2004 Prius JBL Audio system response, specs, and photos

Written 7/05, revised 10/07, by Tom Morrow

This document presents what I found out about the 2004 Toyota Prius JBL sound system while deciding how to improve upon it. Frequency response of the signal delivered to the drivers was measured for the various channels. Pictures and other accompanying information are included. Various information I gathered from the internet and elsewhere is also consolidated here. Note that Toyota has two different audio systems available on the 2004 Prius, the Basic system with 6 channels and the JBL Premium system with 9 channels. This document only concerns the JBL premium system, although there may also be information here of use to people with the Basic system.


Electrical specifications

The nine speakers are: (This information copied from VFAQ )

  1. Front driver's tweeter (top front corner of front door) - 2.0 cm (0.8") 6ohm
  2. Front driver's woofer (bottom front corner of front door) - 16 cm (6.3") 2ohm
  3. Front passenger tweeter (top front corner of front door) - 2.0 cm (0.8") 6ohm
  4. Front passenger woofer (bottom front corner of front door) - 16 cm (6.3") 2ohm
  5. Rear driver's tweeter (top front corner of rear door) - 2.0 cm (0.8") 6ohm
  6. Rear driver's woofer (bottom front corner of rear door) - 16 cm (6.3") 3ohm
  7. Rear passenger tweeter (bottom front corner of rear door) - 2.0 cm (0.8") 6ohm
  8. Rear passenger woofer (bottom front corner of rear door) - 16 cm (6.3") 3ohm
  9. Center dash (above screen) midrange (center channel, not subwoofer) - 6.5 cm (2.6") 2ohm

The head unit includes 6 disc CD changer, radio, and cassette player. The JBL amplifier is separate from the radio; it lives under the passenger seat. It is rated at 45 watts per channel to each of 4 channels by Toyota. That rating is curious since the amp has seven speaker level outputs not four:

The fact that Toyota says the amplifier has four channels might mean that the Front Woofer/Tweeter/Center channels are split out via passive crossover located inside the amplifier housing. Or it could just have been an error on Toyota's part; perhaps there are more than 4 channels of amplification.

The front woofers and tweeters as well as the center speaker are each connected directly to the amplifier with nothing between. The rear woofer and tweeter are connected in parallel to a single amplifier connection at the tweeter. The rear tweeters have a capacitor in series (mounted right on the tweeter in the door), which acts as a high pass filter to keep some of the lows from reaching it. The rear woofer is daisy-chained in parallel off of the tweeter/capacitor connector. All drivers have quick-disconnects on the drivers themselves which allow them to be disconnected and/or reconnected easily.

The amplifier is very much customized for the Prius. It controls the volume of the outputs, as well as the fade/balance. It has a computer network connection to the radio, where digital volume, fade, and balance data is sent to the amplifier. The radio sends two channels (Right and Left) of balanced analog audio to the amplifier, at a fixed level (not changed by the user's volume control). The amplifier performs the crossover function for the front channels, sending lows to the woofer and highs to the tweeter. In addition, there is equalization in the amplifier that reinforces some frequency ranges and lowers others, and the equalization changes depending on volume setting.

The fact that the amplifier takes a fixed level input and controls the volume itself means that the amplifier cannot simply be replaced by an aftermarket amplifier, because aftermarket amplifiers need a variable level input that varies with the volume setting the user has chosen

The JBL amp produces all the beep tones that happen when you press the buttons next to the LCD or on the LCD itself. It sends the beeps to all speakers simultaneously, including to the center speaker, and the beep amplitude seems to be constant, not related to the volume control setting.. Disconnecting the JBL amp outputs will mean you will no longer hear those beeps. The following beeps are not from the JBL amp and happen even with the JBL Amp completely disconnected: Seatbelt, Lock/Unlock with keyfob, Reverse.

The JBL amp mutes the audio when voice command is used. The analog signal coming into the JBL amp from the head unit is muted during bluetooth phone conversations. So if you disconnect the JBL amp you will have to turn the music off manually when using speech recognition, but you won't have to do so for bluetooth telephone calls if you are using the factory head unit.

