Anechoic chambers

Anechoic Chambers are a really weird thing. They're basically a place where as a person you can't hear a thing. They have noise values of -dB (which took me a bit to get my head round but it's like -centigrade it is actually a "positive" number it's just below our scale) so are totally silent to humans, which can apparently drive people completely insane and they start hallucinating. They're also fairly radioactive, basically you don't wanna be in one.

In terms of scientific use they're mostly used for measurements of waves. They don't let any waves out, not just sound waves. It's due to the lack of interference which I can imagine is useful for scientific accuracy. Apparently also useful for testing stuff like antennae or radar, which makes sense too. They come in many shapes and sizes (tiny box to aircraft hanger) and are also meant to be a fire hazard. When it comes to using an anechoic chamber equipment is usually made of wood or plastic and not metal so as to reduce reflection.

In summary an anechoic chamber is a room in which sound is completely absorbed instead of reflected, they come in many sizes and are a huge fire hazard. It's not a place you want to put people but they're great for scientific experiments regarding waves due to the lack of interference.

Image Lab!

Back again, same format. This Lab is about images though which made a pretty cool change. It was quite straight forward though. 

Manipulating Images Using Arithmetic

Image filtering, enhancement and general manipulation using GIMP

1) Download the image file CH_Tor.jpg from Moodle.

Done.

2) Find and run GIMP.

Yay, freeware! 

3) Use File > Open... to load and display CH_Tor.jpg.

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4) Explore some of the filters (and their options) in the Filters menu (particularly the range of Blur and Enhance filters).

This is gaussian blur. It looks like a kinda mild blur I guess? I know Gauss did a lot of work with magnets and such, possibly related? 

Censorship and PC gone mad! 

5) Suggest possible uses for some of these filters for processing digital photographs.

No idea what you could use the former for? Maybe if you wanted to take something out of focus. The latter is used for censorship though, obviously. 

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6) Select Filters > Generic > Convolution Matrix...

7) Click the Reset button to reset the matrix to its default (with only a 1 in the centre cell of the matrix) then click OK. What does this filter do and why?

No idea what it did. Doesn't look like anything noticeable? 

8) Create a new filter with a three by three matrix of 1’s at its centre. Also tick the Normalise box. What does this filter do and why?

It looks kind of.... stretched? Disproportionate certainly. Not sure what's going on honestly. 

9) Create a new filter with a three by three matrix with +1’s on one diagonal and –1’s on the other. Like this:

1 0 -1

0 1 0

-1 0 1

What does this filter do and why?

Now it looks a little bit blurred. I still have no idea what this filter actually does though! It remains a mystery to me. :) 

Fairly straightforward lab though. 

Same format as last time, I'm going to be talking through the lab as I do it. This lab wasn't overly difficult but it did teach me you can do some interesting things with functions and waves regarding sound. It kind of makes me wonder if you could write an algorithm for making music. 

Lab 3 Audio Signal Processing – Generating Signals with GoldWave.

Use your blog to keep notes, sketches, interesting Q and A etc.

Tones, Harmonics, Spectra and Spectrograms

1. Run GoldWave. Ensure the system sound volume is low before you put your headphones on.

Lessons were learned from last time! Duly noted! 

2. Use GoldWave Help at any time.

3. Set the vertical scale (Y-axis) format to Normalized values.

I totally didn't have to check the last lab to do this. That's progress! :) 

4. Work out the duration (time) you will need in order to display 8 cycles of a 400Hz sine wave. Note down this duration. Now use File > New... to create a new mono sound window of this duration with a sampling rate of 44100Hz.

Tada! The duration I used was 0.2s. 

4. Use Tool > Expression Evaluator to generate a sinusoid (single pure tone) wave as follows. Select Waves from the Presets then Sine, f = Hz from the Waves submenu. The expression sin(2*pi*f*t) should appear in the Expression edit box. At the bottom of the dialogue box enter 400 for the frequency in the f = edit box then click OK. Check that you now have eight full cycles of a sine wave displayed. Sketch this waveform. Save the pure tone as an *.wav file. Play and listen to the tone (play it in a loop so that you have time to hear it).

