Broadcast Engineering and Acoustics

SOUND SYSTEMS

ENGR. JOMER V. CATIPON

jvcatipon@yahoo.com

0919-8718913

Sound Systems

• Working with audio means working with sound systems. • Naturally, the range of systems available for different applications is

enormous. • However, all electronic audio systems are based around one very

simple concept: To take sound waves, convert them into an electric current and manipulate them as desired, then convert them back into sound waves.

BASIC COMPONENTS OF A SOUND SYSTEM

A very simple sound system is shown in the diagram below. It is made up of two types of component:

• Transducer - A device which converts energy from one form into another. The two types of transducers we will deal with are microphones (which convert acoustical energy into electrical energy) and speakers (which convert electrical energy into acoustical energy).

• Amplifier - A device which takes a signal and increases it's power (i.e. it increases the amplitude).

• The process begins with a sound source (such as a human voice), which creates waves of sound (acoustical energy).

• These waves are detected by a transducer (microphone), which converts them to electrical energy.

• The electrical signal from the microphone is very weak, and must be fed to an amplifier before anything serious can be done with it.

• The loudspeaker converts the electrical signal back into sound waves, which are heard by human ears.

The next diagram shows a slightly more elaborate system, which includes:

• Signal processors - devices and software which allow the manipulation of the signal in various ways. The most common processors are tonal adjusters such as bass and treble controls.

• Record and playback section - devices which convert a signal to a storage format for later reproduction. Recorders are available in many different forms, including magnetic tape, optical CD, computer hard drive, etc.

• The audio signal from the transducer (microphone) is passed through one or more processing units, which prepare it for recording (or directly for amplification).

• The signal is fed to a recording device for storage.• The stored signal is played back and fed to more processors.• The signal is amplified and fed to a loudspeaker.

The 3-part audio model

One simple way of visualizing any audio system is by dividing it up into three sections: the source(s), processor(s) and output(s).

• The source is where the electronic audio signal is generated. This could be a "live" source such as a microphone or electric musical instrument, or a "playback" source such as a tape deck, CD, etc.

• The processing section is where the signal is manipulated. For our purposes, we will include the amplifiers in this section.

• The output section is where the signal is converted into sound waves (by loudspeakers), so that it can be heard by humans.

• Sources: There are three sources - two tape machines and one radio aerial (technically the radio source is actually at the radio station).

• Processors: Includes a graphic equalizer, left/right stereo balance, and amplifiers.

• Outputs: There are two speaker cabinets (one at each end), each containing two speakers. Note that there are also two alternative outputs: A headphone socket (which drives the small speakers inside a headphone set) and twin "line out" sockets (which supply a feed for an external audio system).

Now imagine a multi-kilowatt sound system used for stadium concerts. Although this is a complex system, at it's heart are the same three sections: Sources (microphones, instruments, etc), processors and speakers.

Whatever the scale of the project, the same underlying principles of sound reproduction apply.

Microphones

• Microphones are transducers which detect sound signals and produce an electrical image of the sound, i.e., they produce a voltage or a current which is proportional to the sound signal. The most common microphones for musical use are dynamic, ribbon, or condenser microphones. Besides the variety of basic mechanisms, microphones can be designed with different directional patterns and different impedances.

Dynamic Microphones

• Principle: sound moves the cone and the attached coil of wire moves in the field of a magnet. The generator effect produces a voltage which "images" the sound pressure variation - characterized as a pressure microphone.

Advantages:• Relatively cheap and rugged.• Can be easily miniaturized.

Disadvantages:• The uniformity of response to• different frequencies does not• match that of the ribbon or• condenser microphones.

Ribbon Microphones

• Principle: the air movement associated with the sound moves the metallic ribbon in the magnetic field, generating an imaging voltage between the ends of the ribbon which is proportional to the velocity of the ribbon - characterized as a "velocity" microphone.

Advantages:• Adds "warmth" to the tone by accenting lows when close‐miked.• Can be used to discriminate against distant low frequency noise in

its most common gradient form.

Disadvantages:• Accenting lows sometimes produces "boomy" bass.• Very susceptible to wind noise. Not suitable for outside use unless

very well shielded.

Ribbon microphone

Condenser Microphones

• Principle: sound pressure changes the spacing between a thin metallic membrane and the stationary back plate. The plates are charged to a total charge

where C is the capacitance and V the voltage of the biasing battery.

Advantages:• Best overall frequency response makes this the microphone of

choice for many recording applications.

Disadvantages:• Expensive• May pop and crack when close miked • Requires a battery or external power supply to bias the plates.• A change in plate spacing will cause a change in charge Q and

force a current through resistance R. This current "images" the sound pressure, making this a "pressure" microphone.

Condenser Microphones

• Because the sensing element of a condenser microphone is a light membrane, it is capable of excellent transient response.

• The fact that the condenser has excellent high frequency response implies good transient response, since sharp transients have more high frequency content than the sustained sounds which follow them.

• Because the condenser microphone must have a continuous, stable DC voltage to bias the membrane, it is common practice to supply that voltage from the sound mixing board.

• The voltage is applied via one of the microphone leads, typically 48 volts, and is commonly referred to as "phantom power".

• Since the alternative is a battery supplied bias, with the risk that a battery can go out in mid performance, the phantom power provision from mixing boards is useful.

Electret Condenser Microphone• Electret condenser microphones are not to be compared with the

studio standard condenser microphones which have such excellent frequency response characteristics.

• The electret class of microphones are condenser microphones which use a permanently polarized electret material for their diaphragms, thus avoiding the necessity for the biasing DC voltage required for the conventional condenser.

• They can be made very inexpensively and are the typical microphones on portable tape recorders.

• Better quality electret condensers incorporate a field‐effect transistor (FET) preamplifier to match their extremely high impedance and boost the signal.

• The flat, faithful frequency response of the condenser microphone arises from its mechanism.

• The charge on the membrane depends only upon the spacing and shows no appreciable resonances to skew the frequency response.

• The capacitance of the parallel plate membrane structure is given by When the spacing changes, the charge changes, giving an electric current through the resistor R.

• The voltage measured across the resistor is an electrical image of the sound pressure which moves the membrane.

END