Home theater and music installation cover a lot of ground — too much, in fact, to present reasonably in one article. Therefore, we are going to cover one part only in this article — sound systems. Residential sound systems come in a variety of types, but their operating characteristics are nearly identical from one to another.

Once you decide which rooms will get sound, you must mount the speakers in each room so they produce the best effect. The general rules for placing speakers in rooms are as follows:

  1. Do not place speakers in the ceilings unless you have no other choice. Sound emitted downward will not easily fill a room. Balance between the right and left channels will be heard poorly also, depending on the listener's position in the room.

  2. Two stereo speakers should be placed with the distance between each of the two being equal to the distance between the listener and each speaker. For example, if your couch is 7 ft from bookshelf-mounted speakers, the speakers should be mounted 7 ft apart.

  3. Place in-wall speakers at about 5 ft above the finished floor (standing ear level). If the ceiling in the room is more than 10 ft high, mount the speaker at about half the height.

  4. Two-way speakers (those with separate woofers and tweeters) should generally be mounted so that the woofer (which reproduces the low-pitched sounds) is above the tweeter (which reproduces the high-pitched sounds).

  5. Make sure that you use weather-resistant speakers (sometimes called weatherized speakers) in areas such as garages, greenhouses or similar areas.

IMPEDANCE MATCHING

Audio amplifiers can produce only a limited amount of current to drive speakers. If too many speakers are connected in parallel (which is the normal connection) to an amplifier, it will be overloaded, and either function poorly or be damaged.

Audio amplifiers have their output limits measured in ohms. For example, an amplifier may have an output rating of 3 ohms. This indicates that the circuit you connect to the output terminals (the speaker connections) must have an impedance (total resistance, including inductive and capacitive reactance) of no less than 3 ohms.

Most speakers are rated at 8 ohms. Thus, two such speakers connected in parallel would give the circuits an impedance of 4 ohms (8 ohms per branch, divided by the number of equal branches). Three such speakers connected to a circuit would yield an impedance of 2.67 ohms (8 ohms, three branches). In these calculations, we are ignoring the resistance value of the conductors, which is negligible except when very long runs are required.

AUTOTRANSFORMERS

While some amplifiers can handle loads with impedances as low as 1¾ ohms, they are not common. In most cases, if you wish to connect more than two speakers (8 ohms) on a circuit, you will have to install an autotransformer to adjust the impedance of multiple speaker pairs, and present a sufficiently high impedance to the amplifier. You will often find these audio autotransformers sold as impedance matching transformers.

Volume controls are often autotransformers (they can also be potentiometers). They can reduce the level of signal sent to the speakers, while still presenting the same circuit impedance to the amplifier.

Components

Before we finish with the basics of sound systems, we should define the following terms:

Tuner. The AM and FM receiver that is used to tune in the channel frequencies. These are frequently combined with an amplifier and enclosed in the same unit. Nonetheless, do not confuse the issue — a tuner cannot amplify the signal, only receive it. If you plugged speakers into a tuner (rather than a combination tuner/amp), they would not produce sound.

Amplifier. A transistor-based signal amplification unit. Amps are used to take the low-powered signals generated by tuners or turntables and raise them to high enough levels to run powerful speakers.

Pre-amp. A pre-amplifier. It is used to boost a signal. Not as powerful as a normal amplifier, but necessary in some systems or to drive certain speakers. Normally used as an input to the power amplifier.

Equalizer. An equalizer is a group of tuned circuits, used to color the sound produced by the system. By using an equalizer, it is possible to add to or diminish certain frequencies or frequency bands. For instance, if a sound system is producing a lot of hiss, using an equalizer to cut back on the high frequencies will eliminate it. The two main types of equalizers are graphic and parametric. Graphic equalizers are the most common type. They modify bands of frequencies, such as from 30Hz to 800Hz, from 1,000Hz to 500Hz, and from 7,000Hz to 1,2000Hz. Parametric equalizers, on the other hand, boost or cut one specific frequency, or a very narrow band of frequencies. Graphic equalizers are most commonly used for home installations, and parametric equalizers are used for recording or special applications.

Tweeter. A speaker specifically designed to reproduce high frequencies. Usually used in combinations with other types of speakers.

Woofer. A speaker specifically designed to reproduce low frequencies. Usually used in combination with other types of speakers.

Subwoofer. An unusual type of speaker used to reproduce very low frequencies, in the range of 30Hz to 125Hz. Unlike normal speakers, subwoofers do not have to be aimed at the listener. Since such very low frequencies propagate (spread out) their sound over a very wide area, they can be placed in almost any part of a room with equally good effects. In other words, sub-woofers tend to spill their sound, rather than radiating the sound in one direction only.

RCA jack or RCA connector. The standard type of stereo jacks and connectors; standard equipment on nearly every piece of stereo equipment sold. They have a central pin or sleeve connected to one conductor, and concentric flanges connected to a second conductor. Jacks are the female piece, and connectors are the male piece.

Signal-to-noise. The generally used measure of signal quality for stereo systems. It is a comparison of the signal level to the noise level. It is expressed in decibels; the higher the better.

Decibels. The measure of signal strength for all types of communication and sound systems. Originally developed for the telephone system. (Hence the name deciBEL, after Alexander Graham Bell, the prefix “deci” meaning 10.) Usually shown written as dB.

Phase-locked-loop, or PLL. A special type of tuning circuit that uses a quartz crystal vibrating at a specific reference frequency. It is used for digital tuning, and generally produces more accurate results than other methods.

Watts per channel. The amount of speaker-driving power that an amplifier can produce. A stereo system has two channels.

Ambient noise. The background noise in a room or area.

White noise. A sound signal containing an even distribution of all frequency levels; used to test a room, to determine its acoustical characteristics.

Dolby. A noise reduction system used in many sound systems. There are several types, often designated Dolby A, B or C.

DBX. Another noise reduction system. Similar in concept to Dolby, but operating differently.