Lecture 16Electro-acousticsMicrophonesAmplifiersTunersLoudspeakers

Instructor: David Kirkby (dkirkby@uci.edu)Guest Lecturer: David Casper (dcasper@uci.edu)

                                                                                 

Physics of Music, Lecture 16, D. Casper 2

The Need for Electro-acousticsThe intensity of sound decreases rapidly with distance, especially if the source radiates sound in all directions.

If a conductor normally hears a 100-voice choir from 3.2m away, it only sounds as loud as 10 voices from 10m away, and the same as just one voice from a distance of 32m.

1m10.0

3.2m1.0

10m0.1

32m0.01

DistanceIntensity

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Transducers and “Pipelines”A device that converts one form of energy into another is called a transducer.The process of converting sound energy to electrical energy, modifying and amplifying the electrical signal, and then converting the electrical energy back to sound is called an audio pipeline, or sound reinforcement.

SoundSources Sound

Output

Transducer(Speaker)

AmplifierEffects,Equalizer

Mixer

EffectsTransducers(Microphones,Pickups)

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Sound and Analog SignalsSound can be described as a superposition of waves:•At any time, a wave has an associated amplitude•A wave’s amplitude can be positive or negative•The sum of two waves is just the sum of their amplitudes

The electric currents in our audio pipeline have all the same properties!

In a purely “analog” sound system, we simply translate sound amplitudes into equivalent electric currents and (eventually) back again.

An ideal system where this works perfectly is said to be “linear”

Sound wave

Current

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Converting Sound to ElectricityA Microphone converts sound vibrations to electric signals.

The first microphone was part of Alexander Graham Bell’s telephone (1876) and used a liquid.

Two years later, the carbon microphone was invented. It is still used in some telephones today.

The ribbon microphone was developed for radio and is sometimes used for studio recording of vocals.

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Dynamic MicrophonesThe dynamic microphone is one of the most commonly used today.

It relies on the fact that a current is produced by moving a coil of wire in a magnetic field (“induction”).

The coil is attached to a diaphragm, which picks up vibrations in the air

Dynamic microphones are rugged and relatively inexpensive. They are also able to handle high sound volume.

Their frequency response is not as linear as condenser (or ribbon) microphones

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Condenser Microphone BasicsCondenser microphones are the other commonly used type.

Condenser microphones rely on electric forces rather than magnetic.

Parallel plates are charged by a battery; the incoming sound causes the distance between plates to change slightly, altering the amount of charge on the plates and creating a current.

Condenser microphones have more uniform frequency response, but are expensive and require a battery. They can also “pop” when very near the source.

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Omnidirectional and Cardioid Microphones

Not all microphones are equally sensitive to sound from all directions (“omnidirectional”). Omnidirectional mic’s have a diaphragm open only at the front.

The Cardioid microphone is so called because its directional sensitivity resembles a heart shape. Cardiod mic’s have a diaphragm exposed to air front and back. Vibrations from the back partially cancel those from the front.

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Directional Microphones“Shotgun” or linear microphones are often used for recording video in noisy/crowded areas.

They use a tubular design which accepts sound coming along the direction of the tube, while sound from the side cancels. These microphones have a relatively limited frequency range.

Dish microphones are used to increase the total intensity of sound (up to 75 times) by focusing it with a parabolic reflector

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Converting Electricity to SoundLoudspeakers are transducers which perform the opposite task of microphones – they convert electrical signals into sound.

Most home stereo systems use dynamic loudspeakers.

In their most basic version, a diaphragm vibrates to produce sound waves in the air.

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Dynamic Loudspeaker OperationDynamic speakers take the alternating electric current of the audio pipeline and use it to vibrate a diaphragm.

The principle is the opposite of the dynamic microphone: the electronic currents in a coil attached to the cone make it an alternating electromagnet, which is attracted and repulsed, in turn, by a stationary magnet.

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BafflesWhen the wavelength of the signal is the same or larger than the diameter of the speaker cone, a dynamic speaker by itself does a very poor job of getting air moving.

Waves from the back of the cone interfere destructively with those from the front.

Even for a large 16” speaker, the wavelength equals the diameter at around 850 Hz.

Baffles prevent waves from back from reaching the front.

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Simple EnclosuresFor lower tones, the efficiency of a speaker is greatly improved by enclosing it in a resonating cavity

In the closed baffle, or air suspension configuration, compression of the air trapped in the cavity adds to the restoring force on the speaker cone, allowing it to move further in and out.

This set-up allows good bass response from a relatively small speaker.

