I believe point-source together with acoustic centers alignment are very important things for speakers hi-fi playback. Without them I believe there is a cap - it can be good but not very good.
I would like to ask for some acoustic theory advice (and practice if available) about comparing two design options to achieve at least some degree of point-source. One obvious is coxial speaker but there is limited choice of them and typically they are rather costly. The other option is MTM and that’s where I am asking for advice and support in comparative analysis. I have read coaxials in terms of point-source characteristics are much better than MTM. I understand the main thing is lobing. But does it apply irrespective of how this MTM is actually built in practice ? I was thinking about two 3-inch fullranges and a ¾ inch tweeter. If it is a no flange build then I can place the mids as close as 3 cm apart.
Their size is around 7cm which gives us 17cm altogether which is almost the same as a typical coax driver with an 18cm midbass. And I have also precise control over the acostic centers alignment as I can put the tweeter in between a few milimeters to the back.
Are there then still significant acoustic advantages of these two options – an 18cm coax and a 3” +3/4” MTM ?
I would like to ask for some acoustic theory advice (and practice if available) about comparing two design options to achieve at least some degree of point-source. One obvious is coxial speaker but there is limited choice of them and typically they are rather costly. The other option is MTM and that’s where I am asking for advice and support in comparative analysis. I have read coaxials in terms of point-source characteristics are much better than MTM. I understand the main thing is lobing. But does it apply irrespective of how this MTM is actually built in practice ? I was thinking about two 3-inch fullranges and a ¾ inch tweeter. If it is a no flange build then I can place the mids as close as 3 cm apart.
Their size is around 7cm which gives us 17cm altogether which is almost the same as a typical coax driver with an 18cm midbass. And I have also precise control over the acostic centers alignment as I can put the tweeter in between a few milimeters to the back.
Are there then still significant acoustic advantages of these two options – an 18cm coax and a 3” +3/4” MTM ?
Yes, the lobing means the MTM is not point source like. Crossing it low takes care of that, and that way it doesn't exactly matter whether it is a TM or an MTM.
Coaxials have discontinuity issues around the throat that cause axial FR irregularities. There is a range of solutions available, all with their own compromises. With proper crossover, it might be possible to achieve a workable final speaker.
MTMs on the other hand offer limited vertical dispersion, even if you take care of driver spacing. Thus limiting the range of applications quite severely. The higher you cross, the worse it gets - so while a MTM crossed over at 500Hz will be pretty good, a 2KHz XO will be substantially worse. In the specific driver sizes you're looking at, the MTM wil be a poor design choice. Consider 2.5 way TMM with properly compensated acoustic centers, this is what I did with a 2x B3S and a BC25SC horn tweeter. The horn aligns the acoustic centers somewhat, and a 2kHz crossover becomes possible. Of course you can compensate them in the crossover as well.
A large horn crossed low (500Hz) also seems to be a valid design choice for point-source target, however a driver that can go that low may have questionable HF characteristics. Plus, it's not going to fit in the same sort of space.
I don't consider point source a typically important target except in nearfield applications where driver summing may be an issue. With proper acoustic design, it is possible to integrate disparate drivers into a fairly cohesive image, fully time aligned even if the centers are not coincident.
MTMs on the other hand offer limited vertical dispersion, even if you take care of driver spacing. Thus limiting the range of applications quite severely. The higher you cross, the worse it gets - so while a MTM crossed over at 500Hz will be pretty good, a 2KHz XO will be substantially worse. In the specific driver sizes you're looking at, the MTM wil be a poor design choice. Consider 2.5 way TMM with properly compensated acoustic centers, this is what I did with a 2x B3S and a BC25SC horn tweeter. The horn aligns the acoustic centers somewhat, and a 2kHz crossover becomes possible. Of course you can compensate them in the crossover as well.
A large horn crossed low (500Hz) also seems to be a valid design choice for point-source target, however a driver that can go that low may have questionable HF characteristics. Plus, it's not going to fit in the same sort of space.
I don't consider point source a typically important target except in nearfield applications where driver summing may be an issue. With proper acoustic design, it is possible to integrate disparate drivers into a fairly cohesive image, fully time aligned even if the centers are not coincident.
When I have seen comments about MTM issues the key parameter was the mutual physical distance of the drivers vs the wavelength. So why minimizing physical spacing matters/helps little ? I am actually looking for hints what physical phenomena are at work here that severely negatively impact the acoustic quality of the MTM design irrespecitive of its actual physical space setup ? Some insight that could allow to also obejctively quantify the magnitude of the problems when comparing MTM vs coaxial.
To be a true point source the drivers need to be withing a quarter wavelength centre to centre.
For an MTM to be a point source the XO should be less than the quarter wavelength at that frequency, with a coax you will need some time delay on the tweeter (the issues as Sangram describes has been somethign coax builders have been working to solve for a long time.
dave
For an MTM to be a point source the XO should be less than the quarter wavelength at that frequency, with a coax you will need some time delay on the tweeter (the issues as Sangram describes has been somethign coax builders have been working to solve for a long time.
dave
Having close drivers does not make a point source on its own.
pawel, can you be more specific? Do you know what lobing is?
pawel, can you be more specific? Do you know what lobing is?
