Anyone who has been following my build thread will no doubt be familiar with the rather unusual cabinet design / construction method I have been working on. Probably not everyones cup of tea but it is different and does throw up the odd curve ball.
I am trying to work out the best port placement. I see advantages and diadvantages to both.
Inside Pro:
Port outlet is longer giving more opportunity for pressure wave to come out straight from the cabinet.
Inside Con:
The port shares a common wall with the driver cavity.
Outside Pro:
opposite of inside Con.
Outide Con
opposite of inside Pro.
Would appreciate the advise of those with some port knowledge as to the one with the least downside.
There is also a long in and a short in but not sure what the gain or loss would be there... Either design the port size would be equal.
I am trying to work out the best port placement. I see advantages and diadvantages to both.
Inside Pro:
Port outlet is longer giving more opportunity for pressure wave to come out straight from the cabinet.
Inside Con:
The port shares a common wall with the driver cavity.
Outside Pro:
opposite of inside Con.
Outide Con
opposite of inside Pro.
Would appreciate the advise of those with some port knowledge as to the one with the least downside.
There is also a long in and a short in but not sure what the gain or loss would be there... Either design the port size would be equal.
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Hi silent screamer;
If you really, really want a midrange port, your crossover frequencies are high enough that the port should be in front for proper SPL summation with the woofers for controlled dispersion, including the 2-PI to 4-PI baffle step transistion. Fbst ~ (4560/Baffle_width_inches)
I have gotten the best midrange sound from a large sealed or aperiodic loaded cabinet with extensive stuffing along the rear cabinet area to absorb the rear cone wave from reflection. It is very common for midrange rear cone vibrations to leak through a port and ruin high SPL listening. I favor a Qts between 0.5 and 0.6 with an aperiodic tuning for the midrange. From your recent pictures it appears that you could make two equal midrange cabinets volumes, each with either a sealed box, or a small rear aperiodic vent at the rear. B&W and Humble Home HIFI use aperiodic type midrange loading.
I have never been able to get a MTM with a ribbon tweeter to sound great due to lobing effects from the large M-T and M-M spacing. Only small diameter dome tweeters like the SB29RDCN and small 4"-5.5" midranges measured and sounded great. The published MTM spacing math is on the web. Since you already appear to own a RAAL 150-10, an TMW is probably in your future. I now only build TMW.
For improved edge diffraction reduction you could make your MTM sides 2-3 boards wider and use a 1.5" quarter round router bit (~$130 in USA) along the entire edge perimeter. Some would say round edges are more organic, as well as delivering improved diffraction control.
If you really, really want a midrange port, your crossover frequencies are high enough that the port should be in front for proper SPL summation with the woofers for controlled dispersion, including the 2-PI to 4-PI baffle step transistion. Fbst ~ (4560/Baffle_width_inches)
I have gotten the best midrange sound from a large sealed or aperiodic loaded cabinet with extensive stuffing along the rear cabinet area to absorb the rear cone wave from reflection. It is very common for midrange rear cone vibrations to leak through a port and ruin high SPL listening. I favor a Qts between 0.5 and 0.6 with an aperiodic tuning for the midrange. From your recent pictures it appears that you could make two equal midrange cabinets volumes, each with either a sealed box, or a small rear aperiodic vent at the rear. B&W and Humble Home HIFI use aperiodic type midrange loading.
I have never been able to get a MTM with a ribbon tweeter to sound great due to lobing effects from the large M-T and M-M spacing. Only small diameter dome tweeters like the SB29RDCN and small 4"-5.5" midranges measured and sounded great. The published MTM spacing math is on the web. Since you already appear to own a RAAL 150-10, an TMW is probably in your future. I now only build TMW.
For improved edge diffraction reduction you could make your MTM sides 2-3 boards wider and use a 1.5" quarter round router bit (~$130 in USA) along the entire edge perimeter. Some would say round edges are more organic, as well as delivering improved diffraction control.
Thanks LineSource for your thoughts and input. The RAAL I'm using is a 140-15D AM with a couple of ScanSpeak 12MU8731's in parallel. So the mids fit your ideal size at 120mm or 4.75"
RAAL claim that the ribbon in question doesn't suffer badly from the large centre to centre due to the design, but time will tell. I will endeavor to keep the spacing down to a minimum.