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Bass Reflex SpeakersBass reflex speakers have a vent (hole) in the front, which allows vibrations from the back of the chamber to produce sound as well as the cone.

The result is lower power consumption than an air suspension speaker, and smoother, extended bass response.

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Efficiency of Dynamic LoudspeakersAlmost all the energy used to drive a dynamic loudspeaker is “wasted” – not converted to sound energy.

This inefficiency is due to the relative lightness of air – imagine trying to punch a handkerchief – you cannot transfer much energy to it.

Because dynamic speakers are very inefficient, they require a large amount of power (from an amplifier) to operate.

Their low efficiency also makes them poor choices for public address systems, where sound is to be projected over a very large area

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Horn LoudspeakersHorn loudspeakers have a moving voice coil driving a membrane which resonates in a horn.

The horn results in far more efficient use of energy (40-50%) than a dynamic loudspeaker, hence these speakers are typically used for public-address and concert sound systems (also bullhorns)

The geometry of the horn must be chosen to match the wavelength of sound it will reproduce; for low frequency, folded or more complicated shapes are required

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Woofers, Tweeters and MidrangeTo achieve the highest quality response over all frequencies, audio systems use two or more speakers of differing sizes.

A small horn speaker is often chosen as the “tweeter” for highest frequencies.

A large dynamic (or bass reflex) speaker serves as the “woofer” for bass.

An intermediate midrange is sometimes added.

The frequencies driving each speaker are controlled by a “cross-over unit” inside the speaker cabinet.

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Noise-Cancelling HeadphonesBose Corporation has invented a technology which uses the principle of superposition to reduce low-frequency noise

The headsets contain a small microphone that measures the ambient sound-level near your head and generates an inverted (out of phase) wave to cancel it before it reaches your ears.

The circumaural seal filters out higher frequencies.

Similar technology is now being used by some rock drummers to reduce ear damage in concert.

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AmplifiersAmplifiers take low-power signals produced by microphones and other transducers and increase the power to levels sufficient to drive speakers.

Amplifiers work by using the low-power input signal to control a higher power output signal, like controlling a knob on a dial.

Transistors (or vacuum tubes) serve as the switches that make amplifiers work. They generate large amounts of heat in the process.

“Pre-amplifiers” are sometimes used in the pipeline, but they boost only voltage (to make the signal larger compared to noise introduced afterward), not power.

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Harmonic DistortionIf an amplifier is not linear, we say it introduces distortion.

Harmonic distortion occurs when the output pulse is clipped due to overloading some component.

In that case, higher harmonics of the original tone(s) are introduced.

If harmonic distortion is small, it may be unnoticeable, since most music contains several harmonics anyway.

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Intermodulation (IM) DistortionWhen an amplifier is truly non-linear (not just overloaded), it introduces new frequencies in the waveform which are not harmonics of any already present. IM distortion is likely to be more noticeable.

Transient IM (TIM) distortion occurs when the amplifier cannot respond to rapid changes in the input signal, and has similar effects on sound quality.

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Radio BroadcastsTwo different methods are used to transmit commercial radio broadcasts

AM (Amplitude Modulation):Amplitude of a carrier wave (540-1600 kHz) is modulated with the signal. May only transmit frequencies up to 5000 Hz, limiting audio quality.

FM (Frequency Modulation):Frequency of carrier wave(88-108 MHz) is modulated with signal. Due to higher frequency of carrier, better audio quality but shorter range.

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The Electromagnetic SpectrumAM and FM radio broadcasts are electromagnetic radiation, in other words light.Radio waves are a color of light that our eyes can’t see, like the rest of the electromagnetic spectrum. The wavelength of an FM wave is a few meters. The wavelength of an AM wave is a few hundreds of meters.

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Stereo BroadcastsBroadcasts in FM stereo represent a challenge because two signals must be encoded, but the transmission must also be compatible with monaural reception by non-stereo devices.

The solution is to use one band of frequencies to store the L+R channel, and a different band to store the L-R.

L+R and L-R are picked up by all receivers; L-R channel is too high frequency to hear with mono. Stereo receivers reconstruct L and R for playback.

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Surround-SoundEver since Walt Disney’s Fantasia (1940), movie studios have tried to create a more 3-d listening experience in the theatre.

Today, many home stereo systems have 5+1-channel audio

(center, left, right, surround left, surround right plus bass).

Tricks like L+R, L-R (see below right) are no longer necessary since all channels are encoded on DVDs.

Both volume levels and phase differences can provide the convincing illusion of 3D.