Papers in Physics, vol. 10, art. 100006 (2018)

Received: 5 January 2018, Accepted: 20 April 2018
Edited by: A. Mart́ı, M. Monteiro
Reviewed by: P. Jeanjacquot, École Normale Supérieure de Lyon –

Institut Français de l’Éducation, France
Licence: Creative Commons Attribution 4.0
DOI: http://dx.doi.org/10.4279/PIP.100006

www.papersinphysics.org

ISSN 1852-4249

Smartphone audio port data collection cookbook

Kyle Forinash,1∗ Raymond Wisman1†

The audio port of a smartphone is designed to send and receive audio but can be harnessed
for portable, economical, and accurate data collection from a variety of sources. While
smartphones have internal sensors to measure a number of physical phenomena such as
acceleration, magnetism and illumination levels, measurement of other phenomena such
as voltage, external temperature, or accurate timing of moving objects are excluded. The
audio port cannot be only employed to sense external phenomena. It has the additional
advantage of timing precision; because audio is recorded or played at a controlled rate
separated from other smartphone activities, timings based on audio can be highly accurate.
The following outlines unpublished details of the audio port technical elements for data
collection, a general data collection recipe and an example timing application for Android
devices.

I. Audio port technical elements for
data collection

The audio port physical interface to the smart-
phone is a 4-pole jack connecting to some exter-
nal device. Table 1 presents the American Headset
Jack (AHJ) standard used by many smartphones
for connections to the 4-pole jack. Figures 1 and
2 present two similar measurement circuits based
upon the same resistor-capacitor network, Fig. 1
circuit for temperature measurements [1] and Fig.
2 circuit with a pair of photoresistors acting as
gates for timing measurements of a moving object
[2].

Although timing and temperature measures are
obviously different, the circuits and data collec-
tion methods are quite similar. In each circuit, a
variable resistor connects the speaker audio out-

∗E-mail: kforinas@ius.edu
†E-mail: rwisman@ius.edu

1 Indiana University Southeast, 4201 Grantline Road, New
Albany, Indiana 47150, USA.

Table 1: Audio connections to 4-pole jack.

Tip Left speaker
1 Right speaker
2 Ground
3 Microphone

Figure 1: Temperature measurement circuit.

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Papers in Physics, vol. 10, art. 100006 (2018) / K. Forinash et al.

Figure 2: Time measurement circuit.

put to the microphone input; the speaker output
is a wave which peak amplitude at the microphone
input varies with the variable resistor, greater re-
sistance resulting in lower peak amplitude. Fig-
ure 3 illustrates the effect of a photoresistor on the
microphone signal amplitude while a moving ob-
ject momentarily blocks and reduces the illumina-
tion reaching the photoresistor - as the illumination
decreases, resistance increases and peak amplitude
decreases.

The microphone input amplitude is digitally
sampled (44100 samples per second is common)
over time as displayed in Fig. 4. Each digital
value represents the speaker output amplitude as
changed by the circuit, then received and digitized
as the microphone input.

A recipe for data collection with Fig. 1 and 2
type circuits generally is:

1. Choose a variable resistor to measure some
phenomena such as force, humidity, etc.

2. Generate a wave of fixed frequency and peak
amplitude through the speaker output and
sample it as the microphone input as illus-
trated in Fig. 4.

3. Detect the significant elements of the micro-
phone input. Significant for timing applica-
tions, wave peaks can be detected by the sim-
ple method described in Fig. 4.

4. Convert the significant elements to corre-
sponding data. For example, by recording a

Figure 3: Microphone input signal amplitude versus
time graph from circuit in Fig. 2 when measuring an
object falling from 1 meter. The amplitude of the wave
input at the microphone drops as the object entering
the first photoresistor (PR1) at time 6.2041 s and again
at time 6.6607 s when entering the second photoresistor
(PR2). The measured time of 0.456 s compares favor-
ably with the predicted time of 0.451 s.

Figure 4: Speaker output of a 4410 Hz sine wave sam-
pled at 44100 samples per second at the microphone,
producing the highlighted 10 samples per wave. Simple
peak detection is possible by comparing three consecu-
tive samples (x0, x1, x2) where: 0 < x0 < x1 > x2 > 0
the wave peak occurs at sample x1.

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Papers in Physics, vol. 10, art. 100006 (2018) / K. Forinash et al.

range of peak amplitudes produced by the cir-
cuit of Fig. 1 and the corresponding temper-
atures, an equation fitted to that amplitude
vs. temperature data can convert the circuit
amplitude output to temperature.

II. Timing data collection recipe

For timing of a moving object, some form of gate is
needed to detect entry and exit. Ideally, gates are
binary, they only open or close with no intermediate
states, unfortunately reality does not represent the
ideal case. For timing applications, the circuit of
Fig. 2 includes one or more photoresistors (gates)
connected serially so that when all are illuminated
(closed) resistance is at its minimum and the peak
signal amplitude input to the microphone is at its
maximum; hence, when illumination of any one of
the photoresistors is reduced, circuit resistance in-
creases and the signal amplitude reduces.