Good question Allen, it may be that my assumptions are too simplified here. Yes, I actually thought point source has mostly to do with the area that emits the sound.
So not going into lobing details yet I I thought that it applies similarly to both cases (MTM and coax) if area is similar.
So if I am missing fundamental physics /acoustics knowledge then just please shortly explain what different phenomena are at play here so that I can study it somewhere.
For now from my intuitive perspective the quarter wavelength applies similarly to 2 sepearate cones within a radius of 17 cm and a cone of this diameter. The same XO issues. But maybe I am wrong and that's what is behind the main thread question
And probably my question can also be phrased is coax is more 'point source' than colocated MTM and if so why ?
So not going into lobing details yet I I thought that it applies similarly to both cases (MTM and coax) if area is similar.
So if I am missing fundamental physics /acoustics knowledge then just please shortly explain what different phenomena are at play here so that I can study it somewhere.
For now from my intuitive perspective the quarter wavelength applies similarly to 2 sepearate cones within a radius of 17 cm and a cone of this diameter. The same XO issues. But maybe I am wrong and that's what is behind the main thread question
And probably my question can also be phrased is coax is more 'point source' than colocated MTM and if so why ?
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The lobing that happens when a tweeter and a mid are separated also happens between the two mids, except they are even further apart so it starts at a lower frequency. It also happens outside the crossover region.
As Sangram says a coaxial has discontinuities.. and the irregularities these create are not point source like.
As Sangram says a coaxial has discontinuities.. and the irregularities these create are not point source like.
There are a few coaxial that are pretty good and use the cone as a continuation of the tweeter horn. Tannoys do this well, there are others. If you’re biamping with DSP it’s a lot easier to get right. As was said in a MTM you have to cross the tweeter low enough to make sure the woofers are not doing freqs outside of their 1/4 wavelength band. The vertical dispersion is better on the coax too.
This is a diagram from Taylor showing the issue graphically (at a single frequency 1.5x the wavelength of the C-C)
(the paper for those interested, it is on line arrays, http://p10hifi.net/TLS/downloads/taylor-line-array.pdf)
If you have the drivers within a quarter wavelength they are essentially coincident. You need a crossover that maintains phase to complete the picture. In practise that means low XOs where wavelengths are long.
This is an MTM point source. First order XO at 250 Hz (active). The lowness of the XO also means that the 360° rotation with the 4th order passive order passive is not very noticable.
dave
(the paper for those interested, it is on line arrays, http://p10hifi.net/TLS/downloads/taylor-line-array.pdf)
If you have the drivers within a quarter wavelength they are essentially coincident. You need a crossover that maintains phase to complete the picture. In practise that means low XOs where wavelengths are long.
This is an MTM point source. First order XO at 250 Hz (active). The lowness of the XO also means that the 360° rotation with the 4th order passive order passive is not very noticable.
dave
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So from what has been said so far it seems that whether it is a single diaphragm (as in coaxials) or two diaphragms as in MTM located super-close does not bring important acoustic differences ?
Is it just that it is typically assumed that a coax diaphragm is less separated from the tweeter than is practice achievable when creating a sort of 'virtual coax' from two small mids and the same tweeter ?
Is it just that it is typically assumed that a coax diaphragm is less separated from the tweeter than is practice achievable when creating a sort of 'virtual coax' from two small mids and the same tweeter ?
There would be alot of difference between MTM and coaxial most notably its possible to get narrow vertical directivity and wide horizontal directivity with MTM.
Then there is fullrange drivers and multiple entry horns playing the same game of point source. All these systems come with their own specialties and issues.
You should figure out which one serves purpose to you and then find out how to optimize and make good system for you. Here some thoughts: For example small fullrange driver can be very good point source, very easy and cheap to implement but is SPL limited and very narrow dispersion on top frequencies. If you need wider response, for example not having dedicated listening chair in the sweetspot, or more SPL capability to throw occasional party, it is ruled out. If you need somewhat wide vertical dispersion as well, listen by sitting / standing then perhaps the MTM is also ruled out, no party. If you need SPL capability then pick the multiple entry horn, party time! Of course all these would work for many things its just possible to somewhat optimize by picking right system for given application.
If it is just you listening in dedicated space then a fullrange driver can make many boxes ticked, augment with bass system like any other main speaker. Don't forget to mitigate diffraction as it can somewhat unpointsource your system! 🙂
Then there is fullrange drivers and multiple entry horns playing the same game of point source. All these systems come with their own specialties and issues.
You should figure out which one serves purpose to you and then find out how to optimize and make good system for you. Here some thoughts: For example small fullrange driver can be very good point source, very easy and cheap to implement but is SPL limited and very narrow dispersion on top frequencies. If you need wider response, for example not having dedicated listening chair in the sweetspot, or more SPL capability to throw occasional party, it is ruled out. If you need somewhat wide vertical dispersion as well, listen by sitting / standing then perhaps the MTM is also ruled out, no party. If you need SPL capability then pick the multiple entry horn, party time! Of course all these would work for many things its just possible to somewhat optimize by picking right system for given application.