There are signal wires feeding into the JBL amp that are labelled MUTE and N-MU on Toyota's schematic. I did not measure any voltages on these wires even when the audio signal was muted for voice command or bluetooth phone. But it is possible I might not have had a good connection in my test, so test yourself if these signals are of interest to you. It is possible that these wires may use grounding to indicate muting.

The LCD displays take into consideration whether the amplifier is connected to the head unit. When the JBL amp is disconnected the volume control on the LCD does not display a level number, and the Sound controls such as Bass, Mid, Treble and Fader/Balance are greyed out so that the user knows they cannot adjust them.

I measured the levels of the balanced signal from the head unit to the amplifier using track 21 of the Stereophile Test CD 3. This signal is a 1khz sine wave tone at -20db relative to digital clipping. I measured 200mVac with my digital multimeter between Left Positive and Left Negative. I measured 100mV between Left Positive and Ground (or Shield), which confirms that this is a balanced signal. In order to get the max output level, it is necessary to multiply the 200mV value by 10 according to Stereophile, so the max signal level is 2 Volts balanced. This sort of balanced signal should generally not be directly connected to an aftermarket amp or other device; a special line output converter should be used such the Soundgate LOCPREA or LOCB.2 balanced line output converters if you want to tap into that signal. However most people will not use that signal because it's not controlled by the volume control.

Physical measurements

All of these measurements were made with a ruler by myself so should not be considered official specs.

All woofers are mounted low and towards the front of the their doors. There is a black plastic ring riveted to the door sheet metal, extending approx 0.5" past the sheet metal. The metal flange of the speaker is screwed into that plastic ring with four screws, and a sticky rubber gasket sits between.

Front Left Woofer (right assumed to be the same)


Rear Left Woofer (right assumed to be the same)


Front Left Tweeter (right assumed to be same, rear assumed to be the same except for addition of capacitor)


JBL Amplifier


Under the Passenger Seat

The following information is provided to be helpful to someone who might want to install an aftermarket amplifier or other device under the passenger seat. This could be a good mounting location for a very small factory amp because it leaves the rear hatch area free to hold batteries to convert the Prius to an EV later on, and the necessary connections are available under the seat. Ventilation is also probably better with underseat mounting than in the lower hatch cargo compartment. The floor is not flat under the seat, so measurements of the height reported here are very approximate and should not be relied upon for any important purposes.


Rear cargo box

The covered cargo box under the hatch area has the following maximum interior dimensions: 33" x 21.5" x 5.5". Note that because the corners are not square the actual usable space is slight less. This seems like a good place for amplifiers, although heat would be a consideration.


Amplifier frequency response measurements

In order to determine the frequency response for the amplifier channels, I used two different test CDs: Stereophile Test CD 3 has full range of warble test signals in 1/3 octave increments from 20Hz to 20kHz, tracks 17-19. I used that for most of the graphs. Autosound 2000 CD-101 is a test CD that has individual tracks with sine wave frequencies from 10 to 99 Hz for the graph of below-100Hz response. Both discs were played through the dash head unit CD player.

For each channel tested, I disconnected the driver(s) and replaced with a 3.3 Ohm resistor. Using a Fluke 10 digital multimeter, I measured the AC voltage across the resistor in millivolts. Volume was fixed and tone controls were centered (except for the charts that display tone control results). Results were entered into an Excel spreadsheet and graphed with Excel. If you want access to the raw data you can download the spreadsheet.

As a control, to make sure the test is valid, I did similar tests on my Home stereo: I used a portable Panasonic CD player feeding into a Harmon Kardon home amplifier, connected to the same 3.3 Ohm resistor. The home stereo gave basically flat frequency response, except for some high frequency rolloff, as seen in Graphs 4 and 5. Since the Fluke 10 multimeter I used is only rated to have a bandwidth of 5Khz, the high frequency rolloff above 5kHz shown in the graphs is likely due to the meter not the actual signals.


