The thread is here, for members: https://www.harbeth.co.uk/usergroup/threads/measuring-the-bass-output-of-loudspeakers.80244/
After the first post, when Shaw said, among other things:
I asked, immediately, what he thought of the Klippel system. For some reason this question was killed by the moderator (or Shaw himself). Then:
Thoughts?
After the first post, when Shaw said, among other things:
Here is another subject that you won't read about in audio forums, but is of paramount importance.
Once upon a time when engineers ruled rather than marketing people, and facts were respected as facts, engineering budgets were directed at the construction of anechoic chambers or dead rooms. These 'free-field' environments were for all practical purposes reflection (echo) free, and when a loudspeaker was measured in such chamber it was possible to say with a high degree of confidence that the frequency response measurement was that of the speaker alone, not a mix of the speaker and the environment in which it was being tested.
Sadly, the engineers lost the argument, and now magazines such as Stereophile and other have created their own methods for measuring loudspeakers 'on the kitchen table' or reviewer's back yard. The acoustic conditions there clearly are far from echo free, and hence, the measurements of tested speakers are necessarily corrupted by echoes from the environment. The human eye is not capable of 'removing' the echoes from a frequency response plot: there is no practical way of accurately removing the corrupting effect of the reflections from a wide-band measurement made in a reverberant space.
But speaker designers face the same problem as the technical reviewer does. I wrote to John Atkinson of Stereophile some years ago draw attention to my observation that most of his review frequency response curves showed a significant boost in the low frequencies which appeared to be consistent and therefore related to his methodology, and not really present on measurements made under truly free-field conditions. I suggested why it might be occurring. No reply was forthcoming, but this issue of excessive bass in Stereophile speaker measurements seems to be well known in the speaker industry. I didn't pursue it as I have the greatest sympathy for those trying to make accurate - or even repeatably inaccurate - technical measurements of loudspeakers without a handy anechoic chamber. It's an issue that dogs all objective speaker designers who must know the relative energy balance between bass. mid and top in their new designs. That's because the bass output of the speaker is the pivotal point in the audio spectrum against which the ear judges the overall correct balance.
Over the last twenty years, I have lost track of how many hours I've invested in this subject of making-do without a real anechoic chamber. I believe that there are just a handful in the world now, and it's not relevant that Microsoft Corp. or other large corporates have a huge ones of impressive performance as they're not available to the audio industry. Ditto those used by defence contractors and (I assume) those likely to be operated by universities for teaching and research, and even audio-centric businesses such as Dolby or Apple are not going to share their resources with speaker tinkerers. So what can we do?
Well, some thirty or so years ago it was suggested that the measurement of the entire bass, mid, top frequency response of a loudspeaker system could be broken down into a measurement of the bass output with the microphone very close to the woofer, and then an additional second measurement of the mid/HF with the mic 1m or so away. This would allow 'windowing' of the mid/high frequencies to catch them before their echoes arrived at the microphone, and by placing the mic close to the woofer, the long-wavelength, long period low frequencies could be measured with enough energy that the inevitable echoes would be swamped. So that opened up the possibility of measuring speakers in relatively ordinary non-anechoic rooms, just as Stereophile does in the back yard.
But - big but: different techniques for measuring the bass output of a speaker yield significantly different results when there can be only one "anechoic" truth.
I've been working on this subject again over the past few weeks, and here are a group of measurements made on the bass output of the same speaker within minutes, at 20 degrees C, using several different techniques, a lot of work. I've overlaid the many different measurements into one graph, below.
Which one do you believe? BTW: one of the curves was the output of a laser pointed at a shiny spot on the cone. Can we assume or not that the no-contact laser method must (surely) give the 'truest' results given the very high financial investment?
Sorry but considering that making these measurement, post processing them and graphing them has taken around 15 hours, I have obscured the precise details for public viewing.
I asked, immediately, what he thought of the Klippel system. For some reason this question was killed by the moderator (or Shaw himself). Then:
All measurement systems, including the Klippel one that do not make use of a real, physical anechoic chamber have to mathematically manipulate two frequency response measurements: one made very close to the woofer (say, 5mm away from the dust cap - almost touching) and another made, say 1m away (the standard anechoic distance from speaker to mic).
All these twin-measurement approached rely on combining these two very different measurements into one wideband 20Hz - 20/40kHz conventional anechoic frequency response measurement. The maths is simple - too simple - and in the many published papers it relies on finding a single frequency where the two curves can be cut and past together, typically around 200-300Hz. But as we'll see, that is a gross simplification of a much more complex issue.
In my long experience with this matter, the outcome for the cut and paste can be whatever you want it to be according to the frequency you chose to make the junction between the measurements. If you have a wideband anechoic-chamber measurement of the same speaker (at 1m) you can literally printout and slide your two DIY measurement curves up and down in level and frequency until you have picked a transition frequency which visually matches the overall real anechoic curve. But if you didn't have that anechoic curve as a guide, should you give ten speaker engineers the near field and the far field measurements and asked them to cut and paste them into a wideband measurement, you'd get ten very different takes on at what frequency and what level to join them together. It's a fuzzy solution to a hard acoustic problem.
And that is a nonsense.
Thoughts?