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From: email@example.com (john jardine)
Subject: Re: Loudspeaker and microcontroller
Date: 25 Oct 2002 13:20:33 -0700
References: <firstname.lastname@example.org> <3DB65042.66FB6FCB@scazon.com> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 25 Oct 2002 20:20:33 GMT
John Woodgate wrote in message news:...
> I read in sci.electronics.design that john jardine
> wrote (in <871568ec.0210240918.63df3a68@pos
> ting.google.com>) about 'Loudspeaker and microcontroller', on Thu, 24
> Oct 2002:
> > Which
> >would give in turn a max of about 3/4watt and 1.5watts, which is still
> >not a lot.
> Yes it is. Remember that the 'loudness' of an audio signal roughly
> correlates with its r.m.s. voltage, and audio programme signals have a
> peak to r.m.s. ratio of 12 to 20 dB (4 to 10 times). But tone signals
> don't! A typical (lowish) loudspeaker sensitivity is 85 dB SPL at 1
> metre with 1 W input. Anywhere inside a normal room, the 1 m figure
> applies everywhere, due to 'room reinforcement' - reflections and
> reverberation. To be loud enough to hear clearly, a tone signal needs to
> be only about 75 dB SPL, which is produced by that loudspeaker with only
> 100 mW input.
> > The best way to up the power is to go for increasing the amp supply
> >voltage to say 24V, use a standard 8ohm speaker and feed the audio
> >amp from the PIC, via a volume control (max 8watts).
> That would be very loud indeed, 94 dB SPL.
Yes. you're quite right of course. 1.5watts (or less) would normally
be *very* 'hearable' using a bog standard (or even insensitive)
speaker. Especially as it's pumping out fixed level tones.
(I'll bet you yourself have the hard won experience to accurately
'meter' the spl you hear, straight into DB's. Jools wont have that
just yet ;-)
In this instance though and for practical reasons. I was trying to
keep a couple of steps ahead of jools' project work. To whit ...
There's nothing more disheartening than getting half way through a job
to find you've hit the "Bloody hell, I'm stuck, I need more
In jools' case I'm simply applying the more is better rule ... Start
off with lots and lots of watts and a volume control to set a level
from 0 to max.
He'll no doubt prog the PIC to create a number of square tones also
possibly noticing he can programme a chorussed melody in as well.
Maybe he will then sum these tones using some resistors. Also as he
says, add some tone shaping caps ect.
After this is done he'll try the unit out with a speaker with no
infinite baffle fitted and still find he can't hear anything. The sig
processing losses are so great that the 1.5Watt amp has virtually
nothing to work with. Have lots of gain available in the first place
and he'll get the sound level he wants just by turning up the pot.
First noticed that this 'sound' thing can have odd aspects, when I
installed an RS components red, 6" square, external burglar alarm.
Staggeringly noisy!. Had it apart of course looking for some weird
electronics/transducer. Couldn't find anything other than a telephone
earpiece stuck onto the plastic casing. It was years before I came
across the magic of folded horns!.
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