Everything you ever cared to know about modifying the Miata

by Matthew Brock mattb@romnet.com

A little background: I currently have Boston Acoustics Professional Series 6.4 and 4.4's in my front and headrests (components in the headrests!), two 6.5" Kicker subs in the rear deck, crossovers all around, and two amps (Sony 300W and Pioneer 100W bridged @ 1 Ohm) powering the whole thing. The face is stock, because I am concerned with security and car theft (amps have speaker-level inputs). After writing up my installation, I recieved a few compliments and a number of inquiries into specifics. Finally after recieving one such email, I decided to write up a very detailed version that would be useful to anyone interested in upgrading from stock. The initial question and my detailed answer follow:

> Hi Matthew,
> I am just another Miata fans who wants to improve the sound system in the car. I can totally agree with your appraoch in upgrading the > > Miata's sound system.... that is, clear, loud sound but without too much bass.
> I've just finished a DIY sound system upgrade but I found that the results are not that astonishing. There is certainly something that I've miss > > out and the only solution is to ask some experts (like you) who has experienced with miata before.
> My system consists of:
> - Alpine CDM 7829E CD/Turner with 25W x 4 and Dual pre-out
> - MTX 2 channels amp
> - A pair of Alpine 6 1/2 inch DDDrive coxial (150W peak 50W RMS Frequncy Response: 40Hz - 32kHz)
> - Four 3 1/2 Kenwood KFC -835C dual cone speakers in the 2 headrest (40W peak only, Frequency Response: 96Hz - 20,000Hz)
> The problems are:
> 1. The sound of DDDrive coxials seems to be easily distorted at mid to high volume. The bass is not as strong as I think and they rattle when the volume is up. Is it due to the fact that the Miata door is too loose? Or just the speaker itself can't handle the power? Or it it becasue of the > wire I use? I use the orginal wire supply with the DDDrive...
> 2. Will the situation improve if I use cross-over to cut the bass below 40Hz?
> 3. Is a splits better than a coxial? Since there is another model of DDDrive split which is also 6 1/2 inch with all features same as mine coxial.....
> 4. Does the MTX amp match well with the DDDrive coaxials?
> 5. The four Kenwood's speakers are driven by the head unit's 4 x 25w. Do u think that's enough?
> 6. Those Kenwood's head rest sounds just ok. But not clear enough. Especially when some music which have low bass. Can I use a capacitors  to cut those basses?
> 7. Do u think that I need to purchase a crossover sys. for my situation?
> 8. How do u comment on my system? What do u think I 've left out or is there any modification u can suggest? (But like u did, I don't want a  woofer in the boot....)
> Ah..... it's just too much questions in my head and I don't know what's the reason that I can't achieve my expected results......
Hmmmm.... > quite nervious .... Hope u can give me a hand :)
> Thanks

Hope I got the spelling of that correct -- it's probably more like g'donya. You get the idea... Ok, let me see if I can help you out here. Let me first commend you on your desire for clarity instead of bass -- in all honesty, you really don't have much of a choice on this matter with the Miata. You probably knew that already. The car is simply not designed to hold bass very well. Unfortunately, this is an issue you're going to have to attack at some point, because regardless of your system (bass thumping or acoustic perfection), you will need a good, resonating bass that's free from distortion in order to reproduce the music accurately. All right, let's look at your system:

Alpine CDM 7829E - Funny, it looks like they ONLY offer this model in Oz. But it appears to be a very solid unit, you got the two preouts, the CD player. And Alpine makes great headunits, so no worries there. Observing the power output, however, I am dubious as to whether 25x4 means continuous power (I think you call it 'Nominal' power, it's also reffered to as RMS) or if it is peak power. I'm thinking peak. I have a sneaking suspicion that it's actually more like an honest 15W per channel, but that's not really a problem as long as you don't try to drive your speakers with it. (Although, it looks like you ARE trying to do this with the rear channels. That should be fine, but I'll examine it more closely in a second)

MTX 2 Amp -- Well, you don't mention how many watts the amp is, and this is important. The more the merrier. As long as it can handle at least 100 watts continuous power (this would probably translate into around 150-175 watts peak power) you'll be fine. I say 100 watts because your speakers are rated at 50 watts continuous. 50 watts per speaker = 100 watts.

