sub freq sweeping

Ok, i'll make this the shorter version of a perplexing question. I have two 12's in the back of a 95' Chev Monte Carlo. They are on individual amps and are recieving about 550rms each from their respective amps; the amps are set pretty close to the same freq. range at about 50hz. The subs are sealed (individualy chambered), rear firing, and the timing is a little off (slow). Also the polarity on both subs is positive.

In the front of the car, in the passenger floorboard, I have a sealed enclosure with an 8' sub in it. Foreward firing toward the firewall recieving about 250rms, and set at about 120hz. This sub was added to give me lost mid-bass to the front stage in an SQ car. I have to say it is doing it's job perfectly, giving me an impresive fullness and depth at about eye  level, about 3" up the front glass, and appears to come from dead center at about 2ft. deeper than the windshield. There is a gain knob on this front amp, to adjust to taste.

That's the set up. Here's the part I hope can be explained to me. When being judged; I rarely ever run my rear subs. I decided to try them both on at the same time. When I do; it seems that the deeper freq. in the rear seem to sweep past me to the front dash when I increase the gain on the front sub. I'm not complaining because I get the mid-bass and it seems to correct my timing problems with my rear subs. I'm happier with my stage than ever!  There is a sweet spot with that dial though. Can someone explain this phenom. to me? Not shure how I accomplished this...

Thanks in advance!  Dave

Sounds like you have an unussual setup, but I agree your soundstage is key. It appears that the rear subs could be slightly out of phase (compared to front) on the timing.

Considering that lower frequencies (from e.g. a 12") travels slightly differently to frequencies produced by the 8", this might be the source of your problem, besides the distance from your listening position.

If you had the means to messure timing at a fixed listening position, and run tests from individual speakers respectively, this would probably lead you to the right solution. In addition, if you do manage to messure any phase differences you will require some special circuitry or box modifications to correct the issue. (An assumption I'm making is that you don't have any reverse wires, which will cause major head-aches and phase)

Perhaps someone can suggest the best approach to messure timing/phase ?

Thanks. I'm actually in phase all around. I'm not trying to fix the timing. It's fixed. I just want to know why the sweeping is going on or what it is. I'm not having any bass cancelation (a sign of out-of-phase).

Thanks for the advise though.

Thanks so much for the precise details. This is exactly what I was looking for. My crossover adjustment is probably the best option. You see, I am a SONY S.Q. demo car. Sony hasn't made any time alignment equipment since the earlier days of the "Mobile ES" equipment. Sony has given me by way of "full ride" sponsorship, anything (current product line) I want for the car. If I continue my relationship with them; I must correct timing and phase manually, not electronically by way of processors. They don't make processors anymore.

Thank you for explaining to me what I needed to understand.

PS. My rear enclusures and front enclosure are built to the specs of the subs in them using Bass Box Pro.

Stereo. Hmmm. Two 8's? Is that what you're saying? I think that the rear wave and the front wave are meeting in the place that i've adjusted. That's just me. I appreciate your advice and respect your knowledge in this field totally! Be assured of that. I just feel that I think that SQ can be adjusted by an ear. What did people do before processors? I am trying to do this by mine; which may be a waste of time!  I don't think SONY deserves the rap they get. I think that a SQ. system can be made by ear; that's all. Pics of my system are on the sonyxplode web site in the "My Ride" section, under David Fancher- 1995 Chevrolet Monte Carlo. I even widened my stage using tweets in the door pannels at about knee level pointing toward the side windows. A former ISACA judge, now in USACi, thought they did the trick? Wish you could hear it. Sorry if you don't think it's possible.

Dave

Thanks Speakermakers. I hope the Box I made is the one I need. I am in regular contact with my Vendor an Rep for Sony (Burke McKay), and he feels like i'm on what I need to be on. I hope he's right. I just want to, I guess, prove that you can make SQ. without anything but a good ear. I may fail, but not without a fight. My ear could allways be wrong. I hope not. I have been in this for 17 yrs. I hope I can still hear. Thanks for being supportive.

Dave

Hi Dave, I had a look at your pics and I must say, GOOD JOB!!

