How to choose audio equipment

The most important specifications

• Frequency response measures the range of tones that your equipment is able to reproduce. Most people are able to hear tones as low as 30 Hz and as high as 15,000 Hz, though many can hear 16,000 - 18,000 Hz and a few 20,000 Hz or higher. If you are buying your equipment to play back or record music, you will find a wider range makes a noticeable difference in the quality of what you hear. On the other hand, if you’re buying a cassette deck mainly to do recordings of people speaking, you’ll find such ranges unnecessary, as most voices, in speaking, span a range of only 200 - 2,000 Hz.

• It’s important to note that the frequency response of an audio system is limited to its weakest component. If you use a cassette recorder that produces a 20,000 Hz high tone with a microphone that will do no better than 10,000 Hz, you’ve wasted the extra quality of the recorder.

• Signal to noise ratio measures the most intense video or audio signal the piece of equipment can reproduce versus its background noise level. An S/N ratio of 58 dB, for example, means there can be up to 58 decibels of signal for every decibel of noise. Higher ratios mean a cleaner signal, and this will be particularly important in situations where you need to reproduce a pure tone interspersed with silence. (Thus someone listening to someone speaking will be much more aware of equipment noise than someone listening to music.)

• Total harmonic distortion measures all of the unwanted changes in frequency (tone) in a recorded signal made by the recording or playback device.

• Wow and flutter measures unwanted variation in the speed of a tape recorder or record player, which shows up as changes in pitch. Wow and flutter exists in CD players, but is so low as not to be measurable.

• Dynamic range is the ratio of the loudest to the softest sound that can be reproduced by a piece of equipment. A wide dynamic range will produce a more lifelike sound only in applications, such as recording classical music, where there’s a large variation in volume.

• Impedance, expressed in ohms, measures electrical resistance in an AC system. There was a time when impedance was important to selecting a microphone, but today, virtually all audio and video equipment have low-impedance inputs. The only reason to buy a high impedance mic (or a switchable mic) would be to use with older equipment.

  For headphones, impedance still plays a role. Headset jacks are 8 ohms. You’ll get better volume with a lower-impedance headset if used alone, but if you have listening centers, you need 600-ohm headsets, as grouping 8-ohm sets on a listening center can damage the amp in the source you are using.

In a cassette deck, frequency response, signal to noise ratio, and wow and flutter are the most critical measures of quality.

In an amplifier, frequency response, signal to noise ratio, dynamic range, harmonic distortion, and maximum output per channel are the critical measures of quality.

In a loudspeaker, frequency response, freedom from distortion, and maximum power it can accept from an amplifier are critical. When good specifications are absent, it’s reasonable to expect larger speakers to sound better. The best speakers use a crossover circuit to divide the frequency range: a two-way speaker uses a woofer (for low-frequencies) and a tweeter (for high), a three-way a woofer, midrange and tweeter.

Powered speakers have an amplifier built in, the more commonly-used unpowered speakers do not. You’ll want a powered speaker for portability or to use in cases where an amplifier would be expensive or inconvenient (such as with a personal computer or an engineer’s station in a studio).

In a CD player, sampling rate is the key quality indicator, since the frequency response of all players is consistently high and distortion almost nonexistent.

In nature, and in analog systems, sound is produced as a continuously varying set of tones. A digital system, however, can only capture discrete tones and reproduce them one at a time. The sampling rate denotes the number of discreet "samples" played per second. A higher rate is better, though many people will never notice the difference between high and low sampling players.

In a microphone, frequency response, pickup pattern, and whether or not the mic is powered are the critical specifications.

You’ll most often find omnidirectional mics, which have a spherical pickup pattern, and unidirectional mics, which are intended to focus on your speaker or vocalist. Surface-mounted mics have an extremely wide pickup pattern and are used to mic two or more people. Shotgun mics are simply unidirectionals with very tight pickup patterns.

Dynamic microphones are unpowered, but condenser mics use a battery or phantom power from the mic mixer. If you want to mic a live speaker, you’d be most likely to use a dynamic mic, because they are easier to handle and less costly. If you were recording the speaker, however, or recording music, you’d want to use a condenser mics, because they reproduce sound more accurately.

Because of cost concerns, most stationary mics are wired, but a wireless system is advantageous if your speaker will be moving around your stage or set. In a wireless system, the critical specifications are available frequencies (UHF offering better range, less interference, better frequency selectivity and better audio than VHF) and whether or not the system is using diversity antennas (which minimize radio frequency noise).

A mixer controls the audio inputs going into your sound system. It can switch between inputs, control the volume of each input, and control the outputs to which each input is sent. In a mixer, the number of inputs and outputs, whether the unit is powered or unpowered (that is, whether it has its own amplifier built in), its control features, whether it has phantom power, and its sound quality (shown by frequency response and signal to noise ratio) are all critical.

An automatic mic mixer is a specialized item used in conferences where a large number of mics are present. An automatic mixer turns mics on as people begin speaking, thus minimizing feedback and background noise which would result from having a large number of mics open.

Useful features
Beyond the basics, you’ll want to consider if your audio equipment includes:

• Multiple inputs and outputs. It’s very important, for example, that an amplifier has the connections you need for the components you plan to use it with, or that a cassette recorder the microphone or auxiliary jacks you need for the uses you intend.

• Headphone jacks, which will be important anywhere people need to listen privately–for instance, with students in a classroom or library.

• PA capability on a cassette deck or record player allows you to use a microphone and the unit’s amplifier and speaker (or external speakers) as a public address system.

• Equalization fine tunes your sound system’s frequency response to a given room. You can use it to adjust bass, treble and midrange to provide the most pleasing sound and also to minimize feedback.

• Reverb is sometimes used to provide artificial echo effects, but it’s real purpose is to provide extra depth and clarity to your sound.

• Delay controls when the sound gets from the amplifier to each set of speakers. It should be used whenever you have multiple speakers in different places in a room, to ensure that the sound from each speaker reaches the audience at the same time.

• Compression, or audio limiting circuitry, limits the volume of input sound to what the system can handle, thus helping to provide an even output sound and to protect the equipment from damage due to overload.

• Variable speed playback allows you to speed up or slow down a cassette or record. You would use this in a dance class (usually to slow down the music to allow beginners to keep up with it), in an aerobics class (to increase your exercise tempo), or in a music class (to fine-adjust the pitch of what you’re playing).

  
  

Beyond the specifications
There are some comparisons that are very difficult to make from a spec sheet. You’ll want to talk to your United account manager to get a feel for:

• Reliability. Will the component or system work through a critical presentation? Does its manufacturer have a good reputation? What’s the warranty? Is it UL listed for business or educational use?

• Serviceability. How long will you have to do without your system if it breaks? How difficult is it to get parts?