'99 Bose

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.

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

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.

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.

Some comments on the below websites:

"Natman" wrote in message news: snipped-for-privacy@netnews.comcast.net...

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.

Well I agree with you that DC resistance is not the same as impedance. However in most cases it is in the ballpark. A "4 ohm" driver will have a DC around 3.6, an "8 ohm" driver will have a DC of 7.2 and so forth. The .5 ohm DC resistance of the 8" BOSE woofer under discussion certainly gives the appearance that somewhere in its impedance curve it will have a lower impedance than most amps can handle.

So far your response has been to say that anyone who says that the BOSE woofers in the Miata are low impedance either:

1) doesn't know what they are talking about.

2) is lying in order to sell something.

or both.

However you have not yet offered any proof to back your assertions that the BOSE woofer is not low impedance or that conventional 4 ohm speakers will work and provide satisfactory sound levels. Until such proof is forthcoming I'm going to keep these theories in the "possible, but not probable" pile.

If it is possible to use 4 ohm woofers and you find the BOSE woofer so deficient, it begs the question: Why didn't you replace your defective BOSE driver with a pair of 4 ohm woofers instead of shelling out $175 for one factory BOSE woofer?

Natman,

I did give you the dimensions of the voice coil. Anyone with basic algebra skills can calculate the inductive reactance of the voice coil. Using the discovered "L" you can calculate the impedance. If you have a Miata with a Bose system, it is easy to do a dynamic impedance test. Use a test CD with your choice of audio tone. Disconnect the door speaker and insert an eight ohm resistor in it's place. Play the tone and set a reference level on an AC voltmeter of eight volts. Remove the resistor and reconnect the speaker. Read the voltage at the same setting. You will be reading the impedance directly for the frequency you are playing off your test disk.

Low power speakers use small gauge round wire for the voice coil. When small wire is used, the DC resistance is higher and more of the audio current is dissapated in heat rather than motion. In higher power applications where the voice coil is mostly resistive the speaker efficiency is low, and the heat generated can easily cause speaker failure.

James Lansing with his JBL company pioneered the edge wound voice coil that was introduced in the model LE8-T eight inch extended range driver over forty years ago. The wire was flattened and varnished before winding on the sleeve. The advantage of using flat wire for the voice coil is that you can get the same number of turns per inch as smaller wire, and the same load impedance with a much higher current and power handling ability. The DC resistance of an edge wound voice coil is much lower than those using conventional wound coils.

One thing that is not obvious is that the impedance of the speaker goes very high at low frequencies. The effect happens as the coil resonates. I didn't measure it on the Bose, but I suspect that the impedance rises to over 20 ohms. An increase can also occurs at the second harmonic of voice coil resonance. Very little audio with content below 30 hz is available off CD or radio broadcast. If the amplifier is DC coupled it might deliver audio down to about 10 hz, but the Bose door speaker that is in the Miata presents a safe impedance to any amplifier that is capable of reproducing sub-audible frequencies.

Jason Cuadra did a good job of explaining why the Bose door speaker is not the choice of an audiophile. It's partly the speaker's fault, but the door makes a poor enclosure. An acoustic suspension enclosure would offer much smoother response and better overall fidelity. The new Miata system with speakers on either side of the wind blocker has to be a big improvement. I have wondered why Mazda dropped the idea of headrest speakers.

I didn't install another brand of replacements speakers because:

1. I didn't want to take the time to disassemble the passenger door to install a matching unit.
2. I didn't want to design and build a baffle/enclosure to install in the doors
3. Once the Miata is rolling, the audio fidelity can't be appreciated!
4. In case I sell the car I can tell the buyer that he has a Bose Audio System
5. Some passengers are impressed by that thumping noise as I roll down the road

I do have a pair of JBL GTO625 that are rated a 60 watts RMS. They sound great on the Bose head, but I am using them in my studio for recording monitors and didn't want to waste them in the Miata. These speakers are very efficient and more than loud enough for use in my car.

frequencies.

So, other than replacing the speakers, it seems like this is an ideal application of a parametric equalizer between the tuner head and the Bose amplifiers. Has anyone else considered/tried this ?

Cheers, Dana

I have not heard of anyone doing it. Anything that would flatten the response curve would improve the sound. I also wonder about using something to deaden the door resonance. Maybe a foam or undercoating spray would work.

