July 19, 2013

Altec A7 Voice of the Theatre horns measured

How well do those old Altec Voice of the Theatre horns measure? Here I put some Altec A7 horns to the test. The passive crossover was not available for testing.


 Tested:

Compression driver: Altec 908-8A 8 ohm
Horn:  Altec 511B horn
Midbass driver:  Altec 416-8B
Cabinet: Altec A7

It should be noted that these are not the drivers used in the original Altec VOTT A7. The diaphragm on the compression drivers is most likely a replacement different to the original. 






First the A7 cabinet was tested for frequency response. This is a half space measurement in less than ideal conditions. The horn was sitting in front of the garage in which it had been stored. The mic was placed at the mouth.


  

The response apart from this peak, is +/- 4 dB below 500 Hz. This cabinet was used up to 800 Hz in the original A7. The peak at 200 Hz suggests a poor horn/driver combination. It could be related to the wrong driver being used, or one that has suffered deterioration. This problem may not be present in all of them. 

Bass extends to around 60 Hz. In its intended application, this speaker would have been placed against a wall in a commercial theatre, providing additional low frequency reinforcement.

The compression driver is intended to operate above 800 Hz. However, in the testing a cap was used that I had on hand. Its value was less than ideal and so the extension of this horn/driver combination isn't represented in these measurements. 

Compression driver/horn gated frequency response:


There is a major problem with the horn. The response is reasonable flat at +/- 3 dB below 4.3k but then there is a major dip that is 45 dB deep. This hole exists for almost an entire octave. The top end extends only to 12k. The frequency response is very problemmatic and we can see that it isn't really suitable for music reproduction. This particular combination should be considered a vocal PA system at best. 

This measurement most likely shows a problem with the driver.

The horn was elevated on my directivity turntable and measured from 0 to 90 degrees. 

The HF horn had a serious problem with ringing and required damping. Knocking on the horn caused it to ring like a bell for an extended period. A steel horn requires significant damping but none was used here.

Directivity sonogram:

 
Horizontal dispersion was measured. From 1k to 6k, constant directivity is achieved ~ 90 degrees. Above this point the pattern starts to narrow before severe beaming occurs above 8k. Below 1k pattern control beams due to the inadequate mouth termination. Since this plot has been normalised, we are simply looking at the dispersion behaviour. The major hole centred around 5.6k is not evident in this measurement as a result - it does not affect the dispersion.

This is a quick snapshot of one particular speaker that might be called the Altec A7 although this would be a little unfair, given the use of the wrong drivers. The results suggest problems with both drivers. This should serve as a warning to anyone considering buying them. These speakers are typically sold without any data to confirm their performance. The measurements don't represent all of them.












5 comments:

  1. Only wish I had the same skills to and tools to test audio equipment as such a in depth level. Awesome stuff.

    Thanks Paul.

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  2. It looks like a modern waveguide would be a great upgrade to this setup.

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  3. The 802 8G was the hifi compression driver from memory, the model tested here was for PA. It is true the A7 was not designed for anything serious below 45hz and therefor for ground shaking effects a sub is required so while I have never tested mine I have trouble listening to other speakers due to their poor dynamics and poor crisp response and with two beefy subs crossed over at 40hz I still love mine. Very interesting. The horn ring was also recognised back in the 70's although I cant here it during playback or dont notice / detect it.

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  4. First. Thanks.
    Second. Can't write notes.
    Getting bug

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  5. Listen to a good pair.
    Trane, Davis, Hendrix
    Incomparable.
    Had them since 1970.
    Trouble writing
    But set up right -wow

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