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EXPERT OPINION: Home audio, with Jill Linz
Photo by Mark McCarty
Q. What is at the heart of a good home audio system?
A. For bona fide audiophiles, there is nothing like a good component system. A good amplifier and good speakers make all the difference in the world. A typical component system consists of a preamp, an amp, and speakers (in two channels for stereo systems). The music source -- whether a CD, MP3, tape, radio, or old-fashioned vinyl record -- starts out as a low-amplitude (that is, quiet) signal that is fed into the preamp, where it is slightly amplified; then through the amplifier, where the signal is amplified fully; and from there to the speakers. Many off-the-shelf home systems are integrated into one unit, usually with a CD player and radio receiver as well as the preamp and amp. With a separate-component system, you can include a player for music from different sources, such as that favorite old cassette or album (though some of the higher-end all-in-one units also have additional inputs for these). The amplifiers in standard units are often not as strong as in the component systems, and they are always solid-state, usings transistors to amplify the signal. By contrast, amplifiers with tubes (yes, like the tubes in your grandparents’ old TV) cost more but generate a warmer, fuller sound.
My own audio system pumps 150 watts per channel. In addition to the preamp, it has an equalizer where I can control the amplitude of specific frequency ranges, which helps me adjust for the room the music is being played in.
No matter how good or bad the system is, the room you are listening in can make a big difference. A large open room with high ceilings can tend to sound bassy, while an overly small room with lots of padding in the form of carpet, furniture, etc., can absorb too much of the sound. This is something that you can control by adjusting what is or isn’t in the room. The problem in most rooms is too much reflection. Different materials reflect or absorb different frequencies—for instance, typical Sheetrock used on walls absorbs low-pitched sounds but reflect higher frequencies.
Q. What about speakers?
A. Most people place a speaker on each side of their system, often facing straight out. This is the worst possible setup, since the sound waves coming out of the speakers can mix and accentuate or cancel out certain frequencies. Theoretically if you could hang your speakers from the center of the room, that would give the best coverage. But who wants their speakers hanging in the center of the room? The next best place would be in a corner, at the midpoint from floor to ceiling, facing at a 45-degree angle toward the center of the room.
Size does matter. You must have large bass speakers in order to truly hear and feel those deep bass sounds. Really good speakers will contain a midrange speaker, a large bass “woofer,” and a small “tweeter.” When the signal is sent from the amplifier, it is a combined signal containing all the frequencies. As signals are sent from the amp to the speakers, filters block certain frequencies, sending only the midrange tones to the midrange speaker, the low to the woofer, and the high to the high to the tweeter. Once all of these frequencies enter the air, they again combine into one complex sound wave.
Except in a professional environment, this type of speaker is disappearing. Ear buds, tiny computer speakers, and “bookshelf” speakers are replacing them. In the same way, separate-component systems for the home are giving way to smaller and smaller units. The iPod seems to be taking over the home audio world.
Q. How can iPods work in such miniature?
A. The secret to the small size of the iPod lies in the way the music is stored. With old-fashioned analog media such as a tapes and LPs, the signal is stored exactly as it was produced in real life. On CDs, that signal is stored digitally. For digital recording, the amplitude of the signal is sampled every 1/44,000th of a second. How quickly or slowly this amplitude changes is a measure of the frequency at that point, and this information is translated into binary code and stored on the CD. There is a minimal amount of loss in this process. The iPod’s MP3 (or newer MP4) format re-samples the digital signal and stores it in groups of frequencies. What comes through the earphones is an approximation of the original signal. This format is getting better, but when an MP3 is played on a component system, it still sounds muffled -- the subtleties in a musical piece are often lost -- in comparison to the CD version.
Most people don’t hear the difference because they are listening to the music through ear buds or small computer speakers, which compensate by relying on something called “virtual tones.” Since small speakers can only produce high frequencies, MP4 code provides the higher harmonics of the bass tones. If a tone of 100 Hz is present in the original signal, the MP4 code will contain 200 Hz, 300 Hz, 400 Hz and so on, which our brains recognize as a harmonic series and respond to by “adding in” the missing 100 Hz tone. Through earbuds, psychoacoustics allow us to “hear” the bass tone that is not physically present.
A self-described music and physics geek, Jill Linz teaches acoustics and other physics courses at Skidmore. She enjoys window-shopping for audio gear at www.sweetwater.com.