Graph 1



Graph 2

Graphs 1 and 2 show the rear channel output at different settings of the volume control. They are both created from the same set of measurement data; Graph 2 incorporates scaling to factor out the level changes. These graphs clearly show that at low volumes the JBL amp is adding bass boost at 70Hz, and treble boost around 5kHz. The effect is that the frequency response is much closer to flat at high volumes than at low volumes. The volume control has a range of 0 to 62 plus "MAX". I typically listen to the factory system at volumes between 30-40.

It seems like one approach for getting flatter frequency response would be to somehow make normal listening level be 50 or so. This could be done by feeding the JBL outputs into an aftermarket amplifier and turning the gain down lower. Or it might be doable by adding heavy duty resistors to the factory system to soak up power, but that solution could increase distortion, heat, and possibly change crossover behavior.

In order to get an idea of whether the upper volume ranges introduce excessive distortion, I listened to track 99 of the Autosound 2000 CD-101 test CD. That has a signal that slowly rises into digital clipping and then stays just below the clipping level. Playing that track I didn't hear any additional clipping even with volume turned up to 62, so on first glance it seems that the upper volume level JBL amp outputs might be clean enough for use fed into an aftermarket amp.


Graph 3

Based on graph 2, Volume level 50 seems like a good level to get a clean signal, even though it's way too loud for actual listening in the factory system. It's almost as flat as the Volume 60 signal, and has some headroom to make sure we're not pushing up against clipping. Graph 3 shows the frequency response of the JBL amp outputs at Volume 50. The result "TheoreticalSumFront" is the result of mathematically summing the front woofer and tweeter voltages. This summing function could be done with a box like the Audiocontrol LC6, and then we would have almost-full range signals for the front and rear channels to feed into an aftermarket amp, and we would retain the factory fader control functionality. Note that the Audiocontrol LC6 website doesn't explain the summing functionality very well, but from reading the sales note I believe it's done by joining two outputs with a RCA "Y" cable. I did some research into summing and found various summing circuits on the web that just use simple resistors, like this one from Rane and another one. The interesting thing though is that most passive line output converters already have the requisite10 kOhm resistors incorporated into their designs, meaning that they can be used for summing with just a Y cable. David Navone confirmed to me that his NE-774V can be used for summing signals with just a Y cable on the outputs, which provides a cheaper alternative than the Audiocontrol LC6.

Note that the "TheoreticalSumFront" signal could probably be made flatter by adding some gain to the tweeter signal before the summing. The treble rolloff in the rear channel is actually desirable to me... I prefer the rear channels to just add fill and bass; having too much treble in the back tends to ruin imaging.


Graph 4


At volume level 30 (a normal to low listening level with the factory system), as shown in graph 4, the 70Hz hump in bass output becomes evident, as well as the steep rolloff below 70Hz. This is undesirable to me, and so I would want some way to get a flatter frequency response. Note that for comparison I have shown the Toyota Head Unit signal that feeds into the amplifier. That has a nice flat response (except for some gradual high frequency rolloff). Tapping into that signal would clearly give the flattest signal of all. This graph also shows that the center channel speaker is band limited, dropping off below about 500Hz and above about 7kHz.

In home theater systems simply disconnecting the center speaker will result in significantly lower levels for dialog and other "centered" sounds. I wanted to make sure this effect was would not occur with the JBL system. To test this, I used the "Stereo Review/Chesky Gold Stereo and Surround Sound Set-up Disc" Track 11. This is a "stepped stereo pan test", where a sound moves between right and left. I listened to see if it would be significantly quieter when the sound was in the center with the center channel disconnected. It was not; the sound retained roughly equal volume all the way right to left whether the center was connected or not. This indicates to me that the center signal is driven off a simple sum of front right and left (with bandpass of course).

Note that the high frequency rolloff shown for the ToyotaHeadUnit data (as well as everything else) is likely the frequency response of my Fluke 10 meter, so it may even be flat up into the higher frequencies.


Graph 5

Graph 5 simply shows higher resolution data for the poor bass response at Volume 30. It shows that the peak is at 70Hz. At least the response is smooth and therefore amenable to parametric equalization.



Graph 6

One thought was "I wonder if I could use the factory tone controls to smooth out the frequency response". This graph seems to indicate that the tone controls have generally the wrong center frequencies to make much improvement.



