Audio Lab 2

Task 1 

Run Audacity. Select Tracks-> Add New-> Audio Track which as you can see has defaults setting of MONO, Sampling Rate 44,100 Hz, 32 bit float. Click on the inverted triangle on the audio track to change the setting to 16 bit PCM What do each of these setting mean? 

MONO means that there is only one sound output. So in a standard pair of headphones you will have stereo because there is a right and left speaker but with mono there is only one channel instead of two. 

Sampling rate is when a sound wave is converted into a sequence of samples. A CD will have a sampling rate of 44,100 HZ and a DVD will have a sampling rate of 90,000 HZ.

PCM is when an audio track is taken and then converted into binary. This can be either 16 bit, 24 bit or 32 bit float. Once in binary the audio is represented digitally and is then converted back into the audio. 

Task 2

 Use the Generate->Tone option to generate 1 second of a sinusoid (single pure tone) of frequency 440 Hz at amplitude 1, mono, 16bit, sample rate (frequency) 44100 kHz. Save the pure tone as a *.wav file in C:\TEMP or on your pen drive if you have one. 

Task 3

By using the same option, create similar tracks for each of the harmonics up to and including the 9^th in the proportions shown below. Add the fundamental and each of the harmonics to each other by selecting all waves (CTRL A) and pressing CTRL SHIFT M. Sketch or cut and paste the result into your lab document and describe what you see.

Task 4 

What shape is the waveform gradually approaching? Listen to the wave as you add more harmonics. Does

the timbre change?

The shape of the waveform is slowly becoming squared off and less of a wave shape. The change is at first a fast change to the first harmonic but after then each sequential harmonic changes slightly. The timbre, or tonation, of each wave changes as we progress through the harmonics. The sound of each harmonic is slightly higher than the last.

Task 5

 Similarly to the previous lab view the frequency content (Magnitude Spectrum) of the waveform using the Analyse->Plot Spectrum option. Compare the peaks in this display with the fundamental and the harmonics you have added to it. Sketch or cut and paste the spectrum in your lab note and describe it.

When looking at each harmonic in comparison with the last we can see that a pattern of sorts emerges. The wave is at 0dB and has a slope that goes towards 2000 Hz. When we look at the second peak we see that is starts at a lower -10dB and slopes down to 2500 Hz. This trend of increasing Hertz and decreasing Decibels continues to the last harmonic which peaks at -18dB and slopes down towards 4200 Hz. We can also see that the curve of the first peak and then the curve of the last peak are completely different. This first peak has a much smoother curve whereas the last peak has an almost straight line.

Task 6
Now view the Spectrogram of the waveform using the audio track triangle and selecting spectrum setting. Describe and sketch this result in your note. Save the final waveform as a *.wav file in C:\TEMP. Listen to the waveform and compare it with the sound of the original pure tone sinusoid.

The spectogram shows an intense bright mix of colours. These are to show the varying degrees of harmonics and amplitude of each track. Each track changes in brightness meaning that the brighter ones have a more intense sound whereas the duller tracks.

Task 7

 Now start afresh and add to the Fundamental pure tone the harmonics up to you and including the 5th in the proportions shown below. Display and sketch the waveform each time you add another harmonic.

Task 8

What shape is the waveform gradually approaching?

The wave beings to slowly take on a sawtooth shape as each harmonic is added to the fundamental and as it approaches the fifth harmonic it becomes a full sawtooth wave.

Task 9

View the frequency content (Magnitude Spectrum) of the waveform as previously. Identify the peaks in this display with the fundamental and the harmonics you have added to it. Sketch it in your lab note.

Task 10

Save the final waveform as a *.wav file in C:\TEMP. Listen to the waveform and compare it with the sound of the pure tone, and the previous waveform.

The pure tone sound has a much lower sounding pitch to it. It doesn't sound as harsh and has a lower volume to it whereas the sawtooth tone is a much higher sounding tone. 

Noise, Mixing, Signal-to-noise ratio, and filtering

Task 1

Open the waveform noise1.wav. This is a white noise fule. Listen to this nuisance file. View and sketch this waveform in the time domain and in the frequency domain.

Task 2

Add a sinusoid of amplitude 0.02, 1kHz frequency of 1 sec duration. Does the resulting waveform look sinusoidal? How does it sound? How does it look in the frequency domain?

As the above image shows the wave and tone produced are a sine wave. However when looking at where the frequency peaks it is clear that the white noise is not sinusoidal as it peaks at -36dB whereas all the other sinusoidal waves have peaked at 0dB.

Task 3

View and sketch the spectrum, view the spectrogram, and listen to the waveform. Locate the pure tone if possible. Save the mixed waveform as an *.wav file in C:\TEMP.

Task 4

Try using the Effect Graphic Equaliser options of Audacity to select the tone and reject the noise in the waveform (we need a slider at max at 1000kHz and sliders at zero elsewhere is possible). Does the waveform look more sinusoidal than before? If so is the period of the waveform approaching that of the original pure tone? How does it sound? To what extent did this filtering work?

Looking at both waves side by side it is clear that after using the equaliser the wave is becoming more sinusoidal than before. The white noise is now less visible and prominent than before and overall when looking and listening to both waves it becomes clear than the the filtering has worked. 

Task 5

On waveforms of your choice from freesound.org explore the effect of the other filter options that are available. 

Below is a sound that I downloaded from the website and then added a 'wah' filter to. The first image is the original and the second image is the altered sound. The 'wah' effect works well if the user is listening to headphones as it resonates throughout the left and right ear.