Parabolic Microphone: Theory, Use, and Performance
With many years of outdoor recording experience, excellent audio and industrial electronic and mechanical engineering backgrounds, resources in Physics, machining, and manufacturing, and knowledge of wildlife; we may be able to help you with our series of Articles. See our other Articles as well. The Article on Audio Noise will explain dBs and many other topics that will help you understand this Article. Hopefully, after studying this information, you will find something of benefit, and learn more about the Wildtronics products as well. Yes, we love to talk about them. Parabolic microphones are a type of microphone used when sounds are too weak for normal microphones, when a highly directional mic is needed, or to boost sounds so they can be clearly heard by the human ear. Common uses are nature recording, sporting event sound reinforcement, and surveillance.
Types of Microphones
First, we will briefly discuss some of the different types of microphones used in audio recording. Capturing audio for sound reinforcement, broadcast TV, live shows, video projects, and natural backgrounds requires the use of different types of microphones. Ideally, an audio source should be very close to the microphone to obtain the largest signal level without interference from surrounding sounds or the microphones own self-noise. The desired SNR is usually in the 40-60dB range. The audio source will change what type of microphone is best to use. For close work, lapel or hand held, cardioid-type microphones are typical. For 360 degree pickup, an omni-directional type is preferred. If the sound source is from a fixed point in the distance, a shotgun microphone may work if the signal level is sufficient. However, shotgun mics do not increase signal levels much, their main purpose is to minimize unwanted sound sources since they offer a narrower polar angle. For distant or low level sound sources, microphone self noise should be low, below 10dBA is recommended. In sporting events, surveillance, and nature sound recording, getting close is not an option, and only a properly designed, quality parabolic microphone can offer suitable SNR, Signal to Noise Ratio. Remember that audio signal level is reduced in half (6dB) every time the distance is doubled. There are also many more extraneous sound sources that you do not want to pick up as the distance to your subject increases. The parabolic microphone solves both issues since it combines a very narrow polar angle and high forward gain. A Parabolic dish is much like the analogy of a telephoto camera lens, at greater distances, more magnification is offered, but with a narrower field of view.
Here are some of the general characteristics of (ideal) properly designed parabolic microphones, not all of them will measure up as we will see in another section. Only parabolic microphones have the amazing ability to actually amplify the audio signal by concentrating all of the sonic energy, collected in the large dish, in phase at the focal point. This concentration is made possible by the unique geometry of the paraboloid shape. This dish gain is “free” gain, as it doesn't add any noise or require electronic amplifiers. Electronic amplifiers can only increase the overall level of both audio and noise, and can do nothing to improve the critical signal to noise ratio (SNR) of the microphone. Both the audio gain and directionality of a parabolic microphone increase as the frequency increases. Gain, for an ideal 22-inch dish with a perfect parabolic shape and focus, is characterized by a curve starting at 0dB at 200Hz, and increasing 6db/octave, to the 35db range at 10Khz and above. Polar angle is about 90 degrees at 500Hz, closing down proportional with frequency to only 4-5 degrees at 10KHz. You will need to point the dish within 2 degrees (10 feet at 100 yards) if you want to retain the high frequency response. Lower frequency sources don't require as accurate an aim. The gain of a parabolic dish is exactly proportion to its diameter. You will want a reasonably sized one, but don't be obsessed over a couple inches. Example, 22in compared to 20in, 22/20 = 1.1, converting to dBs, that is <1dB different and therefore unnoticeable. Inaccurately shaped dishes, off-focus microphones, pointing off-angle, and many other constructional errors will cause a loss in gain as frequency increases (roll-off). At lower frequencies, errors are less important, and primarily dish size, microphone quality, and the mounting method are most critical.
