Measuring Loudspeakers, Part Three:
Page 4 By contrast, Fig.25 shows a similar measurement for a loudspeaker that a Stereophile blind listening panel thought sounded quite colored, with a skewed presentation of instrumental timbres. The speaker is quite flat in the midrange and has reasonably good bass extension. However, there is a severe suckout in the mid-treble. In all but large rooms, this will produce a characteristic, hollow-sounding coloration. There's some peakiness in the mid-treble region above the suckout that will add a little bit of "zing" to the perceived balance. The designer presumably was trying to compensate for one with the other, as the mean tweeter level is about 5dB below that of the woofer. I conjecture that the designer was probably bothered by the peak at 3.5kHz, which would make the speaker sound bright, all things being equal. Thinking "brightness" correlated with too much tweeter energy, he padded down the tweeter. This was to no end, however, because the real problem appears to be a vicious resonance or breakup mode in the woofer cone. (This can be seen in fig.19 on p.85 of the December '98 issue.)

Fig.25 MLS-derived response of a poor-sounding loudspeaker at 50", averaged across a 30 degrees horizontal window on the tweeter axis, and spliced to the nearfield LF response.

The bargraph in Fig.26 tabulates the quasi-anechoic response measurements of 320 of the loudspeakers that I have measured since that 1991 article. Again they are grouped by the standard deviation of their responses over the two decades from 170Hz to 17kHz, weighted to compensate for the fact that an FFT-derived amplitude response has a linear frequency spacing, but is conventionally plotted on a logarithmic frequency scale. The mean deviation from a flat response is 3dB, with only a few loudspeakers having a weighted standard deviation of 1dB or less. On the other hand, only a few loudspeakers have weighted standard deviations greater than 5dB. The overall level of attainment featured by loudspeaker designers is quite good, at least regarding flatness of on-axis response.

Fig.26 320 loudspeakers, range of frequency-weighted standard deviations, 170Hz-17kHz.

Table 1: Frequency-weighted response standard deviation vs recommendation in Stereophile's "Recommended Components."

Response Standard Deviation	Number in Class	Number Not Recommended in Stereophile
1.0	15	1	7%
1.5	39	2	5%
2.0	73	8	11%
2.5	56	8	14%
3.0-4.0	70	20	29%
4.0-5.0	32	14	44%
>5.0	33	14	42%

Is there a correlation between this arbitrarily chosen measure of response flatness and whether or not a loudspeaker sounds good—or at least gets a good review in Stereophile? Looking at the raw data used to compile Table 1, there appears to be no correspondence between flatness of on-axis response and price. But Table 1 does demonstrate that it appears that the chance of being recommended by this magazine increases the flatter a loudspeaker's on-axis response becomes.

However, while flatness of midrange and treble response is a good thing to have in a loudspeaker, it doesn't in itself mean that the speaker will sound good. The model with the overall flattest response in fig.26 was not recommended by the magazine because of problems in other areas of performance. On the other hand, once the response flatness deviates above a certain level—a frequency-weighted standard deviation between 170Hz and 17kHz of approximately 3.5dB, for example—it's unlikely that the speaker will either sound good or be recommended. Exceptions are: a) if the speaker is very cheap or its errors are in a relatively innocuous part of the frequency spectrum or b) if the standard deviation is high because of an overall smoothly tilted response; or c) if the speaker is otherwise superb in almost every respect.