Alpine DDS-R17G 6.5 Coax's - The DDDrive series is a nice group of speakers, but their real superiority is in their bass driver's design. I'm not referring to the coaxials, now. I'm talking about the separates, the component line of the DDDrive series. I'll talk about this as well...

Four Kenwood 835C 3.5's - These are fine for headrests. When I talk about imaging inside the Miata later on, you'll see that when you have proper sound in the Miata, you rarely, if ever use the headrest speakers. (Ok, maybe on a tops-down driving day when you're feeling REALLY good about life you might turn them up a bit).

I'll first give you my recommendations, and then I'll answer any remaining questions you had that weren't addressed.

It looks as if your system should be adequate for generally 'ok' sound. But with a few improvements, you'll astonish yourself. First off, I'd ditch the coax speakers in the front and get a component speaker system like the DDDrive DDC-F17A 6.5's. I can assure you that they'll cost more, how much I don't know. You don't have to stick with the Alpine's, but Alpine does make good audio gear, so if you can scrape together another $100 or so, you can probably trade your old ones in and upgrade to the components.

I can't stress enough how important component speakers are in the Miata. If you look at yourself in relation to the speakers when you're sitting in your car, it should be obvious that your ears aren't getting ANY of the highs. Your knees are getting them, fine. Unfortunately for our body's design, this doesn't help us out much. There's a second issue, as well. When you deal with coax speakers, they must compromise something in responsiveness in order to offer an 'all-in-one' package of bass, midrange, and highs. Unfortunately, what you end up getting is a muddied bass, a blah midrange, and OK highs (that are aimed at your legs). The single best way to improve the bass response is to get a dedicated woofer and have it connected to a crossover network that filters all the highs away to your tweeters. These tweeters will be mounted closer to your ears, and you'll already have an infinitely better sounding system than before. Speakers are a lot like tires in this way. Just get good ones for the snow, good ones for the rain, and good ones for the other days. Or buy one set of all-seasons and drive around hating life.

As for crossovers, if you get a component system, they nearly ALWAYS come with some kind of crossover network. So no problems with a muddy bass or crippled highs. They're usually somewhat adjustable (+/- 3db for the tweeters) in case the tweeters are producing 'too much' highs. This was the case in my system, so I simply cut off a few watts from the tweeters via the supplied crossover. No more problems. The last note of concern when installing separates is WHERE? I probably put more thought into that question than any other. I did research on Miata.net, I annoyed practically anybody who had ever even MENTIONED owning separates, I even rode around for a few hours with the tweeters velcro'd to different parts of the car to test it for myself. In the end, I like having the tweeters mounted high on the door panel, in the soft squishy part at the top. There are a few reasons for this:

1. It's VERY easy to install them there
2. They look very custom (I believe the M editions have the tweeters mounted here)
3. They don't scream 'EXPENSIVE CUSTOM STEREO INSIDE' like before when I had them mounted near the A-pillar on the sides of the dash
4. It made the mounting of the crossover 'box' very simple
5. Imaging isn't that bad

Imaging, as you may or may not know, is how accurately the sound reproduced resembles the actual intention of the musicians when they recorded the music. In general this means a pretty flat response curve (just enough bass, mids and highs). It also means everything getting to your left and right ear at the same time. Unfortunately, this is nearly impossible in automobiles. The left channel is just too close to your left ear, and the right channel is all the way over in the passenger side (oh, I forgot, it's reversed for you in Australia, isn't it?) You get the idea. So the problem is, when you mount the tweeters inside the  door, they're even CLOSER to your ears than coax's. For me, it didn't matter. For you it might. As they say, your milage may vary. But what I found was:

1. In general, speakers sound best when placed close to each other (normal speaker box construction will tell you never to place separates more than 6" apart from each other). Problem is, this means that your tweeters are aiming at your knees. This will not cut it.

2. Car speakers sound best in reference to driver when placed in upper-dash, near A-pillar of windshield -- that is, put the tweeter in the driver's side as far away from that ear as possible. Problem is, the separation of the frequencies is going to be terrible, (i.e., you'll say "THAT'S where my highs are coming from" and point to the window, and "THAT'S where my bass is coming from" and point to the doors). To put it mildly, that will suck a lot. Also, you now advertise your custom audio system to any potential car thief passing by. This, too, will not cut it.

