First recordings through a parabolic dish tend to surprise people: the bird is gloriously loud and close, but everything sounds too bright and thin. Nothing is wrong at this point - the issue fixable with EQ applied in software after the fact.
Why parabolic recordings sound bright
A dish's gain isn't flat. On-axis, it rises roughly 6 dB per octave above its cutoff, because higher frequencies see a proportionally larger reflector. Here's the response of both our 300 and 580 microphones - gain relative to the same microphone with no dish:
Two things to notice here:
- The rising tilt. The 580 picks up about 13 dB at 1 kHz but about 30 dB at 8 kHz, so recordings arrive tilted roughly 15 dB across three octaves. That's the brightness you're hearing.
- The low-frequency dip. Every parabolic dish has an interference notch below its working band - 283 Hz on the 580, 635 Hz on the 300 - where the direct sound and the focused reflection arrive out of phase and partially cancel. Physics, not a defect.
What happens if I didn't perfectly aim the microphone at the subject?
If the microphone is not pointed at the subject, then the frequency response changes, and there is less gain at high frequencies in particular. How much this occurs depends on the size of the microphone and the degree to which it is off axis:
Not a such a big deal for a 300mm microphone, but it has a significant effect in the top octave for the TalonFab 580mm mic where the right equalisation to apply depends more on how closely it was aimed at the target. So in practise some judgement is probably required in terms of how aggressively to EQ the highest frequencies. This information also tells you that live monitoring is more valuable for larger microphones, that listening to the top-end sparkle is most important when aiming and that smaller microphones are probably a better choice for high frequency sound.
In TalonFab's case, since the geometry is known we can simulate and then measure the response of the exact production shape, and avoid guesswork. We provide an easy way to apply the required equalisation - TalonFab Editor and Recorder has curves built right in. We even include the ability to adjust for an off-axis target. These filters are usually best combined with a high pass filter to remove wind noise.
If you're using a generic dish and response curves or equalisation software are not provided, then something like the following is recommended:
- Apply a high pass filter (12-24 dB per octave). Pick the frequency based on the spectrum of the signal you're interested in. For example if a bird call is primarily above 1 kHz, you might try 900Hz. It's rare that you'll get useful signal below say 120Hz, so this could be a good starting point if you don't want to analyse the spectrum of the recording.
- Tilt the top down. Apply a broad cut that increases with frequency above about 1 kHz for a small 30cm dish or about 500hz if you have a bigger 50 or 60cm one - roughly 6 dB per octave of correction, reaching about −15 dB by 8 kHz on a big dish, less on a small one. Cut rather than boost; it preserves headroom and never amplifies noise. Note if that if your dish has loose manufacturing tolerances, or you you think didn't point it exactly at your target, then at the highest frequencies the amount of cut should be reduced to compensate - 3-4db per octave from 5k for a 50 or 60cm dish and from 8k for a 30cm dish would be a good starting point.
- Leave the notch alone. Don't try to boost the low-frequency dip back to flat. The dish genuinely captured little signal there, and a full boost mostly amplifies wind. If you must, half-filling it with a broad gentle bell is reasonable.
Summary
To get really good results from a parabolic microphone equalisation is required. But the exact curve to apply depends on the geometry of your dish and whether you pointed it exactly at your target. For this reason some listening and adjustment is required in practise, particularly for the top octave. TalonFab Editor and Recorder lets you do this with TalonFab parabolic dishes at the click of a button, but it's possible to do a reasonable job with parabolic equalisation if you're using another another microphone.
