SOUND

1. SOUND Part 7

2. Types of Microphone • A microphone is an input transducer converting air pressure variations of sound into an electrical signal… converting sound energy to electrical energy • The electrical signal has the same frequency as the sound and ideally, its electrical signal amplitude is proportional to the amplitude of pressure variation in the sound.

3. Loud Speakers • • A loudspeaker is an output transducer that converts an electrical signal into sound … changing electrical energy into sound energy

4. BAFFLES • • When the front of a speaker cone is producing a compression, the back of it is producing a rarefaction … the two sounds are 180o out of phase and will destructively interfere when they meet. • • If a free speaker were used to play music the sound would be very poor due to destructive interference between sound waves produced by the front of the speaker cone and the sound waves produced by the back of the cone. • • To prevent this, a baffle, such as a speaker cabinet or wall, is used to greatly increase the distance between the two sound waves produced by the speaker cone. This limits their chances of interaction. An airtight cabinet is called an infinite baffle as it prevents the two waves from interacting.

5. FREQUENCY RESPONSE • The frequency response curve (characteristics) of a microphone show how well the microphone responds to sounds of equal intensity but different frequency.

6. Frequency Response of Microphones • The graph enables you to read the variations in power gain or loss of the microphone at different frequencies. • The diaphragm of the microphone has its own resonant frequencies… it will vibrate best at these frequencies and produce a stronger electrical output signal.

7. Fidelity • The fidelity is the degree to which the microphone accurately reproduces the essential qualities of an input sound signal • A high fidelity microphone should respond equally well to a wide range of frequencies from 20-20,000 Hz (the human range of frequency) … its frequency response curve should be essentially flat in the frequency range it is recording.

8. Frequency Response and Fidelity - Speakers • The frequency response curve (characteristics) of a speaker show how well the speaker reproduces sounds of equal intensity but different frequency. • The diaphragms of the loudspeakers have natural resonant frequencies at which they vibrate best … small tweeter speakers respond quickly to high frequency signals … larger woofer speakers respond better to lower frequency. • The fidelity of a speaker is the degree to which it accurately reproduces the original sound. http://www.bcae1.com/speaker.htm speaker design http://electronics.howstuffworks.com/speaker5.htm

9. Diffraction • Diffraction is the spreading out of sound from a speaker, involving the bending of sound waves as they pass through a small opening or past the edge of an obstacle. • Diffraction is a wave phenomenon and is easily observed in water waves … • The region where no waves travel is called a shadow. • The amount of diffraction depends on the wavelength of the sound wave … the longer the wavelength, the more diffraction occurs • When waves diffract through a gap, significant diffraction occurs as the ratio ʎ/d gets higher http://www.falstad.com/ripple/

10. Noise Reduction • Walls are built along freeways to protect residents from high frequency traffic noise. • High frequency traffic noise has a smaller ʎ and undergoes little diffraction … lower frequency sounds from have a longer ʎand can be diffracted round the barriers.

11. Directional Spread • The size of the loudspeaker opening effects the amount of diffraction (sound spreading) that occurs … diffraction is most significant when ʎ ≥ d • Higher frequency sounds (low wavelength) do not diffract as much and are better heard directly in front of the speaker. • Lower frequency sounds (higher wavelength) diffract significantly. • Tweeter speakers are small diameter speakers used to reproduce high frequency sounds, as they resonate at higher frequency. They have to be fan-shaped to assist in the dispersion of the high frequency sound … as diffraction [spreading] of high frequency from a small diameter speaker is not significant.

12. Binaural Hearing • Humans use the differences in sound reaching 2 ears to locate the source of a sound. • The differences in the sound reaching the ear include a time delay [sound travels longer to reach one ear], a phase difference [a different part of the sound wave will reach each ear], a difference in intensity [different distances from the source] as well as the amount of diffraction around the head. • The head has a diameter of about 20 cm, so lower frequency sounds with a wavelength of 0.20 m or more will diffract round the head more significantly

13. Diffraction Pattern • Just like light or water waves, sound passing through a gap forms a diffraction pattern. • There will be a central maximum sound ( more intense .. louder ) and lower intensity ( softer ) minima on either side http://www.falstad.com/ripple/

14. Interference Patterns Central Maximum Sound Intensity 1st Minimum Distance of point in front of gap

15. First Minimum • The angle θbetween the central maximum and the 1st minimum is given by … sinθ= ʎ/w where w = gap width (m)