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Text file about car audio speakers, mainly from Bl
CUSTOMER GUIDANCE is an outline of some basic qualifying
questions one needs to ask the customer ( in a professional
manner ) in order to fit the woofer box to that person's
wants and needs.
1. What kind of car is it? With a sedan, you usually have
considerable room for a sub box behind the rear seat.
For a hatchback, you may have similar room but may be
limited by height.
2. How much money do they want to spend? This determines
if you can build an isobarik box with two woofers.
3. Do they want "boomy" bass? This isn't necessarily a
negative if the person listens only to Rap music. A
moderate peak in the frequency response around 60 Hz
will accentuate this Rap bass.
4. Are they looking for an "unseen" box? A bandpass box
emits all of its sound from the port only so this is
a good solution for a "hidden" box without grilles.
5. How much power do they need or want? This is a
function of how loud they listen. If this person is
a real "pounder," a sealed box or single vented band-
pass box should be used to improve the power handling
of the woofer. A simple vented, or double vented
bandpass box, will unload the woofer thus allowing it
to bottom and eventually destroy itself.
WOOFER CHOICE will assist in answering the options that the
customer should be aware of regarding the benefits or
losses in using a particular woofer.
1. Woofer size: What does the customer want? The mistaken
concept of "bigger's better" remains horribly rampant.
Very often, a smaller woofer in a given box will provide
a lower cutoff frequency than the larger one. This can
be beautifully modelled on BLAUBOX's PLOT RESPONSE
screen.
2. To guide the customer into the best box for their
installation, the DESIGN feature of BLAUBOX is very use-
ful at comparing different woofer/box combinations in
hopes of coming up with a good compromise in performance,
complexity, and price.
3. Be smart and start your designs with 10" woofers; they
are very forgiving of small, and overly large, boxes.
4. Try to use woofers with high Qts values ( 0.5 - 0.8 )
for sealed boxes and low Qts values for vented boxes
( 0.25 - 0.5 ). Bandpass designs like the Qts values
around 0.5 for optimum performance.
5. Try to maintain 4 ohm loads on the amplifiers through
series or parallel connection of the speakers. Even
though most amplifiers today are 2 ohm stable, it
still strains the amps due to the significant heat
dissipation.
6. Try to match the power output of the amp with the
woofer's power handling values. Huge amplifiers on
50 watt rms woofers is kind of "boneheaded."
BOX STYLE discusses the various styles that can be used in
a given installation and the pro's and con's of each.
1. A sealed box is simple to build, will take lots of pow-
er because of the sealed box's "air spring," and sounds
reasonably good in most installations. But, the -3db
frequency is much higher than a vented box. This is
the best design for "beginners" but also for many
installers unsure of woofer box theory.
2. A vented box will often play much lower than a sealed
box but the woofer can become unloaded and begin to
move excessively below its -3db frequency because of
the lack of an "air spring" as in the sealed box.
3. The single vented bandpass box is a fantastic box in
many ways because it offers a sealed chamber on one
side of the woofer ( therefore an air spring ) and a
vented chamber on the other ( therefore a lower -3db
frequency ). But, it is a more complicated box to
build and is less tolerant to BoneHead construction
mistakes such as air leaks.
4. The double vented bandpass box offers even lower
frequency response, and higher efficiency than a simple
vented box, but, like a vented box, the woofer becomes
unloaded at some low frequency and begins to flop
around. The biggest problem with this design is the
very complex and unforgiving construction techniques
which are required.
5. The fifth-order single vented bandpass box is very
similar to the single vented bandpass box but requires
the use of an inductor in series with the woofer.
The inductor increases the rate of the high frequency
rolloff of the system.
FABRICATION is a discussion of the various box designs and
their final fabrication.
1. First, measure the trunk volume that is available
( measure L" x W" x H" ). Use MATH CALCULATIONS if
necessary to convert from cubic inches to cubic feet.
Use the volume measurement ( cubic feet ) in all of
your BLAUBOX calculations.
2. The rectangular box design is the simplest box to
build and is recommended for most installers. The
tubular design is becoming popular due to the cos-
metics and general ease of construction. The wedge
shape maximizes the volume in most trunk areas but
becomes more complex to build with the various angled
cuts.
3. All of the above designs can be modified using the
MODIFY DIMENSIONS command under Screen #2. The
dimensions can be changed until a box fitting the
exact trunk area is designed. By working through
BLAUBOX backwards, you can then determine the frequency
response once the box volume is determined.
4. The PORT command under Screen #2 allows for changing
the port tube lengths and diameters hopefully using
what PVC stock is available in house.
5. Most importantly, using DRAW BOX, you are able to
print out a drawing of the box. Verify all dimensions
before you start to make sure the box will fit the
speaker. A 15" woofer with 12" side walls is going to
require substantial creativity to make it fit!
ACOUSTICS is a discussion of some basic acoustics
fundamentals.
1. Most people can hear sound over frequencies of 20 Hz to
20,000 Hz. A piano will reproduce sound from about 50
Hz to over 10,000 Hz in the harmonics.
