Article on Parabolic Microphone Gain (1 Viewer)

[Lerxst]

I wrote this last month and forgot about it. Simply meant to be a reference for anyone bored enough to read it. Some years ago I was looking for papers on this and only found one that really touched on the topic (it is cited) in a non-trivial way. But it was also pretty dense at points and hard to follow. Moreover, I wanted an excuse to do some animations/simulations. If you do read this and have comments for improving it, I am all ears. It is probably way too long.

The Physics of Parabolic Microphones: Frequency Dependence of Gain - Michael Hurben, PhD

Introduction [latexpage] Parabolic microphones are known for their extreme sensitivity, and the origin of their acuity isn't difficult to guess. It is the most obvious thing about them, which can also make them a liability for field use, namely, their considerable size. Just as a large amount of...

legallyblindbirding.net

 

[Jon.Bryant]

I will read the article more thoroughly, but note that in the introduction you mention how parabolas provide more gain at higher frequencies as if this is a negative, and that recordings are more tinny. This is true, but one of the counters to this argument, is that as higher frequency sound attenuates more quickly than lower frequency sound, and that this attenuation is inverse to the gain provided by a parabola. Therefore the unequal gain of the parabola can be beneficial by 'recreating nearness'. Under this argument, the 'tinny' sound that some people don't like, is then the natural sound of the bird when up close and personal - we then get into the argument of whether we want a 'human ear' or 'birds ear' recording.

I find it interesting that if we are in the rain forest and point a shotgun at a bird singing in the canopy 30m overhead, physics dictate that it will always have the frequency balance as if it is 30m away, no matter how much gain we apply or how loud we play the recording. If we want to recreate nearness, and hear what the bird would sound like, if we were perched on a branch close by, then we need to either apply equalizer or alternatively use a parabola.

I recall that attenuation of higher frequencies is temperature and humidity dependent. Frequency gain from a parabola is fixed for a particular dish. I have therefore been thinking for a while of doing some approximate math to work our the 'impact' of a parabola at different distances, and weather conditions - i.e. what is the apparent shortening of distance to the target, that is created when using a dish? I also think that there must be a minimum distance we should use a parabola, so that you are only recreating nearness, as opposed to over emphasizing the high frequencies. However, as the main reason for using a parabola is for distant work, a 'minimum distance' may not be that important (and I suppose you can always try to fix the an over tinny recording by applying equaliser in post).

 

[Lerxst]

I will read the article more thoroughly, but note that in the introduction you mention how parabolas provide more gain at higher frequencies as if this is a negative, and that recordings are more tinny. This is true, but one of the counters to this argument, is that as higher frequency sound attenuates more quickly than lower frequency sound, and that this attenuation is inverse to the gain provided by a parabola. Therefore the unequal gain of the parabola can be beneficial by 'recreating nearness'. Under this argument, the 'tinny' sound that some people don't like, is then the natural sound of the bird when up close and personal - we then get into the argument of whether we want a 'human ear' or 'birds ear' recording.

I find it interesting that if we are in the rain forest and point a shotgun at a bird singing in the canopy 30m overhead, physics dictate that it will always have the frequency balance as if it is 30m away, no matter how much gain we apply or how loud we play the recording. If we want to recreate nearness, and hear what the bird would sound like, if we were perched on a branch close by, then we need to either apply equalizer or alternatively use a parabola.

I recall that attenuation of higher frequencies is temperature and humidity dependent. Frequency gain from a parabola is fixed for a particular dish. I have therefore been thinking for a while of doing some approximate math to work our the 'impact' of a parabola at different distances, and weather conditions - i.e. what is the apparent shortening of distance to the target, that is created when using a dish? I also think that there must be a minimum distance we should use a parabola, so that you are only recreating nearness, as opposed to over emphasizing the high frequencies. However, as the main reason for using a parabola is for distant work, a 'minimum distance' may not be that important (and I suppose you can always try to fix the an over tinny recording by applying equaliser in post).

Thank you Jon, those are all very good points, and I will work to incorporate discussion of them into my rev2.

Myself I like the "tinniness" because at my age my high frequency content is disappearing. And even the best ears are not immune to the nonlinearity of the Fletcher Munson curves! Years ago I played with equalization, just for fun, that tried to undo those effects, in order to hear what sounds "really sound like" without having to go through the very imperfect filtering and distortion innate to human ears. This has always fascinated me.