It's clear that you have passion for perfection and I think it came out real nice. I've been thinking about your sweeping description and was wondering if you could describe it more precisely; is it air movement, a noise or is the centre of the sound moving while you adjust gain, etc ?

In the pic of the woofers, there appears to be a glass reflection (looks like a cover {I don't mean at/over the amps, higher up}) - is this glass on there permanently ? Also, is there any air flow between the boot and the cabin ?

Another question re your settings:

On the GTR9001::   off/12/24db - subsonic filter; 40hz boost; off/12/24db - low-pass filter

On the GTR1252:: 40hz boost; 80hz - LP/off/HP

Are you running any active x-over in your system, or anything else important to know about the sub setup ?

Thanks bro.

To give some anwsers: The GTR9001's are one to each sub, set at LP, no subsonic (off), Subs are divided in air space inside the box, at the top of the box there is a boot between the box and top of trunk that has an opening about 3" tall and 18" wide to allow opening into the cabin, and the glass is there permanently but about 4" out from subs to allow excursion, and is open on all sides. for air flow. I know it probably affects my waves but the USACi requirements say that the judge can't touch my subs anywhere with a ball-point pen or the subs aren't protected.

The 1252's are on the highs. 1 2-channel for front, and 1 2-channel rears. The XM1S slimline amp is on the front 8" sub at 2 ohms. The 3 amps are running LP on all subs but with nothing external just the F/R/S adjustments and parametric on the head unit. Doing everything else with speaker placement and enclosure volume and position. Notice the tweeters in the doors? I used that placement to widen my stage. They hit the side windows and open the stage WAY boyond the A pillars! There are also tweets in the A pillars aimed up and toward the center of the galss raising my stage to the center glass about 8" above the dash.

As for the sweeping: It's not air; it seems to be moving the bass from behind me to infront of the headrests toward the hood. I think, now, from what has been explained to me; i'm completing a wave between the two sub positions (F/R). A friend helped me test it with a mic and an oscilascope last night; and we think that when the top of the front wave overlaps, ever so slightly, the bottom of the back wave that's what seems to the ear to be that sweet spot. Kind of like filling a hole in the front seat area.

I'm learning as I go; and having fun doing it. I mean; that's what it's all about right? Allways open to suggestions; or anything anyone can toss at me to help me make it better. Thanks for the good word man. I really appreciate it.

Dave

loosingmymind wrote:

---

The subs are sealed (individualy chambered), rear firing, and the timing is a little off (slow).

---

What do you mean by the timing is a little off? 

loosingmymind wrote:

---

In the front of the car, in the passenger floorboard, I have a sealed enclosure with an 8' sub in it. Foreward firing toward the firewall recieving about 250rms, and set at about 120hz. This sub was added to give me lost mid-bass to the front stage in an SQ car. I have to say it is doing it's job perfectly, giving me an impresive fullness and depth at about eye  level, about 3" up the front glass, and appears to come from dead center at about 2ft. deeper than the windshield. There is a gain knob on this front amp, to adjust to taste.

That's the set up. Here's the part I hope can be explained to me. When being judged; I rarely ever run my rear subs. I decided to try them both on at the same time. When I do; it seems that the deeper freq. in the rear seem to sweep past me to the front dash when I increase the gain on the front sub. I'm not complaining because I get the mid-bass and it seems to correct my timing problems with my rear subs. I'm happier with my stage than ever!  There is a sweet spot with that dial though. Can someone explain this phenom. to me? Not shure how I accomplished this...

Thanks in advance!  Dave

---

You say there is a sweet spot with the dial.  What happens when you move past the sweet spot?  Does it move to the back once more?  What frequency is your subs crossed over at?  If I had to guess, the subs are causing resonances within pannels in the rear that you are localizing making it sound like the bass is in the back.  When you turn up the 8, it masks these resonances and now your localization is dominated by frequencies in the front.  There could be other reasons as well, especially if the sub and 8 are overlaping any. 

loosingmymind wrote:

---

Thanks. I'm actually in phase all around. I'm not trying to fix the timing. It's fixed. I just want to know why the sweeping is going on or what it is. I'm not having any bass cancelation (a sign of out-of-phase).

