Loudspeaker Specifications: What Matters?

In a recent answer to a reader’s letter, I somewhat bluntly stated that Stereophile‘s reviewers use “hi-fi” adjectives to describe loudspeaker sound because even good loudspeakers are too far removed from sounding “real” to be compared directly with live music. Upon reflection, this may have sounded too dismissive, so I will elaborate a little in this short essay.

What would be the specifications necessary for a loudspeaker to sound “real?” First, it should have a bandwidth as wide as possible; with the exception of a slight HF roll-off when you sit in the back row of the stalls at an orchestral concert, what you hear live is what the instruments emitted, so let’s go for HF extension well above 20kHz at the top—40kHz, for example, giving us an octave’s margin above the limit of hearing. At the low end, “DC” is not possible, though let’s say a few Hz would be sufficient to minimize the effects of low-frequency group delay. The on-axis response should, of course, be absolutely flat.

There is still plenty of debate over whether a speaker should have a flat on-axis response or a flat power response—the reverberant response in the room should be flat with frequency, which implies a rising on-axis response with frequency in a conventional room. I would suggest that it is sufficient that the on-axis response be flat, the energy falling away smoothly as the listener moves increasingly off-axis, either vertically or horizontally, with no sharp discontinuities.

In live sound every frequency component constituting that sound reaches your ears at the same time, ie, a linear phase response. For the phase response of the loudspeaker, it is sufficient that it be identical to that of an electronic filter in that it should be minimum-phase; ie, having the appropriate relationship to the amplitude response so that flattening the latter flattens the former. For optimum amplitude stereo imaging, the loudspeaker must approximate either a point- or a line-source across the whole audio band.

Finally, measurements of the dynamics of live orchestral sound suggest that a peak spl capability of 115–120dB, unweighted, would suffice. Regarding non-linearity, let’s specify under 1% THD and IMD across the audio band at 96dB average levels.

There isn’t a speaker yet designed that comes anywhere near these specifications!

But don’t despair. By choosing a set of reasonable compromises, a talented designer can end up with a loudspeaker that satisfies musically on an individual basis. The user just mustn’t forget that every area of reproduction has necessarily had to be compromised somewhat in order for the listener to be able to afford to buy something other than a Wilson WAMM or Infinity IRS V.

With a loudspeaker intended to sell for under $1000/pair, the compromises are of necessity greater than usual and it takes a designer of genius to provide a model offering an all-round musical performance in this price range (footnote 1).

This short article was published in January 1989. Later that year I started using DR Labs’ MLSSA system to measure loudpeakers, which I discussed in a February 1990 article. I have been using MLSSA ever since, the result being a unique archive of more than 900 loudspeaker reviews where the measurements are presented in a consistent format. An article comparing the measured frequency responses for the first two years in which I used MLSSA can be found here and “Getting the Best From Your Loudspeakers,” an article on loudspeaker set-up, here.

In 1997 I presented an AES paper on loudspeaker measurements, based on my experience. This paper, split into three parts, can be found here here, and here. A related article can be found here.

Two videos in which I discuss audio measurements in general can be found here and here.

Footnote 1: Stereophile‘s founder J Gordon Holt wrote two articles on Subjective Loudspeaker Testing in the 1960s.

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