Selecting Speakers

How many do I need? Which configuration do I want?
The best place to start when you are upgrading or building a new home theater sound system is to determine the best speaker system for your room and your budget. As the technology has improved we’ve gone from monophonic (single-speaker) record players to stereophonic audio to surround sound. Most recently, the technology has allowed audio engineers to think of each channel that they are mixing as a source that can be positioned anywhere in the room; not just left or right, front or rear, but at ground level or overhead. With these advanced formats like Dolby Atmos and DTS:X, each audio channel can be mixed as an object that can be positioned or move position in the room as needed. Sophisticated processors take this position information, while considering your speaker layout, adjusting the relative volume of each object in each speaker channel to position the source for a realistic simulation of the environment that the movie producer wants to put you in the middle of.
Your choice of speaker systems will first be guided by your budget. Next, you’ll need to determine how many speakers you will be placing in your room, and how they will be mounted or positioned. Some people prefer form over function. Those that are primarily concerned with the aesthetic appearance of the room will prefer speakers mounted in the walls behind acoustically transparent fabric, which can include q projector screen. Audiophiles that love the full sound and look of big speakers will want them sitting on speaker stands or cabinets, or hanging from the ceiling.
Speakers are designed to accurately reproduce the recorded sound, without distortion of any kind. While no speakers are perfect, some are better than others at being able to reproduce all sound frequencies cleanly and accurately. There are several common metrics that audio experts use to measure a speaker’s capabilities.

Frequency Response

The first measurement to understand when selecting speakers is the frequency response. This describes the range of sounds, from low notes to high notes that the speaker can reproduce. Frequency response is expressed in Hertz (Hz), a measure of the cycles per second that a sound wave vibrates, whether that vibration is on a guitar string, a drum head or a singer’s vocal chords. A healthy human can hear sounds from 20 Hz. to 20,000 Hz. The lowest bass notes vibrate at under 100 Hz. The highest frequencies are contained in harmonic vibrations coming from objects like metal drum cymbals.
The word “speaker” is used interchangeably to mean two different things. In this guide, we will use it to refer to a complete audio component. It is commonly used to refer to the individual component that produces sound from the electrical signal coming from an amplifier, but for clarity we’ll refer to those individual components as a transducer.
It is hard to design a single speaker transducer that can accurately reproduce all of the frequencies humans can hear. That is why full-range speakers are usually constructed from multiple transducers. For example, a 3-way speaker can be constructed of a low-frequency transducer (a woofer), a mid-range transducer and a high frequency transducer (a tweeter). A crossover network is used to split the incoming audio signal into different frequency ranges, sending the low frequencies to the woofer, the high frequencies to the tweeter, and for 3-way speakers, the mid frequencies to the midrange.
A speaker’s frequency response can be plotted on a graph, with the Y axis showing the sound amplitude and the X axis showing the frequency. An ideal speaker would have a perfectly flat frequency response over the full range of frequencies that humans can hear. Sadly, there are no ideal speakers in the real world. Individual speaker transducers will reproduce some frequencies more efficiently (more sound output for a given input signal power) than others. It’s hard to describe a speaker’s frequency response accurately without showing this frequency response graph (also known as the frequency response curve). But most often manufacturers will publish only a range of frequencies, for example 40 – 20,000 Hz. A simple range of frequencies isn’t very meaningful, as you aren’t being told how much sound you will hear at all different frequencies; particularly at the extremes of the frequency range, where sound levels tend to drop off sharply. High-end speakers should specify an amplitude range that their frequency response measurement stays inside of; for example, +/- 6 dB, or better yet, +/- 3 dB. The tighter the amplitude range, the better.

Power Handling and Distortion

The larger your room and the higher the sound volume that you want to achieve, the more power you will need in your amplifiers and the more power you will need your speakers to handle.
Speakers always provide a power rating, indicating how much power they can handle continuously from your amplifier. If you drive a speaker with more power than it is designed to handle, the speaker transducers will fail. A speaker transducer has a coil attached to the back of the speaker cone. The coil acts as an electromagnet, turning the electrical signal into a magnetic signal. The coil sits inside a permanent magnet, held by the speaker’s basket. As the magnetic signal varies, it drives the speaker’s cone inward and outward. The speaker coil is made of relatively thin and lightweight copper wire, coated with a special substance that provides electrical insulation. When you overpower a speaker, even momentarily, the resulting electrical current will overheat the voice coil, causing damage. If you push your speakers too hard, the voice coils could burn up, and the copper wire could melt and separate. So, don’t fry your speakers! It is important to always use speakers that are designed to handle as much or more than the maximum power that your amplifier can deliver.
The goal of every audiophile is to build a system capable of full, accurate sound reproduction. Distortion occurs when one or more components can’t accurately reproduce the signal from the audio source. While electric guitar players often love distortion in their sound, and even buy distortion effects (fuzz boxes, etc.) to get that sound without having to drive their amplifier hard, audiophiles only want to hear distortion if it was in the recording. Speakers will distort the sound if they are driven too hard. For example, a speaker cone is suspended with a flexible surround, and it is designed to be able to travel in and out over a certain distance. If a speaker is pushed beyond its limits, it won’t be able to create a sound pressure wave that matches the electrical signal, and so the sound will be a distorted version of the signal. If you want big sound volume, you will need big speakers (or, more accurately, speakers capable of converting big electrical power into big sound pressure waves).

