STG FOR PC™ SOUND ANALYSIS TUTORIAL


I. What is sound spectrum?

Let us start with a single tone sound:

1. Open the file named tone800Hz.wav from C:\Program Files\Stethographics\STG1\STG Examples directory.

2. Listen to the sound.

3. Switch to Spectrogram Display. The spectrogram shows a single line at 800Hz. This tells us that all the energy of the signal is concentrated at 800Hz.

4. Sounds you normally hear are composed of a number of tones played simultaneously. Record the sound of you, speaking into the electronic stethoscope or a microphone. Switch to the Spectrogram Display. The spectrogram shows a number of high energy bands. These bands correspond to a number of tones of different pitch and amplitude that combined together make the sound of your voice. Sound spectrum is just a decomposition of the sound into a number of tones, the process that allows us to find out the pitch and the amplitude of these tones.

II. Sound pitch

Pitch or frequency is expressed in Hertz (Hz), named after German physicist Heinrich Rudolf Hertz (1857 to 1894), who demonstrated the nature of electromagnetic waves propagation. Our ears can hear tones from 15Hz to 20,000Hz. What we usually call ‘low pitch tone’ corresponds to approximately 15Hz to 300Hz. To play a low frequency tone:

1. Open the file named tone150Hz.wav from C:\Program Files\Stethographics\STG1\STG Examples directory

2. Listen to the sound

Tones higher than 300Hz are usually referred to as high pitch tones. To play a high frequency tone:

1. Open a file named tone800Hz.wav from C:\Program Files\Stethographics\STG1\STG Examples directory

2. Listen to the sound

Which sound ‘sounds higher’?

III. Crackles and Wheezes visualization

STG can visualize sounds in time and frequency domains.

Crackles are intermittent ‘explosive’ sounds that have been described as being similar to the crackling sound heard as wood burns. Crackles appear in the time domain as intermittent spike-like deflections. They can be seen in the unexpanded display but are usually seen more readily in the expanded mode. Crackles are divided into two types, ‘fine’ and ‘coarse’ by their acoustic properties. On auscultation fine crackles are in general higher pitched, less intense and of shorter duration than coarse crackles.

Wheezes are described as relatively ‘continuous’ sounds as compared to crackles. They usually last for more than 200 milliseconds and have a musical quality. On time expanded waveform analysis they can be seen to have a sinusoidal pattern.

It is generally accepted that time domain is convenient for visualization of the short-lived sounds like crackles and short wheezes, while the frequency domain is is more useful for observing sounds of long duration, like wheezes.

To visualize crackles:
1. Open the file named CHF_Crackles.wav from Examples directory
2. Listen to the sound
3. Click on any of the crackles to position cursor.
4. Click the Zoom In button to expand the sound horizontally. Observe the waveform of individual crackle.
Click to see full view

To visualize wheezes and rhonchi:
1. Open the file named CHF_Rhonchi.wav from C:\Program Files\Stethographics\STG1\STG Examples Examples directory.
2. Listen to the sound.
3. Expand the sound horizontally. Observe sinusoidal waveform of the wheeze.
4. Switch to theSpectrogram Display.
5. Observe dark bands corresponding to the high energy of sound at that particular time (horizontal axis and frequency (vertical axis).
6. Play back the sound again, while relating the sound to the dark bands on the spectrogram.

IV. Summary of Crackles and Wheezes Recorded from Multiple Sites

The software contains the algorithm that identifies and counts crackles and wheezes. The count is indicated at the bottom of the STG screen. Fine and Coarse counts are presented as crackles per second; the wheeze rate is the percentage of the time in which wheezes were detected. In default crackles and wheezes are counted over 10 seconds and the average count is indicated. When any region of waveform display is highlighted, the count is recalculated to represent the average crackle and wheeze count over the highlighted region.

The Summary display assists in visualizing crackle and wheeze counts from multiple sites. Since you are using 1 channel STG, the data collection process is sequential, from one site at a time. We, at Stethographics, have developed a 16 channel STG system that collects data from 16 chest sites simultaneously. Lung sounds recording from multiple sites not only streamlines the auscultation but also greatly improves the diagnostic process. For example 16 channel STG automatically calculates the location of the origin of crackles and wheezes inside the chest. You can see examples of crackles and wheezes localization at Physiology section of this site.

Click to see full view Summary display indicates crackle and wheeze rate with circles. Green circle diameter is proportional to the fine crackle rate, blue circle diameter is proportional to the coarse crackle rate, red circle diameter is proportional to the wheeze and rhonchi rate, and black circle diameter is proportional to sound loudness or RMS. Append crackle and wheeze count to the summary display with Append Crackles and Wheezes to the Summary button.

To indicate crackle and wheeze count from a new location first, move stethoscope to a new location by clicking on the summary display, then click Append Crackles and Wheezes Count to the Summary button.