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Sound
What we hear as sound is caused by rapid changes in air pressure!
It is thought of as a wave, but that can be confusing since we are immersed in the medium. A wave seems to be on the surface.
Sound is 3D
Sound sources is at the centre of a sphere.
Pressure changes (higher, lower, higher …) move outwards from the source.
Pressure and Amplitude
Loudness is a matter of how big the pressure change is.
If we let normal ambient air pressure be 0, then sound amplitude is +x to –x for some way to measure pressure. Air pumps at gas stations here use Pascals, but we can use whatever we like.
PHYSICS ALERT !!
1 Pascal = 1 Newton per square metreStandardized normal air pressure is:P0 = 2 X 10-5 N/m2 = 0.00002 N/m2 = 20μ Pa. Why do we care? Because a decibel, the
standard measure of sound level, is:
10 log10 where p is the rms pressure
of the sound.
Who cares?
All sound folks use decibels, or db. You get to know (feel) how loud things are after a while.
Threshold of Hearing (TOH) 0 dB
Rustling Leaves 10 dB
Busy Street Traffic 70 dB
Front Rows of Rock Concert 110 dB
Military Jet Takeoff 140 dB
Instant Perforation of Eardrum 160 dB
How loud is it?
The sound of leaves is 10 db.The sound of a pencil dropping is 20 db.
How much louder is that ?
10 times!Decibels are based on powers of 10.If one sound is 10x times more intense than
another sound, then it has a sound level which is 10*x more decibels than the less intense sound.
Frequency
If you stand in one place, the pressure waves will pass you. The number of peaks that pass per second is the frequency.
Measured in Hertz (Hz), formerly cycles per second.
60 Hz = low hum
440 Hz = ‘A’ on piano
4000 Hz = limit of telephone (voices)
Frequency
If we draw a curve that represents pressure VS distance we can see that sound does look like a wave.
Frequency
Since the speed of sound in air is a constant (pretty much) then the number of peaks that pass is a second is related to the distance between them is a fixed way.
Also, the horizontal axis could be distance just as easily as time.
Frequency
Frequency is a precise way to specify pitch.A 440E above CSummed Oboe AOboe EPhantom B 3960
Interesting sidebar
The speed of sound is different in different media (air and water). When a sound moves from one medium to another, part of the sound is transmitted, part is reflected back!
Sound Systems/Tech
A sound system is a collection of electronic components designed to record and display sound. Details depend on applications:
Theatre: large output, many speakers
Studio: large input, many microphones
Amplifier
Changes volume (voltage/current/power) levels of a signal. Needed to get good sound levels from a speaker.
Recording
How can sound be recorded?
How can it be stored on a computer?
What tools do we use for this?
Where do the wires go?
Why am I doing this?
It is still sound if it is on a hard drive?
How much disk space do sounds need?
Recording
To record sound on a computer is very simple.
Recall that microphones change sound (pressure) into voltage (electricity).
A computer ‘sound card’ can change voltages into numbers (just as we do: 12 volts, for instance) which can be stored in a computer memory.
A sound is therefore a sequence of numbers that represents voltages (that represent pressure).
Digital Sound
How BIG the numbers can be dictate accuracy of voltage samples ->volume or amplitude.
8 Bits (binary digits) can store numbers between 0 and 255 (256 different frequencies)
16 bits has 32768 different frequencies.
Called depth or quantization.
Digital recording
How fast you play the samples back dictates frequency.
Obviously one plays them back as fast as they were recorded. However, more samples per second means higher frequencies can be recorded.
Quantization Distortion
Quantization is the process of selecting whole numbers to represent the voltage level of each sample. The A/D converter must select a whole number that is closest to the signal level at the instant it’s sampled. This produces small rounding errors that cause distortion.
Quantization distortion increases at lower levels because the signal is using a smaller portion of the available dynamic range, so any errors are a greater percentage of the signal.
File Sizes
Sampling Rate x Resolution x Number of Channels x Time in Seconds / 8 = File Size (in Bytes)
44100 x 16 x 2 x 60/8=10,584,000 /minute
Sizes
Bytes per minute, uncompressed.
Sampling Rate Resolution Number of Channels File Size
44,100 16 2 10,584,000 44,100 16 1 5,292,000
22.050 16 1 2,646,000
11.025 16 1 1,323,000 11.025 8 1 616,000
Recording Sound – Windows XP
Sound recorder
•To open Sound Recorder, click Start, point to All Programs, point to Accessories, point to Entertainment, and then click Sound Recorder.