High Frequency Response and Correction
A parabolic dish will always have more gain as frequency increases, this is a fundamental property that can not be changed. Higher gain at higher frequencies can be useful, since higher frequencies suffer greater loss with distance due to atmospheric attenuation. Human voice is also much more recognizable with boosted harmonics, so even though a typical dish is way too small to boost male human voice (100Hz), the large boost in harmonic content allows a dish to intercept speech at an amazing distance. However, if you want to preserve perfect fidelity, since a properly made, truly parabolic dish response is 6dB/octave, the recording may be de-emphasized during editing, with a first-order low-pass filter starting at the frequency of interest. The resulting gain response will now be flat and equal to the gain of the dish where the filter's response begins. This technique will only work well with an ideal parabolic response, like the Wildtronics models have. You can even put in a small allowance for atmospheric roll-off if you note the distance of the subject and weather conditions. Look up “atmospheric attenuation” on the Internet for charts and calculators to do this. Example, if your recording is a bird with a 2-16Khz voice range from 200 feet, and atmospheric losses are 6dB more at 16KHz, start a 4dB/octave low-pass response at 2Khz where dish gain is already 25dB. The resulting gain curve will be from 25dB at 2KHz to 31dB at 16KHz, so the recording is not colored by excessive harmonics, and the bird will sound as it would at short range. At higher frequencies, such as birds, dish gain is typically 20dB, this is a linear increase of 10 times! This means a bird at 300 feet can sound as if it is only 30 feet away – it is truly amazing. With quality microphones (<10dBA) and preamps that add no additional noise, the addition of the dish gain and focusing ability allows a recording to offer great improvements over human hearing. A high frequency boost is also particularly valuable for older people since age reduces the ear's frequency response. By the way, Wildtronics Parabolic Mics exceed the theoretical expectations with their Patent Pending Enhancements.
Parabolic Mic Capabilities Example
Your sound source will need to be in the 40-60dB SPL range above your mic self-noise and unwanted background noises for a good recording. If your subject is quieter than that and you can not get closer, a parabolic dish may be the only answer, even though they alter the frequency fidelity. Parabolic mics are also the only way to dramatically improve human hearing. The Wildtronics Parabolic Microphones offer a noise-free boost of 10-35+dB from 400Hz to 20KHz, and use special techniques, and circuits to achieve a super low apparent self-noise as well. A bird with a 2KHz voice will appear almost 20X closer, and an insect or bird with a 5KHZ voice will appear 40X closer, so good results can be obtained with distances up to 500 feet! Species can be recognized even further until overwhelmed by background sounds.
Here is a typical example, and a good time to make an important point: You want to record a typical 2KHz tone bird with an SPL of 70dB at 1m that is 100m away in a treetop, so its SPL is 30dB, you can just hear it but it is whisper quiet, with 10dB of mic self noise and a background pickup of 25dB using your standard high quality cardioid type mic, your SNR would be only 5dB, so recordings would be very poor.
With the 25dB of gain @ 2KHz from the boosted Wildtronics Dish, the bird SPL is now 55dB, some of the 360 degree background pickup (mostly other birds) has been reduced by 10-15dB, combined with the super low self-noise array mic, SNR is now a respectable 40-45dB, and you actually have a good recording of something you can barely hear.
Other 20'' or so parabolic dishes will not have this performance due to several major factors: Noisier microphones, lack of booster plates, improper polar pattern microphones, and almost no off-angle rejection so the background sounds are not reduced at all. Using the same bird above with a typical “standard” parabolic dish fitted with an otherwise excellent MKH20 mic pointed outward, the bird SPL will be increased to maybe 47dB if you are lucky, but the background will certainly remain at 25dB, your SNR is therefore only 22dB, you will hear the bird, but it will be flooded out by all the other birds, and you will not have the recording you hoped for or should have gotten.
Important Parabolic Mic Details
Constructional details, things to look for, and differences between various Parabolic mics will be explained next. The performance of a parabolic microphone depends on it's physical diameter, specific shape and paraboloid accuracy, its focal point and depth, microphone quality, and several critical constructional details. It is surprising how much difference small details make in performance, when two dishes with the same basic size are compared. As said before, the diameter sets the frequency response and gain, just putting a microphone near any curved shape will result in some gain, just like cupping your ears with your hands, but nothing like a properly designed system that measures up to its theoretical expectations as discussed in the last section. Let's examine some common constructional methods and their effects on performance.
A low-noise microphone is critical for good performance, but not just any low-noise mic will give you good results, in fact any standard mic will not. The microphone's polar pattern must match the dish aperture and focal point. If the microphone is improperly matched, as many commercially made units are, two outcomes result: If the mic pattern is too narrow for a deeper dish with a short focal length, there will be a loss of gain, since only a portion of the dish will be utilized. If the mic pattern is too wide for a shallow dish or picks up sound from the front, unwanted stray pickup will occur. The stray pickup will flood your subject with background sounds and also cause destructive interference with the dish reflections resulting in gain loss as well. Some manufacturers of very costly units don't seem to understand the principle of matching the microphone to the dish yet. We will go into more depth in the next section so you can recognize these constructional errors for yourself.