If it is just you listening in dedicated space then a fullrange driver can make many boxes ticked, augment with bass system like any other main speaker. Don't forget to mitigate diffraction as it can somewhat unpointsource your system! 🙂
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You're welcome!
100%! 😉 'Duh'
Yes, it was the first useful paper I saw online on the subject, though having learned how to design large prosound and ceiling arrays (using co/tri-ax drivers), decades before it has saved me lots of typing over the decades online. 😉
In an MTM you get that from the lobing, so not a real point source. But the case with most common MTM these days.
All systems are about juggling compromises.
dave
From the materials it seems that what actually counts is the distance from the acoustic centers of the midbasses and tweeters. I would like to tell shortly what design has been on my mind for some time - two fullranges and a 3/4 no flange tweeter. I am thinking about 2xPeerless 3" 830986 and e.g. Vifa OX20SC02. It is doable mechanically to place them ca. 6cm apart ( M-T centers).
What kind of problems would u see in this practical design ? Vifa would be recessed in relation to the mids so that the peak of their impulse responses is coincident. For the on-axis FR I can compensate/linearize it with custom DSP. Although I am rather sceptical as for the audibility of some FR irregularities especially in the top octave.
What kind of problems would u see in this practical design ? Vifa would be recessed in relation to the mids so that the peak of their impulse responses is coincident. For the on-axis FR I can compensate/linearize it with custom DSP. Although I am rather sceptical as for the audibility of some FR irregularities especially in the top octave.
A subject near and dear to my heart...
I've played a lot with MTM and WMTMW arrays including the XA series at Snell (XA90, XA75, XA Reference). One thing you get with symmetrical arrays is that it forces symmetry in the polar pattern. You can achieve lobe free performance if you can get the highest crossover point to work. The ratio between element spacing and crossover point determines everything. I found the best way to reduce the "wavelengths of separation at crossover" was to use smaller mids. a typical 5" or 6 1/2" pair of mids will be too far apart. What you really need is a pair of 2 to 3" diameter drivers and then a dome tweeter with good low frequency response and very short overall dimensions. It turns out once you can get the highest transition to work (dome to smaller mids) then the lower transitions (in, say, a WMTMW form) have no spacing challenges. You can even space units out.
I did a series of simulations of systems with 3, 4 or 5 ways and you can take a "log periodic" approach (like the antenna) and achieve fully lobe free constant directivity over a wide range. This means that each section can have the same number of Octaves bandwidth and should be set to a length in proportion to the center frequency (the Log Periodic part).
I have also played with post mounted coaxes. These were made mostly as inwall or ceiling speakers (designed for Snell then PSB). The typical tweeter on a post has very crappy response, primarily from tweeter reflections off of the cone behind. I found that if the tweeter was built into a directional flare (I used a 2 slope approximation to a JBL CD profile) that the tweeter directivity increased to the point where it wasn't reflecting off the cone and had very nice response. The larger flare didn't seem to effect the woofer response so it was a win-win.
A good coax with a pair of mids around it, possibly using the same cone assemblies as the coax unit, could be made to be lobe free. As always, the trick with coaxes is getting smooth enough response (which was always the UniQ or Tannoy challenge).
I've played a lot with MTM and WMTMW arrays including the XA series at Snell (XA90, XA75, XA Reference). One thing you get with symmetrical arrays is that it forces symmetry in the polar pattern. You can achieve lobe free performance if you can get the highest crossover point to work. The ratio between element spacing and crossover point determines everything. I found the best way to reduce the "wavelengths of separation at crossover" was to use smaller mids. a typical 5" or 6 1/2" pair of mids will be too far apart. What you really need is a pair of 2 to 3" diameter drivers and then a dome tweeter with good low frequency response and very short overall dimensions. It turns out once you can get the highest transition to work (dome to smaller mids) then the lower transitions (in, say, a WMTMW form) have no spacing challenges. You can even space units out.
I did a series of simulations of systems with 3, 4 or 5 ways and you can take a "log periodic" approach (like the antenna) and achieve fully lobe free constant directivity over a wide range. This means that each section can have the same number of Octaves bandwidth and should be set to a length in proportion to the center frequency (the Log Periodic part).
I have also played with post mounted coaxes. These were made mostly as inwall or ceiling speakers (designed for Snell then PSB). The typical tweeter on a post has very crappy response, primarily from tweeter reflections off of the cone behind. I found that if the tweeter was built into a directional flare (I used a 2 slope approximation to a JBL CD profile) that the tweeter directivity increased to the point where it wasn't reflecting off the cone and had very nice response. The larger flare didn't seem to effect the woofer response so it was a win-win.
A good coax with a pair of mids around it, possibly using the same cone assemblies as the coax unit, could be made to be lobe free. As always, the trick with coaxes is getting smooth enough response (which was always the UniQ or Tannoy challenge).