How to Measure Frequency and Sampling Rates in Adobe Audition

Complete all of the steps below using Adobe Audition and complete your answers on the answer sheet provided and the stepped tone_silence.

When you have finished, submit your answer sheet and file on Canvas.

1. Getting started

Effect of sampling rate on file size and frequency ‘response’

The frequency of a sound is the number of times per second that the air pressure goes through maximum (or minimum, or zero) in one second. For a pure tone, frequency can easily be determined using software packages such as Audition, but the fundamental or dominant frequency of more complex sounds can sometimes be more difficult to measure in this way.

As a reminder of what is meant by ‘frequency, measure the frequency of a pure tone.

Open file ‘stepped tone.mp3’ again (‘File’>’open’, then browse for file). Use the horizontal zoom button to zoom in until each cycle of the wave is about 1 - 2 cm on screen. Now drag the cursor to highlight exactly between two successive peaks. Using the indicators at the bottom right of the screen, read off the length of time highlighted in the ‘Selection/View’ window. This represents the time taken for the air pressure to go from maximum positive, to maximum negative and back to maximum positive again.

Use this figure to work out the frequency of the wave in the ‘stepped tone’ file.

3. Sampling rates, frequency response and dynamic range of a compressed file (mp3)

First, some facts:

A stereo file sampled at 16 bit resolution and 16000 sampling rate generates 16000 x 16 x 2 bits per second – ie a bit rate of 512Kb/s.

The commonly used MP3 bit rate is 128 Kb/s – about a quarter of that of the 16000Hz sampled file and about 1/11th of a ‘CD’ quality file. If we assume that the file retains 16 bit sampling resolution, since the bit rate is ¼ of that of the 16000Hz sampled file, this would suggest that the sampling rate has been reduced to about 4000 samples per second.

Apparent sampling rate and dynamic range of mp3 file.

Open file ‘stepped tone.mp3’ again

Insert a silence of 1 second at the point where the amplitude changes. Go to ‘Edit’>>’Insert’>’silence’ and set to 1 second. This establishes a ‘minimum’ signal amplitude.

Save, then close the file, name it ‘stepped tone_silence.mp3’. This is necessary to ensure that the added part has been compressed.

Re-open the file and zoom in horizontally until you see ‘dots’ appear. These indicate actual sample points. Expand until the dots are about 0.5 cm apart and put the cursor at the first section of the file. Now drag the cursor to select exactly 1ms of the file. You can see this by watching the ‘Selection/View’ window at the bottom right of the screen.

Go to ‘View’> ‘Time Display’ and click on ‘samples’. The ‘Selection/View’ window will now indicate how many samples you have selected.

Calculate the effective sampling rate of the file and note it on your results sheet.

Go to ‘View’> ‘Time Display’ and select ‘Decimal’.  Zoom out horizontally until the whole wave form of this file can be seen in the edit window.

Determine the dynamic range of the2 second portion which encompasses the silence. Select the part of the file from 2 seconds to 4 seconds. Use ‘Window’ Amplitude Statistics’>’General’ to find max/min RMS amplitudes.  And then calculate the Dynamic range.  Write the results in your result sheet.