Because the photoresistor resistance does not
change instantaneously when illumination changes,
as illustrated in Fig. 3 by the gradual peak ampli-
tude drop, and in the case where one photoresistor
is illuminated differently than another producing
a different amplitude result for each, a peak am-
plitude threshold must be determined. Above the
threshold a gate is considered closed (exited) and
below the threshold a gate is considered open (en-
tered). To determine the threshold, the peak ampli-
tude is initially calibrated by illuminating (closing)
all gates and incrementally increasing the speaker
volume until the microphone peak amplitude ap-
proaches its maximum, M. After fixing the volume
for M, the microphone peak amplitude for the cir-
cuit when any single gate is open (entered), L, is
determined by blocking (entering) each gate in turn
and recording its lowest peak amplitude. L is set
to the greatest peak amplitude of any blocked gate
to ensure the threshold is crossed when any gate is
blocked. The maximum M and minimum L then
set the upper and lower output limits of the circuit
peak amplitude. The threshold amplitude, TA, the
peak amplitude level that determines when an ob-
ject enters or exits a gate is: TA=(M+L)/2, the
amplitude midpoint between M and L. An ampli-
tude crossing the threshold from above is defined as
gate entered, while crossing it from below is defined
as gate exited.

With the threshold defined, deriving timing data
from the microphone audio is both straightforward
and accurate. From the general recipe, the practi-
calities of timing a moving object reduces to:

1. Choose a photoresistor to measure illumina-
tion.

2. Generate a sine wave of 4410 Hz and volume
setting at M through the speaker and sample
at the microphone at 44100 samples per second
which corresponds to 10 samples per wave.

3. Detect the wave peaks by the method de-
scribed in Fig. 4.

4. Convert wave peaks to corresponding timing
data by noting the sample number at threshold
crossings.

Microphone input sampling occurs at 44100 sam-
ples per second or one sample every 0.000023 s
(equals 1 sample/44100 samples/second). Accurate
timing between events is simply a matter of count-
ing samples between threshold crossings. Gates are
entered when a wave peak crosses the threshold
from above and the sample number of the peak is
recorded at this time. Similarly, gates are exited
when a wave peak crosses the threshold from be-
low. The time between entering two gates is then:

t =
g2 − g1

44100 samples

second

, (1)

where t=time between Gate 1 and 2, g1 = Gate 1
entry sample number, and g2 = Gate 2 entry sample
number.

Figure 3 illustrates the microphone input while
timing the one meter free fall of an object when
entering two photoresistor gates. The amplitude of
the wave input at the microphone drops as the ob-
ject entering the first photoresistor (PR1) at time
6.2041 s and again at time 6.6607 s when enter-
ing the second photoresistor (PR2). The mea-
sured time of 0.456 s = 20110 samples/44100 sam-
ples/second compares favorably with the predicted
time of 0.451 s.

The prescribed recipe above was followed in the
design of the GateTiming [3] timing application
that measures times of entry and exit at multi-
ple gates. Figure 5 presents the times recorded

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Papers in Physics, vol. 10, art. 100006 (2018) / K. Forinash et al.

Figure 5: Sample timing results from GateTiming ap-
plication.

during multiple runs through two gates. The
Date/UTC.ms column is the clock time on entering
the first gate, the Time column is the time between
entering the first and second gate; a more detailed
display of each gate entry and exit times is a view-
ing option. Table 2 lists the times as recorded by
the application of a block falling one meter. The
variation of measurements is primarily due to slight
differences in the height when dropped above the
first gate. For example, dropping from 1/2 inch
(1.27 cm) above the first gate produced a timing
in the range of 0.42 s, the time is lower than the
one predicted because the block is traveling a lit-
tle faster through the gates (about 0.5 mph or 0.80
km/h at the first gate). Other timing variations are
likely due to slight changes to the block profile rel-
ative to the gates, not always maintaining a stable
orientation.

III. Conclusions

Although only timing was closely examined in this
paper, a range of audio port data collection ap-
plications are possible, including our own Android

Table 2: Times of a block falling one meter.

Time [s]
0.453
0.457
0.471
0.451
0.448

Mean 0.456

applications: AudioTime+ [4], a general purpose
tool for signal display and analysis that can exam-
ine the behavior of a variable resistor in the cir-
cuit of Fig. 1 and 2, GateTiming [3], to measure
the time an object passes between or through a se-
ries of gates, DCVoltmeter [5], measurement of 0-
10VDC via voltage-to-frequency conversion of mi-
crophone input, then converting the frequency to a
corresponding voltage value; a simple, active volt-
age controlled oscillator circuit is required for the
conversion. Other applications for data collection
from a variety of sources using the basic recipe are
also possible. By using this method, data can be
collected from almost any sensor based on variable
resistance.

[1] K Forinash, R Wisman, Smartphones - Exper-
iments with an External Thermistor Circuit,
The Physics Teacher 50, 566 (2012).

[2] K Forinash, R Wisman, Photogate Timing with
a Smartphone, The Physics Teacher 53, 234
(2015).

[3] R Wisman, K Forinash, Mobile Science -
GateTiming, December 2017. Google Play:
https://play.google.com/store/apps/details?
id=edu.ius.rwisman.gatetiming

[4] R Wisman, K Forinash, Mobile Science -
AudioTime+, November 2013. Google Play:
https://play.google.com/store/apps/details?
id=edu.ius.audiotimeplus

[5] R Wisman, K Forinash, Mobile Science -
DCVoltmeter, January 2016. Google Play:
https://play.google.com/store/apps/details?
id=edu.ius.dcvoltmeter

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