Date: Wed, 27 Apr 2011 15:43:52

Author: Zani, Gerald

Subject: Re: speakers

Post:

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I have to respectfully disagree with the crowd here. I hope I am wrong.
But I don't think so.

An audio speaker - amplifier system is not a good example of the principle
of impedance matching as used in Physics education. Audio bridging is not
designed for efficient power transfer. It is designed for audio fidelity
which willingly wastes power to achieve good fidelity.

An audio signal is transmitted as a voltage signal. The power transfer is
not important. The undistorted voltage transfer is most important. A powe=
r
source that is impedance matched to a load transmits max power. But the
price costs half the voltage! Audio system bridging seeks to preserve the
voltage.

For an audio systems the term =93impedance matching=94 is a misnomer. It s=
hould
be called =93Max voltage transfer=94. An audio amplifier attempts to prese=
nt
near zero output impedance, typically 0.1 Ohm. But it is labeled
differently because audio bridging is different than impedance matching.

A loudspeaker is a most difficult kind of load because the impedance
changes wildly with frequency. Although a speaker may have a labeled input
impedance of typically 8 Ohms it may be 8 Ohms at 150 Hertz but may become
30 Ohms at 50 hertz and then become 4 Ohms at 10 KHz. The DC impedance of
an 8 Ohm speaker tends to ~ 7 Ohms.

The efficiency of the power transfer between an amplifier and a speaker of =
a
Hi-Fi audio system is poor, by design because the sound would be awful if
one were to maximize the efficiency of the power transfer between the
amplifier and the speaker. This is because of the problems of distortions.

In audio one bridges the source to the load in such a way that the voltage
signal is not distorted. The impedance of an amplifier is low by design.
Much lower than the impedance of the speaker and contrary to common thought
the impedance is not matched as it is with a power supply and a resistive
load.

I am finding zero literature on this concept in the TPT and AJP.
I hope to locate an intro Physics of Sound book which takes up this topic.
But I found this topic only in Sound Recording and Sound Engineering texts.

- Jerry

On Wed, Apr 27, 2011 at 2:51 PM, George Herold wrote=
:

> Jerry, I didn't mean to pick on you. There were several
> mis-conceptions or mis-statements.
>
> I know very little about driving stereo speakers, but I'd guess that the
> proper design of the cross-over network is part of the 'art' of good
> speaker design.
>
> George H.
> > -----Original Message-----
> > From: tap-l-owner@lists.ncsu.edu
> > [mailto:tap-l-owner@lists.ncsu.edu] On Behalf Of Jerry DiMarco
> > Sent: Wednesday, April 27, 2011 12:41 PM
> > To: tap-l@lists.ncsu.edu
> > Subject: Re: [tap-l] speakers
> >
> > You're right, it is a rating - not a spec. Sorry if
> > that mislead anyone. I knew better (long ago), it was just
> > in a memory bank too far. However, I'm curious about the
> > broadband matching circuit Brian mentioned. Is that standard
> > these days?
> >
> >
> > Jerry D
> >
> >
> > At 4/27/2011 10:35 AM, you wrote:
> > > Dear tapers, I'd like to try and correct what appears to
> > be a common
> > >misconception.
> > >Audio amplifiers will be 'rated' by what impedance they are
> > designed to
> > >drive. This has to do with the maximum current/voltage that the
> > >amplifier can produce. This 'rated' impedance has nothing to do with
> > >the output impedance of the amplifier itself. Typically audio
> > >amplifiers will have a very low output impedance. (After
> > all you don't
> > >want to be wasting a lot of power in the amplifier, you want it all
> > >dissipated in the speaker or load.)
> > >
> > >Now the only time you will find distortion problems when
> > using speakers
> > >that have a different impedance than the 'rated' impedance of the
> > >amplifier is when you are driving them really hard. When
> > you start to
> > >push either the maximum current or voltage that the amp can
> > produce. (Of
> > >course even when the speaker is matched to the rating of the
> > amp, if you
> > >drive it hard you can still get distortion.)
> > >
> > >George H.
> >
> >
> >
>
>


--=20
Gerald Zani
Demonstration Manager
Physics
Brown University
(401) 863-3964

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I have to respectfully disagree with the crowd here.=A0 I hope I am wrong.=
=A0 But I don't think so.An audio speaker - amplifier system is=
not a good example of the principle of impedance matching as used in Physi=
cs education.=A0 Audio bridging is not designed for efficient power transfe=
r.=A0 It is designed for audio fidelity which willingly wastes power to ach=
ieve good fidelity.

