Sennheiser MKH-110;s , MKH-20’s and more recently, the same mics Curt has been using, the Audio Technica AT-3032’s which seem to perform very well at a much lower cost. He also positions ho rigs on top of a hefty 17’ high lighting stand that can be elevated with a hand-crank.


Here are some mp3’s of Walt’s modified SASS rigs. This set of four recordings also allows one to compare the performance of the expensive mkh-20 mics with that of the lower-cost AT-3032’s:


Home recording at sunrise:

SASS/AT3032 Mics on 17' high tripod

SASS/MKH-20 Mics on 17' high tripod


Recording at Whitetail Pond Charlie Elliott Wildlife preserve, at midnight:

SASS/AT3032 Mics on 17' high tripod

SASS/MKH-20 Mics on 17' high tripod






Here are two recordings made on the same location one with the modified SASS rig positioned 4 feet above the ground and the other positioned at 17 feet :


SASS/MKH-20 recording of River Frogs:

4’ high tripod

17’ high tripod


17’ Tripod on location for River Frogs Recordings. Photo by Walt Knapp




The Crown SASS® or Stereo Ambient Sampling System


Walter Knapp’s modified Crown fixture with two MKH 110’s mics. Photo by Walt Knapp


A Knapp modified SASS fixture with AT-3032 Mics installed. Photo by Walt Knapp



Here are some “to scale”  printable templates of the SASS Fixture based on the above Crown Diagram


11” X 17” paper  (top and front views)

 8.5“ X 11” paper (top view only)

8.5“ X 11” paper (front view only)


One way to record in stereo with PZM’s is to mount two PZM’s on a wedge: two 2'-square panels, angled apart to form a “V.” The Crown SASS fixture does this on a smaller head-size boundary.


SASS uses two small microphones spaced a few inches apart. Each microphone is on a surface that blocks sound from the rear, and these surfaces are angled apart. The SASS is like a near coincident pair, in which two directional mics are angled apart and spaced horizontally a few inches.


The surfaces make the microphones directional only at mid-to-high frequencies. At low frequencies, the microphones pick up all around them—they are omni-directional. The SASS produces stereo in different ways at different frequencies. At low frequencies, the SASS acts like a spaced pair, producing time differences between channels to make a stereo effect. At high frequencies, the SASS acts like coincident pair, producing mostly loudness differences between channels to make a stereo effect. At mid-frequencies, the SASS acts like a near-coincident pair, using both loudness and time differences to make stereo. This is the same way the human hearing system works. Our ears are omni-directional at low frequencies, directional at high frequencies (because the head blocks sounds), and are spaced apart a few inches. Since the SASS hears sounds the same way our ears do, it produces very natural stereo with easy-to-localize images. It also gives a pleasing sense of spaciousness, a sense of the environment in which the sound was recorded. The fixture uses a block of dense foam between the mic capsules. This foam barrier absorbs sound. It prevents sound from the right side from reaching the left microphone, and vice versa. Thus, the signal is much louder in one channel than the other. For a phase cancellation to be complete when two channels are combined to mono, the levels in both channels must be about the same. But the levels in both channels are different in the SASS (due to the foam barrier between capsules), so phase cancellation in mono is relatively slight (Fig. 57). Thus the tone quality stays the same in stereo or mono with the SASS.


SASS-P MKII Polar pattern for Right Mic






Other Stereo Arrays

A double-boundary monaural “rig” designed in 1998 by George W. Swenson, Jr. of the US Army Corp. Swenson describes it as a,“…flat-reflector microphone designed for the frequency band [from] 10 to 40 Hz [to capture] sounds produced by explosions [at] a military artillery training facility.”  The dimensions of the reflector are 4.5 x 9.0 meters.  A single microphone is mounted on the ground right in front of the vertical wall to achieve the double boundary effect.




Furthering Our Study


Here’s DIY Stereo Boundary Mic Rigs blog starting with our first stereo localization test of three boundary mic fixtures we did in B-18 together.  Curt Olson has taken a look at it and has suggested experimenting more with the distance from the leading edge.


