Speaker design, X-Over, and MTM Questions - Please help a noob!

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Hi!

A couple of questions from a noob designer for the speaker design gurus here - hope you can help me understand a couple concepts.
-I have read several places that the baffle affects the x-over (or usually that the crossover has been designed to take the baffle into account). What does this mean? What is done to the crossover (specifically) to account for the baffle affects?
-I have also read a little about the MTM configuration, and how it is supposed to use a odd-order crossover, usually a 3rd order. Why is that? Will it not work with a 2nd or 4th? What about a 1st order?
- Also regarding the MTM, what is the benefit to this configuration? I have seen many projects use it, but have not found any really good explaination.
- When one speaks of 'tweeking' a x-over, what exactly does that mean? Does it mean using a standard 'text book' cross over, and slightly varying some of the component values? Or actually changing the topology?
- Finally (for now!), can someone expain what the Qes, Qms, and Qts is physically? What does it mean if the Qms (or Qes) is high or low? How does this affect the speaker performance?

Thanks very much for your help!

-SB
 
There is a baffle step in a drivers response. At 1 wavelength across the baffle the radiation characteristic is hemispherical, changing to omnidirectional as the wavelength exceeds the baffle width and wraps around it; when it gets to fully omnidirectional the on axis response is down 6dB, so to compensate one has the first inductor of the midbass low pass filter kick in not at the nominal crossover frequency but at the frequency where the baffle step is fully in place; this way as the on-axis response is climbing above that frequency that inductor compensates for the rising on-axis response.

MTMs will work with any order. 1st order doesn't offer much driver protection and allows a lot of driver passband overlap, so it is seldom used. Joe D'Appolito (the inventor of the MTM) prefers using a 2nd order electrical configured for a total of 4th order when combined with the driver acoustical rolloffs.

MTM has a better polar response pattern, maximizing horizontal and minimizing vertical dispersion, while using two woofers helps get their SPL closer to that of the tweeter, as the woofers are losing SPL due to the baffle step and tweeters are generally more sensitive to begin with.

Tweaking means changing anything or everything to get better response.

Qes is the electrical losses of the driver, Qms is mechanical losses, Qts is total losses. Simply put the lower the Qts the higher the efficiency, usually. But also lower Qts gives less bass response, usually. A full understanding of Q factors is too much for a simple answer.
 
BillFitzmaurice said:
There is a baffle step in a drivers response. At 1 wavelength across the baffle the radiation characteristic is hemispherical, changing to omnidirectional as the wavelength exceeds the baffle width and wraps around it; when it gets to fully omnidirectional the on axis response is down 6dB, so to compensate one has the first inductor of the midbass low pass filter kick in not at the nominal crossover frequency but at the frequency where the baffle step is fully in place; this way as the on-axis response is climbing above that frequency that inductor compensates for the rising on-axis response.

MTMs will work with any order. 1st order doesn't offer much driver protection and allows a lot of driver passband overlap, so it is seldom used. Joe D'Appolito (the inventor of the MTM) prefers using a 2nd order electrical configured for a total of 4th order when combined with the driver acoustical rolloffs.

MTM has a better polar response pattern, maximizing horizontal and minimizing vertical dispersion, while using two woofers helps get their SPL closer to that of the tweeter, as the woofers are losing SPL due to the baffle step and tweeters are generally more sensitive to begin with.

Tweaking means changing anything or everything to get better response.

Qes is the electrical losses of the driver, Qms is mechanical losses, Qts is total losses. Simply put the lower the Qts the higher the efficiency, usually. But also lower Qts gives less bass response, usually. A full understanding of Q factors is too much for a simple answer.


First, thank you very much for your anwser, it helps me a lot...but of course, now I have a few more questions!

For the baffle step, it would help me if I can get a real life example. So if I have a two-way system with a 2nd order crossover of 2,500 Hz, in a box with a 225mm baffle width (about 9 inches), this correponds to a frequency of about 1,500 Hz. So does that mean that that the inductor value for the low pass changes to what it would be for a 1,500 Hz crossover, but the capacitor stays the same? I am really confused on this.