2. We can hear sound over a huge range in loudness. Most
people with good hearing can recognize sound as low as
0 db, the threshold of hearing. At about 120 db perm-
anent hearing loss occurs ( many rock concerts average
this level in the front rows ). The 120 db range of
loudness represents a difference of 1,000,000,000,000
times a noise at 0 db!
3. Technically speaking, a doubling of amplifier power is
3 db, but the human ear often needs volume differences
of nearly 10 db to qualify a sound as "twice" as loud.
( 1 db is the smallest audible increment in loudness ).
4. Our ear requires significantly more bass for low
frequencies to sound as loud as mids and highs at low
listening levels. This need originated the term
"Loudness" for the little circuits providing bass
boost at low levels.
5. Our hearing has a different frequency response for
different angles of listening. Our hearing is most
sensitive to high frequencies at the sides of our head.
We also listen in real time noticing timing differences
as small as 1/1000 of a second. This is why the direct
sound from a dash speaker, and early reflections off
the windshield, are often heard as a "smearing" of the
overall tonal quality. ( Another reason to keep
tweeters away from glass ).
6. Tremendous low frequency SPL's are possible inside a
car due to "small room" compression effects. It's like
sticking your head inside the woofer box!
ELECTRONICS is a discussion of simple relationships with
voltage, current, impedence, and power.
1. Current is the rate of electron flow in a circuit and
is measured in amps ( AC or DC ). Lots of current flow
is designated as high current.
2. Voltage is responsible for pushing electric current
( pressure ) through a circuit and is measured in volts
( AC or DC ). A high voltage wants to push current but
may not be able to if there is limited battery current
capacity.
3. Resistance "resists" the flow of current in a circuit
and is measured in ohms. A large resistor has a high
resistance to electron flow. Resistors in parallel
lower the final resistance; in series raises the
resistance. For example, two four-ohm speakers in
parallel yields a two-ohm load; two four-ohm speakers
in series yields an eight-ohm load.
4. Voltage, current, and resistance are mathematically
linked through Ohm's Law stating: V(voltage) =
I(current) X R(resistance).
5. Power represents the product of pressure (voltage)
times electron movement (current), P = V x I and is
measured in watts.
6. A battery produces direct current (DC) but audio coming
out of an amplifier is alternating current (AC) which
is actually alternating positive and negative voltage
which then induces forward/backward electron movement
at the speaker.
7. Capacitors block the flow of DC, yet pass through AC
signals. They are measured in microfarads (mfd).
Inductors ( aka, coils ) pass DC but block AC signals.
They are measured in milliHenrys (mH).
WOOFER DESIGN is a discussion of a loudspeaker's most
important performance data including Thiele/Small
parameters.
1. A woofer is made up of a voice coil glued to a cone
which moves in and out of the magnet structure.
Changing voltages on the voice coil, provided by the
amplifier, create a magnetic field around the voice
coil which is pulled closer to, or pushed away from,
the magnet.
2. The suspension parts, surround and spider, provide an
excursion centering function, and also keep the voice
coil centered in the magnet structure. Compliance is
the measure of woofer air volume movement for a given
woofer.
3. Frequency response is a function of the cone diameter.
Smaller diameter cones offer higher frequency output.
Dispersion of smaller woofers, the ability of the
woofer to produce highs off axis, is also better.
4. Three parameters of a woofer define 90% of its opera-
tion: "fs" is the natural resonant frequency of the
woofer ( much like a weight hanging on a string);
"Qts" is the measure of the damping of the woofer
( the magnification of movement at resonance); "Vas"
is a function of the amount of air the woofer can
move ( synonymous with compliance ).
5. Power handling is either the amount of power before the
voice coil burns up, or the amount of power before the
woofer reaches its excursion limits. Power handling is
measured in watts rms with "max" values two to three
time rms.
BOX DESIGN explains some basic performance differences,
and slang terms, between the various box styles addressed
by BLAUBOX.
1. A sealed box is simple to build, will take lots of pow-
er because of the sealed box's "air spring," and sounds
reasonably good in most installations. But, the -3db
frequency is much higher than a vented box. This is
the best design for "beginners" but also for many
installers unsure of woofer box theory.
2. A vented box will often play much lower than a sealed
box but the woofer can become unloaded and begin to
move excessively below its -3db frequency because of
the lack of an "air spring" as in the sealed box.
3. The single vented bandpass box is a fantastic box in
many ways because it offers a sealed chamber on one
side of the woofer ( therefore an air spring ) and a
vented chamber on the other ( therefore a lower -3db
frequency ). But, it is a more complicated box to
build and is less tolerant to BoneHead construction
mistakes such as air leaks.
4. The double vented bandpass box offers even lower
frequency response, and higher efficiency than a simple
vented box, but, like a vented box, the woofer becomes
unloaded at some low frequency and begins to flop
around. The biggest problem with this design is the
very complex and unforgiving construction techniques
which are required.
5. The fifth-order single vented bandpass box is very
similar to the single vented bandpass box but requires
the use of an inductor in series with the woofer.
The inductor increases the rate of the high frequency
rolloff of the system.
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