3. Speakers sound 'ok' in reference to each other and 'ok' in reference to driver when mounted in upper door. And if you paint 'em black, they're almost invisible to the outside world. As it should be.

Again, it's a compromise, but it works for me.


The whole idea of headrest speakers flies in the face of reason. If
the sound system on the stock Miata was any good at all, they wouldn't
have even INCLUDED headrest speakers (as evident on the '99 w/ Bose
system, or the M-edition Miatas -- no headrests!) There's a reason for
this. The headrests are only large enough to hold 3.5" speakers, albeit
a LOT of them. But what kind of sound do 3.5" speakers make? Some
midrange, and a WHOLE LOT of highs. Naturally, you don't want highs
coming from behind you! First off, that's not how most music sounds.
If anything, you want rear-fill in the LOW range. But that's it.
Unfortuantely, you can't very well put a subwoofer in the seats.
The truth is, the headrest speakers were designed because Mazda knew
their sound system was so anemic they would HAVE to include extra sound
for top-down driving. The solution was clear: put speakers in the
headrests, RIGHT BEHIND your ears, and you'll have the sound you need.
Of course, what this effectively does is place 2 tweeters behind
your head when you're driving, screwing up the soundstage to all hell,
and giving you an enormous headache in the meantime. Trust me on this,
you don't want to use headrests. I'd almost advise against installing
anything in there, except for the fact that I know there are times when
you're driving with the top down when you really want the extra volume.
Honestly, though, if you have good enough front speakers with a good
amp, you won't touch the headrests, except maybe to turn them off. You
want the music to be coming from the direction you're driving -- from in
front of you. It sounds best this way.
Now, enough of my tirade against headrests. Let's examine the
installation. You have 4 speakers. None of them should be trying to
reproduce bass, so first thing would be to get some bass-blockers on the
speakers. You mention capacitors before in regards to the headrests.
Capacitors were used in the stock headrest speakers to block sound , so
the suggestion was to cut the caps in the STOCK speakers to get more
sound. Of course, you already have new speakers for the headrests, so
this isn't a factor at all. Don't cut anything on your custom speakers!

Bass blockers will prevent your smaller speakers from trying to
reproduce low-frequencies (~120 Hz and below). Since they don't have to
try and do what they were never designed to do, this usually gives you a
little more power to drive the mids and highs (a couple of watts at
least). But how are you trying to drive them? You mention before that
you were using the headunit to power them. Understand this: if you're
driving only two channels (rear left and rear right) of the headunit
into those 4 headrest speakers (which I think is what's happening in
your case), you're only getting something like 7.5 watts per speaker.
This just isn't enough! For your information, that's precisely how much
the STOCK miata radio produced in the headrests! So what can you do
about this?

Unfortunately, I don't know the specifics of your radio. I'll offer
one suggestion, the other being to get a new amplifier (I'll talk about
this later). Or, of course, you could just choose to deal with it :)
What you CAN do is this: you can attempt to bridge the front and
rear channel speaker outputs on the headunit and sending them to your
rears. If you are just using the RCA preouts for the front channel to
send to the amplifier, this should mean that you aren't using the
speaker outputs for the front channel at all. Now, I don't know if your
headunit turns off the signal going to the speaker outputs if it detects
an RCA output being used, but it might not. If not, you can just use
the front channel RCA preouts to power the amp (and thus the front
speakers). Take the normal speaker output for the front channel and
connect it with the rear channel. This will double your power output
for the rear channel. I don't know what this would do to your fader
controls, though!

Remember, you can only do this if the RCA preouts are independent of
the speaker outputs. You can test this easily by connecting two
speakers to the FRONT speaker outputs of the headunit, then connecting
the RCA preouts for the FRONT to the amplifier (where two other speakers
are connected). If you only hear the front speakers, or only hear the
speakers off the amp, it won't work. If it does work, you might have
problems with your fader control. But, with the addition of some bass
blockers, you're looking at about 18 Watts per speaker, which is about
250% better than before.


