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Measurements and a strange room


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In some of the other posts here and abouts there are several discussions on the validity of measurements when discussing power requirements for a given set of speakers in a given room.

Whilst I am not going to enter that fray I will just offer the following rather odd measurements I took recently with Tony at a fellow audiophile's house:

Just as a bit of background the house in question has some rather large, rather old tannoy speakers that I understand to have sensitivity of 96 db/w/m. The guy is running a 45 wpc tube amp and according to Tony at a distance of 18 feet from the speakers they were hitting 113 dB.

This is obviously BS and I told Tony such. We got into an arguement that was settled on a visit of the 2 of us to the house in question.

Turns out - at least according to Tony's digital SPL meter peaks were indeed hitting 113 with an average of about 106. Doesn't matter how you calculate it - on the face of things this is impossible if you crunch the numbers.

However - here we were experiencing it. Now it could be that Tony's meter was off - but we have used it before and judging by the volume in my ears I do not think it was that far out.

What I do think was happening was the effect of a slightly strange room. At one end it was about 10 feet wide. At the other end it was nearer 15 feet - maybe more. The room was very long - guess over 30 feet and I guess it was acting rather like a Horn. Going from about 6 feet away to 18 feet away made no discernable difference to the SPL.

In a nutshell - the measurements and calculations we use are not wrong - but they may not apply in all cases due to the vaguaries of room design. First time I have experienced it personally - but there you go.

Of course I ended up with egg all over my face an a nice little victory for Tony - but at least he didn't rub it in - too much.....

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Standing waves in the room can cause a dramatic increase in output - some claim as much as 10dB...

You also get as much as 6dB by corner loading and another 6dB by having

two speakers in phase. Usually you will realize more like 8dB from the

corner loading and summing of the speakers. It is very common to see

6dB swings from standing waves, so that's like a total of 10-14dB that

can be added to the rated sensitivity of the speaker (10dB if they were

already measured in a corner, and 14dB is measured free-field).

The dimensions of the room however do not lend themselves well to any

kind of "hornloading". There isn't any amount of constriction of air to

increase the power transfer.

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Interesting Mike - that could be it - I was reaching with the horn effect room.


Hard to tell on the distortion levels as I am not accustomed to such volumes but it only seemed to hurt the ears through volume - no obvious clipping apparent.

I dont think the amps in question have enough headroom to do 80 wpc anyway - but I cannot swear to it.

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