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lithium

Where did my midbass go?

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Posted (edited)

So I decided to post this outside my build log to see if anyone might have any ideas to help me solve this issue. 

Equipment

SLS 8inch Midbass Drivers running 60hz - around 200 i think. Mounted in the doors w/ a pretty solid treatment of SDS  MLV, CCF, and CLD. Shitty GM doors aside, its a pretty solid installation. 

kicker 250.2 bridge to each side

Description of problem:

Midbass from left side is quieter by rough 1/2 to the ear w/ an 80Hz sine wave tone (measures around 8dB lower IIRC). I don't remember there being a huge difference between left and right previously, though I didn't really touch the stereo for over a year. My understanding of interaural time difference suggests that I could have overlooked this issue after setting my TA correctly. 

Stuff I've Tried:

  1. The only thing I've done recently with respect to the installation of the drivers was rebuilding the left door baffle after the MDF was exposed to water. I used 3/4 in cutting boards to build the replacement to the exact same shape. Maybe I didn't build a robust replacement but it seems to be fine when I feel for vibration and so forth (feel solid when speakers are playing). 
  2. More CLD in the left door. I had a resonance issue in the door at 75hz so I went after it. Did not improve output. 
  3. swapped drivers around to rule out issues with amps, speakers, difference between doors, etc. 
  4. measured speaker voltage to rule out amps

Ideas

  • acoustic mode
    • maybe I can build a small enclosure that I can move around the foot well to test this out? 
  • I suck at building speaker baffles out of cutting boards (I didn't find a great glue so I used machine screws)
  • fuck IB door speaker, build door enclosure or IB kickpanels
  • Take some nearfield measurement of the midbass to see what they're doing. Probably do these doors open and use a sweep

I'll take some measurement later and post them. 

Edited by lithium

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Switch the RCAs on the amp input and see if the input issue switches side.  If it stays on the same side, the issue is with the amplifier.  Where are you measuring and what are you measuring with?  8dB is a HUGE difference.  I don't see how you could overlook this if it was happening previously. 

 

I have had a similar issue before with a shorted RCA cable.   One side had a 2V voltage drop.  Took me a year to figure it out.  Throw some tones at it with your meter on it and do some measurements with full sweep of the volume control. 

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4 hours ago, Tirefryr said:

Switch the RCAs on the amp input and see if the input issue switches side.  If it stays on the same side, the issue is with the amplifier.  Where are you measuring and what are you measuring with?  8dB is a HUGE difference.  I don't see how you could overlook this if it was happening previously. 

 

I have had a similar issue before with a shorted RCA cable.   One side had a 2V voltage drop.  Took me a year to figure it out.  Throw some tones at it with your meter on it and do some measurements with full sweep of the volume control. 

Measurements where with a dayton emm6 and taken at the driver's head position (no one sitting in the seat), swept the mic around and averaged. I was using pink noise for an RTA measurement. I'll get some measurements today if the weather cooperates.

I'll also disconnect the drivers and measure output at the amps(AC voltage). They're pretty old refurb kicker kx250.2  so it not impossible that one is on the way out.

I've ruled out the drivers at this point (surprised these guy are still working after 10+ years in a car door).

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When you do a major dash install for stage and image, your ultimate goal is to have the entire sound up and out on your dash. Your mid bass is most likely where you left it, just blended with the dash. Once I got dialed in with time alignment, my doors practically disappeared (way they should be).

my guess is your doors are out of phase with your dash drivers (if you have them wired exact negative and positive). First thing I would try is locating an audio test disc. If I can remember correctly, you can order one from USACI. 

Change the polarities on the midbass drivers to phase them up with the dash. Hope this helps !!!

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1 hour ago, Randal Johnson said:

When you do a major dash install for stage and image, your ultimate goal is to have the entire sound up and out on your dash. Your mid bass is most likely where you left it, just blended with the dash. Once I got dialed in with time alignment, my doors practically disappeared (way they should be).

my guess is your doors are out of phase with your dash drivers (if you have them wired exact negative and positive). First thing I would try is locating an audio test disc. If I can remember correctly, you can order one from USACI. 