To make this section easier to understand well, I'm going to take a
second to explain why you want to have the most power amplifier you can
buy. First, let's define power. Have you ever wondered why some amps
are rated at 400 Watts but only cost $150, while a Rockford Fosgate 40
Watt amp costs almost $400?? The reason is because a lot of amplifier
manufacturers try to inflate their claims of power. The really
important factor is the THD, or total harmonic distortion, at those high
power values. Distortion is measured on a curve, and is generally
exponential. What this means is that your stereo at volume '5' is going
to sound 80% clearer than at volume '6', and 200% clearer than at '7'.
This is just an example distortion curve. A REALLY good amp, on the
other hand, has a very, very low rate of change, until you get to about
the peak amounts. In other words, volume 5 sounds the same as 6, 7, 8,
and 9. Only at 9-10 does it get even a little distorted.
What you have to understand is that I'm not talking about distortion
in terms of speaker distortion, or car vibration. I'm talking about the
actual signal that's SENT to the speakers. Get a crappy amp, and the
signal that's being SENT to the speakers will be distorted at
unreasonably low levels. Naturally, if you have crappy speakers it will
only get worse. And if you have a speaker box in the shape of the Miata
door (with it's own share of vibrating mechanisms that I'll talk about
later), you can only imagine how wonderfully BAD it can sound.
Now, those really expensive amps will give you a clear amplified
signal no matter the volume that it's asked to drive. And when they say
'40 watts' they are actually being conservative, unlike the 'Discount
Audio Club Special 500 Watt Amplifier' that's probably not useful after

In conjunction with the amplifier, the speakers must be rated to
handle the amount of power you'll be recieving from the amp. Your
headrests, for example, can probably handle 25-30W continuous power.
Again, peak power doesn't mean squat. Your Alpine coax speakers can
handle 50 watts, as can their component counterparts. That means you
must get an amp that can producte 50W of continuous power per channel,
or a 100W amp. In general, you want to match the amp with the
speakers. So, don't get 25W speakers and match them with a 500W amp.
(you'll get speaker distortion at volume level 2!!) Similarly, don't
get 100W Boston Acoustics component speakers and drive them with the
headunit (or you'll be under-powering them, which means you'll start to
hear amp distortion at around volume level 5). You want a good match
because you want to be able to turn up the volume and get a clear
increase without audible distortion. If it distorts a little at volume
8 or 9, no big deal.

This is my primer for amps. In your case, MTX makes decent amps, so
as long as they're rated at 100W continuous power, you probably won't
have a problem here. If your problem is a rattling or vibration in the
door, that means the speakers are getting the signal just fine (a good
thing), but your doors have a problem (I'll address this next). If you
can hear the speaker actually distorting when you turn it up to around 5
or 6, that means something can't handle the power. If you don't have
too crappy an amp, that usually means it's the speaker -- again, get

You can tell if you have a 'good' amp by looking at the following:
Brand Name: There are no 'steals' or 'great deals' to be had here;
you definately get what you pay for, so look for a good name. Examples
would be Clarion, Infinity, Kenwood, Kicker, Pioneer, Rockford Fosgate,

High-Current: An amp is considered high-current (not to be confused
with high-power) if it can handle bridgeable channels starting at 1
Ohm. If they can go down to .5 Ohm or less, don't even worry about it
-- it's a good amp! 2 Ohm or higher means it's not high-current, though
this doesn't necessarily mean a bad amp.
Low THD: Make sure it's low at HIGH power outputs -- they usually
won't tell you this if they suck.

Again, your mileage may vary.


You mention at one point trying to cut the lower bass frequencies in
order to prevent that annoying vibration in the door. DON'T. You
shouldn't be crippling your system to make it sound better! That
doesn't make any sense! The Miata door is INFAMOUS for it's buzzes and
rattles. In general, though, they can be traced to the following two

The door lock: get oil, grease, K-Y jelly, anything and everything
you can to make this as smoothly operating as possible. Ditto for the
next thing:
The windows: make sure all the bolts are tightened, screws are
secured, the actuator moves smoothly and the window doesn't have too
much free play.
If anything is allowed to move very much in that door, it will

vibrate like all hell. Just secure everything tight, and grease up all
the moving parts. This should fix the problem. Be thorough. Even if
it takes you 5 hours to find what's doing it, it'll be worth the
hundreds of hours you'll spend swearing at your car if you don't.
By the way, you should generally NOT use any dampening material like
Dynomat or NoizeKiller in your doors. The reason is simple: in a
standard speaker, the speakers are mounted in a wooden BOX, because the
box allows the resonation to produce more volume. Normally, bass
frequencies produced by speakers are just too low-volumed to hear -- the
speaker enclosure amplifies it. In this case, your DOOR is the speaker
box. Dampen the sound, and you're killing your bass. You don't want
that. What you want is a correctly operating door, so fix the problem
by attacking the source of the vibrations. If you see that something is
vibrating against the metal, by all means put something in there to stop
it (dynomat, noizekiller, foam, styrofoam, etc.). Otherwise, don't mess
with the stuff...


