Speakers interest your students. They have them in their cars, dorm rooms, apartments, and homes. The popularity of 5.1 surround sound also renewed interest in having a good system in the home. If you want to take students beyond the basics in the chapter here is additional information on monitor speakers.

If a perfect speaker system existed, the sound reproduced would be an analogous copy (exact duplicate) of the original acoustic sound. We already know this is not a perfect world, and that is why we have benchmark specifications to evaluate speakers.

However, selecting a monitor speaker is not strictly a numbers game. If it were, you could just pick the speaker with the best numbers and go home. A critical factor in how a speaker sounds is the acoustical environment the speaker is placed in. Regardless of the manufacturer, each speaker will sound different in different rooms.

Technicians test monitor speakers in anechoic chambers. The term anechoic means "without echo." Since the room is free from echo and reverberation, the only sound in the chamber comes from the speaker, and thus, the sound can be accurately measured. Anechoic chambers are designed so that the test instruments register zero sound-pressure level the moment the speaker is turned off.

Naturally, speaker test measurements taken in an anechoic chamber are taken in a theoretically "perfect" listening environment. Unfortunately, a studio does not have the same acoustical qualities and specifications as the chamber the speaker was tested and developed in. This is why speaker specifications are merely a guide to the speaker's performance and cannot tell you how the speaker is going to sound in a particular studio.

There is, of course, one other factor in selecting monitor speakers—your ears. No two people have matched sets of ears. Each person perceives the sound of even the best monitor speakers differently. The only objective guide to go by is the speaker's technical specification sheet.

Frequency Response

A speaker's frequency response is the range of frequencies that the speaker can reproduce from the lowest to the highest, the wider the range the better. Likewise, a frequency response that is as flat as possible over the frequency response range is best. Generally, a speaker with a flat frequency-response curve from 50 Hz to 20 kHz should be sufficient for most work.

Efficiency

A speaker consumes a specific amount of power to produce a specific sound-pressure level. Some speakers, by their design, consume more power to produce the same sound pressure as a speaker designed to be more efficient and use less power. There is an old myth that more power is better. For example, if 35 watts of power is good, then 70 watts are not only better, but it also will make the speaker twice as loud. The myth is wrong. The extra power provides additional headroom for the amplifier, which allows it to operate with less noise and distortion. There only is a small increase in sound-pressure output.

A power amplifier should be matched to the power range of the speaker with allowance for some extra headroom. As a rule, studio monitor speakers should be capable of reproducing from 105 to 115 dB-SPL. Most studio monitoring is done at 80 to 90 dB-SPL, and, just as with the power amplifier, the extra headroom is desirable for those times when it is necessary to turn the speakers up to hear a quiet passage in detail.

A monitor speaker matched to an amplifier with too little power will lose efficiency and, in some cases, cause the amplifier to go into clipping and distortion trying to power the speaker.

Distortion

Distortion occurs when elements of the speaker design combine or add audio frequencies to the reproduced sound frequencies that were not present in the original audio. A monitor speaker, like every component in the audio chain, can add distortion to a signal if not properly designed. There are four types of distortion to look for in speaker specifications; however, not all manufacturers publish such figures.

Harmonic distortion occurs when the drivers or crossover networks of the speaker introduce harmonics into the sound produced by the speaker. Harmonic distortion is expressed as a percentage of the total signal and labeled as total harmonic distortion or THD. Ideally, such THD should be below one percent at normal operating levels; lower is better.

Transient distortion occurs when a speaker fails to respond quickly enough to a fast attack, slow-to-release sound such as the hit on a snare drum. In serious cases, the speaker lags behind the transient, causing a ringing sound that can be quite annoying.

Intermodulation distortion is something speakers are susceptible to just by their basic physical design and operation. IM, as it is called, occurs when two frequencies coming from the speaker combine to form a third frequency that has no relation to the music or sounds coming from the monitor speaker. This often happens at the two ends of the frequency spectrum. For example, a high-pitched flute is playing at the same time a bass drum is playing and all of a sudden the flute takes on a fluttering sound due to IM distortion in the speaker. This type of distortion is very annoying. Even people who are not aware of IM distortion experience listener fatigue. Just as with total harmonic distortion, the IM distortion should be less than one percent; lower is better.

