Acoustical  Testing  I  Black  Box  

 Submitted  to:  

Dr.  Dominique  Cheene  &  Dr.  Lauren  Ronsse  Columbia  College  Chicago  

October  8th,  2014    

                   

   

By:  Andrew  Hulva,  Student    Columbia  College  Chicago  

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Table  of  Contents    

Abstract  .......................................................................................................................................  3  

Introduction  ..............................................................................................................................  3  

Methods  .......................................................................................................................................  4  Initial  Observations  ................................................................................................................  4  1-­‐B  ..........................................................................................................................................................  4  2-­‐A  ..........................................................................................................................................................  5  

Data  Analysis  .............................................................................................................................  6  1-­‐B  ..........................................................................................................................................................  7  2-­‐A  ..........................................................................................................................................................  8  Tones  Generated  by  the  Black  Box  1B  .......................................................................................  9  Dynamics  .............................................................................................................................................  9  1-­‐B  .....................................................................................................................................................  9  2-­‐A  ...................................................................................................................................................  10  

Conclusions  .............................................................................................................................  11  1-­‐B  ........................................................................................................................................................  11  2-­‐A  ........................................................................................................................................................  11  

Comparison  .............................................................................................................................  12  1B  .........................................................................................................................................................  12  2A  .........................................................................................................................................................  13  

Discussion  ...............................................................................................................................  13    

                                           

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Abstract  A  “black  box,”  namely  a  4U  rack-­‐mountable  gear  hard  case,  was  said  to  

contain  audio  effects  gear  of  any  sort.  The  purpose  of  this  study  was  investigate  and  

determine  signal  flow  as  well  as  the  components  contained  therein  without  any  sort  

of  visual  confirmation  prior  to  completion.  It  was  determined  the  box  contained  an  

Apex  Aural  Exciter,  parametric  equalizer,  graphic  equalizer,  and  a  combination  

dynamics  modifier.    

Introduction  The  “Black  Box,”  a  regularly  used  tool  to  help  teach  students  about  signal  

flow  and  investigative  techniques  was  the  subject  of  this  study.  The  goal  was  to  

determine,  without  any  visual  confirmation,  what  the  Black  Box  contained.  A  group  

of  four  students  in  Acoustical  Testing  I  (Andrew  Hulva,  Chris  Kezon,  Cody  Elston,  

and  Luc  Schutz),  were  those  that  conducted  this  study.    

It  was  given  that  each  of  the  two  inputs  had  a  unique  output  in  order  to  

ensure  signal  would  always  flow  from  input  to  output,  preventing  damage  to  the  

internal  components.  There  were  two  inputs,  labeled  “1”  and  “2”,  and  two  outputs,  

labeled  “A”  and  “B.”  This  can  be  seen  in  Figure  1.  There  were  no  other  instructions  

or  limitations  placed  on  the  group,  as  any  equipment  and  software,  within  reason,  

would  be  provided  by  the  engineering  staff  at  Columbia  College  Chicago.  

 

 

Figure  1:  Black  Box  Faceplate  

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Methods     Due  to  the  then  current  lack  of  efficient  data  acquisition  software,  the  test  

environment  was  initially  configured  as  such:  analog  equipment  at  each  end,  using  a  

tone  generator  for  stimuli  and  both  an  oscilloscope  and  powered  speaker  (QSC  K8)  

as  receivers  of  Black  Box  output.  Qualitative  observations  were  made  but  not  

recorded,  as  the  usefulness  and  reproducibility  was  limited  due  to  the  uncertainty  of  

operation  efficacy.    

Introducing  digital  audio  analysis  software  at  each  end  allowed  for  the  entire  

spectrum  to  be  analyzed  and  presented  on  screen,  increasing  initial  understanding  

by  providing  a  holistic  view  of  the  Black  Box’s  effect.  A  desktop  workstation  running  

the  Windows  XP  operating  system  utilizing  a  Sound  Devices  USB  Pre2  as  its  sound  

device  for  both  input  and  output  replaced  the  tone  generator,  oscilloscope,  and  

powered  speaker.  To  confirm  the  unique  input/output  signal  paths,  output  L  

(default  for  most  software  suites)  of  the  Pre2  was  sequentially  connected  to  the  

assumed  inputs  (female  connecters)  of  the  black  box,  and  their  alphabetically  

equivalent  assumed  outputs  (male  connectors)  were  connected  to  the  line-­‐level  

input  of  the  Pre2.  Attempting  to  pass  a  1  kHz  sine  tone  through  each  configuration  

yielded  the  presumed  composite  noise  floor  of  the  devices.  Switching  to  the  other  

possible  combinations  yielded  spectrums  dominated  by  the  input  signal,  resulting  in  

the  input/output  combinations,  “1  –  B,”  and  “2  –  A,”  as  seen  in  Figure  2.    

