Art 321 Sound, Audio, Acoustics Dr. J. Parker

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 metre Standardized normal air pressure is: P 0 = 2 X N/m 2 = N/m 2 = 20μ Pa. Why do we care? Because a decibel, the standard measure of sound level, is: 10 log 10 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 10 x 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 V = /f where v is speed of sound, f is frequency.

Amplitude

Frequency Frequency is a precise way to specify pitch. A 440 E above C Summed Oboe A Oboe E Phantom B 3960

Frequency

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 can reflect

Multiple sounds What we hear with multiple sounds is the sum of all of them.

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

Microphone Converts air pressure differences into electrical voltage.

Microphone

Speakers For display of audio.

Amplifier Changes volume (voltage/current/power) levels of a signal. Needed to get good sound levels from a speaker.

Mixer Control volumes of multiple inputs into (multiple) outputs.

Complete Sound System

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 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.

Sampling

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.

Clipping

File Sizes Sampling Rate x Resolution x Number of Channels x Time in Seconds / 8 = File Size (in Bytes) 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, ,584,000 44, ,292, ,646, ,323, ,000

Recording sound – MAX OS X

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 V/index.html