Rear deck speakers in, improvement in imaging. ?

[Tode9]

All theories welcome, explanations would be better though.

Have an 2005 Accord, using stock locations - Seas neo Al on dash, RS-180 in door. Stage is definitely on the left and somewhat low. Still playing with crossover and TA (not using TA for driver's seat only) - I'm new to this.

I have some drivers lying around and feel that midbass is lacking, so I wanted to see how it would sound if I threw a pair of speakers dedicated to midbass on the rear deck. Prior to this, only speaker mounted behind me was a sub.

I turn everything on and stage has been pulled higher and slightly more to the right. The LP is currently set to 125 Hz so there is overlap with the door mounted drivers which HP at 80 Hz, but even if I set the LP to ~60 Hz the effect remains, though very little output. Only other bit of info I have is that the read deck speakers are wired in reverse polarity or the effect is lost (probably just needs TA, haven't played much, did it today)and are receiving a stereo signal - no mono midbass.

I've read some of the articles on this site concerning rear midbass, but it seemed that the deck was a poor place for them. Am I simply confusing imaging and therefore 'improving' the poor, but distinct imaging, by blurring it?

[bikinpunk]

First thing right off to me is the localization of the midbass. Usually midbasses still play up to 200hz or so, and at this range are likely going to be located a bit more easily than something playing 80hz or below.

Going along with that, if you have the ability to put bass up front, then do it. I've spent the last few months trying to get all my bass up front, and that's with midbasses next to my legs.

Then you've got issues with aiming, etc etc. If you listen to only the midbasses (assuming they do play up to 200hz or so) you can hear vocals, though faintly.

Don't get me wrong, I'm not saying you can't do it because I've personally never tried, and I have a lot of faith in man's ability to try, try, try. I'm just giving you my feelings on the subject.



Oh, and going a bit off topic, have you put your LP on the sub below 125hz? That seems pretty high to me. But, if it works...

[Tode9]

Just to clarify, if I've caused any confusion, I'm running two way (up until today, still kinda). The sub is disconnected as I'm trying to ascertain the benefit/detriment of rear deck midbass. I'm only referring to the rear deck midbass in reference to the LP.

Re: bikinpunk

Yes, the sub LP is ~60 Hz at 12 dB/octave to compensate for my disappointment with midbass output. However, it is currently not connected, sorry, should have been more specific.

[FunkDrSpot]

Not to hijack but to supplement your rear speaker imaging questions: I heard that keeping the tweeters ( high pitched directional sound ) out of the rears improves sound imaging....correct?

[Abmolech]

Not to hijack but to supplement your rear speaker imaging questions: I heard that keeping the tweeters ( high pitched directional sound ) out of the rears improves sound imaging....correct?

Normally yes, it depends if you use time alignment.

No doubt using rear mounted mid bass like this will improve imaging, assuming you deal with resonance.

This is the basic premise of two channel stereophonic.
When listening to a pair of speakers in a room both ears will hear the signals produced by both the left and right loudspeakers. The timing differences associated with this acoustic 'crosstalk' between the two channels and each ear lie at the core of the 'stereo illusion'. This is what allows us to perceive phantom images between the speakers, and coincident-microphone and stereo-panning techniques (which employ only level differences between the two channels to convey the spatial information) rely entirely on this acoustic crosstalk to work properly. A large portion of the left speaker signal will go to the right ear of the listener, and similarly a large portion of the right speaker signal will go to the left ear of the listener When listening to ordinary stereo material via headphones, this interaural timing information is missing — we have only the differences in level between the two channels to go on — and hence the stereo images become non-linear and ill-defined. In fact, most people perceive the individual sound sources to lie on a line running directly through the centre of the head, instead of being portrayed in front of us as they would be with loudspeakers.
While it is true that if coincident microphones of 20 cm apart, you should have 580 millisecond delay between left and right microphone, to simulate a 90 degree azimuth, crosstalk will reduce this to 250 Milli seconds. This results in a 30 degree loss in separation down to the recommended 60 degree speaker position, allowing the correct placement but with the loss in depth. Some have tried to reduce this crosstalk with various DSP logarithms, producing an increase depth, but at a serious loss with a VERY tiny sweet spot and any head movement completely collapses the illusion.

An increase in crosstalk facilitates more width and depth because the apposing ear receives a much move diffuse signal, with the perception of increased depth and width.This is the reason for the "precise" conditions for stereo, too much cross talk in the intensity sensitive frequencies and the illusion collapses (70 percent) and too little, (55 percent) time sensitive frequencies landscape diminishes. (Easy to prove with headphones)

By using rear mid bass you have increased the acoustic crosstalk, in the most important frequency range for stereophonic. Time sensitive frequency's. This should help to centre the image and create more depth. If you use time alignment on the rear mid basses with a 20 millisecond delay, you should gain even more.

[FunkDrSpot]

I'll take all of that as a 'yes'. Cause my head would explode from trying to read that paragraph.

[Tode9]

Originally Posted by Abmolech

By using rear mid bass you have increased the acoustic crosstalk, in the most important frequency range for stereophonic. Time sensitive frequency's. This should help to centre the image and create more depth. If you use time alignment on the rear mid basses with a 20 millisecond delay, you should gain even more.

I remember reading a werewolf thread (which of course, i can't find now) in which he stated that mounting on the read deck was a non-ideal choice, being that one speaker is essentially, directly behind your head and the crosstalk benefit diminished. Would you agree, and if so, in your estimation would mounting the drivers in the rear doors be a better location (provided the doors don't ring)?

I have minimal processing capabilities, can't delay more that 9.9 ms. Why a 20 ms delay as opposed to a shorter delay based on the physical location of the drivers? I've seen threads with high delay for rear fill ambience applications, but I'm only running these under 200 Hz.

[Tode9]

Just to update, after playing with it for a week, I can say the imaging effect are only present in the lower octaves. With high frequency content, say female vocals, the stage pulls to the left, but male vocals are more centered. Guess I need to work on tweeter aiming.

[Abmolech]

The question with rear mounting speakers is what is their purpose?
First they SHOULD be decorrelated, (IE left and right) and use the stereophonic effect. By using the speakers closer to an equilateral triangle, we gain a much better image for stereo.
Point
The speakers are better as far back and out to the sides as practical.

A room could be acoustically defined by these three terms.Diffusion, Reflection and absorption. Especially if the phase is known for the listening position. The secondary benefit of rear speakers is to emulate a room. Since a diffusion is simply a correlation of reflection and absorption, a speaker could emulate a rear wall etc. In this case, we would use a time delay to simulate the delayed effect of a diffusion from a rear wall.

Point
The car could now have a sense of space (ambiance) of a much larger room, because we use the precedences effect to judge reflections. (The higher amplitude giving us the "true" representation of the rooms size, rather than the various reflections that might follow)

The prime directive of stereophonic is to force an acoustic recording space into or onto, a listening space that does not resemble the recording space. (For example Gothic cathedral versus your car) Rear speaker properly used, will help facilitate this action.