Parabolic microphones - reality, mono and stereo sound.

A bird is singing in a tree, 50 meters away from a wildlife recordist.

The key question is:

Does he want to record the bird as it sounds where the microphone is, - or does he want to record it as it sounds where the bird is? As soon as he wants to record the bird as it sounds where the bird is, (that is what most wildlife recordists want) - then what he does will have little to do with theory and academic discussion about flat frequency curves, accuracy and "natural".

How could it be otherwise? Suppose that he has an excellent microphone, the most "true to life" there is. The only "life" that this microphone knows anything about, is the "life" nearby. It has no comprehension at all about the "life" 50 meters away, how it sounds where the bird is. Therefore, any "true to life" microphone will record the reality where it is placed, not the reality 50 meters away. Conversely, a microphone which records something, as it sounds 50 meters away from it, is a microphone which is actively doing something with the incoming sound. It's output is not the same as the input.

We must then forget about flat frequency curves as a factor of quality. "Quality" must instead be described in terms of wanted change of reality, perhaps not even a linear change.

The shot-gun.

Roughly, a high-class shot-gun ( more than 2.000 USD) has a more or less frequency independent acoustical amplification of what is in front of it, and a more or less frequency independent attenuation of what is around it. Using such a shot-gun creates a recording similar to what the bird sounds like where the microphone is, but with an attenuation of environmental sounds.

This creates an illusion: If the bird is 50 meters away, one can hear on the recording that it was 50 meters away, but the surrounding acoustics are gone, as if the microphone was very close. If such a recording will be used as a background to a close-up telephoto film sequence, the sound engineer, who finds the sound too "distant", will start to adjust his equalizer controls on the sound mixer. When he has finished doing so, and is pleased with "the nearness" he has achieved by changing the sound, one can see on his equalizer controllers that they roughly describe the typical curve of the acoustical amplification of a parabolic dish. But in the process he has also amplified the inherent noise of the electronics by some 10 dB.

The parabolic dish.

A parabolic dish has an acoustical amplification. It "gathers" the sound together, and spots it on one place, the focus. If a dish amplifies 10 dB at a certain frequency range, it means 10 dB less of necessary electronic amplification within that range, and thereby 10 dB less of electrical noise - "hiss".

But that is not all. The amplification also increases with increasing frequency, with a lower knee depending on the diameter of the dish. This creates the "nearness" that the engineer has to turn his knobs to get. The following experiment was made in 1984:
A loudspeaker producing pink noise (like noise between the stations on the FM-band) was placed five meters up in a tree, with other high trees around. The pink noise was picked-up by a Telinga 55 cm dish 30 meters away from the loudspeaker. The frequency spectrum was monitored in 32 bands on a computer.

The same microphone, without a parabola, performed a similar frequency spectrum at a distance of 3 meters away from the loudspeaker! Judging from frequency spectrum, the dish had "shortened" the distance by a factor of 10!

For many ears, the parabolic dish has been described as an "acoustical amplifier." But the word "amplifier" has also exclusively been used on electronics with an equal amplification over frequency. The word "amplifier", in fact, implies that it should have a flat frequency curve. An amplifier designed to have an uneven frequency curve is called an "active filter", or for example "RIAA-amplifier" - the letters "RIAA" describing the way the frequency curve is unlinear.

Therefore, we re-phrased the definition of a parabolic dish to "an acoustical filter, approximately compensating for distance." As far as we know, nobody has ever argued against this definition.

A parabolic dish is "an acoustical filter, approximately compensating for distance."

So what do we have? We have a recordist with a microphone 50 meters away from a bird, and he wants to reproduce the sounds of the bird as it sounds where the bird is. He cannot move closer - which is the only correct way - because he fears that the bird will fly away. If he adds a parabolic dish to his microphone, this dish will - because it is a filter - somewhat reshape the sound so that it resembles the sound where the bird is. He has - in fact - made the same kind of manipulation of the sound as the sound-engineer, with his equalizing filters, but acoustically, without contributing inherent electrical noise!

This is the advantage by using a parabolic dish. By manipulation it creates "nearness" without contribution of electrical noise! This "nearness" is not more true, or more false, than the "nearness" the engineer achieves.

Why does a parabola do this? First of all we have to consider why it is frequency dependant, - why it amplifies more at high frequencies.