Sensitivity

Some speakers are more sensitive than others. More sensitive speakers will produce a higher sound amplitude for a given power input. Speaker sensitivity is measured in dBA (sound amplitude or volume) at 1 watt input, 1 meter from the speaker. Similar to other measurements, sensitivity should really be measured as a graph, showing sound amplitude versus frequency. Most speakers simply show a single number, which will give you a general idea. Less sensitive speakers will require higher power to achieve the same volume as a more sensitive speaker.

Impedance

Simple electrical circuits involve a source of electromotive force (voltage), and a source of resistance, like an incandescent light bulb. Ohm’s law tells us that V = I * R (voltage = current * resistance). So, if we have a 100 volt power supply and a light bulb that has 10 ohms of resistance, we know the current running through the circuit is 10 amperes, or amps. We also know that power = voltage * current, so we can calculate that the light bulb is consuming 100 watts of power. This example assumes a steady voltage, and this is known as a Direct Current, or DC signal. Audio amplifiers drive speakers with a constantly varying electrical signal with a voltage that averages zero volts (also known as Alternating Current, or AC). Speakers don’t act like simple resistors. Speakers convert electrical energy into a magnetic field, which in turn drives the speaker cone or diaphragm forward and backward. Impedance is a measure similar to resistance, but it measures the combined effect of the electrical resistance and the reactance to the changing magnetic field. We can still use Ohm’s law to calculate the electrical current that will result or the power we will consume when we drive the speaker with a signal having a particular voltage, but the effective voltage of an AC signal is measured as the Root Mean Square of the peak voltage.
You don’t need to know all of the details of speaker impedance described above. However, you should know that 8 ohm speakers consume less power from an amplifier at a given volume setting than 4 ohm speakers, because the lower impedance speaker draws more current from the same voltage. If you wire 2 speakers in parallel, they will act as a single speaker with half the impedance. If you wire 2 speakers in series, they will act as a single speaker with twice the impedance. Amplifiers will specify the minimum impedance that they are capable of driving. Amplifier power ratings must mention the speaker for that rating. Many amplifiers are rated to deliver higher power to 4 ohm speakers than to 8 ohm speakers.

Dispersion

Speakers have different dispersion patterns. Some focus the sound energy in the direction that the speaker is facing, and some are designed to spread the sound out, so that listeners who may be off the axis of the speaker can hear the sound almost as well as a listener whom the speaker is directly facing. Speakers with a wide dispersion pattern will also generate more sound reflecting off the walls, ceiling and floor.

Subwoofers

In the early days of home audio, speakers were full range; designed to produce the full range of frequencies. Over time, several trends changed the way speakers are designed and installed in consumers’ homes. Experts recognized that humans are less sensitive to the direction of the source of low sound frequencies. Companies capitalized on this by providing a single subwoofer for low frequencies, and smaller “satellite” speakers for the mid and high frequencies. One of the first popular designs was the Bose Acoustimass 5, launched in 1987. This subwoofer + satellite design made speakers less obvious and easier to place in a home without rearranging the furniture. In the mid-90’s when surround sound was introduced, the subwoofer + satellite design became the dominant trend, as placing five large, full-range speakers was much less practical and feasible than moving the responsibility for the low frequencies to a subwoofer, enabling smaller speakers to handle the left, right, center, left rear and right rear channels. As we’ve evolved from 5 to 7 and more channels, the subwoofer + satellite design has become fairly standard. Still, many audiophiles and music enthusiasts want their main left and right speakers to be able to handle the full frequency range without any assistance from a subwoofer, to ensure the most accurate reproduction from the highest highs to the lowest lows.

Low Frequency Effects (LFE) Channel

Surround sound formats usually include a Low Frequency Effects channel, containing the low rumbles of explosions, collisions, vehicle engines, and other scary or violent things that happen in movies. The sound from the LFE channel is usually mapped to the subwoofer channel, if it is configured in the AV Receiver or Audio Processor, but it will be mapped to the other speakers if a subwoofer channel isn’t configured.

Levels and Types of Surround Sound

In the beginning, there was monophonic sound: a single channel. In the late ’50s stereophonic LP records were produced, and technical advances in recording, manufacturing and playback were made over the next decade leading stereo LP records becoming the dominant format for music distribution. Stereo cassette tapes and 8 track tapes were somewhat popular, but in the early ’80s the compact disc was launched, and it soon dominated music distribution.
Home video distribution started with VHS and Betamax tape, and these formats included stereo audio. Dolby developed the Dolby Digital (AC-3) surround sound format in 1986, but it wasn’t until the introduction of DVDs in 1996 that consumers could get an inexpensive, accurate surround-sound format in their homes. Dolby Digital led to a revolution in amplifiers, receivers and speakers, as consumers recognized that their home video experience was much better with surround sound. Dolby Digital supports up to 6 discrete channels, and these are normally organized as 5.1 channels, with 5 full-range audio channels (left, right, center, left rear and right rear) and a subwoofer channel (denoted as .1).
 
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