[rollei_list] Re: Completely OT- Loudspeaker info
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 15 Feb 2005 20:27:41 -0800
----- Original Message -----
From: "Jerry Lehrer" <jerryleh@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, February 15, 2005 6:48 PM
Subject: [rollei_list] Re: Completely OT- Loudspeaker info
> Richard
>
> The people in the audio world refer to the planar
> speakers, such
> as the ones made by SME , Magnaplanar, and Martin-Logan,
> as
> dipoles. Some "dipole" speakers have the backwave
> absorbed
> almost completely. Then, are ribbon speakers "dipoles"?
> All
> that I have used, absorbed the backwave.
>
> Jerry
>
> Richard Knoppow wrote:
Loudspeakers of the diaphragm type are all dipoles. That
is energy is radiated from both sides with a 180 degree
shift of polarity from one side to the other. Ribbon and
electrostatic speakers are dipoles as or common moving coil
and cone speakers.
The loudspeaker inclosure does two things. First, it
prevents destructive interference from the backwave
radiating into the hemisphere as the front. Because most
loudspeakers are small compared to the wavelength at low
frequencies each side is strongly non-directional. If the
speaker is placed in a baffle which is large enough the path
length around it will prevent this kind of interference down
to the frequency were the sound waves are able to bend
around it.
A fully enclosed box will completely prevent destructive
interference by the back wave.
The second purpose of at least some enclosures is to
provide a better acoustical impedance match of the air to
the loudspeaker. This is a somewhat complex subject. The air
has a characteristic impedance. Just as in an electrical
circuit the maximum transfer of energy takes place when the
source impedance matches the sink impedance, and both are
resistive. A small cone in air is both mismatched and very
reactive. A closed box will improve matters somewhat but
there is a still a rapid fall off of transferred energy
below a wavelength approximately equal to the circumference
of the loudspeaker. In order to improve this at low
frequencies some sort of impedance matching device must be
used. The two types most often encountered are the
bass-reflex and the horn. A bass reflex enclosure is the
analogue of an impedance matching filter section. A labrynth
type box is the equivalent of a 1/4 wave matching section as
used in radio frequency transmission lines. The Bass reflex
is essentially a lumped constant version of this. A properly
designed bass reflex enclosure will substantially improve
the match over a fairly narrow range of frequencies. An
acoustical horn is the analogue of an electrical
transformer. One can see it as enlarging the size of the
loudspeaker diaphragm to the size of the horn mouth although
this is a gross simplification. When used for mid or high
frequencies horns can be close to their ideal. This can
yield a very great increase in the efficiency of power
transfer and also be used to control distribution of the
sound. At low frequencies an ideal horn is usually too large
to be practical but a smaller horn can be used and still
have considerable improvement in efficiency. For the minimum
frequency possible a close box enclosure with a properly
designed loudspeaker and lots of equalization is superior to
the other forms due to size but is very ineffient.
In the past large horn systems were use almost
exclusively in motion picture theaters because electronic
amplification was expensive. In more recent years large
closed box systems have taken over for bass reproduction
because power is now cheap and they deliver better, or
deeper, bass. Horn midrange systems are still used because
of their advantage in controlling distribution.
Up to maybe the 1950's a typical 500 seat theater had
about a 10 watt amplifier. Amplifiers as large as 50 watts
could be found only in the largest theaters. Now one can
have kilowatts if desired so speaker efficiency is no longer
so important.
All this takes me back to what seems like a previous
life.
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