 

[pdterp]

You have created a very comprehensive paper on parabolic microphones. I found it especially interesting that your article mentions parabolic microphones with an 8-inch diameter. Our company manufactures parabolic microphones with 24-inch, 14-inch, and 8-inch active diameters (excluding the front flange).

I would be interested in your thoughts on my blog post/video addressing the question, "How Big Does a Parabolic Microphone Need To Be?" It would seem that the parabolic mic provided some amplification compared to a bare lapel mic from 6 feet away.

Since we are on a birding forum, you might also be interested in this compilation of bird clips captured by our customers using our smallest parabolic mics.

I look forward to your reply. I have been designing parabolic microphones for 12 years and have yet to find a reasonable estimation of a design's performance. If I had followed the science, we would have never released our smallest parabolic, but it has proven very useful for our customers. (I'm a mechanical engineer, not a scientist.)

Thank you for your time.

 

[3dpan]

I wrote this last month and forgot about it. Simply meant to be a reference for anyone bored enough to read it. Some years ago I was looking for papers on this and only found one that really touched on the topic (it is cited) in a non-trivial way. But it was also pretty dense at points and hard to follow. Moreover, I wanted an excuse to do some animations/simulations. If you do read this and have comments for improving it, I am all ears. It is probably way too long.

The Physics of Parabolic Microphones: Frequency Dependence of Gain - Michael Hurben, PhD

Introduction [latexpage] Parabolic microphones are known for their extreme sensitivity, and the origin of their acuity isn't difficult to guess. It is the most obvious thing about them, which can also make them a liability for field use, namely, their considerable size. Just as a large amount of...

legallyblindbirding.net

Really interesting. The animated images are very useful in understanding the text.
However, in my download, the equations were not reproduced, just a gap in the text.

 

[Lerxst]

Really interesting. The animated images are very useful in understanding the text.
However, in my download, the equations were not reproduced, just a gap in the text.

Thank you. Oddly enough I noticed earlier today on another post that the equations were not showing up. I've now fixed the problem, and if you reload the page, they should all be there now.

I am in the process of revamping many of the animations for the articles I did on roof prisms / phase coatings. I've thought about turning such material into videos at some point.

 

[3dpan]

Thank you. Oddly enough I noticed earlier today on another post that the equations were not showing up. I've now fixed the problem, and if you reload the page, they should all be there now.

I am in the process of revamping many of the animations for the articles I did on roof prisms / phase coatings. I've thought about turning such material into videos at some point.

Aha, that's better. Now another challenge, following your equations.

I notice you have a Wildtronics parabolic mic. Something that has mystified me is the "barrier disc" in the centre of the parabola.
Can you explain why one would place a disc there ? It seems to be obscuring the incoming sound waves.
Or is it acting as a boundary mic ?

PS, congrats on your Okarito Kiwi hunt.

 

[Lerxst]

Aha, that's better. Now another challenge, following your equations.

I notice you have a Wildtronics parabolic mic. Something that has mystified me is the "barrier disc" in the centre of the parabola.
Can you explain why one would place a disc there ? It seems to be obscuring the incoming sound waves.
Or is it acting as a boundary mic ?

PS, congrats on your Okarito Kiwi hunt.

The disc you mention is what the manufacturer refers to as a "booster plate." I have reached out to them for more details on this component but they will divulge nothing, claiming it is entirely proprietary. I've been looking for the patent application but have not found it yet. And I'm loathe to do anything by the way of deconstruction of the component.

I measured the diameter and found that it reduces the cross-sectional area of the dish by about 8%. On the website they state a sound level reduction of 5%. My initial guess for the difference is that diffraction effects from the disk help mitigate the loss in area, but not by a lot. Incidentally the size of the native microphone they use would present a barrier a bit under 1% of the total area.

The manufacturer claims that the booster plate serves to improve isolation and boost the low end. The latter I can see as resulting from a change to the diffraction pattern at low frequency. As for isolation, the plate is perhaps spreading the foci from the off-axis reflection even further than they are for a plain paraboloid.

I had not really thought much about the role of this feature and I'm now intrigued by it and will try to see if the literature turns up anything when I have a chance to look. The only other recourse is to procure (or write) a simple finite element model and try to emulate the design, but that is going to be a bigger time investment for me than I have right now.