Next, if I have some kind of impedience compensation circuit in parallel with the crossover, do I treat these as totally separate? Or (in the case of a zobel on the low pass filter) would the capacitor in the zobel need to be added to the capacitor in the crossover (since they operate in parallel)? Same with a notch filter, I guess.

Back to the MTM, if I have two 8 ohm woofer's operating in parallel, the net resistance will be 4 ohms. Does this mean the tweeter must also be 4 ohms or 8 ohms is ok. If 8 ohms, do I need to put a resistor in parallel with the tweeter to make it an equivalent of 4 ohms?

I really appreciate your time in anwsering my questions, I am very interested in this.
 
So does that mean that that the inductor value for the low pass changes to what it would be for a 1,500 Hz crossover, but the capacitor stays the same?

Yes, that's pretty much correct, so that when you hit the intended corner frequency you go from a 6dB to a 12dB rolloff, or more if required. You have to actually measure the woofer response to see at what frequency the SPL starts to rise.

if I have some kind of impedience compensation circuit in parallel with the crossover, do I treat these as totally separate?

Yes, but don't automatically use one until you're sure that it's necessary.

if I have two 8 ohm woofer's operating in parallel, the net resistance will be 4 ohms. Does this mean the tweeter must also be 4 ohms or 8 ohms is ok.

Go 8 ohms with the tweeter; the lower impedance of the woofers raises their voltage SPL closer to that of the tweeter, which is usually going to have a higher SPL than the woofer section. You only use resistors if in testing you find that the tweeter level has to be padded down.
 
So does that mean that that the inductor value for the low pass changes to what it would be for a 1,500 Hz crossover, but the capacitor stays the same?

Actually in this case 1.5kHz is where the baffle step is completely realized; the rise in response more likely starts at around 400Hz. To tame the rising SPL above that an inductor value between 1 and 2 mH is likely for a 4 ohm load.
 
baffle step

Its also important to realize that compensating for the baffle step isnt for every one. this only provides optimal results in anechoic testing. Many well built systems dont compensate for this and here is why. If the listening room requires the speakers to be near the back wall or worse in the corner then the reflection will keep the response step from happening completely. That means if someone is using, say, bookshelf speakers near a wall without sound absorbsion then compensating for the step will actualy yeald a boost at the step frequency. So it is important to know the acoustice of the room also. An extreme example is in wall speakers. There is never a step compensation for these because what is the step (a 16 foot wall?)
 
Re: baffle step

piro said:
I am also trying to get an answer aboud baffle step compensation.
How did you get 1.5KHz from 225 mm?
and this is an aritcle I found that I hope will help me too.

Piro- Thanks for the info, it helped quite a bit. I also found the True Audio website helpful too. The 1,500 Hz came from a single cycle at 225mm (345m/s / .225m). This is not correct, I know now.

planet10 said:


This article outlines ways of dealing with bafflestep.

http://www.t-linespeakers.org/tech/bafflestep/intro-bds.html

If i've got 2 drivers i like putting the 2nd one on the back.

dave

Well, two speakers is an expensive solution to the problem.

BillFitzmaurice said:


Actually in this case 1.5kHz is where the baffle step is completely realized; the rise in response more likely starts at around 400Hz. To tame the rising SPL above that an inductor value between 1 and 2 mH is likely for a 4 ohm load.

Yes, from the above reading, I understand the baffle response more. I thought it was a singular affect at a frequency corresponding to the baffle width, but that was totally wrong, it is actually an attenuation of all low frequencies below (about) the baffle width. From the True Audio Website, they have a simple L-R circuit (the L and R in parallel) in series with the speaker (I assume that it is before any other crossover components), and for a 0.25m baffle (similar to what we have been discussing) they came up with a 1mH inductor. This is similar to what you say, so I believe this is the same thing. I understand that this simple attenuates the high frequencies above the desired baffle frequency.

eric180db said:
Its also important to realize that compensating for the baffle step isnt for every one. this only provides optimal results in anechoic testing. Many well built systems dont compensate for this and here is why. If the listening room requires the speakers to be near the back wall or worse in the corner then the reflection will keep the response step from happening completely. That means if someone is using, say, bookshelf speakers near a wall without sound absorbsion then compensating for the step will actualy yeald a boost at the step frequency. So it is important to know the acoustice of the room also. An extreme example is in wall speakers. There is never a step compensation for these because what is the step (a 16 foot wall?)