Graph 7

Graph 7 was made from the same data as graph 6, by scaling it so that zero is the no-tone-control signal. From this graph we can see that the tone control center frequencies are approximately: Bass 80Hz, Mid 2kHz, Treble 7KHz.


Summary of measurement results

The main observations are:


Possible ways to improve sound

It is clear that there is a need to add more low frequency response.

My suggestion for most people wanting to improve the sound quality with minimal effort is to install a high quality front component set (woofers, tweeters, and crossovers), and drive them off of the rear output of the JBL amp (which has better bass). The rewiring might be accomplished under the seat on the outputs from the JBL amplifier. Many people don't need the rear speakers at all so they can be disconnected, but if you insist on having rear speakers for occasional rear passengers then you can probably remount the front woofers and tweeters in the rear, rewiring so that they are driven off the JBL front woofer and tweeter outputs.

Adding a subwoofer is the most common approach to getting more bass, mostly because audio installers don't need to understand the details of the Toyota audio system in detail to implement it. Connecting it to the rear outputs would give it the best signal to work with. But note that you would still be sending it a signal that is not flat. This might not be a significant problem since in the real world resonances and other environmental factors often change the bass response by much more than that, and you can always crank up the level of the subwoofer. Many subwoofers have "Bass Boost" circuitry that boosts around 50Hz or so, and that might be useful for evening out the response. It might also be useful to set the subwoofer crossover frequency lower than 70Hz, perhaps 50 Hz, in order to counteract the effect of the sloping input to the sub.

Another approach might be to employ a parametric equalizer to smooth out the 80HZ boost. The Rockford Fosgate Power series amplifiers have the "Para Punch" bass boost on the rear channels, which has a variable center frequency of 35-70Hz and 0 to 18db boost. This type of control could help flatten the bass response without a subwoofer.

The JL Audio CleanSweep is designed expressly to clean up the frequency response of systems like this by equalizing the JBL amp outputs back to flat.

I'm not a big fan of subwoofers in cars myself. They mess up the impulse response by delaying bass events however long it takes the sound to come from the rear of the car to your ears. I prefer to have my bass as crisp as possible, even at the expense of missing the lowest octave of bass. A great 6.5" speaker in a door panel that is well damped with Dynamat type products and sealed up can sound much better than one of the compact 5.25" or 6.5" subwoofers whose design goal was to make it small enough, light enough, and cheap enough that everyone would buy it. The door itself becomes a subwoofer, with greater interior volume than you would ever find in a small sub enclosure, and the leaks in the door essentially turn it into a ported design.


Pictures of the factory system

The JBL amp has mounting brackets integrated into the casing.

Left: The JBL Amp, as seen with the black plastic cover removed and the passenger seat removed. Right: JBL amp removed, exposing air vent.


Left: connectors to amp plugged in. Right: connectors unplugged.


Left: the Speaker/Power connector on the amp. Right: the Head Unit connector on the amp.


Some shots of the interior of the amplifier... not a lot of circuitry in there. Right picture: you can barely see the power transistors (or whatever they are) heatsinked and screwed to the case to conduct heat away.


Left: holding the sail panel so you can see the silver clip and white snap on the bottom that attach it to the door. Right: holding the tweeter under the back of the sail panel where the tweeter fits in.

Left and Right front doors

Location of the woofer in the front door


Front door woofer off. Window rolled all the way down. Notice glass 3" away from mounting lip would prevent very large magnet speakers from being used.


Left and Right rear doors

Left: the back of the tweeter assembly. Notice the black capacitor on the rear tweeter. Right: Right Rear tweeter connector


Rear speaker. Window rolled all the way down is nowhere near the speaker well. The side impact bar is a good 4"+ behind, so big speakers would fit back here.


The center speaker.



Disassembly information

If you want to replace the speakers, I suggest you first read Sparky's guide. That has lots of pictures and will help you understand how to disassemble the rear doors much better than my discussion below.