Proportionally, the larger the dish, the lower the frequency response, and higher the gain – the practical limit being what you are willing to transport, and what can be reasonably manufactured. Doubling the size will require double the thickness to retain rigidity, that is already 4X the weight plus that of supporting structures. In addition, the focus and parabolic accuracy still needs to be within 2mm to retain a 20KHz response – try to do that with a hypothetical 4-foot behemoth! That is why most commercial parabolic dishes are in the 18-24 inch range, and there is little difference between them based on their exact size (+-1.5dB) or unnoticeable. Other factors will be much greater in comparison.
In addition to performance, some things to look for in a practical parabolic microphone include: Lightweight, rugged construction, without protruding parts or cables that can be dangerous or snag easily. Connectors need to be compatible with your various equipment. A tripod mount will be very useful for any application of more than a couple minutes. The type of handle is often very important, noise from your grip shifting commonly ruins an otherwise good recording. A clear dish is very handy for general awareness and a good visual aim of the subject. Outdoors, wind is always an issue, so look for a specified windscreen system or provisions to add them built in. Portability while traveling may be important to you, so a unit that can be disassembled or even curled to fit into standard luggage may be a priority.
Parabolic dish microphones are available with varying degrees of performance. On the low end of the scale are toy-like 12 to 15 inch dishes with junk microphones that only offer improvement to the hearing impaired. Mid-range, lightweight, so called “professional” units that are better, but far from optimal due to a multitude of design misconceptions. At the other extreme are large heavy units that require a separate microphone and cost over $3000 for some reason, but at least don't suffer from as many design issues, and offer better performance than some mid-range units. We will discuss the Wildtronics versions in a separate section, and how they solve all the problems, including price.
Why Some Parabolic Mics Work Poorly
The Wildtronics Microphone Patent Pending features offer a 20dB advantage or 10x better SNR than other parabolic microphones in the example above! Let's analyze what is wrong with the other designs, because they are falling far short of the theoretical expectations of a parabolic dish:
On types with the mic pointed outward, high frequency loss is high, due to the improper polar pattern omnidirectional mics have from the backside where the dish is located. Looking at the MKH20 data sheet, you can see that you already loose 2-3dB at 2KHz, 10dB at 8KHz, 17dB by 16KHz, and more than the dish gain by 20KHz. These high losses are because the mic is mounted BACKWARDS for use in a dish – this is ridiculous! If you are really interested in buying such an arrangement, print out the MKH20 data sheet, and ask them how in the world this is going to work well backwards, when you are loosing much of the signal that the dish is providing. They will probably tell you that many have used it with good results instead of a proper explanation.
Another major problem with any open mic design is the signal cancellation that results when the incoming forward wave interferes with the reflected wave off the dish. This problem is even worse when the mic points out since the forward wave is enhanced even more. At a frequency where the wavelength is 4x the focal distance (600-800Hz), gain will drop dramatically over a 1KHz wide bandwidth, resulting in a poor bass response just when the dish gain should be beginning. The response will then peak at twice that frequency and undulate back and forth in a wild fashion. No matter how flat and perfect the response of the installed microphone is, when mounted incorrectly in the dish as above, it will be become horrible. In all cases that I have reviewed, where high quality open mics are NOT used, the mics offered instead are about 10-15dB noisier than the Wildtronics internal array mic system. Off-angle and forward wave rejection is only possible when special techniques are used to reduce the incoming forward wave, this important feature is not available in any other standard parabolic mic, so they will pick up all the background noise just like any other omnidirectional mic would.
The dish must also retain its shape to less than 2mm to avoid roll-off at the higher frequencies. Use this test: Measure your dish diameter at 45 degree intervals, you should have <5mm difference or your high frequencies will roll off prematurely. The assumption is that we are talking about an actual parabolic dish here, before buying one, ask the maker how accurate the dish is compared to a true paraboloid and insist on an answer. Wildtronics true-paraboloid dish molds were CNC mastered, and the dishes are accurate to well under 1mm. When the error exceeds about 2mm, high frequency roll off will begin. With an error of 8mm, even the frequencies of the birds you are trying to record are not being amplified by the dish properly anymore. Some of these dishes are actually just spherical sections, or molds made from whatever, which have minimal focusing ability. After using a Wildtronics Parabolic Mic with its Patent Pending features never available before, you will gain a new appreciation of what a properly designed, quality parabolic mic can do for you, especially if you have used other so-called “professional” parabolic mics in the past.