The focus of the parabola can be thought of as a a region, - a "globe" of energy, compressing and decompressing. This globe gets smaller and smaller with increasing frequency and the energy gets more and more concentrated. When the frequencies are really high, as with light, the focus is extremely well defined, and every smallest error on the shape of the telescope mirror will be critical. This is not the case with the sound parabola; it is not at all a precision instrument. Sten Wahlström described in an article long ago, how he successfully recorded birds with a microphone and a plastic umbrella!

A wawe-length of 10 kHz means a "globe" of about 35 mm, and using a truly omni-directional microphone, it is of small importance where it is, as long as it is somewhere inside this globe. At 3 kHz the "globe is 110 mm. But the smaller the globe is, the bigger is the microphone membrane in relation and amplification increases. Pink noise, all frequencies at the same time, is the ideal sound for a parabol to handle. With so many frequencies to work with, it can do a lot of "coming closer". The opposite is when a bird sings a pure tone, almost like a sine wave, with few overtones. The parabol will amplify the tone, depending on which frequency it is, but it cannot create any "coming closer". There is no point in recording a dove, for example, or a cuckoo, using a parabol. There are no overtones to "reshape".

Parabolic microphones and stereo sound.

A simple stereo effect can be achieved by placing two small microphones on each side of a head-like construction. Sometimes this is called "dummy head" or binaural stereo. Personally, I prefer to put the microphones on my glasses, close to my ears. A recording made this way can be very nice when listened to through head-phones, but not so impressive when loudspeakers are used. Several such designs have been made for example by Sennheiser, and Neuman makes a very sophisticated dummy-head. Another common design is to use a plate, "Jecklin-plate", big or small, and place the two microphones on each side of it. The function of this plate is to shield off the sounds from one another in a left and a right channel. "State-of-the-art" recordings of classical music have been made this simple way.

The Pressure Zone Microphone: The PZM principle uses the compression and decompression of air between a plate and a membrane in parallell with the plate, very close to it, a millimetre or less. This arrangement gives about 6 dB extra amplification of the signal. This gives not only 6 dB less inherent electronic noise, but also that sound at the other side of the plate will be screened off.

The Pressure Zone Principle:

In a stereo arrangement, it looks like this:

What will happen if such a design is placed in a parabolic dish?? Look at this:

Conclusion: The "parabol signal" - the sound "gathered together" by the parabol, and focused on the same point - will affect the two membranes equally. The effect of two capsules, mounted PZM, on each side of a plate and used in a parabol, will create a mono centre channel, amplified by the parabol, and a background of binaural stereo.

The focused sound will be recorded mono, while the environment will split into a left and a right channel!

This is easily tested: If the two microphones are connected with one another, and the plate between them removed, the design can only work as a conventional parabola with a conventional mono microphone. If the plate is put back again, between the microphone capsules, one can neither hear, nor measure any significant difference.

Some of the environmental sounds will get attenuated by anti-phase effects, but this is often exactly what we want.

The "hole in the middle", and "the ping pong effect".

Some people claim that "there must be a hole in the middle" of the sound picture from a stereo parabol. Others complaint that there must be "a ping-pong effect. The "hole" statement is wrong, unless one claims that all stereo sound has a hole in the middle. On the contrary, the PZM design described above will add a centre channel to the middle. The statement about ping-pong effect is correct, as it is correct with all stereo at very short distances:

In an environment with many insects, the Telinga design, as well as X-Y and M/S stereo will make a mosquito which is moving an inch, move several meters between loudspeakers. This stereo problem can be solved only by using two microphones placed as far away from one another as the replaying loudspeakers.

The future:

Hand-noise and wind-noise are very practical problems which often gives surprising results: Even a very careful design can result in a total disaster at low frequencies. Also electrical noise is sometimes "paradoxical": If a noise is "scratchy" it will be more disturbing for the ears than a "soft" noise, even if it measures lower in voltage. As the stereo DATmic has an inherent noise level which is (in voltage) beaten only by Sennheiser MKH-20, and also quite "soft," we do not have any illusions to make a lot of progress on that parameter.

When improving reliability, we have to compromise with costs and weight. The future is full of such balance problems.

What we would like from our users, in the meantime, is that they get better earphones. All the time we meet nice people with nice recording equipment, but using the cheapest earphones for pop-music. This makes them totally unable to locate whether a bird sounds "best" when it is focused, or a bit out of focus. Neither can they with such earphones decide about the best recording distance, or hear distortion, as the ear-phones distort all the time anyway.

Not to speak about the problem that the frequency reproduction of such ear-phones more or less is a question about the listeners head-size.....

Best regards,
K G Strandberg,
Telinga.