Here’s what I’d like you to do to follow-up on the questions/directions you feel our test poses. Take your double parallel boundary rig and your Primo EM158 capsules and build at least one more rig so that you can perform your own test of stereo depth in a location of your choice. In addition to the second rig of your design that you build, also include in your comparison test, at least three of the five that are highlighted in red in the following chart.



Table of Stereo Mic Arrays Using Omni-Directional Microphones



Array Name

Array Sub-Category
















6” / 20cm separation






No Barrier or Boundary.

Omni-Opposing Capsules (180 degrees)



















6” separation











Approx. 12” x 12” Barrier

Omni Capsules Opposing




Approx. 12” x 12” Barrier

Omni Capsules Opposing













6” separation

Approx. 12” x 12” Barrier

Omni Capsules ORTF (110 degrees)



















Approx. 12” x 12” Barrier

Omni Capsules Front Facing







































Omni-Opposing Capsules (180 degrees)






Neumann’s Dummy Head









Styrofoam Wig Stand (life-like ears are often added)









Binaural Headphones









Rich Peet’s Blockhead









Rich Peet’s Cube

(4 channel)












Parallel Boundary






Front Facing (both omni capsules parallel to head spaced boundaries)






Crown PCC (mono)









Curt Olson “Head-Spaced Barrier Rig”









06420 “Parallel Boundary”












Angled Single Boundary

ORTF (mixed angles)
















Curt Olson Pseudo SASS ~110 degrees









06420 Pseudo SASS ~110 degrees









Crown’s Wedge Drawing









Olson Wedge ~60 degrees

















Angled, Double Boundary






ORTF - Angled approx 110 degrees, capsules flush to one boundary






Crown’s SASS (technically, a partial double boundary too)



















With Barrier



















With Double Boundaries






Could be done,..














With Triple Boundaries






Could be done,..
























ORTF Mixed angles






Chris Burmajster --Blumlein Difference Technique (BDT)












 13” +







13” separation






No Barrier or Boundary.

Omni Opposing Capsules














No Barrier or Boundary.

Omni Front-Facing













13” Separation

No Barrier or Boundary.

ORTF Facing (110 degrees) Capsules














21” separation






No Barrier or Boundary.

Omni Opposing Capsules

Generic –

Pictured is a part of Rob D’s backpack mount with opposing omni capsules spread at 21”












Jecklin  13” Separation






Approx 12” X 12” (or larger) Barrier ORTF (110 degree angled) capsules




13” Separation










Approx 12” X 12” (or larger) Barrier Front Facing Capsules



















Down Firing Omni Capsule with gap 1/10” to 1/50”



Design and Make Your Own PZM Boundary Stereo Rig



Single Boundaries






Crown- A pair spread on common plane 8’ or wider


















Double Boundaries






Crown’s “Wedge on Floor” drawing.








Triple Boundaries






Crown’s Lamm & Lehmann L2 Floor Array version #1









Rob D’s LAMM L2




Rob D’s PZM capsule holders for L2









Crown’s Lamm & Lehmann L2 Floor Array version #2









Rob D’s PZM capsule holders for L2









PZM Pyramid – recommended for speech, “highly-directional.”








Quadruple Boundaries






Crown #1560 Recommended for when rejection of sounds from the side is required.








Curvilinear Boundaries






PZM Cone









PZM Dish







Other Non-Boundary Designs

Walter Knapp’s Parabolic Mic

Quad Pac” 4 channel Rig (cold be used with double-deck Hi-MD)

Richard’s Contact Mic Page

Electret Tri Capsule Stereo Mic Heads using 6- Rapid 35-0190 Capsules (Richard)

Tom Robinson’s Thin Boundary Mic Rig

Stereo Parabolic Mic Rigs (Rich Peet)

Stereo Shotgun Mic Rigs


Mic Power Supply  (If the Hi-MD mic input does not provide enough voltage)

Source for small order Primo EM-158’s Gene Dorcas