Change the polarities on the midbass drivers to phase them up with the dash. Hope this helps !!!

I'm just comparing the midbass drivers by themselves. 

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Just checked the voltage at the amps and at the driver, measured the same. So amps are good, drivers are good, so now its probably installation issues... 

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In order to get my 2 way active setup in phase was to change the polarities on the tweeters. 

Beings you have a 3 way, there’s no telling which drivers are out of phase. 

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Measure in the near field first.  ie, jam the mic in the speaker.  See if they are balanced at the source.  Troublle shooting at the ear isn't always the easiest.

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30 minutes ago, ///M5 said:

Measure in the near field first.  ie, jam the mic in the speaker.  See if they are balanced at the source.  Troublle shooting at the ear isn't always the easiest.

This is where I was going next.  If your voltage is equal, you're only left with phase/cancellation/ or TA, which will affect phase and cancellation as well. 

The only other thing I can think of is driver wear or short.  Once the coil warms up, something can be going awry and causing a resistance increase, effectively dropping power. It can read properly at the terminals, but effectively leak past that point.  Odd, but just throwing possibilities out there.

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Posted (edited)

Oh man, I got some data. I'll try not to make a giant post here. 

So at this point I know the amps and drivers are fine. I matched the amp output voltage and disabled EQ. 

Lets start with some 1/12th octave RTA pink noise measurements. Mic at driver head position and was pointing straight forward (dayton ecc6). Just playing the individual drivers with xover at 50hz and 175hz (4th order i think) everything else is disabled. 

5 - teal is left driver, 6 -gold is right driver. Have some pretty big nulls at 75 and 150hz when playing the left midbass.

No idea what the 35hz peak is, cabin mode or something. When I measured with the sub ON it doesn't show up. [I reviewed some backround measurements, this is noise from the car running at idle]

image.thumb.png.0bb6c6371495c65771f1a8f1f88e762d.png

Near field measurements. I believe the settings I used were 30 Hz to 200 Hz over 5 seconds.  I think we can ignore the level difference, I might have changed the output level to avoid clipping the microphone. I didn't take time to measure out the distance or anything. Just got the microphone a few inches from the cone and measured. 

pink - 9 is left, blue - 10 is right. This measurement was taken with the doors open as well. 

image.thumb.png.b16ab72a4e7288a87e3bbc6640ab9118.png

I repeated the near field measurements with the doors closed. I thought that the latched door might constrain the doors differently, 

green 12 is right, dark blue 13 is left. Im not sure what to make of this. Maybe I should have shortened the sweep length.. I any case, I think we can see that the midbass drivers are performing fine. 

image.thumb.png.ff85bb6e915b4e28c5ec556cef72870e.png

Next I moved the mic around the cabin a bit and measured some additional 1/12th octave rta pink noise responses. Only left midbass is playing. 

Dark blue/green - 15 is measured 2 ft forward of the driver (just above the steering wheel almost). Gold - 16 is measured at the front passenger head. Green - 17 measured between front seat at head level. Purple - 18 Driver position baseline measurement.  

plots  with 1/12 oct smoothing. Easier to read IMO

image.thumb.png.ec0293e91f296c3b441e5b7db4d045a0.png

So I'm starting to get the idea that this is some kind of cabin mode or some structure in the cabin is creating this null. Perhaps something in/under the dash.

Here is one more plot that might demonstrate this the most. These are all measurements with only the left driver playing. 

purple - 18  measured at driver head. Red - 21 measured at driver head and right door is open. Green - 22 measured at driver head with all doors closed and Left driver window rolled down. 

I think its very interesting that null at 75 hz is complete missing with the Rt door open. Rolling down the driver window didn't help too much but I think the window glass is very close to the rear of the driver so perhaps that is not helping and this is a bad measurement. 

