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AR3a Measurements by Dennis Murphy

Guest amerchant

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>For the 3a fans out there - Dennis Murphy has, on his website,

>measurements that he made on the 3a. Lots of other

>interesting stuff on his site as well.







This is an excellent example of speaker-measurement confusion. It is an attempt by the tester to seem scientific and objective, yet the procedure appears flawed. There is no reference as to how the measurements were made, if gated, actually anechoic, or outdoors. There is no mention of microphone position, distance or measurement type. The biggest problem, however, is that the curves appear to show gross interference effects from reflections from cabinet-edge molding, floor and wall boundaries and so forth, rather than the true performance of the speaker itself. These interference effects appear to color the response measurements in both the anechoic- and the room-measurement charts shown. I am guessing that an ungated microphone was positioned about 1 meter on-axis with the middle of the speaker for both the anechoic (likely done outdoors) and room response (indoors). To demonstrate how this can be almost meaningless, all one has to do is move the microphone a few inches in any direction and the wild dips and peaks move around in frequency.

The woofer response only vaguely resembles any response measurements done on the AR-3a by AR, NHT for SR Magazine, Hirsch-Houck Labs or CBS Labs. The AR-3a woofer, when measured properly, is +/- 1.5 dB from 38-575Hz, and there are no wild 10dB swings in the response as shown in these measurements. The tester most likely accurately measured boundary reflections. Furthermore, the “big hump” described by the tester starts at 600Hz and peaks at 800Hz, and this is outside the operating range of the AR-3a woofer. It is likely mutual-radiation effects or interference effects, both of which will be swamped in the far field. It could also be the midrange if the control was set to maximum, as there is a jump in response at crossover if the control is full clockwise.

This is precisely why AR developed the reverberant test chamber, in order to capture the total acoustic energy of its speakers in the reverberant field. The performance of each individual driver at AR was measured precisely on- and off-axis in anechoic chambers in the usual fashion, but the acoustic-power response of each speaker system has to be measured in the far field, either by using a thousand measurement points or, more practically, by using the reverberant chamber. This is far more representative of the speaker’s sonic accuracy.

Therefore, unless you listen to music on your AR-3as from a distance of 39.37 inches away, those response curves (and the tester’s editorial comments) are not representative of the AR-3a’s performance, and mean little to the final sound of the speaker.

--Tom Tyson

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