Mazda Miata: '99 Bose

My 99 has 6 X 8 inch speakers. There don't seem to be very many choices in
that size. Are your speakers 8 inch round? Has anyone tried Pioneer 6 X 8
as a replacement? Walmart has them for $56... here's the link...
http://www.walmart.com/catalog/search-ng.gsp?search_constraint=0&search_query=car+speake rs&ics=20&ico=0&Continue.x=19&Continue.y=9
"Larry Gadbois" <net> wrote in message
news:UrkFc.5321$news.atl.earthlink.net... 
past 
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cup 
I 
the 
purchased 
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will 

Yes, it is a round speaker with four screw mounting holes. The speaker
measures 8" in diameter between holes. The design is a little strange. It
was designed to maximize bass response. It has a cheap plastic tube about
the size of the speaker at the back to increase low frequency resonance.
The cone does not have any hard surfaces to enhance mid-range. The voice
coil is edge-wound and flat to handle high current. The magnet is located on
the front of the speaker instead of the back. I guess they save a little
space that way.

If one or both speakers fail again I will replace them with a couple of
JBLs. I can get a pair for what one Bose replacement costs.

"jimuntch" <com> wrote in message
news:supernews.com... 
in 
8 
http://www.walmart.com/catalog/search-ng.gsp?search_constraint=0&search_query=car+speake rs&ics=20&ico=0&Continue.x=19&Continue.y=9 
hold 
out. 
The 
was 
$175. 
without 
end 

On Sat, 03 Jul 2004 18:46:37 GMT, "Larry Gadbois"
<net> wrote:

The "cheap plastic tube" is to keep water off the speaker. The BOSE
speaker in the 99 Miata is a special 1/2 ohm unit and is mated with
the BOSE amp. Using conventional 4 ohm speakers would probably give a
low max volume.
 

I don't know how the "1/2 ohm theory" originated. I have seen it floating
around the internet in other places. The statement is incorrect. The plastic
tube on the back of the Bose might provide some protection from water
running down the inside of the door, but the speaker cone is not exposed to
the inside of the door. The water resistant spider (the accordian voice
coil suspension) is all that might be exposed to water.

The speaker impedance is not the DC resistance. The voice coil on this unit
consists of about 80 turns of flattened wire, .75 inches in length, with a
radius of .68 inches. This calculates out to about 217 microhenries. I do
not know at what frequency the Bose speaker impedance is figured, but at 300
hertz it works out to be about 4 ohms. At 1 khz it is about 14 ohms.

Bipolar transistor power amps loose efficiency when connected to loads below
2 ohms. Four ohms minimum seems to be the design standard in the industry.
As for volume loss when substituting high impedance speakers, the volume
reduction on high efficiency speakers is nominal. When you half the power
you are down by 3 db.

Miata owners that have Bose systems can install other 8" round speakers
without loosing volume. One benefit of speaker replacement would be to
improve mid-range volume, and improve the balance between the bass and mid
frequencies.


"Natman" <com> wrote in message
news:comcast.net... 
It 
on 

On Sun, 04 Jul 2004 21:11:49 GMT, "Larry Gadbois"
<net> wrote:

I've seen it on the internet too. Here are some of the places:

http://members.fortunecity.com/jasoncuadra/audio/bose/bose_rta.html

http://stephen.fosketts.net/miata/nbaudio/index.html

http://www.clearwateraudio.com/cwc8b.htm

An awful lot of people seem to believe it. That's hardly proof of
course, but until I see an actual frequency / impedance graph that
says otherwise I'm going to stay with the 1/2 ohm theory.

 

You forgot about the mechanical coupling effect of the speaker and it's
surroundings. The only way to settle such things is to actually measure
them. (Been there, done that!). Don't forget the wiring size and length
from the speaker to the amp.

"Larry Gadbois" <net> wrote in message
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Mechanical coupling and wiring size and length has nothing to do with rated
speaker impedance. The impedance of the speaker is determined by the
electrical properties of the voice coil. It is the alternating current
resistance.

What you are describing is the amplifier load impedance, which is a
different issue.


"chuckk" <compuserve.com> wrote in message
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a 
do 
industry. 
power 
mid 
strange. 
resonance. 
little 
of 

Some comments on the below websites:

"Natman" <com> wrote in message
news:comcast.net... 

Jason has done a good job in displaying the frequency response of the system
using his Real Time Analyzer, and has accurately described the acoustics of
the speaker and it's door enclosure. The technical analysis of the Bose
power amplifier and speaker system is not exactly right.