Going forward, I think I will try filling some cavities with some denim insulation material I have lying around. Open to any suggestions... image.thumb.png.8dba05573f2fd06daa64312f92c4f2a5.png

 

 

 

Edited by lithium

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Posted (edited)

15 dB of attenuation at 75 hz with a pretty narrow Q. Hopefully its not my HVAC acting as a resonator. I guess I can remove my dash trim and block all the vents. 

Edited by lithium

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1am, I am exhausted, just got to my hotel in Boston...barely read this, but have one concern before even trying to.  How many averages did you take and how many averages did it take before the curve settled down into what you have above?  I want to make sure we aren't chasing variations on the noise excitation signal you are using.  I'd also be curious on all the narrow band FFT settings.

Generally I assume you measured everything right and have found your cabin modes, but would like to digest the plots with more than a 20 second look and need sleep.  I also want to make sure the data is real which is why I asked the questions above.

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Posted (edited)
53 minutes ago, ///M5 said:

1am, I am exhausted, just got to my hotel in Boston...barely read this, but have one concern before even trying to.  How many averages did you take and how many averages did it take before the curve settled down into what you have above?  I want to make sure we aren't chasing variations on the noise excitation signal you are using.  I'd also be curious on all the narrow band FFT settings.

Generally I assume you measured everything right and have found your cabin modes, but would like to digest the plots with more than a 20 second look and need sleep.  I also want to make sure the data is real which is why I asked the questions above.

That's a good point. I think I just rolled with the defaults in REW. Probably should have spent some time thinking about how to take good measurements with REW before hand. I did have fairly good agreement with repeated measurement. 

Edit: After some reading I think I should have performed this test with some different settings. If the current data looks questionable I fine with repeating the test later. 

Sample rate is 48kHz so I have 6hz resolution. 

image.png.2cd51c929fabec3e86960e587a5e3770.png

Orange is full range with all left side speakers and sub enabled.  Green, blue, and purple are just the LT mid bass driver. Blue and Purple are taken at a higher signal level obviously. 

image.thumb.png.7b5fd67cc165acaa82daf97908e75d3a.png

 

Edited by lithium

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Come up with a solution ??

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Posted (edited)

I've been doing some reading. If this is just a cabin mode there is little I can do to fix it. 

Some solutions like boosting 75hz and 150hz with eq have some obvious drawbacks. 

Another idea would be to xover the midbass/sub at 80hz but that defeats the purpose of running large midbass drivers. 

I'm thinking about building a small enclosure for a smaller woofer to try out other locations for midbass in the car, like kick panels. If a kick panel location doesn't excite the same mode then I could move the 8s there. I've seen people vent kick panels thru the tire well. 

Edited by lithium
Fixed typo

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My biggest concern is that you were measuring the variation in the excitation signal and not the response of the speaker.  Measurements need to stabilize with noise so however many averages that takes you need.  Normally 100 covers it, but not always.

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36 minutes ago, ///M5 said:

My biggest concern is that you were measuring the variation in the excitation signal and not the response of the speaker.  Measurements need to stabilize with noise so however many averages that takes you need.  Normally 100 covers it, but not always.

gotcha. I'll repeat the measurements with significantly more averages. Would you recommend changing any other settings or measurement signal?

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Just noticed you posted the settings.  Yes, some changes.

100 averages

Hanning window

67% overlap 

Personally I have no need for 1/12 octave, 1/3 or narrow band is enough ... I am a narrow band guy, but I can calculated the octave bins in my head.

If there are any other settings you have a question on send me a text to peak so I don't miss them.

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How did the mid bass drivers sound before you did the upgrade ??

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Just as a clarifier with only 8 averages and using a white noise signal what you measured was the variation in the excitation not the response.  That being said, the valleys are surely indicative or where the modes of your car are but the variation, width, and values are dubious at best with only 8.  General rule of thumb with an FFT and noise signals are to watch the response as it averages and keep averaging until you see no further changes.  

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I would start with playing just the tweeters first and see how low they can play without stress.  Then I would listen to the mids alone and see how high they can play without breaking up and how low without stress.