The larger the wire, the more clear the signal. Simple. For the
speakers, try 16-20 guage wire. For the headunit to the amp, get the
best RCA cable money can buy, complete with radiation shielding and
kryptonite connector. I'm serious. If you scrimp here, you'll get some
annoying electrical hum that will drive you insane and it'll be your own
stupid fault. Monster cable makes great wire, for $30.00 (US dollar),
you can wire up your car. It's worth it.

Get a really, really THICK cable for the wire from the amp to the
battery. We're talking 8-10 guage. If you use thinner wire, it'll heat
up easier, and cause an electrical fire, and toast your lovely Miata,
and you'll have to walk everywhere and explain that you burned your
Miata because you didn't listen to Matt. You don't want that, so use
thick cable. Get one of those special connectors for hooking up power
lines to your battery, don't just solder it on to the terminal (I know
you're not an idiot, I just feel I should mention the obvious). Cut the
power wire going to the amp at some point near the battery and hook up a
40A fuse inline, just in case. You just never know -- and like we went
over before, Miata's dislike fire.

Use the same thick guage cable for the ground connection to the
car. Try grounding it to the battery ground -- this minimizes the
amounts of grounds your car uses, which is good for a couple of reasons:

1. Less signal noise (that annoying hummmm in the background of your
radio music)
2. Less likely to electrocute yourself when you close the car door
on those cold, dry days (my Miata is ALWAYS shocking me!)

The single biggest problem with the Miata sound system is its
complete lack of bass. Putting a good component system in the front
will allieviate a lot of this, but for those of us who enjoy moving our
cars without turning on the ignition, a better bass system is required.
But how? Or rather, to translate into typical Miata-owner language:
'Where?' Good question. I suppose you could put it in the trunk
(boot), but we know how much this would suck. First off, you're cutting
down on the already Liliputan amount of storage space given. Second,
you don't want your bass to sound like it's coming from THAT far behind
you. It will end up sounding like there's some ghetto-mobile riding
behind you that's playing the exact same music you're listening to. No

Of course, if you don't care about passenger room, or all your
friends are under 5'5", you could throw a small 6.5" Bazooka tube in the
passenger footwell. Again, not generally a practical solution.
Or you could do what looks like Mazda SHOULD have done when they
designed the Miata: put the subs in the rear deck. "What rear deck?'
you scream, 'You mean that stupid LEDGE behind my seats. There's NO
ROOM!' To the contrary! If you actually go through the pain-in-the-ass
procedure to remove the rear deck, you'll find the mystical room of
which I speak. In all honesty, it's not THAT hard to get to the rear
deck, but it involves taking off around 15 10mm heavily torqued bolts,
not to mention the seatbelt cover (a bitch in itself) and carpeting.
What you'll find is that the rear deck is just a piece of folded
metal that comes out quite easily (after removing aforementioned
bolts). Underneath this, though, is NOTHING. I mean, it's just this
blank SPACE staring out at you, screaming 'PLEASE FILL ME UP!'. All you
have to do is cut two 6.5" holes in the deck plate and mount the
speakers. There's MORE than enough room to accomodate the depth. I've
seen one crazy nut who put two 8" speakers in there! I think two 6.5"
will do fine, thank you. When you mount the carpet back over the
speakers, put some foam in between or risk the dreaded carpet buzz (only
slightly more annoying than the electrical hum).
Also, remember that they must be free-air speakers. If they're not
specifically designated 'Free-Air', assume they're NOT. What this means
is that instead of requiring an enclosure of some sort (that is, a BOX
that has to sit in your trunk), they can use the empty space behind them
to generate the 'thump, thump' of a good sounding bass speaker. Kicker
makes some cheap, good FA subs, (that's what I'm using). If you do
this, just remember to cross them over at around 80-100Hz. Any higher
and they're not doing their job correctly. Some nice amplifiers have
built-in crossovers for subs, otherwise just get a simple $20 low-pass