 

[Lerxst]

Following up on the Wildtronics "booster disc"... I was looking at the microphone/disc enclosure and noticed it does have a factory seal, so it made me reconsider the idea that there are active elements residing throughout the structure. I then looked through the website a bit more and found:

"Integral booster discs are designed into the microphone assembly to increase the audio gain, boost low frequency response to that of a 30-inch parabolic dish, and help isolate the sound pickup to only the focused target and avoid other stray sounds. This amazing booster disc technology further sets the Wildtronics Parabolic Microphone apart from all others. The addition of the booster disc gives approximately a 6dB advantage over conventionally designed parabolic microphones-- that is like being at half the distance to the subject. Not just one, but an array of low noise microphone elements are used at the focal point of the parabola."

Distributing additional transducers so as to pick up reflections that don't find the nominal focal point would mitigate the diffraction effects that worsen at lower frequencies.

When I have time I plan to remove the entire microphone/booster assembly and map out its angle dependence; that might give some clue as to how this array is laid out. I'm not yet curious enough to break the seal and look inside.

They microphone-less versions of their dish don't include any kind of "plate", another reason to conclude that they've wired up the main element at the main focus with additional electronics/transducers.

 

[3dpan]

Following up on the Wildtronics "booster disc"... I was looking at the microphone/disc enclosure and noticed it does have a factory seal, so it made me reconsider the idea that there are active elements residing throughout the structure. I then looked through the website a bit more and found:

"Integral booster discs are designed into the microphone assembly to increase the audio gain, boost low frequency response to that of a 30-inch parabolic dish, and help isolate the sound pickup to only the focused target and avoid other stray sounds. This amazing booster disc technology further sets the Wildtronics Parabolic Microphone apart from all others. The addition of the booster disc gives approximately a 6dB advantage over conventionally designed parabolic microphones-- that is like being at half the distance to the subject. Not just one, but an array of low noise microphone elements are used at the focal point of the parabola."

Distributing additional transducers so as to pick up reflections that don't find the nominal focal point would mitigate the diffraction effects that worsen at lower frequencies.

When I have time I plan to remove the entire microphone/booster assembly and map out its angle dependence; that might give some clue as to how this array is laid out. I'm not yet curious enough to break the seal and look inside.

They microphone-less versions of their dish don't include any kind of "plate", another reason to conclude that they've wired up the main element at the main focus with additional electronics/transducers.

Very interesting, and very clever.

 

[vupham]

I'm curious how you store this disc when traveling and birding. I find it uncomfortable and difficult to carry because it’s so fragile and easy to break. I’ve been using a parabolic recorder, which I love—the sound quality through a field recorder like the Tascam DR40 is outstanding. However, it’s challenging to maneuver in the field, especially under dense canopies. Rotating around the tripod and camera with the disc attached is tricky, and I’m always worried about its fragility.

 

[Lerxst]

I'm curious how you store this disc when traveling and birding. I find it uncomfortable and difficult to carry because it’s so fragile and easy to break. I’ve been using a parabolic recorder, which I love—the sound quality through a field recorder like the Tascam DR40 is outstanding. However, it’s challenging to maneuver in the field, especially under dense canopies. Rotating around the tripod and camera with the disc attached is tricky, and I’m always worried about its fragility.

Not sure who this question is directed to, but in the case of the Wildtronics, it is anything but fragile. It is easy to remove the dish itself from the handle/microphone unit, and roll into into a more compact cylindrical shape. It pops right back into form when you unroll it and there seems to be no hysteresis from such treatment. It takes a real beating and shows no effects at all. There is likely more information about its robust character at their website. In the field I clip it to my belt and it isn't a chore to carry it at all.

 

[vupham]

Not sure who this question is directed to, but in the case of the Wildtronics, it is anything but fragile. It is easy to remove the dish itself from the handle/microphone unit, and roll into into a more compact cylindrical shape. It pops right back into form when you unroll it and there seems to be no hysteresis from such treatment. It takes a real beating and shows no effects at all. There is likely more information about its robust character at their website. In the field I clip it to my belt and it isn't a chore to carry it at all.

I see, thank you for reply

 

[Montereyman]

I went with a Telinga kit that has a large folding dish (actually it rolls up and goes into a sleeve) and a much smaller rigid dish that is roughly 13 inches in diameter. The Telinga mic can be used with either dish.