Here's the cliff notes on removing the interior door panels:

The speakers are then visible and can be unscrewed by removing the four philips head screws. They will still be held in place by the very sticky rubber gasketing between the speaker flange and the black plastic mounting ring. Use a screwdriver to pry the speaker out; it takes more force than you might expect. Do it slowly so that there is time for the rubber gasket to unadhere gradually, and try to keep the gasket sticking to the speaker basket so it will be easier to reinstall.

The tweeters just press fit into place and are easy to pop out.

The center speaker can be removed easily without disassembling anything else. The speaker is mounted to the grill assembly. The edge towards the back of the car swings up. I found I could work my fingernails and then fingers under the corner nearest the passenger seat, and just pull up to pop it out. Don't try to pull it from the edge towards the front of the car because that's the "hinge" side. Some others have suggested a butter knife wrapped in fabric, but if you can get it with your fingers only that's preferable. Be careful because that area will show marks easily.



My system

After immersing myself in the world of car audio while researching this document, I decided it would be fun to enter my vehicle in some car audio competitions. As I was putting my system together I decided to make the effort to comply with the rules of the car audio competitions. I was so inspired by the local qualifying competitions that I decided to try for the world finals. I qualified and I drove from California to Atlanta and back for the 2006 IASCA world finals car audio competition. I was a bit disappointed that I came in last place (8th out of 8 in Rookie), but most of the other competitors, even in the Rookie division, had much more experience with other competitions than I did. Most of my competitors completely gutted their vehicles and replaced panels with new fiberglass to optimize sound quality, spending tens of thousands of dollars on dedicated vehicles and top of the line exotic electronics to wow the judges. So I was proud of my $2500 system with used scratched head unit being in the same league. My goal was to have fun competing for a year and then end up with a practical great sounding daily driver.

Competition setup

This install is completely stealth, so that no car functionality is lost to the system. The hatch area remains fully usable, and the system is invisible when closed up. The electronics are mounted above the fully usable spare tire. The subwoofers are in the rear doors. The factory head unit remains usable for the in dash CD changer and cassette, and an aftermarket head unit is mounted below it. The processor controller is in the glove compartment. Front speakers are mounted in the factory locations.

The centerpiece of the system is the Alpine H700 processor which provides equalization, crossover, time alignment, switching, digital and analog inputs, and a volume control, allowing me to get a pure digital signal path completely independent of the factory system for my mp3 player. The H700 has a separate controller mounted up front that allows me to tweak everything easily.

Changes to be made after the competiton for daily driving:

To meet the competition rules I had to use an external Alpine head unit, but after the competition I will remove that and mount the Alpine H700 processor controller under the factory head unit, so I can use it to switch between the factory head unit sound (connected via the rear speaker outputs to the H700 RCA inputs with a voltage dividing resistor network) and the digital SPDIF output of my Archos Jukebox 20 mp3 player (which receives charging power from a DC/DC installed under the amps). I'll use the H700 equalizer to correct the bass rolloff of the factory JBL amp.

Note that these diagrams and pictures below are of the competition setup which ignores the factory system entirely, so most people would not find this the best setup.




Most of the system is mounted in the cargo box under the hatch area, all under the hatch carpet floor. You see the amplifiers, fuses, and H700 processor, as well as the Maxxbass processor in the lower right corner which was removed because it degraded the sound a little.


The underside of the amp rack shows the connections, and the cooling fan. Everything has quick disconnects so it can be removed for service.


The Focal TN52 tweeters were installed in the factory positions with some cutting and held in place with mortite weather seal.


The front speakers were mounted in the factory rings, and lots of dynamat type material deadens the door.


The rear 8" JL audio subwoofers required custom plywood mounting rings epoxied onto the rear doors, and I also had to use a heat gun on the interior plastic panels to expand the speaker grille shape and give them room to boom.


The system in the cargo box lifts out to access the spare tire, but is secured for theft and accident prevention with a bolt through the center into a custom bracket on the spare tire bolt.


The competition head unit was installed below the factory head unit, but after competition that will be replaced by the H700 controller which was in the glove compartment.




Sparky's guide to replacing the rear speakers in the Basic (non-JBL) system.

Schematic of the Audio system

VFAQ has some basic facts on the audio system.


Please contact me with errors, omissions, and compliments.