Loudness distortion is the simplest form of distortion. It is the result of sending too much power to the speaker, which overloads the crossover network and drivers. The result is that the crossover network and drivers go into hard clipping, creating a nasty, rough, metallic sound. Pushed to the extreme, the crossover network overheats and its components fail, which sends too much voltage to the drivers, causing the drivers to fail. In the worst cases, the crossover network or the drivers catch fire. Some speakers incorporate internal fuses or circuit breakers that prevent damage to the speaker components. This type of distortion is easily preventable by simply observing the manufacturer's power handling limits for the speaker.

Sensitivity

Sensitivity is related, in some degree, to the efficiency of the speaker. Sensitivity is the sound-pressure level, measured one meter directly in front of the speaker, the speaker generates when it receives one watt of amplifier power. Generally, a speaker sensitivity of 90 dB-SPL or higher is considered good sensitivity for a monitor speaker.

Polar Response

Monitor speakers have a polar frequency response chart just as a microphone does. The microphone chart indicates where the microphone is sensitive to sound. A polar chart for a monitor speaker shows you the speaker's sound pattern or the speaker's coverage angles. There are charts for the vertical and horizontal planes, and this information allows you to aim the speaker for its most effective use. As with the microphone, the term "on axis" means directly in front of the speaker.

Time Alignment

With professional monitors, one of the more critical elements of the design is time alignment of the drivers and the crossover network. Looking at the face of most professional monitors reveals that not all of the drivers are mounted flush with the face of the speaker. Typically, the woofer is flush with the face of the speaker and the midrange and tweeter are set back from the face of the speaker. This is so that the sound leaving each driver will be precisely in time, or in phase, with the other two speakers in the cabinet. Remember from chapter 2, phase is a difference in time measured in degrees. If the sound from each of the three drivers does not leave the monitor speaker in phase, then the sound is not an accurate representation of the audio.

While driver alignment is critical, so is the time alignment of the crossover network. The components chosen for each frequency range can induce time delays in the audio to the drivers.

Monitor Speaker Placement

Once a user selects a good monitor speaker, it must be properly placed within the studio's acoustical environment. The exact placement will affect the quality of sound reproduction. The guidelines offered in this chapter are general guidelines, and the mathematical calculations to determine the precise placement of a speaker within a studio are beyond the scope of the text. Usually compromises have to be made since you likely will not be allowed to move doors, walls, or the ceiling to accommodate speaker placement.

In theory, the best place for a monitor speaker is in the center of the room hanging equidistant between the floor and the ceiling. This position allows the speaker to radiate in a full sphere in all directions from the speaker. Under these ideal conditions, the sound will be the same at any point within a specific distance from the speaker. However, this is not very practical, so a compromise placement solution has to be selected.

On The Wall

Hanging a speaker flush on a wall, or mounting it flush in the wall, cuts the speaker's sphere in half and the wall reflects the sound into a hemisphere. This hemisphere, in theory, doubles the apparent sound level of the speaker. The apparent increase in volume will come in the low frequency range, typically below 150 Hz.

On the Wall, in the Corner

By hanging a speaker in a corner, at the junction of two walls, the speaker's sphere is cut to one-quarter and the walls reflect the sound into a quarter-sphere. This quarter-sphere doubles the apparent sound level of the speaker compared to a speaker hanging flat on a wall. The apparent increase in volume will come in the low frequency range, typically below 150 Hz.

On the Wall, in the Corner, On the Floor, or At The Ceiling

Placing a speaker in the corner at the junction of three surfaces, two walls and the ceiling, or two walls and the floor, cuts the speaker's sphere to one-eighth of what it was, or an eighth-sphere. This eighth-sphere, in theory, again increases the apparent sound level of the speaker compared to a speaker placed in the corner on the wall.

Professional monitor speakers have bass output adjustments and specific instructions to compensate for the increased bass and the effects of speaker placement in the studio.

As a final note on placement, do not hang or place the speaker on a stand that is just a few inches or a foot or so from the wall. Sound reflecting off of the speaker cabinet edges and the wall behind the speaker will destructively interfere with the direct sound from the face of the speaker, and this can cause a significant decrease in the total bass energy of the room. When it is not possible to mount the speaker flush to the wall or well into the room, cover the back of the speaker with such sound absorbent material as rock wool or acoustic foam. Most speaker manufacturers provide speaker-placement guidelines as a part of their instruction manual.