Speaker Xover design............center/front

[JKnPA]

I am trying to understand the design behind the Xover points of my center and front speakers! They are both listed as 2.3khz.
Are Xover points stated at the 3db. points, or can they differ according to the manufacturer? I did the calculations for both speakers, the low pass center(mid-woofer)section seems about correct(-3db.@1.8khz) , but the high pass center(tweeter) section seems too high(-3db.@5.5khz).
I believe the center speaker should range between 300hz.-3.5khz. without too much loss.
See links below on Polk specs........
http://www.polkaudio.com/homeaudio/specs/cs1/
http://www.polkaudio.com/homeaudio/specs/monitor40/

Jk.

 

[j_garcia]

What you are asking doesn't make any sense??? The x-over point is chosen to allow the drivers to adequately cross to each other with the smoothest response. Ideally, you choose a point that keeps each driver operating in the range that it performs best in. A tweeter may be -3dB at a given frequency, but the response below some other point may be a little uneven, so filtering it higher may give you better overall response. Just looking at the x-over point really doesn't tell you anything, as you need to know the order, slope, actual response of each driver and then the response of both combined, etc... Those x-over points weren't arbitrarily chosen, they were chosen for a reason.

 

[JKnPA]

Links..............

The links give the 'order' and the 'nominal impedance'. I'll include the schematics for both speakers.
I was under the impression the 'Xover point' was basically defined as a -3db. point; that was one of the questions!
See link............. graph
http://en.wikipedia.org/wiki/Audio_crossover

These are the schematics for the two speakers in question.The nominal impedance( 8ohms), should be the driver impedance at the 'flat portion' of the driver impedance/freq. response graph( not shown, but stated by Polk in the links).


Schematic for center speaker Cs-1......................
http://www.polkaudio.com/forums/attachment.php?attachmentid=18796&d=1155785350
schematic for front speaker M40......................
http://www.polkaudio.com/forums/attachment.php?attachmentid=18724&d=1155782337

 

[j_garcia]

The x-over point should be the -3dB point in the example, but what exactly is your question or what are you getting at?

Without a response curve for the overall speaker and/or the individual drivers, I'm not sure what you want to know or accomplish?

 

[jeffsg4mac]

I think you are getting xover confused with a typical response. A typical response is usually measured with a -3db down point, like a sub-woofer may be rated to 25hz, meaning it will be down 3db at 25hz. A crossover is defined as an order. First order, second order, third all being the most common. A first order has a 6db per octave slope, a second has a 12, and a third has an 18. Then you might have a combo crossover. with a third on the tweeter but a first or second on the other drivers. It all depends on the drivers being used. The end goal is to achieve a flat response with minimal phasing problems. This is something you really only need to worry about if you are designing a speaker. If you bought one that you like, then it makes no difference what crossover points or slopes the manufacture used. If you like the sound, then they did something right.

 

[JKnPA]

Calculations..........

Sometimes technical questions(calculations) are difficult to put into words. In an effort to understand the audio response of my center and front speakers, I tried to calculate the 3db. points of both speakers( Xovers included). I am trying to get a graph of the voltage vs. freq. of both speakers( center & front) over the audio range. I've been working on this for most of the week.
1. Is the Xover point( 2.3kz, as stated in link by Polk) the 3db. points for the center or front speaker ??? If not, what are the 3db.points ?
2. If the Xover points.(center) are below the -3db. level in the vocal range( 300-3khz.), doesn't that create an audio dip that the listener would not be able to hear ?
* The designer is looking for a flat response across the vocal range, or am I wrong?

 

[jeffsg4mac]

Sometimes technical questions(calculations) are difficult to put into words. In an effort to understand the audio response of my center and front speakers, I tried to calculate the 3db. points of both speakers( Xovers included). I am trying to get a graph of the voltage vs. freq. of both speakers( center & front) over the audio range. I've been working on this for most of the week.
1. Is the Xover point( 2.3kz, as stated in link by Polk) the 3db. points for the center or front speaker ??? If not, what are the 3db.points ?
2. If the Xover points.(center) are below the -3db. level in the vocal range( 300-3khz.), doesn't that create an audio dip that the listener would not be able to hear ?
* The designer is looking for a flat response across the vocal range, or am I wrong?

I believe you are still confused. A crossover is not a 3db point, a crossover point is where the drivers seperate in their frequencies. ie, where a tweeter starts fading out and the midrange takes over. The crossover point is where the slope starts.