Many people might try to convince you that the rear deck is not a
good place for subs, because they're effectively aimed upwards to the
sky instead of actually AT you. Forget about these annoying people.
They're just jealous that you can fit a duffel bag inside your trunk and
they can't. Anyway, the bass will be coming from the trunk, behind you,
and everyone knows directionality isn't nearly as important in bass
thumpers as it is in tweeters. So there.

Ok, that's about all the advice I can think up right now. It should
be more than you need, but I know how it feels to be starting out with
audio mods to the Miata. It's addictive, and there aren't enough
reliable sources out there to get info from. For what it's worth,
here's my system:

Stock headunit (have issues with my car getting stolen again)
2 x Boston Acoustics Pro. Series 6.4 (6.5" woofer, 1" tweeter) in
2 x Boston Acoustics Pro Series 4.4 (4" woofer, 1" tweeter) in
Sony XM 754HX 300W 4-channel Amp
2 x 6.5" Kicker free-air subs in rear deck
Pioneer 100W Amp @ 1 Ohm.

Just in case you were interested in knowing, my system is LOUD and
CLEAR. I'm putting in the extra bass because I think I could go for
some more rear-fill, but it really doesn't need more than its got. I
just enjoy car audio as a hobby, and don't mind putting in the effort
for that 'extra custom' sound. The Sony Amp is wonderful, balancing out
the 100W RMS Boston Acoustics speakers almost perfectly (75 watts x 4).
It has a built-in crossover network (just a simple high-pass/low-pass/no
change circuit, but nice to have), has speaker-level inputs (great for
the stock stereo that doesn't have RCA preouts), and is high-current
(bridges down to 1 Ohm). It cost $200 and is available from
Also FYI, I rarely use the headrest speakers. I have them, and have
used them on occasion with the top down, but once I get the rear-fill
set up, I doubt I'll ever use them again. With the fader set to middle,
they're too loud for me, and screw up the soundstage. I have them set
to about 10% on (middle = 50%).
Final FYI - The best thing about my system is how stock it looks.
My friends sit in my car and look at the CRAPPY head unit and say,
'Where are you hiding the real headunit?' because they can't believe
such a pile of dogshit could produce such clear and loud sound. I'll
tell them what I tell you: my headunit doesn't have to do anything at
all except play tapes and tune radio stations. The Amp in the back does
ALL the work. In fact, I wired up an aux. input to the stock headunit
(a very, very easy mod) so I can play MiniDiscs or CD's in my stock
headunit no problem. I have only once turned up the volume in my car to
halfway, and that was to show off to some friends who were standing 20
ft. away from my car. They were impressed. At 1/3 power, the volume is
high enough to be heard from outside the vehicle with the top up. I
imagine at 2/3 the bass would be enough to set off car alarms, and this
is without the subs. That's the power of quality component speakers for
ya'! The sound is wonderful, although it was a bit raspy before I cut
the front tweeters back 3db, but now it's fine. The best part: total
cost (since I installed everything myself) is $350 for the speakers (I
splurged here, but they were a GREAT deal that I got used from a friend
getting out of audio competition), $200 for the amp (best deal I've seen
on it), $50 for assorted cables and connectors. The subs are another
$100 each, the amp another $130. That's a total of about $850 for a
system that could have cost me couple of thousand to get custom
installed. If the thought of spending $850 for a car audio system
sounds crazy, think about this:
1. The cost of the fancy factory Miata system is at LEAST that, and
sounds worse
2. Some people shell out $400 for a headunit, $200 for speakers,
$100 for an amp, and another $300 to get it installed, and wonder why it
sounds like dung.
3. Hey, it's only half a month's wages!

I hope this helps you out, and isn't too daunting. I've been asked
questions like yours before, so I wanted to make this as in-depth as I
could so that I can just edit it a bit and post it up to Miata.net. At
least you get the personalized version... :)
If you have any other questions, feel free to ask...

Matt Brock, the Miateer mattb@romnet.com

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01 April, 1999