He says, "The fact that the pink line is around 2dB down in places means
that the wire resistance to the drivers is significant. It's losing about
2dB of power, which is something like a 30% loss of power in the wires. The
Bose system uses 0.5 ohm drivers so that the power amp, with no switching
power supply, delivering 12V peak to the drivers, can theoretically deliver 
getting to the speakers."

Here Jason exposes his lack of technical understanding. The pink line is his
graph is down 2 db because the amplifier load causes a drop due to all
resistance in the coupling, not just the wire, but the amplifier outputs
also. The speaker is not 100% resistive and causes further variations from
the -2 db attenuation. If he had used a non-inductive load to graph the
output he could have determined the coupling losses and damping factor of
the system.

Jason may have measured the DC resistance of the speaker with an ohm meter.
It does measure .5 ohms. A speaker is an inductive load and the speaker
impedance can not be determined by measuring the DC resistance. As the audio
signal increases in frequency the impedance of the speaker increases, though
the increase is not quite linear due to the mechanical design of the
speaker. Direct coupled audio power amplifier outputs are capacitive in
nature, which assists in the coupling to an inductive load (speaker).
Anyway, loosing 2 db isn't a problem. The Bose system is capable of
delivering in excess of 100db SPL at power levels of less than 50 watts.
 

Fosketts says on his website, "special low-resistance (.5-Ohm) speakers."
Resistance and impedance are not the same. He says that the tweeter is 4
ohm. I don't know if the tweeter says 4 ohms on it. With some tweeters the
DC resistance and the AC impedance may be close, as a tweeter is capable of
delivering high volume levels at relatively low power. Fosketts also states,
"This means that removing the Bose system (say, to replace the speakers)
requires replacing all of the speakers and wiring past or removing the Bose
amplifier." This is not true. Many Miata owners have upgraded to a better
quality door speaker that improves mid-frequency response and system
linearity. I suspect he is in the business of selling replacement autosound
system.

 

Here is a company that is clearly in the business of selling new systems for
the Miata. They state, "We manufacture a system to accommodate this need.
The Bose® replacement system is designed to work seamlessly with the factory
amplifier. The impedance of this woofer is ½ ohm, just like the factory
speaker." Here they say their replacement is 1/2 ohm impedance. I doubt if
they manufacture these speakers, and if their replacement is the same as the
Bose speaker it isn't 1/2 ohm impedance. This is a sales organization that
is not careful about providing technical information on the products they
sell. The Bose door speaker is not just a woofer. It also is responsible
for reproduction of the mid-range frequencies. 

The reason there is so much mis-information and confusion on Bose systems is
because Bose does not publish specifications on their systems. In answer to
the FAQ, "Can I get a list of technical specifications?", Bose answers, "No,
because our custom-engineered, complete system solutions largely supersede
conventional audio measurements. Even some basic building blocks have been
redefined-including amplifiers, speakers and equalizers. But most
importantly, our integrated systems deliver results significantly different
from those of conventional systems with individual, unmatched components.
With a Bose automotive sound system, the location and design of the speakers
can be much more important factors in determining output than a wattage
power rating. The bottom line? The best way to test a Bose system's sound
quality is to use your ears, not a list of specifications. "

I met Dr. Amar G. Bose in 1970. He was a Professor at M.I.T. He had started
his company in 1964 and by 1968 introduced the Bose 901 Direct/Reflecting
speaker system. The Bose 901 used nine 4" speakers in a small acoustic
suspension cabinet. Eight of the speakers faced forward, while a ninth was
mounted on the back of the cabinet. Each speaker was rated at 30 watts. The
system used no internal frequency compensation networks and would handle 270
watts RMS. The small speakers with low mass provided excellent transient
response. The rear mounted speaker provided a reflected wave off walls or
surfaces behind the speakers which added a reverb to the sounds. When
properly placed the reverb would be 10 to 20 milliseconds. The speaker
system came with an equalization box to boost the level of low frequencies.
The box needed to be connected using tape-in/tape-out connections on the
amplifier, or to be placed between preamp and amp. The equalization box
offered few adjustments, and the bass boost was about 24 db. The high amount
of bass boost more than compensated for the bass attenuation from the small
speaker boxes. This was the first hi-fi speaker system designed for the
customer that liked exaggerated bass response. All his speaker systems since
the 901 are designed to accentuate the low frequencies. Serious audiophiles
do not like Bose speaker systems, but it seems to be popular these days to
install a high power amplifier and sub-woofer in your car.