OUTLINE Speaker and mike mechanisms Speaker construction Housing Horns Resonance woofers and tweeters filters Distortion Doppler Effect
Speakers and Mikes Speaker always turns electric signal into vibration/ sound wave. Mike always turns vibration/ sound wave into electric signal. called transducer, because they transform energy. Other energy transducers: motor, generator light bulb, photocell heater, engine
Dynamic Speaker Dynamic speaker works on same principle as electric motor: turns electrical energy into motion energy. For speaker, the motion is sound vibration. A current-carrying coil acts like a magnet. Can attract or repel permanent magnet, making it move. usually voice coil moves, but could be magnet.
Speaker Vibration If the current that flows to the speaker is AC, with frequency f then direction of the current changes with frequency, f ; magnet changes from repelling to attracting with frequency, f ; voice coil moves back and forth with frequency, f. Voice coil is attached to membrane (cone); cone vibration makes a sound with frequency, f.
Dynamic Microphone If a magnet moves near a coil, or vice versa, it will induce current to flow in the coil. If the coil moves back and forth due to sound wave hitting the membrane its attached to, then the induced current will be AC with the same frequency as the sound wave. Same waveform, too. Sound transformed to electrical signal.
Other Mechanisms Other mechanisms for transducers: Piezoelectric materials, which respond to pressure with electric signal and vice versa. crystal mikes and speakers Variable capacitors As charged plates get closer, opposite charges on plates attract more, causing increased capacitance, and vice versa. electrostatic or electret speakers and mikes
Speaker Housing The housing of a speaker is not just for aesthetics. Without the housing, sound wave from back side of speaker will tend to cancel wave from the front. When one is pushing air, other is pulling; high pressure from one canceled by low pressure from other. Housing blocks the wave from back. at least gives a a detour. Demo
Make it an Asset If you block the wave from the back, you lose that sound energy. Bass reflex speakers use that sound energy; comes through opening (a port or duct) or drone cone – a speaker without a coil. The sound from back is brought back into phase, so it won’t cancel by driving reflex port above resonance or by making wave travel half wavelength.
Sound Radiation Low freq sound waves diffract easily, spread out in all directions as they come out of speaker. High freq sound waves don’t, unless they pass through an opening similar in size to their wavelength. Horn speakers have diffractive openings a few cm wide. Diffract wavelength down to few cm, or freq up to f = v/ =(343m/s)/(.03m)=11kHz
Woofer and Tweeter Should cone be large or small? Should be large, because large cone pushes a lot of air, making louder sound. Should be small, because heavy cone is hard to make move quickly. Send low freq components to large woofer since that doesn’t need to move quickly. Send high frequency sound to small tweeter.
Compliance Not only do tweeters tend to be light, they also tend to have stiff suspension. And woofers have soft suspension. As with springs, weight and stiffness determine natural freq Spring constant told how stiff spring is, Compliance tells how soft suspension is. Woofers have higher compliance.
Bose Speakers Small speakers generally good at hi frequency, but don’t sound rich because low freq’s don’t resonate. Bose wave technology, uses a 34” long air column folded up inside to make low frequencies resonate.
Bose Speakers Can you hear the difference between ordinary small speakers and Bose? disclaimer —not meant to fill a room this large
Noise Cancellation How can you block out ambient noise while listening to headphones? Physically block it with foam padding. Cancel it. Ambient sound it picked up by mike a sound wave opposite it is produced electronically. In headphones, these two waves cancel approximately listen to difference cancel-on and off
Distortion Speakers and mikes don’t perfectly reproduce waveforms especially at high amplitude. One reason is at high amplitude, the voice coil moves far perhaps even so far that the magnet’s field begins to drop off. Another reason is the Doppler Effect
Doppler Effect A moving source of sound gives off a different pitch: Moving toward you, pitch is higher, moving away, pitch is lower example: cars speeding by demo
Intermodular Distortion IM occurs when speaker is emitting complex sound: Moving in one direction due to low freq component, while giving off high-freq component The speaker cone is a moving source for that high freq sound. Will be distorted by Doppler effect.
Surround Sound The 5.1 standard for surround sound gives a greater spatial feel because it uses 5 speakers instead of two for spatial localization plus one sub-woofer very low pitch sounds aren’t directional. Required six channels of sound and 6 speakers
Directionality Cues Binaural cues that help us determine directionality: relative phase of low-frequency sound, relative intensity of mid-range sounds, relative time of arrival of high-freq sounds. Non-binaural cue: Which frequencies are strong in the spectrum? Effects like diffraction around the pinna & the head, mean that the ear has a different response to sounds from different directions.
Transfer Function Transfer function tells what fraction of the free sound is actually transferred to the eardrum It is different for sounds from different directions for example, sound from the side has more response from 500Hz-1500Hz and near 8 kHz than sounds from front
SRS 3-D If you increase the amplitude of components from 500Hz-1500Hz and near 8 kHz you can make it sound like the sound came from the side even if it came from speakers in front of you SRS technology uses this idea to create fake surround sound Can also make the sound appear to come from higher than the speakers if speakers are below TV, for ex
SRS TruBass Another problem with speakers is reproducing good bass without using large, heavy speakers TruBass uses the idea of the missing fundamental to create fake bass. For ex, if the sound contains 50 Hz tone but speakers aren’t responsive below 100Hz then have speakers emit 100 Hz, 150 Hz, 200 Hz the mind will fill in the missing fundamental at 50 Hz
Summary Mechanisms for changing sound to electricity and vice versa: magnetic induction, piezoelectric, capacitor. Speaker construction may include housing to enhance sound, horns to diffract high freq sound Woofer and tweeter have different natural freq Distortion due to large motion, Doppler effect
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