The Wildtronics Parabolic Microphones
The goal of Wildtronics was to develop the world's best parabolic microphone with the lowest noise, best frequency response and gain, optimal size and weight, and many other features mentioned below – all while maintaining an affordable price. This process, like any process, begins with a proper theoretical knowledge of the issues. In this case the issues are: Definition of the problems, Parabolic dish Physics, sound, noise, microphones, electronics, mechanical design, molding technology, and manufacturing methods combined with innovation. The Patent Pending Wildtronics Parabolic Microphone is the culmination of these efforts to improve every important aspect of a parabolic microphone.
This includes: A well integrated, trim, light, and snag-free design in textured black for stealth. Solid, reliable mechanical features that allow quick quiet handling, including an almost silent hand-grip. A built in tripod mount, and accessory mount for attachment of our various Accessory Bar Kits to mount additional equipment is provided. A specially designed, built-in, cord-free, high-tech microphone assembly that is resiliently mounted to reduce handling noise, and always precisely located at the focal point. A durable, semi-flexible, clear, highly accurate, truly parabolic dish which is dampened to prevent resonance and easily removed. The innovative acoustical booster plates nearly double the gain, and lower the frequency response compared to any other parabolic microphone, and thereby offer the performance of a 30-inch plus dish. Special techniques and circuits further block the unwanted, out of phase stray pickup from the front, so only the focused sound source you are aiming at is maximized.
A super low-noise mono microphone with special electronics, based on array technology is ruggedly built into each Wildtronics Parabolic Microphone. This array mic alone combined with its booster plate, without even including the dish gain, result in an equivalent mic self noise of an amazing 4dBA – try to buy a microphone that good! The mics are even heated by excess phantom power to reduce condensation problems in high humidity environments. Built-in foam windscreens handle light winds, and an optional secondary wind sock can extend wind tolerance to 20MPH. The rear-facing, 200-degree polar angle mono microphone assembly is a perfect match for the focal point inside the dish. Locating the focal point inside the dish (deep dish design), minimizes the stray side and rear lobe pickup patterns that plague shallow dish or open microphone designs. Minimizing all sources of stray pickup (noise) will always yield the best SNR for your subject. If you want the background pickup, an optional Mono-Stereo model includes forward-facing, low-noise, stereo background microphones, with an integrated booster and separator plate, allowing you to capture both the focused dish sound source and a L/R stereo background channel. The stereo channel is completely separate from the mono channel, so you can use only as much stereo background as you want, unlike other stereo systems where the stereo background can not be separated. The stereo background can also be useful to “find” a hidden bird, and then switched off to record it.
Electronics for phantom powering is built in, or you can use the internal 9V battery if you do not have phantom power. Balanced XLR and 3.5mm outputs, for professional and consumer use, are conveniently located on the back, for both the mono dish and optional L/R stereo outputs. An optional Accessory Bar Kit allows you to use the included dual handles, balanced just right for those that need to hold the unit for long periods. You can also pre-mount and wire all kinds of equipment right on the bar for super fast deployment. A 2oz Mini Accessory Bar is also available, and used to mount a mini-recorder right on the handle for quick one hand use.
And there is more yet! Our incredible Amplified Parabolic versions include integrated very low-noise, variable-gain electronic amplifiers with direct headphone, line, and amplified mic outputs. These 1oz low-cost amps have full professional specifications. The Amplified Mono-Stereo Microphone model even has a built-in mixer to combine the mono and stereo channels in any proportion you want. The Amplified models are easy to use and perfect for everybody, DSLR, headphone use, and making low-cost hand-held recorders perform as well as a very expensive recorder. The best part is that these USA made products are a fraction of the cost of any comparable parabolic mic, of which NONE can offer the performance, features, and professional attributes of the Wildtronics Parabolic Microphone models.
Read our Parabolic Microphone Selection Guide for short descriptions and a comparison table of all Wildtronics products.