Thanks for a this constructive post.

I liked your list of reasons for keeping the stock driver. I especially agree with reasons 3 & 4. There are a lot of people who knee jerk that the only thing to do is to rip out everything BOSE and start over. For *my* use the Miata is just not cut out to be the platform for a really top end system. I also must sadly concur with reason #5. There seems to be an entire generation that is being brainwashed into thinking that bass is *supposed* to sound like that.

Despite its flaws in stock form it's better than the average OEM system. I believe that it could be a good deal better yet with some simple modifications.

First the door IS very resonant, both in the metal and the door panel. Some sound deadener and maybe a few wood ribs should improve things. The nice thing about door improvements is that they will still improve sound even if you switch to different drivers later.

Next is the lack of any crossover for the woofer. An 8" woofer is awfully large to run free. The addition of a coil to limit high frequencies should clean up the mid range.

Finally, as you pointed out in an earlier post, BOSE loves to boost EQ in the bass. It certainly sounds like they have overboosted the bass in this application. Any thoughts about using a notch filter to tone it down to realistic levels?

In order to do the crossover and the notch filter, a frequency / impedance graph would be needed. The method you suggested sounds interesting. Could you provide a bit more detail, especially:

"Play the tone and set a reference level on an AC voltmeter of eight volts. Remove the resistor and reconnect the speaker. Read the voltage at the same setting."

Bingo!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I've tried and tried (I love music) to show my 16 y/o daughter what musical BALANCE is. She's not the least bit interested in listening to any kind of balanced sound. Wonder how many of these stupid kids are going to be 50% deaf in 10 years.

Minor point. I don't think anyone is "brainwashed" into liking a boomy bass-heavy sound. Bass provides the a bone-shaking concert/club feel...and many people like it. No, they are not audiophiles and no the sound is not accurate for acoustic music, but they prefer it that way. Just like some people prefer super sweet breakfast cereal or soda or Red Bull versus those who savor wine or scotch.

There's satisfaction for aficionados and there's primal satisfaction...

-John

"Generic" wrote in news:40edcab2$0$62141$ snipped-for-privacy@news.nntpservers.com:

Gotta disagree, much of todays kids music relies totally on these hugely exagerated bass riffs. Kids that grow up on this crap (not because of the bass) DO become "brainwashed" into thinking that is what music is supposed to sound like.

I don't know what you mean by 'brainwashed.' Just what IS music supposed to sound like? Classical? Choral? Jazz? Industrial? Atonal? Experimental? Vocals? Reggae? Rap? Heavy Metal? Variety is the spice of life...all of these and more are present in my own collection...

It is true that most kids grow up on top 40 radio, which is calculated and targeted for them. There have been many generations of teen boy bands (e.g. New Kids on the Block, N-Sync) that pubescent girls fall for year after year. Teen boys tend to fall for more aggressive music, be it heavy metal or rap/hip-hop.

Many people never explore beyond what's hot at the moment, many never explore beyond the very formulaic top 40. On the other hand, some get stuck on what "music is SUPPOSED to sound like" so they never try anything new. Sometimes it takes years before people understand the new style--then opinions change.

No genre is all "crap," for there are always innovators who create something unique. Certain genres (especially heavy metal and rap) were created in the electronic era and rely on amplified sound. Bass plays a large role because the amplifier is a muscial instrument of the genre. Take time to explore and you may find something you like.

-John

"Generic" wrote in news:40ede9bb$0$62162$ snipped-for-privacy@news.nntpservers.com:

you're talking types of music. I'm talking about cranking the bass knob to full NO MATTER what you are listening to. Or in the case of the this pseudo musical crap they pass off these days, it's encoded with the bass hugely overemphasized on the recording. EIther way, it's a totally unbalanced sound that they DO become brainwashed to expect. That is proven every time I get in my daughters car and move the bass knob off FULL gain. And she isn't always listening to rap. Sorry, but my standards for appreciating music require the artist to either SING or be able to play a musical instrument. :)

In my original post where I said people were being brainwashed in to thinking bass was supposed to sound boomy, I was referring to the quality of the sound. No comment was intended toward any particular style of music. The BOSE system in my Miata is an equal opportunity distorter.