     

Insert  Signal  Flow  diagrams  here  Figure  2:  Generic  Signal  Flow    

 

Initial  Observations  

1-­‐B     Following  no  specific  protocol,  initial  observations  were  recorded  for  the  

input/output  combination  “1  –  B”  (1B)  using  SpectraPlus  for  signal  generation  and  

analysis.  With  pink  noise  as  stimulus,  the  initial  observations  were  as  such  (taken  

from  laboratory  notebook):  equalization  at  8  kHz.  With  no  stimulus  (digital  zero),  

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the  response  was  indicative  of  60  Hz  analog  hum.  These  initial  observations  can  be  

independently  confirmed  in  Figure  3.    

 

 Figure  3:  Pink  Noise  Response  and  Noise  Floor  of  1B  

    In  order  to  add  to  the  initial  observations,  a  microphone  was  used  as  

stimulus  for  aural  testing.  The  signal  chain  was:  microphone,  microphone  

preamplifier,  Black  Box,  and  a  powered  speaker.  The  resultant  output  had  

noticeable  hiss  and  distortion  from  varying  levels  of  input  (speaking  voice)  and  had  

no  audible  output  when  not  being  spoken  into,  signifying  a  noise  gate.  No  other  

concrete  observations  were  made  due  to  the  human  ears  resolution  and  no  

unaffected  signal  to  compare  to.    

2-­‐A  Using  the  human  voice  as  stimulus  again,  no  obvious  defects  in  the  signal  

could  be  determined,  so  initial  observations  were  recorded  using  pink  noise  and  

digital  zero  as  stimuli  for  the  input/output  combination,  “2  –  A”  (2A).  With  pink  

noise  as  stimulus,  there  are  no  obvious  variations  in  response,  resulting  in  the  

assumption  that  the  subtle  variations  are  due  to  equalization.  With  no  stimulus,  the  

same  60  Hz  hum  can  be  seen.  These  observations  can  be  independently  confirmed  

in  Figure  4.  The  signal  flow  for  this  input/output  combination  seemed  to  be  less  

involved  than  the  1B  signal  path.  

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Figure  4:  Pink  Noise  Response  and  Noise  Floor  of  2A  

Data  Analysis    

Following  the  creation  of  an  Excel  macro,  by  Andrew  Hulva,  that  automated  

the  data  acquisition  process  utilizing  the  analyzer  and  automation  library  included  

with  SpectraPlus-­‐SC,  multiple  measurements  could  be  recorded  in  a  short  time  span.  

Averaging  was  set  to  infinite  within  SpectraPlus,  to  overcome  any  variations  in  the  

time  constant  signals,  and  thirty-­‐two  (32)  sub-­‐tests  were  ran  during  each  test  that  

gathered  1/3rd  octave  data.  Each  sub-­‐test  would  run  the  analyzer  and  generator,  

wait  five  (5)  seconds,  place  the  data  in  Excel,  and  reduce  the  level  of  the  generator  

three  (3)  dB  for  the  next  sub-­‐test.  Each  test  began  with  the  generator  at  zero  (0)  

dBFS  and  end  with  it  at  -­‐96  dBFS,  excluding  the  tests  designed  to  assess  the  

threshold  of  the  compressor/expander,  which  began  near  the  assumed  threshold  

and  had  one  (1)  dB  increments.  Stimuli  for  tests:  pink  noise,  sine  sweep  (0  Hz  –  24  

kHz),  16  kHz  tone,  8  kHz  tone,  4  kHz  tone,  2  kHz  tone,  1  kHz  tone,  500  Hz  tone,  250  

tone,  125  Hz  tone,  and  an  80  Hz  tone.  All  testing  moving  forward  will  be  assumed  to  

have  followed  this  procedure.    

 

 

 

 

 

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1-­‐B  

The  stepped  noise  test  showed  a  type  of  compression  that  dynamically  

affected  frequencies  below  500  Hz  and  an  equalization  point  at  8  kHz.  This  can  been  

seen  in  Figure  5.    

 

 Figure  5:  Stepped  Noise  Response  of  1B  

 

Tone  tests  show  compression  above  a  threshold  for  all  frequencies  and  

downward  expansion  for  all  frequencies  below  a  threshold.  This  can  been  seen  in  

Figure  6,  and  note  that  the  frequency  is  irrelevant  as  all  frequencies  showed  same  

compression  ratio.  The  compression  above  a  threshold  seen  in  tone  tests  is  not  

observed  with  pink  noise  as  stimulus,  most  likely  due  to  pink  noise  having  a  lower  

overall  level  in  SpectraPlus  to  account  for  natural  variance  that  would  have  clipped  

the  input  of  the  Pre2  had  it  not  been  lowered.  Tone  tests  also  showed,  where  noise  

tests  did  not,  additional  tonal  components  higher  in  frequency  than  the  input  tone.  

