Crown OC-150A - Page 18

Thhe net resu ll: of this csmplex impedance cau*s t he
voltage and current waveforms to be out of phase by
some angie Bess than 9@, %e figure 2-2.
$300 1800 270-600
(B) Current wavefg~~m with pure is?ductiv@ reac-
tance. Current lags voltage by 90%
(C) Current wavefaam with camplex im-
pedance, a cowbi nation af inductive reactance
and resistance. Current lags voltage by some
angle less than 90).
FIG. 2-2
Now back to our original question, Let's took at a
typical loudspeaker, a complex load, consistiugaf in-
ductive reactance and resistance. it has ken deter-
mined that this speaker repre-nts an 8 ohm im-
pedance a$: IKHz,
We wifl set up our test by using a lKMz input signat
to %he arnpfifier of sufficient !@vet to cause an out-
put voltage of 10 vasts across the swaker terminals,
"8
I
t
I
f
f
1
f
I
With this information, we can compute the power to
Si
be P=F
Z
I
p"@ #,A [.
+A -- 12.5 mtts
8
Oses this mean that the speaker is dissipating the
12,s wa@s as acoustical mwer across the comptex
impedance, in a simplified analysis, we can account
for the total power (12.5 wags] From the amplifier,
being dissipated in four argas.
(15 %me OI the power is dissipated as heat across
the resistive pae of the immdancc. This is es-
sentiall y wastc?c% power since if. dms nat produrn
any acoustical r~soit, Resistance is found in
such places a5 in the wir@ ofthe s~ak~r" waice
csi t .
(2) Same af the po~er is dissipated 8% heat in the :
amplifier itself because of the reactiv~ cornpa-
nent of the impedance, One af the
cha~acteristics of pure reactance is that ikcannat
dissipate any pwer.
Rather it semes as an enera storage device,
This stared energy wili be returned to the
ampiifier and wasted as heat. Lwk again ad:
Figur~ 2-2, Notice that in each cycle &a periods
s;tl time exist when the uoitage is positive while
the curreut is negative. (This is due hcs the
reactive component causing a phase shift.) tt is
during these WQ psriods kaf time that negative
power is produced, and the ~tar~d power in the
~eactance is returned to the amplifier to be
wasted as heat It should be clear from this, that
to drive ;a pure reactive foad weauld be pointless
as it would only cause amplifier heating.
(3) am@ sf the tatat wwer i~ transformed and dis-
sipated as acsusticaf enerB. This is becausethe
ind~aefive reactance (voice coil) BS situated in a
magnetic field, The applied AC WWFC~US~S the
voice csil to move, Since the voice coil is at-
"caehed ts a large diaphram which in turn causes
'
campressions and rarefactions of the surrsun-
ding air, acsustical energy is produced.