Thanks for the advise though.

---

I may be telling you something you already know; but keep in mind that there is a difference between electrical phase and acoustic phase.  Electrically all the speakers might be in phase, but due to their different locations within the vehicle they might be out of phase with one another causing constructive and destructive interference.  This is especially problematic when more than one speaker is producing the same frequencies on one of the stereo channels.  The results are what is known as lobing (or comb filtering).  The best solution is to eliminate these overlaping frequencies, but without causing a dip in the response.  Usually this can be achieved with steeper crossover slopes.

speakermakers wrote:

---

2.     Frequency response of harmonics of fundamental sounds. This is why your subs in the back make the music sound bigger than life. After you have successfully reproduced the initial sound if you also reproduce the lower harmonic on top of that sound you validate that sound for your brain and make it believable. This is why music over a cell phone dose not sound right.

---

Just a quick correction; there is no lower harmonic below the fundamental.  Harmonics only show up higher in frequency than the fundamental.  Or am I misreading what you are saying?

loosingmymind wrote:

---

Sony hasn't made any time alignment equipment since the earlier days of the "Mobile ES" equipment. Sony has given me by way of "full ride" sponsorship, anything (current product line) I want for the car. If I continue my relationship with them; I must correct timing and phase manually, not electronically by way of processors. They don't make processors anymore.

---

It really is too bad too as they used to make some of the best processors on the market.  Now I am forced to run an 8 year old deck just so I can enjoy the 4000X processor. 

loosingmymind wrote:

---

 

I just feel that I think that SQ can be adjusted by an ear. What did people do before processors? I am trying to do this by mine; which may be a waste of time!  I don't think SONY deserves the rap they get. I think that a SQ. system can be made by ear; that's all.  

---

Sure it is possible to use your ears to tune your system.  But you will be your limit on how good it can sound.  Psychoacoustics and other limits of your hearing will effect your results.  And it will take you huge amounts of time and money to get there.  However precision measuring equipment with much higher accuracy than your ears and a good processor can fix things that you would never find by using your ears and I can guarantee you the differences will be very audible and benificial.  And you will do it in MUCH less time.  So to answer your question, before processors they either lived with mediocricy or spent heaps of time training their ears and learning how to change the signal to match what they want to hear.  As an example, I know the great speaker engineers like Arty Newdell (sp?) will spend weeks upon weeks on a crossover design using his highly trained ears and years of experience.  However I've watched my old boss Dan Wiggins many times design incredible crossovers in just a couple hours with the help of measurement equipment.  And this included an impressive imaging speaker setup using a cheap 6$ Goldwood midrange and a $12 Audax tweeter.  I don't mean to sound too negative with this, especially since there is a great deal of satisfaction to do something great yourself after great effort. Just understand processors are tools to make things easier for you if you let them.

There was mention earlier about group delay.  If you are looking into working with this any, it is good to know what to look for ahead of time.  Here's a great link discussing group delay by Dan Wiggins and Dr. David Hyre: https://www.trueaudio.com/basslst2.htm  And here's a quote by Dan Wiggins explaining what to look for with group delay when designing an enclosure:

"it's just a measure of time through the system at a given frequency. How much is too much? I try to keep the product of group delay and frequency below 400, and when it's lower than 3 msec (latent time of fusion) it's a non issue.

Thus at 20 Hz I try to stay at or below 20 msec. At 40 Hz, 10 msec. At 80 Hz, 5 msec. Once I've dropped the group delay below 3 msec or so it no longer matters.

Note that this implies that 50 msec at 8 Hz isn't a problem; 50 msec at 50 Hz is... There isn't one "good" or "bad" number - it's a continuum that you want to stay below.

Dan Wiggins
Adire Audio"

Link for the above quote: https://forum.carstereos.org/group-delay-t33736p4.html

Thanks Steven. As allways, i very much respect your opinion and help. I'll look into some sort of processor and group delay. Any sugestions on a processor (not to outrageous $ please)?