This  will  be  analyzed  separately.    

 Figure  6:  Compression,  Expansion,  and  Extra  Tonal  Components  of  1B  

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2-­‐A       Both  noise  and  tone  tests  showed  compression  above  a  certain  threshold,  

and  since  the  two  stimuli  have  different  overall  levels,  this  is  indicative  of  a  higher  

threshold  than  that  of  1B.  There  also  was  no  evidence  of  the  expander  seen  in  1B,  as  

some  unit  decrease  in  output  produces  the  same  change  in  input.    

 

 Figure  7:  Stepped  Noise  Response  of  2A  

 Noise  tests  also  show,  as  seen  in  Figure  8,  multiple  equalization  points,  as  at  

these  points  the  variance  is  bell  shaped.  The  shape,  or  Q,  of  the  equalization  is  not  

constant.  Tone  tests  do  not  show  additional  generated  tones,  confirmed  at  all  

frequencies  tested.  However,  the  noise  floor  of  2A  does  show  the  60  Hz  hum.  

 

 Figure  8:  2A  Noise  Response  vs.  Unity  Gain  

 

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 Tones  Generated  by  the  Black  Box  1B     Close  inspection  of  the  tones  generated  in  1B  revealed  an  interesting  

mathematical  relationship.  The  equal-­‐temperament  tuning  system,  used  in  Western  

music,  follows  the  following  formula:    

𝑓! = 𝑓 ∗ 2!/!"  where,  𝑓 = 𝑓𝑢𝑛𝑑𝑎𝑚𝑒𝑛𝑡𝑎𝑙  𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦  (𝐻𝑧)  

𝑓! = 𝑟𝑒𝑠𝑢𝑙𝑡𝑎𝑛𝑡  𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦  (𝐻𝑧)    𝑛 = 𝑛𝑢𝑚𝑏𝑒𝑟  𝑜𝑓  𝑠𝑒𝑚𝑖𝑡𝑜𝑛𝑒𝑠  

 For  any  given  tone  f,  the  resultant  tones  satisfied  this  relationship  when  n  =  

1,  12  and  21.  These  musical  intervals,  relative  to  the  fundamental,  are  the  first  

octave  and  a  major  6th  above  that  octave.    

Comparison  of  the  response  of  1B  to  an  Apex  Aural  Exciter  solidifies  the  

hypothesis  that  it  was  causing  the  generated  tones.  As  seen  in  Figure  9,  the  response  

of  both  the  Black  Box  and  the  Aphex  have  similar  contours  and  feature  both  the  60  

Hz  hum  and  the  generated  tones.  Shown  is  a  1  kHz  tone,  but  these  conclusions  are  

supported  for  all  tones  tested.    

 

 Figure  9:  Black  Box  vs.  Aphex  Aural  Exciter  

Dynamics    

1-­‐B     Graphing  the  input  versus  out  gives  a  standard  compression  graph  that  many  

will  be  familiar  will.  This  is  telling  in  that  it  will  visually  show  the  types  of  dynamics  

modification  equipment.    

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 Figure  10:  Input  vs.  Output  for  1B  

      One  can  see  expansion  affecting  the  lower  levels  and  compression  affecting  

the  higher  levels.  The  expansion  ratio,  derived  by  a  linear  regression  of  the  two  

points  denoted  by  the  first  two  arrows  on  Figure  10,  was  1:2.5.  The  compression  

ratio,  derived  by  a  linear  regression  of  the  points  denoted  by  the  second  two  arrows,  

was  2:1.  Thresholds  are  not  provided  due  to  data  being  recorded  relative  to  the  0  

dBFS  of  the  Pre2.  

2-­‐A    

 Figure  11:  Input  vs.  Output  for  2A  

 

                         Following  the  same  procedure  as  1B,  Figure  11  was  generated.  Again,  

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performing  a  linear  regression  on  the  two  points  denoted  by  the  arrows,  a  

compression  ratio  of  3.5:1  was  found.  It  is  important  to  note  the  absence  of  

expansion  on  this  input/output  combination.    

Conclusions     Taking  all  of  the  data  into  consideration,  conclusions  were  made  regarding  

both  the  type  of  equipment  inside  the  Black  Box  and  the  order  in  which  signal  

passed  through  these  devices.    