I am sure this is true, but I don't really agree with it, but only for two reasons. First, in my opionion it is best to try to design a speaker that by itself has a smooth frequency response, and does not rely on room affects. Second, from a design standpoint it doesn't seem like a good idea to depend on something you can't control, such as room placement. What if the person using the speaker moves to a new place, and moves the speaker, or the room size is very different than you expect, or can't place it near a wall, etc etc. I can control (more or less) the speaker's frequency response, but I can't control the room it is put in. Anyway, I agree mostly with what you say, and am sure many speaker designers follow this advise. For my own speakers, I will consider this, as the fewer elements in the crossover the better, plus I know I have designed the speaker in such a way that it is required to place it near a wall. But in general, I don't know about this...
 
piro said:
what drivers do you use?

At the moment, none :(. I am still designing, thus all the questions about the crossover. I have not decided which drivers to use, but I really like the AudioTechnology units (but very expensive), and also the Seas Excel line (expensive, but not so bad). I originally wanted to use Accuton, but it seems they are a bit noisier than their website indicates, and they are also coming out with a new line of speakers, so I will hold off on them. In fact, I really like the Thor speaker kit, it is exactly what I am looking for. Except maybe using the AT drivers...:confused:
 
Re: Re: Re: baffle step

planet10 said:


You already have 2 drivers if you are talking MTM... my feeling is just that it is better on the back than on the front -- an MTM usually forces you to cross the tweeter really low which i try to avoid.

dave

I guess I should say first off that I am not sure I will do an MTM setup, I was interested in it on a conceptual level - I wanted to know what its benefits were compared to a usual configuration. That said...

:confused:

I don't understand what you mean. Being MTM doesn't (I think) require much of a change in where you cross over to the tweeter, usually about 2500-3000Hz from what I have seen, this is the same as most two way systems.

But what I really don't understand is putting the driver on the back. Do you mean having the usual MTM on front, and a single "M" on the rear, or do you mean the T on the front with the "MM" on the rear? Or (worse, money wise) MM on both front and rear? One advantage of the MTM seems to be that since you have two low frequency drivers, you have already offset 3db of the about 6db baffle step response you have with a single driver - or is this wrong?

I don't know if the www.t-linespeakers.org site is yours or not, but I really like it, I like transmission line speakers.
 
frugal-phile™
Joined 2001
Paid Member
Re: Re: Re: Re: baffle step

sbolin said:
But what I really don't understand is putting the driver on the back. Do you mean having the usual MTM on front, and a single "M" on the rear, or do you mean the T on the front with the "MM" on the rear? Or (worse, money wise) MM on both front and rear? One advantage of the MTM seems to be that since you have two low frequency drivers, you have already offset 3db of the about 6db baffle step response you have with a single driver - or is this wrong?

MT on the front, M on the back. No bafflestep, and you can take advantage of push-push loading. The maximum XO in an MTM is determined by the spacing of the 2 Ms. To get an XO as high as 2.5-3k (which i consider too low) you can only use a 5" M or smaller (maybe a 6). With only one driver on the front, you have much more flexibility.

I don't know if the www.t-linespeakers.org site is yours or not, but I really like it, I like transmission line speakers. [/B]

Yes that is mine. Thanx.

dave
 
Re: Re: Re: Re: Re: baffle step

planet10 said:


MT on the front, M on the back. No bafflestep, and you can take advantage of push-push loading. The maximum XO in an MTM is determined by the spacing of the 2 Ms. To get an XO as high as 2.5-3k (which i consider too low) you can only use a 5" M or smaller (maybe a 6). With only one driver on the front, you have much more flexibility.



Yes that is mine. Thanx.

dave

Regarding the x-over in the MTM, does that mean the crossover point should be at the same frequency as the speaker spacing? That would work out to about 138mm spacing for a 2500 Hz crossover. On the other hand, the Seas Thor kit crosses over between two 18W units ( which are 6-7") at 2800 Hz, so maybe it is one of those rules that is broken - probably because it isn't very practical to follow.