Once there depending on mounting locations and aiming anything can happen.  As a first start though I would try to have the tweet take as much mid as it can and the midbass take as much from the sub as it can.  Then compare with less.

Be careful as you adjust to not throw off your baseline, ie, do NOT use any boost in the eq or any voltage gain in the equalization.  All cut.

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1 hour ago, ///M5 said:

Just as a clarifier with only 8 averages and using a white noise signal what you measured was the variation in the excitation not the response.  That being said, the valleys are surely indicative or where the modes of your car are but the variation, width, and values are dubious at best with only 8.  General rule of thumb with an FFT and noise signals are to watch the response as it averages and keep averaging until you see no further changes.  

Thanks for the clarification. I should have known better at the time. 

I'm going to repeat the measurements today. I want to see how severe the mode is. 

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Posted (edited)

Ok, new measurement time. 

These are white noise, 65536-point spectrum using Rectangular window, and around 60 averages per measurement. It settled in about 20-30 averages but I let it run longer. 

4 pink - Left midbass, 5 blue - right midbass 

image.thumb.png.d805c0796112fb887fb13ec128b05201.png

same w/ 1/12 smoothing

image.thumb.png.31e0673428bcb7cab88cd8f784b3499f.png

 

I have a decent amount of amplifier headroom so I tried some pulling down the peaks at 115 hz and 180 hz on the left driver to flatten the response. I also messed around with the gain to level match the right driver. I need some more seat time to see if this sounds ok to the ear. 

Pink is no EQ, Blue is after EQ and bumping up the level. 1/12 smoothing

image.thumb.png.5a942ff768334735ec5bbdd6915e39b1.png

 

Here is the measured comparison between the left and right after some EQ. I took this so I can do some listen later and see if the changes worked as intended. With such aggressive EQ on the Left Midbass it looks like the effective Xover points are different between left in right. Maybe I will not be able to hear a difference or perhaps I'll need to change the xover to better restrict the low and high frequency content played through the driver. 

blue is left after eq, green is right after eq - 1/12 smoothing. 

Additional level matching later by ear. image.thumb.png.55d7151b96e718f53a8d6241188e4a6a.png

 

I think that this addresses the issue unless I'm overlooking something like not having enough amplifier headroom. There's nothing I can really do about speaker placement. I thought it would be fun to do a kick panel IB midbass install but not on this car. The more I listen to the midbass the less happy I am with their current performance. The doors are just not handling the 8s as well as I would like.

After I finish the A pillars I might attempt some door enclosures. I think it would cool to make some door enclosure that are iso mounted on the door frame. I think that would eliminate most of the buzz, squeak, rattle. I'm not sure if 20 year old door hinges will handle more weight but at least I would have an excuse to buy a new car if the fucking doors fall off. 

Edited by lithium

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4 hours ago, ///M5 said:

I would start with playing just the tweeters first and see how low they can play without stress.  Then I would listen to the mids alone and see how high they can play without breaking up and how low without stress.

Once there depending on mounting locations and aiming anything can happen.  As a first start though I would try to have the tweet take as much mid as it can and the midbass take as much from the sub as it can.  Then compare with less.

Be careful as you adjust to not throw off your baseline, ie, do NOT use any boost in the eq or any voltage gain in the equalization.  All cut.

Thanks for this. I was messing around with the tweeter and mid range xover point and started way to high initially. The mark audio drives are pretty decent up high but a  tweeter can do a bit better it seems. I just roughed things in for now and will do a full tune after building the  A pillar enclosures. 

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One other warning, don't use the measurements as gospel as I assume you weren't sitting in the car and measuring at both ears.  Even then your head will influence the measurement.  Of course finding modes and odd response anomalies to try to attack with eq surely works.

12 hours ago, lithium said:

Thanks for this. I was messing around with the tweeter and mid range xover point and started way to high initially. The mark audio drives are pretty decent up high but a  tweeter can do a bit better it seems. I just roughed things in for now and will do a full tune after building the  A pillar enclosures. 

Glad it helped.  Isolation and understanding limits make a big difference in what can be done.

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