Frequency Response of Your Audio System

• Things you need:
  • SPL meter (preferably digital)
  • 1997 IASCA's Setup and Test CD: track 96 or equivalent recording of 1/3 octave bands of pink noise starting at 25Hz and stepping up to 20kHz in 1/3-octave steps.
  • Graph paper
  • Ear plugs
  • SPL meter stand (if you plan to use your car for RTA competition, but if you are only trying to make the car sound better, you and your girlfriend can stay in the car during measurements providing you spend a lot of time with her in the car.
  
  First, set the SPL meter to the ANSI-C weighted scale mode and a slow response. The SPL meter should not be moved during your measurements and should be placed in the driver's seat at your ear's level. All measurements should be taken at 10dB more than the ambient noise. To do that, play the 25Hz band of the track 96 and raise the volume of your deck to at least 10dB higher than the ambient noise. I like to take all the measurements at 75dB (ambient dB level is usually 50-60dB). From this point on, never adjust the volume control of your deck.
  Having adjusted your signal level, play all of the 30 tones off the track 96 recording SPLs in the graph provided below the corresponding frequency of the measurement taken.
  These 30 measurements you've just taken do not compensate for the C-weighted curve imprint. To compensate, you need to add the inverse of the C-weighted scale to the affected frequency bands. For the vertical portion (y-axis) of the graph, change the number scale of the highest measurement to be 3dB higher than your highest SPL measurement and label down in 3dB increments. So, if your highest measurement was 98dB, put that value instead of the 15dB value on the graph.
  To create your car's frequency response curve, place a dash on the grid corresponding to the numerical value of the SPL measurement (which has already been compensated for the C-weighted curve imprint) taken at each corresponding frequency. Thus, if you measured 88 dB at 25Hz, you first add 4dB making it 92dB and place a dash at the location representing 92dB at 25Hz. Connect all the dashes to form a RTA curve. The response curve should have smooth transmissions. If that is not the case, you have system or equipment problems. More on how to fix that later. Good luck.
  
  
  

  May/1998

  
  

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  Email: czarekj@hotmail.com