Here we are. Sounds pretty flat at the moment. Definitely a sine wave though, if maths has taught me anything. 

5. Now add to this Fundamental tone the harmonics up to and including the 9^th in the proportions shown below. You can add a new harmonic by adding an expression to the existing expression. For example, to generate the fundamental tone plus the third harmonic at one third intensity you would use the expression: sin(2*pi*f*t)+sin(2*pi*3*f*t)/3. Display and sketch the waveform, together with the expression that you used to generate it, each time you add another harmonic.

Harmonic Number	1	2	3	4	5	6	7	8	9
Amplitude relative to Fundamental	1	0	1/3	0	1/5	0	1/7	0	1/9

6. What shape is the waveform gradually approaching?

Looks like a sine wave or some kind of parabola thing to me still. 

7. Use Effect > Filter > Spectrum Filter to view the frequency content (Magnitude Spectrum) of the waveform. Compare the peaks in this display with the fundamental and the harmonics you have added to it.
   
   
   
8. Save the final waveform as a *.wav file. Listen to the waveform and compare it with the sound of the original pure tone sinusoid.
9. Now start afresh and add to the Fundamental pure tone the harmonics up to and including the 5^th in the proportions shown below. Display and sketch the waveform each time you add another harmonic.

Harmonic Number	1	2	3	4	5
Amplitude relative to Fundamental	1	1/2	1/3	1/4	1/5

#1

#2

#3

#4

#5

#6

10. What shape is the waveform gradually approaching?

A tan wave. 

10. View the frequency content (Magnitude Spectrum) of the waveform. Identify the peaks in this display with the fundamental and the harmonics you have added to it.

10. Save the final waveform as a *.wav file. Listen to the waveform and compare it with the sound of the pure tone, and the previous waveform.

It sounds a lot more altered than the other clips. It's really quite noticeably different. 

Audio Lab 2

1. Download the “sopran ascenddescend.wav” file from http://aivpsm.blogspot.co.uk and save it to your desktop. Use Windows Sound Recorder (Start Menu / All Programs / Accessories) to play it, note its duration in seconds, and use File\Properties to classify it as Mono or Stereo.

First comment is that windows 8 wont open the file in sound recorded. Off to a good start. No idea why. 

2. Close down Sound Recorder.z

Gladly.

3. Run GoldWave.

Ok.

4. Use GoldWave Help at any time. Make notes in your lab book from the help for next time.

Will bear this in mind.

5. Download to a local drive the "english words2.wav" file from Blackboard. Use File > Open... to read it into GoldWave. Ensure the system sound volume is low before you put your headphones on. Put on your headphones then play the file using the green Play button in the Control window. (If the Control window isn't displayed, select Window > Vertical Control.

Done. Those are a kind of grating selection of words.

6. Put the mouse cursor inside the waveform window and left-click. The section to the right of where you clicked should now be highlighted. Now press the yellow Play button and note that play starts from the new cursor position.

Indeed it does.

7. You can play part of the waveform by highlighting a section using the mouse and buttons just like you do selecting text in a word processor. Left Button Click, hold, drag and release.

It's quite imprecise, but I guess it's not necessarily a bad feature for ballparking things. Misclicked the other play button by accident. Worked the second time. So far so good.

8. You can use File > Save Selection As... to save the highlighted part (Don’t overwrite your original files)

Yep, that's something that will certainly be useful for making specific changes. 

9. Now try out the standard Edit commands: Copy, Cut, Paste, Delete, Trim. Note the Undo command.

Delete was my favourite. Those motivational words are getting under my skin by now. 

10. Use Options > Window... to change the Y amplitude axis numbering between Normalized and dB. Note that this does not change the waveform, just the units you measure it with.

Alright, useful. I prefer dB I think as I understand it better! 

11. Use the View All toolbar button to show all of the waveform. Then use your mouse to select the part of the waveform from 10 seconds to 11 seconds (approximately). Now click the View Selection toolbar button. Try also the other Zoom buttons on the toolbar.