1-­‐B     It  was  known  that  a  compressor,  expander,  and  an  Aural  Exciter  were  

affecting  the  signal.    Order  was  proposed  as  such:  Compressor/Expander  to  an  

Aphex  Aural  Exciter.  This  order  was  based  upon  the  lack  of  compression  on  the  

generated  frequencies  shown  in  Figure  6,  and  this  order  complies  with  traditional  

signal  flow  for  musical  applications.  The  compressor  was  to  have  a  compression  

ratio  of  2:1,  and  the  expander  a  ratio  of  1:2.5.  This  accounts  for  two  (2)  units  of  the  

four  (4)  -­‐rack  spaces  available  in  the  Black  Box.    

2-­‐A     It  was  known  that  a  compressor  and  an  equalizer  were  affecting  this  signal  

path.  These  two  devices,  however,  did  not  explain  the  60  Hz  hum  seen  in  the  noise  

floor.  This  was  attributed  to  the  Aphex  Aural  Exciter  in  1B,  so  it  was  hypothesized  

that  the  Aural  Exciter  was  part  of  the  signal  chain.  Passing  the  signal  through  the  

device  with  “bypass”  engaged,  the  self-­‐noise  would  be  added  to  the  signal  without  

the  additional  harmonic  content.  Taking  all  of  this  into  consideration,  the  order  was  

proposed  to  be:  Compressor,  to  a  15-­‐band  equalizer,  and  to  an  Aphex  Aural  Exciter  

in  “bypass  mode".  The  compressor  was  to  have  a  ratio  of  3.5:1,  and  the  15-­‐band  

equalizer  was  to  have  the  parameters  shown  in  figure  12,  derived  from  the  

difference  from  unity  in  Figure  8.    

 Frequency  (Hz)   63   100   160   250   400   500   630   800   1K   2K   4K   8K   16K  

Gain  (dB)   -­‐4   0   +4   +1.5   -­‐1   +1.5   +4   +1.5   -­‐3   -­‐3   +3   -­‐8   -­‐8  

Figure  12:  EQ  parameters  

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  Since  the  compression  ratio  is  different  from  that  of  1B,  it  meant  either  the  

compressor  housed  two  channels,  or  there  were  two  different  units  in  the  Black  Box.    

The  equalizer  adds  another  rack-­‐space,  totaling  three  (3),  within  the  four  (4)-­‐rack  

spaces  allotted.    

Comparison     The  Black  box  contained  a  5-­‐band  parametric  equalizer,  31-­‐band  graphic  

equalizer,  Aphex  Aural  Exciter,  and  a  Compressor/Expander.  For  input/output  1B,  

signal  flowed  as  such:  5-­‐band  parametric  equalizer,  to  an  Aphex  Aural  Exciter,  and  

to  a  Compressor/Expander.  For  2A,  signal  flowed:  Compressor/Expander  to  the  31-­‐

band  graphic  equalizer.  The  actual  devices  can  be  seen  in  Figure  13.  

 

       Figure  13:  Black  Box  with  Faceplates  removed  

1B     The  group  had  not  recognized  the  5-­‐band  parametric  equalizer  in  the  signal  

chain,  and  this  could  have  been  due  to  its  masking  by  the  Aphex  Aural  Exciter.  The  

Exciter  dynamically  adapts  to  different  signals,  making  the  recognition  of  a  device  

such  as  an  equalizer  exceptionally  difficult.  Also,  the  predicted  order  was  wrong,  

with  the  Exciter  coming  before  the  compressor.  The  logic  behind  the  prediction  

could  have  benefited  from  additional  testing  to  confirm,  but,  again,  the  adaptability  

of  the  Exciter  made  the  prediction  of  order  challenging.    

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2A     The  Aphex  Aural  Exciter  was  proposed  to  be  in  the  signal  chain,  and  it  was  

not.  This  conclusion  was  supported  by  the  similarity  of  noise  floors  of  2A.  The  

possibility  that  the  signal  path  was  affected  from  Exciter  exists,  as  crosstalk  is  

something  not  understood  by  the  group,  and  there  was  no  other  explanation  for  the  

similarity.  The  type  of  equalizer  said  to  be  in  the  Black  Box  was  also  incorrect,  but  

the  recognition  of  an  equalizer  is  what  was  most  important  to  the  group.  

Discussion     The  scope  of  this  project  was  to  identify  the  components  contained  inside  the  

Black  Box,  and  that  was  most  certainly  accomplished.  With  the  creation  of  the  Excel  

macro,  large  amounts  of  data  was  able  to  be  collected  that  not  only  expedited  the  

entire  process,  but  enabled  the  group  to  confirm  all  predictions  from  multiple  

angles.  This  confidence  allowed  the  group  to  take  risks  such  as  naming  the  Aphex  

Aural  Exciter.  This  project  taught  the  group  how  best  to  acquire,  analyze,  and  

synthesize  conclusions  from  data,  and  for  that  they  are  most  appreciative.