 

[MDS]

The F3 or - 3dB point is across the entire frequency range the speaker is supposed to be able to reproduce. I don't think it has anything to do with what xover points were used for the individual drivers.

If the speakers are rated 20 Hz - 20 kHz +/- 3dB that means its response dips no lower than - 3dB and no higher than + 3dB but it does not tell you at which frequencies the low/high points occur.

 

[Swerd]

Sometimes technical questions(calculations) are difficult to put into words.

You got that right .

I am trying to get a graph of the voltage vs. freq. of both speakers( center & front) over the audio range. I've been working on this for most of the week.

You can't calculate that. The only way you can get that is by direct measurement with a microphone and some appropriate computer software.

1. Is the Xover point( 2.3kz, as stated in link by Polk) the 3db. points for the center or front speaker ??? If not, what are the 3db points?

Are the 3 dB points you're talking about the ones in the diagram in the Wikipedia page on crossovers? If so, this refers to an electrical filter that limits the signal going to the speaker, not the speaker response itself.

2. If the Xover points.(center) are below the -3db. level in the vocal range( 300-3khz.), doesn't that create an audio dip that the listener would not be able to hear?

* The designer is looking for a flat response across the vocal range, or am I wrong?

Correct, the designer is looking for a flat response across the crossover range, where both speakers (woofer and tweeter) will contribute to the sound you hear.

 

[JKnPA]

Xover stuff..........

Jeff,
I didn't say the Xover point was the 3db. point!.......
I was asking if the Xover points are designed near the 3db. point or 6db points, so there is no 'dip' in the transistion between the two drivers, in this case the mid-woofer and tweeter!

Others........
I calculated the 3db. points of the low/high pass filters to be
1.8kz. and 5.5kz. perhaps my calculations are WRONG! Can anyone here calculate the above 3db. points with the info provided in the links?
Polk states the Xover points are at 2.3kz., which is no where near the high pass filter 3db. point( 5.5khz.)......
This is a link to a 3way speaker system...... see chart in section 10. Notice the Xover points are at 6db.. I'm looking for a chart that looks likes this, but has two sections not three!
http://www.linkwitzlab.com/crossovers.htm
Does anyone follow me..................

 

[j_garcia]

You really don't need to know the -3dB point for the DRIVERS, that is basically irrelevant. Unless you intend to modify the x-over, this information holds little use for you. The x-over point for the two drivers may not be exactly the -3dB point of the x-over, because it depends on the response of the drivers and the resulting response curve that results from the combination of the drivers together.

I STILL don't see what you are calculating based on, if they say the x-over is 2.3K, then that is what it is, and they chose it because that is what worked.

 

[jeffsg4mac]

Jeff,
I didn't say the Xover point was the 3db. point!.......
I was asking if the Xover points are designed near the 3db. point or 6db points, so there is no 'dip' in the transistion between the two drivers, in this case the mid-woofer and tweeter!

Others........
I calculated the 3db. points of the low/high pass filters to be
1.8kz. and 5.5kz. perhaps my calculations are WRONG! Can anyone here calculate the above 3db. points with the info provided in the links?
Polk states the Xover points are at 2.3kz., which is no where near the high pass filter 3db. point( 5.5khz.)......
This is a link to a 3way speaker system...... see chart in section 10. Notice the Xover points are at 6db.. I'm looking for a chart that looks likes this, but has two sections not three!
http://www.linkwitzlab.com/crossovers.htm
Does anyone follow me..................

You are getting into speaker design territory here. Crossovers are usually designed with the free air resonance, impedance and frequency response of the driver itself in mind. The free air resonance is especially important when it comes to the tweeter. A rule of thumb for a tweeter is for the crossover to be no lower than twice what the free air resonance is.

 

[Swerd]

I was asking if the Xover points are designed near the 3db. point or 6db points, so there is no 'dip' in the transistion between the two drivers, in this case the mid-woofer and tweeter!

This is a link to a 3way speaker system...... see chart in section 10. Notice the Xover points are at 6db.. I'm looking for a chart that looks likes this, but has two sections not three!

I am trying to follow you, but I am having difficulty understanding what you are asking.

Are you trying to graph the electrical output (voltage vs. frequency) of the crossover filters in your speakers? That is what the diagram shows. If so, why? I don't understand how that would be useful.

