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Frequency Response Variation in Allison CD-7

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Folks like Snell and Revel were known for testing production speakers against a design reference to better guarantee that speakers would sound like they were supposed to. But such consistency was never baked into earlier speakers, even those we still regard highly today.

As I've been shuffling surplus speakers around, to decide which I would be keeping and which selling, I decided to put a pair of Allison CD-7s along one living room wall, one of whose positions has always been troublesome because of close reflective surfaces. But this one sounded especially bad during the Audyssey sweep. Was it the crossover? Was it the age of this 1980s vintage speaker? Was it just the fact that speaker builders back then didn't especially concern themselves with variations in drivers and components?

I decided to run a simple test. Yes, I know these were not laboratory conditions, and I can certainly do better, but this was a quick-and-dirty. With each speaker about 20" from the back wall, I placed a microphone at tweeter height about 18" away. I fed a series of sine wave tones generated by Performance Audio Tone Generator Pro through a Denon 2802 receiver. I used a simple plug-in mike to my iPod Touch, using JL Audio's SPL meter for readouts. With just one speaker running at a time, here's what I got:

Frequency Speaker A Speaker B

100 37.3 35.9

250 40.3 43.7

400 48.9 48.5

1k 56.6 53.7

1.6k 49.6 54.4

2.5k 48.4 38.0

10k 55.0 49.7

WHITE 75.0 71.6

PINK 85.1 81.8

So Speaker B - the one that sparked this investigation - registers a bit more bassy at 250 (room reflections due to different placement perhaps), but notably down at 2.5k - just above the 2000 crossover) and also down in the upper treble. White and Pink noise results are also lower for Speaker B.

Assuming these results are replicated under more precise conditions, what conclusions might be drawn? Does the lower tweeter performance suggest the crossover cap be replaced? Or is this just sample variation? Obviously if there is that much variation between the two, it's not surprising the Allisons don't image as well as my Outlaws, my old Celestions or my new Revel M22s.

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I first suggest that you absolutely make sure that you located each speaker in the same place and fed it the same signals from the same channel. That way, you eliminate any boundary-related issues and any issues with your amp, preamp, or source generator.

Assuming you measured correctly, this situation is most likely a problem with driver aging, but the crossover might be having problems, too. A multiple malfunction, so to speak. The pink and white noise broad-spectrum level differences would probably not be related to capacitor deterioration.

I suggest you swap the tweeters and see if the differences change places above 2 kHz. If so, then one tweeter might be having problems (or perhaps even both tweeters, given their respective ages). You could also do the same thing with the woofers. That will cull out any anomalies with the crossover.

Howard Ferstler

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Using sine waves in a "live" enviroment (vs anechoic) AND in two positions makes your results more a function of your test environment than the speakers under test.

I'd bet you'd get MUCH better correlation/matching if you repeated the test using one location and the same amplifier channel (amplifiers can have different frequency responses between channels).

I'd also suggest using 1/3 or 1/2 octave filtered pink noise (as opposed to sine waves). The "narrower" the pink noise, the more accurate the results at/near the crossover frequencies. Readily available on test-cds.

As far as "imaging" is concerned, Allison acoustics never placed much importance on this quality. Instead, the priority was very broad dispersion at all frequencies. To get good "imaging", dispersion has to be tightly controlled, to get some degree of "beaming".

"Intentional beaming" minimizes interference from nearby objects/boundaries, but also limits where one can sit to get the desired results. Allison loudspeakers emphasize uniform "power response" in the "far field". Speakers that image well (Snells) are meant to be listened to in the "near-field".

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