DIY Collapsible Parabolic Dish Microphone
Content
A collapsible and easy to build dish
microphone
Current project state:
finished.
Last update:
20131003
Home
Intro
Calculation and
Construction
The Microphone
email the author
I wanted to have a directional microphone for bird and insect sound recordings in the
field. The local electronics shop offers a small plastic dish reflector (diameter of about
30cm) which I purchased. But the first test was more than disappointing. Both gain
(signal to noise ratio) and directionality effect were miserable with this small dish so I
decided to build something more capable. It should still be easily transportable, though.
The larger dish therefore had to be made collapsible in order to fit into a tramper
backpack. I came up with the following design.
The small
dish is used
as a base to
carry a set
of
two
extension
rings
that
approximate
the parabolic
form
well
enough. The
whole thing
is
held
together by
screws. This
picture
shows the
first
build
which I cut
from
polyacrylic
(Plexiglass®)
sheet which
turned out to
be
an
expensive
mistake.
Polyacryl is
too brittle, it
will
break
and splitter
while
cutting, and
it can not be
bent well enough for the purpose without the risk of breaking it. Polyethylene (aka
polycarbonate) will probably work better, and I found polystyrene most comfortable
to cut and handle (2mm PS sheet can actually be cut using scissors). In my
country, there is plenty of this material used (and dumped) as advertisment panels
on construction sites. Naturally, with the extensions the dish becomes quite deep. I
did not find this to impose negative effects on the frequency response.
If you don't have a base dish available to build upon you can also simply use a flat
plate on the back side. This will lower the performance by only a small fraction.
The extension rings have been cut into pieces of two (inner ring) and three (outer
ring) in order to make them transportable (the assembled outer ring is more than 1m
wide). Assembling it in the wild takes about 5 minutes.
Calculation and Construction
I include a thurough explanation of how to calculate the dimensions of the extensions as
a PDF file. The calculations involve some fairly simple trigonometry. There's also a
spreadsheet available that does the math for you (even in a more exact way than how
it's done in the PDF). You only type in the basic dimensions and how big you want to
have your extension(s). The spreadsheet should work in LibreOffice or in OpenOffice.
Explanation of the calculations, PDF
Explanation of the calculations, OpenDocument source file
Spreadsheet for the calculations, OpenDocument file
The Microphone
Adding to this, I custom built a stereo microphone from arrays of 5 mic capsules in
parallel at each channel. Such an array assembly helps to dramatically raise the S/N
ratio: the audio signals from the capsules add up because of their synchronicity while the
noise from the individual capsules (which is stochastic) tends to cancel out. The mics I
used had a sensitivity of 58dB (IIRC), the best I could get at this size and at a
reasonable cost at that time. I estimated that the sensitivity of the whole assembly
(microphone array and dish) is comparable to that of a human ear. Not bad!
The spacing
between the
two arrays
has
been
made
so
that a wall of
cardboard or
plastic can
be put in
between.
This
wall
would act as
the key element in separating the channels, by practically cutting the dish in halves.
It should fit tightly around the mic area and be the size of the whole crosssection of
the dish. I never got around to making a more permanent solution than crudely
cutting in a piece of cardboard (but it works).
Size matters. Because the mic cartridges are small and because an electret
microphone responds to air pressure waves rather than to motion of the air, they
should practically be no obstacle for sound waves down to a wavelength of double
their size. The ones used are 4mm and therefore respond in an omnidirectional
(spherical) manner at up to 42kHz. Field recordings (with Crickets) show that this
particular type works well above 30kHz (I think the datasheet showed linear
response until 28kHz, sorry I forgot the exact type).
There are also cartridges available that have cardioid caracteristics. Due to the parabolic
approximation of the dish, I expect a "focal spot" rather than a focal point. My feeling told
me to put the capsules closely together, so to have them all in this area. Peter from
South Dakota brought me to the idea that arranging them in a spherical shape (instead of
flat) would make them space a little more and block each other even less. Using
cardioids pointing outward from the focal point would also help in separating the stereo
channels.
(to be continued...)
Collapsible parabolic dish microphone by Markus Petz (Mintaka) is
licensed under a Creative Commons AttributionNonCommercial
ShareAlike 3.0 License. Permissions beyond the scope of this
license may be available at the project website.
microphone
Current project state:
finished.
