Time-based errors are those that affect pitch and tempo. When playing an LP record whose hole is not perfectly centered, you’ll hear the pitch rise and fall with each revolution. This is called wow. The pitch instability of analog tape recorders is called flutter. Unlike the slow, once per revolution pitch change of wow, flutter is much faster and adds a warbling effect.
Digital recorders and sound cards have a type of timing error called jitter, but the pitch deviations are so rapid they instead manifest as added noise. With all modern digital audio gear, jitter is so soft compared to the music that it’s almost always inaudible.
The last type of time-based error is phase shift, but this too is inaudible, even in relatively large amounts, unless the amount of phase shift is different in the left and right channels. In that case the result can be an unnaturally wide sound whose location is difficult to identify.
Room acoustics could be considered an additional audio parameter, but it really isn’t. When strong enough, acoustic reflections from nearby boundaries create the comb filtered frequency response described in Chapter 1. This happens when reflected sound waves combine in the air with the original sound and with other reflections, enhancing some frequencies while canceling others.
Room reflections also create audible echoes, reverb, and resonance. In an acoustics context, resonance is often called modal ringing at bass frequencies, or flutter echo at midrange and treble frequencies. But all of these are time-based phenomena that occur outside the equipment, so they don’t warrant their own category.
Another aspect of equipment quality is channel imbalance, where the left and right channels are amplified by different amounts. I consider this to be a “manufacturing defect” caused by an internal trimmer resistor that’s set incorrectly, or one or more fixed resistors that are out of tolerance. But this isn’t really an audio parameter either, because the audio quality is not affected, only its volume level.
The preceding four parameter categories encompass everything that affects the fidelity of audio equipment. If a device’s noise and distortion are too soft to hear, with a response that’s sufficiently uniform over the full range of audible frequencies, and all time-based errors are too small to hear, then that device is considered audibly transparent to music and other sound passing through it. In this context, a device that is transparent means you will not hear a change in quality after audio has passed through it, even if small differences could be measured.
For this reason, when describing audible coloration, it makes sense to use only words that represent what is actually affected. It makes no sense to say a power amplifier possesses “a pleasant bloom” or has a “forward” sound when “2 dB boost at 5 KHz” is much more accurate and leaves no room for misinterpretation.
Chapter 1 explained the concept of resonance, which encompasses both frequency and time-based effects. Resonance is not so much a parameter as it is a property, but it’s worth repeating here. Resonance mostly affects mechanical transducers—loudspeakers and microphones—that, being mechanical devices, must physically vibrate. Resonance adds a boost at some frequency and also continues a sound’s duration over time after the source has stopped. Resonance in electrical circuits generally affects only one frequency, but resonances in rooms occur at multiple frequencies related to the spacing between opposing surfaces. These topics will be examined in more depth in the sections that cover transducers and room acoustics.
When assessing frequency response and distortion, the finest loudspeakers in the world are far worse than even budget electronic device. However, clarity and stereo imaging are greatly affected by room acoustics. Any room you put the speakers in will exaggerate their response errors further, and reflections that are not absorbed will reduce clarity. Without question, the room you listen in has much more effect on sound quality than any electronic device.
However, the main point is that measuring these four basic parameters is the correct way to assess the quality of amplifiers, preamps, sound cards, loudspeakers, microphones, and every other type of audio equipment. Of course, to make an informed decision, you need all of the relevant specs, which leads us to the following.
“The Audio Expert” by Ethan Winer, published by Focal Press (ISBN: 9780240821009), is available here.