PH 105 Dr. Cecilia Vogel Lecture 24

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.

Dynamic Microphone

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