Not obvious but a dish is more directional that most shotgun mics and so it is important to use headphones and verify the signal strength getting to the recorder.

 

[Lerxst]

You have created a very comprehensive paper on parabolic microphones. I found it especially interesting that your article mentions parabolic microphones with an 8-inch diameter. Our company manufactures parabolic microphones with 24-inch, 14-inch, and 8-inch active diameters (excluding the front flange).

I would be interested in your thoughts on my blog post/video addressing the question, "How Big Does a Parabolic Microphone Need To Be?" It would seem that the parabolic mic provided some amplification compared to a bare lapel mic from 6 feet away.

Since we are on a birding forum, you might also be interested in this compilation of bird clips captured by our customers using our smallest parabolic mics.

I look forward to your reply. I have been designing parabolic microphones for 12 years and have yet to find a reasonable estimation of a design's performance. If I had followed the science, we would have never released our smallest parabolic, but it has proven very useful for our customers. (I'm a mechanical engineer, not a scientist.)

Thank you for your time.

I'm sorry I did not respond sooner to you. I am not on birdforum very often and I missed this post previously.

I suppose my immediate reaction is that I expect an 8-inch dish to perform "reasonably well" in the sense that with a cutoff around 1.5kHz, you'll still be boosting a large part of our innate audio range. Voices have spectral content in that range and so I expect such a device may provide enough gain to make it beneficial for certain applications. Based on the videos you have, the loss in lower frequencies is evident, but that isn't going to ruin one's ability to, say, understand a conversation recorded with an 8-inch dish with some nominal signal level. It will be "colored" but obviously, not unrecognizable. The effect on bird calls may be less (or occasionally more) important; given what I know about the strategy of apps such as Merlin, roll-off in low frequencies is likely not going to impact its ability to identify many calls that have sufficient high-frequency content.

I'd be interested to see/hear broadband audio recordings (say, with a swept tone from 20Hz-20kHz) from a distant source (such that the native microphone alone provides very little or no signal) using both the 8-inch and your largest dish available, with the same mic, for a spectral analysis.

All that being said, if one's goal includes capturing a weak signal with principally low-frequency content, a larger dish is necessary.

 

[WRL]

I'm sorry I did not respond sooner to you. I am not on birdforum very often and I missed this post previously.

I suppose my immediate reaction is that I expect an 8-inch dish to perform "reasonably well" in the sense that with a cutoff around 1.5kHz, you'll still be boosting a large part of our innate audio range. Voices have spectral content in that range and so I expect such a device may provide enough gain to make it beneficial for certain applications. Based on the videos you have, the loss in lower frequencies is evident, but that isn't going to ruin one's ability to, say, understand a conversation recorded with an 8-inch dish with some nominal signal level. It will be "colored" but obviously, not unrecognizable. The effect on bird calls may be less (or occasionally more) important; given what I know about the strategy of apps such as Merlin, roll-off in low frequencies is likely not going to impact its ability to identify many calls that have sufficient high-frequency content.

I'd be interested to see/hear broadband audio recordings (say, with a swept tone from 20Hz-20kHz) from a distant source (such that the native microphone alone provides very little or no signal) using both the 8-inch and your largest dish available, with the same mic, for a spectral analysis.

All that being said, if one's goal includes capturing a weak signal with principally low-frequency content, a larger dish is necessary.

May I reiterate my previous relevant post?-> Microphone for android to amplify sound
And as one source: Parabolic microphone - Wikipedia
So a dish with a diameter of 8 inch (= 20 cm) will give you very little, almost nothing in respect of the desired effect.

 

[Lerxst]

May I reiterate my previous relevant post?-> Microphone for android to amplify sound
And as one source: Parabolic microphone - Wikipedia
So a dish with a diameter of 8 inch (= 20 cm) will give you very little, almost nothing in respect of the desired effect.

Yes, totally agree with what you have written. The post I was responding to has links to video recorded with the 8-inch dish which doesn't sound terrible although it has clearly lost its bottom end. For certain applications such a highly filtered signal might be acceptable, for others, not at all. It appears to be recording a fairly close subject and I expect that even with the dish removed there is enough signal present for the mic alone to be adding whatever low-end is there. If the goal is to capture a weak signal with reasonable fidelity over the nominal auditory range, such a small dish cannot work and isn't an option for a recordist.