I have designed and built my first working prototype of a proximity detection
system integrated into a microphone. My current prototype automatically adjusts
microphone audio gain. The gain will increase when the distance between the
microphone and the user is far or the
microphone is tilted away from the user. The gain will decrease when the
microphone is very close to the user. The hope is that this system
will minimize the effects of using a microphone improperly. The microphone
then becomes more transparent to the user, and has a more
Future prototypes will adjust equalization as well as gain, and will map
distance with various audio effects for expressive purposes.
No, this is just a quick prototype made with the materials immediately on hand.
The sensors and circuitry could be
implemented to fit inside a normal microphone shell. A microphone with
proximity detection could be physically virtually indistinguishable from a
Current Automatic Gain Control (AGC) circuits use only the audio signal when
adjusting audio gain. If the audio signal is low, the gain is boosted.
If the audio signal is high, the gain is cut.
There are numerous problems with this idea:
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If a person chooses to speak loudly or softly for effect, the AGC minimizes
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AGC cannot distinguish between somebody pausing and somebody far away from
the microphone. Therefore, AGC increases gain during pauses. This is called
"pumping." Pumping can be very disturbing to listeners, as they hear
background noise steadily rising during pauses.|
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AGC circuits either have to delay their audio signal coming out or, more
commonly, they adjust audio gain "on the fly." Delaying the audio signal
coming out can be confusing to listeners and users and is not practical in
musical applications. Adjusting on the fly
means that the gain adjustment is always slightly delayed. We have all heard the disturbing negative
effects of this. Every time a person using a microphone takes a breath,
the gain increases and the audience hears the "pumping." When the user again
begins to talk, the amplifier is disturbingly loud for a
brief moment before the circuitry compensates. It may become an annoying
pattern for every sentence the user makes. The only way to compensate is to
increase the time it takes for the gain control to adjust to the audio
signal. Then the positive effects of gain control are lost so if, for
example, the user moves away from the microphone, increased gain to the
amplication is delayed.|
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Microphones loose bass response as a function of distance. It would be difficult for an
AGC circuit to detect this loss and compensate. However, future prototypes of a microphone
with proximity detection could compensate for this easily.|
The current prototype will run for many hours on 2 9V batteries. Future
prototypes will operate even more efficiently.
The current prototype can change adjust gain by about 8 decibels.
Here's a video of my officemate Victor testing
the prototype. He's actually holding two microphones, my prototype and a conventional
microphone, so we can compare the difference.
The thing to notice
is that the oscilloscope has a line which moves up and down as he moves my prototype
toward and away from his face. There's an LED in the circuit whose brightness changes as
well as a function of distance, but it's more fun to watch the oscilloscope.
Here are the resulting wave forms from the demonstration video above. My prototype is the
upper waveform while the conventional microphone is the lower waveform.
The signals are of approximately equivalent strength when the
microphones are close to Victor's mouth. As Victor moves the microphones away,
the conventional microphone's signal drops at a much higher rate than my prototype.
Besides seeing this visually, you can hear this aurally. Listen to the difference between
the two microphones:
Demo audio with my prototype.
Demo audio with conventional microphone.
Here's another example:
fastened a speaker to my mouth and played test tones, comparing the response of the
microphone at various distances. The
solid line shows the results with proximity detection enabled, and the dashed line
shows the results with proximity detection disabled.
Charts 5 and 7 show the RMS amplitude varying as a function of distance.
Charts 6 and 8 show the Decibel difference between distances.
||With the speaker still attached to my mouth, I tilted the microphone toward me and
away while playing test tones.
Chart 3 shows the difference in amplitude for various
tones with proximity detection enabled and disabled. It shows that the difference
in amplitude between on axis (microphone tilted toward the mouth) and off axis
(microphone tilted away from the mouth) is vastly reduced when then sensor is enabled.
Chart 5 shows the same information as a function of difference in Decibels
I'm not a big electronics guy so this was a difficult circuit for me to make.
Fortunately I had help. Twice the circuit itself warned me before I blew myself
up. By chance, a video camera captured these critical moments.
Video of the first warning.
Video of the second warning.
(I don't know why the thing keeps calling me Dave.)
I have already made a Second Version of this
microphone, using ultrasonic sensing monitored and controlled by a DSP
chip. You can read more about it here.
Thank you to Win Craft, formerly of THAT Corporation
in Milford MA, for the initial idea
the sparked the project.
graduate researcher for the media lab's
Viral Communications gave me the initial idea for implementation.
Joe Paradiso taught
Sensors for Interactive Environments, the
course which prompted this project and gave me the knowledge
to create it.
Ari Benbasat was his T.A. I am still in awe of how much
I learned from this course --- they did a wonderful job.
Finally, my officemate Víctor Adán has been a wonderful source for
feedback as we have discussed various approaches to this project. He has
kindly put up with me as I
made a mess of our office, conducted my weird tests, and made him the
perpetual guinea pig. I couldn't hope for a better officemate.