So this just gets rid of all the chaff and "blows up" the selection for more precise work. It's pretty cool to look at in the control panel too, where you can see flares of activity pretty accurately. 

12. Select the part of the waveform containing one of the words then try out the Effect > Invert, Effect > Reverse and Edit > Mute commands. Play and listen to each result.

I wasn't honestly sure what Invert did. I took some screenshots though, it inverts the wave. Reverse reversed it, which sounded a bit creepy. 

13. Note the amplitude (height on the vertical scale) of an easily identifiable point on the wave (e.g. a maximum). Now use Effect > Volume > Change Volume to attenuate (reduce the amplitude) of the waveform. Do this by typing -6 (NB minus six ) into the edit box at the top right of the Change Volume dialogue box window when it pops up then click OK. Use Undo and Redo to toggle between the original and attenuated waveform. After applying the -6 dB amplification, by what percentage has the waveform been attenuated?

Looks like roughly 50% decrease. 

14. With the waveform at its attenuated value use Effect > Volume > Change Volume to amplify (increase the amplitude) of the waveform. Do this by typing 3 into the Change Volume dialogue box and then click OK. Using Undo and Redo observe the change in the maximum amplitude. After applying the +3dB amplification what is the amplitude (percentage) now?

Increased by 25% of the deficiency. 

15. Amplifying the waveform again by +3dB should bring it back to its original value.

This was about three quarters of the deficiency. I.e. the remains of what was missing. 

16. Attenuate the waveform then try Effect > Volume > Maximize Volume, which should make the largest amplitude 100% (or whatever value you put in the Maximum (dB) text box).

Ok. 

17. Now investigate Effect > Volume > Fade In and Effect > Volume > Fade Out.

I tried fade in on the whole thing and fade out on the last part of the file. 

Closing Thoughts: So I guess this lab is really just an intro to working with Goldwave. For that function it's fine and I can totally understand why we'd be asked to do something like this. 

First Impressions

Sound

A fancy looking Time History.

Sound is a physical phenomenon that stimulates the sense of hearing. It's formed of waves, of which there are two types; Transverse and Longitudinal. An example of Transverse waves would be a ripple in water, where the vibrations in the water are at a right angle to the disturbance. Longitudinal waves  work much like Newton's Cradle, where the wave moves along before bouncing back in the other direction in a series of compression and rarefaction.

Some Facts

1. For a transverse wave the Wavelength is the distance between two successive crests or troughs, this defines frequency. 
2. Amplitude is the height of the crests / troughs. This defines how much of the sound there is.
3. The Velocity (v) of sound is 333 meters a second.
4. The frequency (f) of sound is measured in Hertz.
5. The Wavelength of sound is known as the lambda (λ). 
6. The formula for working out any missing variable in terms of sound is v=fλ. 

Harmonics

A Harmonic is an integer multiple of the fundamental frequency. In English (or numeric form I guess) this means that if you're using components of 1000Hz (1kHz from here on out) that 3kHz would be the 3rd Harmonic of 1kHz. In much the same vein 3kHz would be the 6th Harmonic of 500Hz. 

Sound Intensity, Level and Decibels

By far my favourite part of today's lecture this covers Decibels (dB) and how sound intensity is measured. It's measured in decibels which are a non linear logarithmic scale. 

• Sound Intensity Level = 10log10(Isound/Isound)dB

• log10(10)=1
• log10(2)=.3
• 10*0.3=3dB
• 40dB=.666 of 41dB
• 43dB=2*40dB
• This scales linearly.
• 3dB increase = 2*
• 6dB increase = 4*
• 9dB increase = 8*
• 12db increase = 16*

Logarithmic Proof: 

• log1010=1
• log102=0.3
• log104=0.6
• log108=0.9
• 0.9 + 0.1 = 10 (if I cheat and work backwards because I know log1010=1) :D 

Inverse Square Law

• I/R^2
• R = Distance. So the volume of a sound quarters as distance doubles, for example;
• 1m = 1R
• 2m = 0.25R etc