An audio signal is transmitted as a voltage signal.=A0=A0 The power tra=
nsfer is not important.=A0 The undistorted voltage transfer is most importa=
nt.=A0 A power source that is impedance matched to a load transmits max pow=
er.=A0 But the price costs half the voltage!=A0 Audio system bridging seeks=
to preserve the voltage.

For an audio systems the term =93impedance matching=94 is a misnomer.=
=A0 It should be called =93Max voltage transfer=94.=A0 An audio amplifier a=
ttempts to present near zero output impedance, typically 0.1 Ohm.=A0 But it=
is labeled differently because audio bridging is different than impedance =
matching.

A loudspeaker is a most difficult kind of load because the=A0 impedance=
changes wildly with frequency.=A0 Although a speaker may have a labeled in=
put impedance of typically 8 Ohms it may be 8 Ohms at 150 Hertz but may bec=
ome 30 Ohms at 50 hertz and then become 4 Ohms at 10 KHz.=A0 The DC impedan=
ce of an 8 Ohm speaker tends to ~ 7 Ohms.

The efficiency of the power transfer between an amplifier and a speaker=
of a Hi-Fi audio system is poor, by design because the sound would be awfu=
l if one were to maximize the efficiency of the power transfer between the =
amplifier and the speaker. This is because of the problems of distortions.<=
br>

In audio one bridges the source to the load in such a way that the volt=
age signal is not distorted. The impedance of an amplifier is low by design=
.=A0 Much lower than the impedance of the speaker and contrary to common th=
ought the impedance is not matched as it is with a power supply and a resis=
tive load.

I am finding zero literature on this concept in the TPT and AJP. I =
hope to locate an intro Physics of Sound book which takes up this topic.=A0=
But I found this topic only in Sound Recording and Sound Engineering texts=
.

- JerryOn Wed, Apr 27, 2011 at 2:51 =
PM, George Herold wrote:

Jerry, =A0I didn't mean to pick on you. =A0There were several
mis-conceptions or mis-statements.

I know very little about driving stereo speakers, but I'd guess that th=
e
proper design of the cross-over network is part of the 'art' of goo=
d
speaker design.

George H.
> -----Original Message-----
> From: tap-l-owner@lists.=
ncsu.edu
> [mailto:tap-l-owner@list=
s.ncsu.edu] On Behalf Of Jerry DiMarco
> Sent: Wednesday, April 27, 2011 12:41 PM
> To: tap-l@lists.ncsu.edu
> Subject: Re: [tap-l] speakers
>
> =A0 =A0 =A0 You're right, it is a rating - not a spec. =A0Sorry if=

> that mislead anyone. =A0I knew better (long ago), it was just
> in a memory bank too far. =A0However, I'm curious about the
> broadband matching circuit Brian mentioned. =A0Is that standard
> these days?
>
>
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 Jerry D
>
>
> At 4/27/2011 10:35 AM, you wrote:
> > =A0Dear tapers, =A0I'd like to try and correct what appears t=
o
> be a common
> >misconception.
> >Audio amplifiers will be 'rated' by what impedance they ar=
e
> designed to
> >drive. =A0This has to do with the maximum current/voltage that the=

> >amplifier can produce. =A0This 'rated' impedance has nothi=
ng to do with
> >the output impedance of the amplifier itself. =A0Typically audio
> >amplifiers will have a very low output impedance. =A0 (After
> all you don't
> >want to be wasting a lot of power in the amplifier, you want it al=
l
> >dissipated in the speaker =A0or load.)
> >
> >Now the only time you will find distortion problems when
> using speakers
> >that have a different impedance than the 'rated' impedance=
of the
> >amplifier is when you are driving them really hard. =A0When
> you start to
> >push either the maximum current or voltage that the amp can
> produce. (Of
> >course even when the speaker is matched to the rating of the
> amp, if you
> >drive it hard you can still get distortion.)
> >
> >George H.
>
>
>

-- Gerald ZaniDemonstr=
ation ManagerPhysicsBrown University(401) 863-3964

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