MUS 302 ENGINEERING SECTION

Transcription

1 MUS 302 ENGINEERING SECTION Wiley Ross: Recording Studio Coordinator Twitter=> Web page => Youtube Channel=>

2 FREE AUDIO SOFTWARE (30 day trial)

3 LECTURE 1 Sound & Perception

4 A DEFINITION OF SOUND The perception of acoustic stimulus created by a vibration in air. This vibration moves air resulting in air pressure changes, that in turn, moves the ear drum and a sound is heard. Animation courtesy of Dr. Dan Russell, Kettering University

5 SINE WAVE TONE A pure tone consisting of only one frequency (unlike most musical instruments). Amplitude Wave length

6 THE SPEED OF SOUND In air, the speed of a sound wave is approximately 1130 feet per second. So, for every foot a sound wave travels the time required is approximately one milli-second (1/1000th of a second)

7 FREQUENCY OF A SOUND WAVE Frequency is the number of cycles (waves) that occur in 1 second. The units for frequency are either CPS (cycles per second) or Hertz (Hz)so, 1000 CPS = 1000 Hz.

8 FREQUENCY RANGE OF HUMAN HEARING It is generally said that, at best, a human can hear sine wave frequencies from approximately 20 Hz to 20,000 Hz (or 20 khz) 20 HZ 100 HZ 1000 HZ 10,000 HZ 20,000 HZ Sine Sweep

9 THE WAVELENGTH OF A SOUND Wavelength = Speed of sound (ft/sec) / frequency (cycles/sec) Example: For a 1130 Hz sine wave has a wavelength of one foot ft/sec / 1130 Hz = 1 ft 1130 ft/sec / 20 Hz = 56.5 ft 1130 ft/sec / 20,000 Hz =.678 Inch 20 Hz Sine Wave 56.5 foot length

10 DEFINITION OF POLARITY positive Polarity Reversed negative The positive or negative value of the wave (crest or dip). If a wave is polarity inverted 180 degrees the positive & negative parts of the wave are reversed. The term out of phase is often used incorrectly to describe an reversed polarity condition, i.e. when an identical signal is sent to two loudspeakers and the signal wires ( + & -) are reversed on one speaker so that when one speaker moves out the other moves in.

11 Summing Sine waves Summing (adding) Sine Waves in Polarity = Adding Sine Waves Out of Polarity =

12 DEFINITION OF STEREO Two discrete channels of audio to be played on two loud-speakers. The audio in each channel has some information that differs from the other. Left Channel Right Channel

13 PHASE A small difference in time arrival of two copies of the same wave(equal frequency) within the period of one wavelength. Note: a phase shift of 180 degrees is reversed polarity. 180 degrees 360 degrees 0 degrees ø Phase shift angle (0-360 degrees)

14 HARMONICS Musical instruments generally produce the equivalent of many different sine waves with many different phase relationships. Harmonics are sine components of the fundamental (lowest) frequency that are multiples of that frequency. For example, the third harmonic of a 1000 Hz fundamental is 3000 Hz or 3 times 1000Hz.

15 HARMONICS Fundamental Second Third Forth

16 TIMBRE The tonal quality of a sound due to its harmonics and envelope. Envelope = amplitude changes over time. The quality of timbre is why we can tell, for example, a piano from a guitar. Two waveform examples with the same pitch & harmonics but with different envelopes

17 DIFFRACTION The ability for sound waves with large wave length waves (low frequencies) to bend around an object. Sound source Barrier Diffracted waves

18 SHADOWING Frequencies whose wavelength is sufficiently small (high frequency) compared to an obstacle can not bend around the obstacle but instead create an acoustic shadow. Sound Source A view from above of a human head. Head Shadowing on this side of the head causes the this sound to be softer in this ear.

19 THE MASKING EFFECT When two sounds occur nearly simultaneously the louder sound will prevent the perception of softer the sound. The Masking effect is greatest when the louder & softer sounds are close to the same frequency. MP3 s and other compressed audio use the masking effect to allow part of the audio data to be discarded. ie: that which is masked.

20 THE PERCEPTION OF DIRECTION The Perception of the direction at which a sound is coming from is determined by : 1. Differences in sound intensity between ears. 2. Differences in sound time arrival between ears.(delay) 3. Head shadowing, i.e.: changes in timbre between ears. 4. Pinna (the outer ear) response timbre differences. 5. To some extent, the direction of sound can be determined by just one ear.

21 LOUDNESS & DECIBELS Loudness is the perceived intensity of a sound. Loudness is not a linear (one to one relationship) to measured intensity. Which means, a sound of twice the measured intensity is not perceived to be twice as loud. This is why the decibel units were created. Decibels scale loudness measure into more meaningful units. For every doubling of acoustic sound intensity there is a 3 decibel increase in loudness.

22 INTENSITIES & DECIBELS Range of intensities (in units of power) of human hearing is approximately 10 to the 13th power! ( 1 trillion to 1). The Decibel relates this huge range to a logarithmic scale where the numbers are relatively small.

23 UNITS OF THE DECIBEL Decibels can measure the intensity of not only acoustic waves but electrical waves too. db S.P.L. is a unit of measure of acoustic loudness. S.P.L. = Sound pressure level dbv is a unit of electrical voltage or level (what you see on a meter). dbm is a unit of electrical power.

24 ACOUSTIC DECIBEL 0 db SPL = the threshold of hearing 120 db SPL = the threshold of pain db S.P.L. = 10 log P/P ref. where P is the new loudness & P ref. is the old loudness. dbv = 20 log v/v ref. where v is the new voltage & V ref. is the reference voltage. dbm = 10 log P/P ref. where P is the new power level & P ref. is the reference power level.

25 DOUBLING INTENSITY When the sound pressure level is doubled, as when two equally loud sound occur together, there is a + 3 db SPL increase in acoustic level. When two electrical signals are doubled in an audio mixing console, the meter shows a 6 dbv increase level. This is a doubling of an a electrical quantity called voltage.

26 SOUND TRANSMISSION Sound intensity diminishes with distance. For every doubling of distance the sound intensity is 6 db SPL lower. (not including reflections from room surfaces) Sound Source 90 db SPL d = distance 2d d 84 db SPL

27 THE PERCEPTION OF EQUAL LOUDNESS WITH FREQUENCY The ear/brain does not hear equal intensities of different frequencies to be equal at all listening levels or volumes. At low volumes you don t hear the extreme high & low frequencies to be as loud as the middle frequencies. As the volume is raised you will tend to hear a more equal relative balance between low, middle & high frequencies. Listening volume affects perceived tonal balance. As an engineer / producer it is important to listen at volume levels similar to those which the product will most likely be played. If not the tonal balance may not sound correct when played on a consumers home equipment. (for example)

28 ASSIGNMENTS Cancel the mono component of a favorite stereo recording (see youtube video ) Learn about the Haas or Precedence effect (see youtube video ) Listen to your stereo speakers at low and loud levels and notice the difference in perceived bass & treble. If you have an ipad or a smart phone download a free decibel app and measure different acoustic sound levels