Last update:
20131003
Home
Intro
Calculation and
Construction
The Microphone
email the author
I wanted to have a directional microphone for bird and insect sound recordings in the
field. The local electronics shop offers a small plastic dish reflector (diameter of about
30cm) which I purchased. But the first test was more than disappointing. Both gain
(signal to noise ratio) and directionality effect were miserable with this small dish so I
decided to build something more capable. It should still be easily transportable, though.
The larger dish therefore had to be made collapsible in order to fit into a tramper
backpack. I came up with the following design.
The small
dish is used
as a base to
carry a set
of
two
extension
rings
that
approximate
the parabolic
form
well
enough. The
whole thing
is
held
together by
screws. This
picture
shows the
first
build
which I cut
from
polyacrylic
(Plexiglass®)
sheet which
turned out to
be
an
expensive
mistake.
Polyacryl is
too brittle, it
will
break
and splitter
while
cutting, and
it can not be
bent well enough for the purpose without the risk of breaking it. Polyethylene (aka
polycarbonate) will probably work better, and I found polystyrene most comfortable
to cut and handle (2mm PS sheet can actually be cut using scissors). In my
country, there is plenty of this material used (and dumped) as advertisment panels
on construction sites. Naturally, with the extensions the dish becomes quite deep. I
did not find this to impose negative effects on the frequency response.
If you don't have a base dish available to build upon you can also simply use a flat
plate on the back side. This will lower the performance by only a small fraction.
The extension rings have been cut into pieces of two (inner ring) and three (outer
ring) in order to make them transportable (the assembled outer ring is more than 1m
wide). Assembling it in the wild takes about 5 minutes.
Calculation and Construction
I include a thurough explanation of how to calculate the dimensions of the extensions as
a PDF file. The calculations involve some fairly simple trigonometry. There's also a
spreadsheet available that does the math for you (even in a more exact way than how
it's done in the PDF). You only type in the basic dimensions and how big you want to
have your extension(s). The spreadsheet should work in LibreOffice or in OpenOffice.
Explanation of the calculations, PDF
Explanation of the calculations, OpenDocument source file
Spreadsheet for the calculations, OpenDocument file
The Microphone
Adding to this, I custom built a stereo microphone from arrays of 5 mic capsules in
parallel at each channel. Such an array assembly helps to dramatically raise the S/N
ratio: the audio signals from the capsules add up because of their synchronicity while the
noise from the individual capsules (which is stochastic) tends to cancel out. The mics I
used had a sensitivity of 58dB (IIRC), the best I could get at this size and at a
reasonable cost at that time. I estimated that the sensitivity of the whole assembly
(microphone array and dish) is comparable to that of a human ear. Not bad!
The spacing
between the
two arrays
has
been
made
so
that a wall of
cardboard or
plastic can
be put in
between.
This
wall
would act as
the key element in separating the channels, by practically cutting the dish in halves.
It should fit tightly around the mic area and be the size of the whole crosssection of
the dish. I never got around to making a more permanent solution than crudely
cutting in a piece of cardboard (but it works).
Size matters. Because the mic cartridges are small and because an electret
microphone responds to air pressure waves rather than to motion of the air, they
should practically be no obstacle for sound waves down to a wavelength of double
their size. The ones used are 4mm and therefore respond in an omnidirectional
(spherical) manner at up to 42kHz. Field recordings (with Crickets) show that this
particular type works well above 30kHz (I think the datasheet showed linear
response until 28kHz, sorry I forgot the exact type).
There are also cartridges available that have cardioid caracteristics. Due to the parabolic
approximation of the dish, I expect a "focal spot" rather than a focal point. My feeling told
me to put the capsules closely together, so to have them all in this area. Peter from
South Dakota brought me to the idea that arranging them in a spherical shape (instead of
flat) would make them space a little more and block each other even less. Using
cardioids pointing outward from the focal point would also help in separating the stereo
channels.
(to be continued...)
Collapsible parabolic dish microphone by Markus Petz (Mintaka) is
licensed under a Creative Commons AttributionNonCommercial
ShareAlike 3.0 License. Permissions beyond the scope of this
license may be available at the project website.
