From: DPierce@world.std.com (Richard d Pierce)
Subject: Re: Amp/speakers cut out -- Why?
Organization: The World Public Access UNIX, Brookline, MA
Date: Wed, 9 Sep 1992 14:56:27 GMT

In article <1992Sep8.144559.5402@julian.uwo.ca> mjscott@valve.heart.rri.uwo.ca (Mike J. Scott) writes:
>Hi,
>   Perhaps one of you rec.audio.wizards can answer my question.
>Either my amp, or my speakers have an automatic cut-out circuit
>in them.  My amp is rated at 100 watts and the speakers have 100w
>power handling, but the cut-out generally kicks in when the amp is
>only at half volume.  How come?  
>   Relevant parts of the system are:
>	Pioneer VSX-4400 amp
>	18 gauge copper cable (~20 foot runs)
>	dB PLUS 990e 2-way speakers (Canadian brand)
>		- Power Handling max. 100w rms
>		- min. power 10w rms
>		- efficiency 98dB(avg. room)
>		- freq. response 30H - 22kHz
>   Oh, by cut-out I mean that sound stops for 3-5 seconds then

WEll, if they are the speakers I think they are, they have a relatively
unsophisticated tweeter crossover (s simple capacitor). This provides almost
no protection against relatively large amounts of low frequency information
from destroying the tweeter. 

The manufacturers insert a thermal cut-out device (a simple bi-metallic switch
device) which simply disconnects the speaker if too much long-term signal
drives the speaker.

There are a couple of anomolies with the specification. First, the sensitivity
is rated at "98 dB(avg. room)". If that's to be believed, it suggests that with
a 100 watt RMS power rating, the speaker is capable of producing 118 dB of
acoustical output. I think that you will find that this is far from the case.
118 dB is pretty godamm loud. A sensitivity of 98 dB directly translates into
an efficiency of 10%, damn near impossibly to achieve with direct-radiator (ie
normal-looking boxes and drivers) systems. 

Secondly, the specification of 30-22kHz suggest, in combination with the
sensitivity figure, a rather large system. Without going through the details
of the math involved, it suggest that, if the speaker is a bass-reflex system,
then it has to be at least 44 cubic feet (the size of three average
refrigerators). If it's a closed box system, then it's got to have at least
90 cubic feet, the size of 6 average refrigerators! This presumes that the
sepcifications really mean -3 dB @ 30 Hz, and a sensitivity of 98 dB measured
1 meter from the loudspeaker with an input of 1 watt (equivalent to 2.83V if
it's an 8 ohm system).

So waht's the point of all this? Why should I try to shoot down Mike and his
poor system? 

Well, Mike, I am not trying to shoot down you or your system. But it's
necessary to expose the specifications for what they are: bunk! The system, I
assure you, does not have a sensitivity of 98 dB. I can probably guess pretty
well that, while it may have some output at 30 Hz, it's very far down from
what it's capable of at much higher frequencies. ANd it can't handle 100 watts
RMS.

What you've probably discovered is that the system is behaving exactly as it
was designed, and that it's specification has little relation with the design
or the system's performance. Also, it's important to remember that the position
of the volume control does not have a strong correlation with the actual power
output in watts. As a simple though experiment, how much power is the amplifier
putting out with the volume control all the way up and your not playing any
music? (answer: none!).

.





-- 
|                Dick Pierce                |
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Organization: Doctoral student, Urban and Public Affairs, Carnegie Mellon, Pittsburgh, PA