Regarding the two speakers - now I understand, that is a good idea. You lose some of the benefits to the MTM configuration, but gain others. But how to put it in a t-line?
 
Re: Re: Re: Re: Re: baffle step

planet10 said:


MT on the front, M on the back. No bafflestep, and you can take advantage of push-push loading. The maximum XO in an MTM is determined by the spacing of the 2 Ms. To get an XO as high as 2.5-3k (which i consider too low) you can only use a 5" M or smaller (maybe a 6). With only one driver on the front, you have much more flexibility.

dave


Dave,

Why does moving one M to the back give more flexibility when designing the XO? Wouldn't you cross over at the Baffle Step frequency anyhow, or would you design as an "ideal" 2-way (TM on the front) as the drivers allow, and then use the second M on the back with a totally independent LP filter (which is dependent of the BS frequency)?

Jennice
 
frugal-phile™
Joined 2001
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Re: Re: Re: Re: Re: Re: baffle step

Jennice said:
Why does moving one M to the back give more flexibility when designing the XO?

Because you are no longer forced to XO really low.

Take for example a pair of Vifa P13s and the matching tweeter. If in an MTM you are going to need to XO at something like 2.5k. This low you are going to need a 3rd or 4th order XO + baffle step compensation. Move the M to the back and you can use my favourite XO for this combo -- a 5 uF cap on the tweeter. You can roll the back P13 off at the bafflestep with an inductor if you want (ie 2.5 way without the inherent phase issues) or let it run full-range if your room will support it.

Now your amp is much-much happier without all the reactive components between it and the speaker and it all sounds more lively and dynamic.

dave
 
Dave,

I like the way you think (and your illustrative way of explaining things).

Originally I tyhought of making a MTM or TMM, in order to make a slim floor speaker without the need for a wide baffle and a 8" or bigger woofer. My concern was that a single driver (maybe 5" or 6") would not move enough air for the room size.
If I understand your concept correctly (with the second M moved to the back), I will have the same bass capability as a baffle-step corrected MTM, but with more freedom to design simpler and better XO. Is that correctly understood?

I recon that the overall sensitivity will be a little lower? My M's aren't working in parallel, and the tweeter will need a series resistor to match it's sensitivity to the single M, as it would when designing an ordinary 2-way speaker, right?

Also, if I understand you correctly, I can use a lower order ( == simpler) XO when crossing higher, or even use the M's natural roll-off, which will make the entire XO and BSC circuit much simpler and cheaper. Consequently, I think a second M isn't that much more expensive than all the (otherwise needed) extra XO omponents... Am I on the right track?

Jennice
 
frugal-phile™
Joined 2001
Paid Member
Jennice said:
If I understand your concept correctly (with the second M moved to the back), I will have the same bass capability as a baffle-step corrected MTM, but with more freedom to design simpler and better XO. Is that correctly understood?

I recon that the overall sensitivity will be a little lower? My M's aren't working in parallel, and the tweeter will need a series resistor to match it's sensitivity to the single M, as it would when designing an ordinary 2-way speaker, right?

Also, if I understand you correctly, I can use a lower order ( == simpler) XO when crossing higher, or even use the M's natural roll-off, which will make the entire XO and BSC circuit much simpler and cheaper. Consequently, I think a second M isn't that much more expensive than all the (otherwise needed) extra XO omponents... Am I on the right track?

Same bass capabilities, same sensitivity (ignoring any losses in the more complex XO), no BSC required at all -- but the tweeter padded instead (the BSC in an MTM essentially pads the tweeter along with the upper range of the mids).

More flexibility with XO, including using Ms natural roll-off and lower order XOs.

Plus you gain with an easier load for the amp (not to be underestimated), and the benefits of push-push (more downward dynamic range & less energy transfer to the cabinet reducing its colorations.

dave
 
planet10 said:

...
Plus you gain with an easier load for the amp (not to be underestimated), and the benefits of push-push (more downward dynamic range & less energy transfer to the cabinet reducing its colorations.

dave

Why should less energy be transferred to the cabinet with this push-push design? If I understand you correctly, the speakers are electrically in phase, so there's the same air preassure change in the cabinet, so the sides should have the same preassure that causes them to vibrate... :confused: I don't get the point!?

Jennice
 
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