Audacity
Sound Forge
Sound Effects
http://www.pacdv.com/sounds/index.htmlhttp://freesoundfiles.tintagel.net/Audio/http://www.a1freesoundeffects.com/http://www.pachd.com/sounds.htmlhttp://www.soundhunter.com/http://simplythebest.net/sounds/WAV/sound_effects_WAV/index.htmlhttp://www.acoustica.com/mp3-audio-mixer/sounds.htmhttp://ljudo.com/default.asp?lang=tEnglish&do=it
Edit: To alter to fit a purpose
Sound files contain audio stored as samples.
We can change the value of any sample or group.
We can copy samples from one place (time) to another.
We can delete samples.
We need an interface
Samples are averages of sound values over time. Not a good abstraction.
Originally, editing was done using audio tape. Tape was physically cut with a blade and then taped back together.
Old StyleTape Editing
Cut editing (also known as destructive editing) consists of physically cutting magnetic recording tape, removing an unwanted portion and rejoining the
recording tape with a special adhesive backed splicing tape. Monophonic and stereophonic 1/4 inch tapes were traditionally edited in this
way. Cassette tapes can be cut edited but it is difficult to locate the precise edit point. The technique is sometimes useful for retrieving damaged cassettes.
Multitrack tapes (4 or more tracks) are not cut edited. .
Tape Editing
Removing a section of 1/4 inch tape1. Play the tape until you hear the first cue,
that is, the sound at the beginning of the section you intend to remove.
2. Put the tape recorder into edit mode. (See the operator's manual.)
3. Rotate the tape spools by hand with the tape against the replay head, (usually the one on the right).
4. Using a chinagraph pencil, mark the tape at the beginning of the section to be removed.
5. Mark the tape at the second cue, e.g. at the end of the section to be removed.
6. Pull the tape away from the replay head and lay it in the guide of the editing block, oxide side down.
7 Line up your chinagraph mark with one of the diagonal slits in the editing block. Choose the 450 (45 degree) slit for a stereo recording, the 600 slit for a mono recording or the 900 slit if you are removing clicks.
8 Cut the tape by running the corner of a single-sided razor blade along the slot. The corner of the blade should run along the bottom of the slot in the editing block. It does not matter that the corner of the blade becomes blunt because that is not the part of the blade that slices the tape.
9 Now cut the tape at the other chinagraph mark 10 Remove the unwanted section of tape but keep it safe in
case you need to replace some or all of it 11 Slide the two ends of the tape together in the editing
block. The two cut ends should just butt together. There should be no overlap and no gap between the ends.
12. Cut about 2cm of adhesive splicing tape. Touch one end of the splicing tape on to the side of the razor blade, so that it lightly sticks there.
14. Stick the length of splicing tape over the butt join and press out any air bubbles with your finger nail. .
15. Remove the magnetic tape from the splicing block.16. Play the tape at a good listening level and listen to the edit point. As the edit passes the replay
head there should be • no change in the level (loudness) or quality of the foreground speech, music , e.t.c. • no change in the level or quality of the background noise • no change in the rhythm of the speech or music
So, how does this translate into Digital??
Let’s use GoldWave: it is free (evaluation version), and is relatively full featured.
http://www.goldwave.com/
The sound (tape) is in a windowTime advances from left to right; volume (voltage, intensity) is up/down. There Are two channels here (stereo)
Time (in seconds)
The controlsTime advances from left to right; volume (voltage, intensity) is up/down. There Are two channels here (stereo)
NewFile
OpenFile Copy
To Clipboard
Save selection Delect
Selected data
Paste
PasteInto new window
Which channel
Scale
The device controlsTime advances from left to right; volume (voltage, intensity) is up/down. There Are two channels here (stereo)
Play
Rewind
Record
Left Channel
User play(looping)
Volume
Balance
Speed
Right channel
Editing – Grab, cut, pasteRight Click on right sideOf sound to be copied/
deleted
Left Click on left sideOf sound to be copied/
deleted
Then select cut or copy or delete,or use del or ctl-x or ctl-c
Editing – ‘Colors’From the computer commercial on TV
Kira Willey - Colors.MP3
Final edit
Colors – edit 1
Recording – Audacity
Available at http://audacity.sourceforge.net/Open source!