Or are you interested in the frequency response (sound pressure level vs. frequency) of your speakers? That is what you cannot calculate. It must be measured. Since Polk does not publish that we have to assume that they did that and were satisfied with what they saw.

 

[JKnPA]

Crossover design.................

You got that right .
You can't calculate that. The only way you can get that is by direct measurement with a microphone and some appropriate computer software.

******************************************************
Well, I guess I disagree. You can(IMO) design Xover filters if you have the freq. response vs. impedance curve of the drivers you plan on using!
As I stated earlier........... the 'nominal impedance' is taken from the flat portion of the curve, where you design the filter about those frequencies! See links......................
Nominal impedance(Znom.)...... taken from the flat portion of the curve, see graph!
http://en.wikipedia.org/wiki/Electrical_characteristics_of_a_dynamic_loudspeaker
Article........ choosing a Xover.........
http://www.speakerbuilder.net/web_files/Articles/xover article/xpointmain.htm

 

[j_garcia]

The x-over point doesn't NEED to be -3dB!! It CAN be, but it depends on the specific combination of drivers/response and the x-over used. These are 2nd order, so they are 12dB/octave slopes, one driver wired out of phase. That will affect the combined response also, and ultimately, it is a MEASURED RESPONSE for the speaker as a whole that determines the x-over design and where and how the two drivers blend at the x-over point. Trying to look at charts and specs to design a x-over simply isn't going to give you the best sounding speakers - you need to measure and listen, and since you don't seem to be trying to design anything, I fail to grasp where you are going with this...

Unless you are trying to modify the x-over, what you are trying to find out seems quite pointless.

SO WHAT EXACTLY ARE YOU TRYING TO DO????

 

[JKnPA]

My point....................

I am trying to get a graph of the voltage vs. freq. of both speakers( center & front) over the audio range. I've been working on this for most of the week.

****************************************************

I am trying to verify the Polk specs. by doing the calculations and plotting a graph of the speakers response thru the audio range!
For those not interested in the 'Math' and how the filters/drivers react.................. that's fine! But don't tell me its 'pointless'! I posted the question for a reason!
Knowing the 'nominal impedance' of the speakers and the filter components, the 3db. points and xover points s/b able to be calculated!
I'm surprised no one here can do the calculations!

 

[j_garcia]

I guess if you have nothing better to do....whatever floats your boat man.

The question was, WHY? FOR WHAT PURPOSE? What is the math going to tell you that LISTENING doesn't? What is reverse engineering an average speaker going to tell you?

 

[JKnPA]

I am trying to follow you, but I am having difficulty understanding what you are asking.

Are you trying to graph the electrical output (voltage vs. frequency) of the crossover filters in your speakers? That is what the diagram shows. If so, why? I don't understand how that would be useful.

*******************************************************
Swerd.........
Yes........ I am trying to plot the voltage vs. freq. response( transfer characteristic) for my center & front speakers( Xover filters & driver).
Reason....... verify Polk's specs.
learning process........ doing the math!

Thanks.....

 

[Nick250]

I'm surprised no one here can do the calculations!

It seems to be the case. Time to move on IMO.

Nick

 

[Mark Seaton]

Sometimes technical questions(calculations) are difficult to put into words. In an effort to understand the audio response of my center and front speakers, I tried to calculate the 3db. points of both speakers( Xovers included). I am trying to get a graph of the voltage vs. freq. of both speakers( center & front) over the audio range. I've been working on this for most of the week.
1. Is the Xover point( 2.3kz, as stated in link by Polk) the 3db. points for the center or front speaker ??? If not, what are the 3db.points ?
2. If the Xover points.(center) are below the -3db. level in the vocal range( 300-3khz.), doesn't that create an audio dip that the listener would not be able to hear ?
* The designer is looking for a flat response across the vocal range, or am I wrong?

Hi JKnPA,

I believe you should start searching the DIY forums on speaker and crossover design. While the concept starts off very simply, the execution is much less so.

The "nominal" impedance of a driver is of minimal use in evaluating what a crossover will actually do when connected to the driver. There really isn't anything to "calculate" in your question above, other than guessing at what the relative level of the drivers are at the crossover frequency. Real speakers in real boxes do not have flat frequency responses, nor do they have constant impedances. The crossover must compliment these two varying parameters to result in a smooth response from the speaker.

There is no correct answer to the electrical or acoustic level of the individual drivers at the crossover frequency as they can range from +3dB in level to -6dB in level, depending on the chosen design.

Hopefully that helps with some of the confusion.