For edge diffraction I have already ordered a couple of no name 3" rounding over bits from China to try out. They have a radius of about 28mm so hopefully that is enough to help out.
Tend to agree that taking the edge off the corners does make it more pleasing to the eye, but I am doing it for sound...
Can you dumb this down a bit more please too much unknown jargon for me.
"If you really, really want a midrange port, your crossover frequencies are high enough that the port should be in front for proper SPL summation with the woofers for controlled dispersion, including the 2-PI to 4-PI baffle step transistion. Fbst ~ (4560/Baffle_width_inches)"
The SS drivers being used have a Qts of 0.29 so they really dont lend themselves to sealed.
I have abandoned the idea of trying to weave the port across the back of the cabinet and have decided to build the port into the speaker holder, extending the speaker cabinet on the 4 centre slices.
The 2 x 18mm centre slices will be the port making it 36mm wide, and the two adjacent 18mm slices will form the sides of the port and be the structural strength of the holder, along with the top and bottom pieces from the port.
RAAL claim that the ribbon in question doesn't suffer badly from the large centre to centre due to the design, but time will tell. I will endeavor to keep the spacing down to a minimum.
For edge diffraction I have already ordered a couple of no name 3" rounding over bits from China to try out. They have a radius of about 28mm so hopefully that is enough to help out.
Tend to agree that taking the edge off the corners does make it more pleasing to the eye, but I am doing it for sound...
Can you dumb this down a bit more please too much unknown jargon for me.
"If you really, really want a midrange port, your crossover frequencies are high enough that the port should be in front for proper SPL summation with the woofers for controlled dispersion, including the 2-PI to 4-PI baffle step transistion. Fbst ~ (4560/Baffle_width_inches)"
The SS drivers being used have a Qts of 0.29 so they really dont lend themselves to sealed.
I have abandoned the idea of trying to weave the port across the back of the cabinet and have decided to build the port into the speaker holder, extending the speaker cabinet on the 4 centre slices.
The 2 x 18mm centre slices will be the port making it 36mm wide, and the two adjacent 18mm slices will form the sides of the port and be the structural strength of the holder, along with the top and bottom pieces from the port.
Attachments
Using 4560/X_wide is a common approximation for a floor length X_wide speaker cabinet baffle step compensation.
I think the main advantage of a rear port on your MTM would be to keep midrange frequencies from leaking through the port into the front soundstage. Extensive cabinet stuffing also minimizes midrange port leakage. A rear port could also help cabinet art requirements.
If you put the midrange port on the rear, then rear wall spacing will affect the total SPL.
MJK's Mathcad worksheets show a modest SPL difference between front and rear ports(looks like 5%) from upper frequency port output and non-symmetric diffraction effects. If your mid-woofer crossover occurs at a frequency where the rear port is generating SPL, then the crossover and BSC must take this into account.
There are several good papers on MTM spacing. Both M-T and M-M spacing are important. Biro Technology
I think the main advantage of a rear port on your MTM would be to keep midrange frequencies from leaking through the port into the front soundstage. Extensive cabinet stuffing also minimizes midrange port leakage. A rear port could also help cabinet art requirements.
If you put the midrange port on the rear, then rear wall spacing will affect the total SPL.
MJK's Mathcad worksheets show a modest SPL difference between front and rear ports(looks like 5%) from upper frequency port output and non-symmetric diffraction effects. If your mid-woofer crossover occurs at a frequency where the rear port is generating SPL, then the crossover and BSC must take this into account.
There are several good papers on MTM spacing. Both M-T and M-M spacing are important. Biro Technology
Did a bit of a mockup today to see if the rear port idea works. I have to offset the tweeter by 18+mm from centre to allow the port to be central.
Which is fine since the frequency response looks flatter with the mids not in line with the tweeter anyway. Still plenty of room for generous radius on the sides.
I could put a port where the X is but think I prefer to port to be rear facing, as trying to get length and exit the front would be challenging.
Which is fine since the frequency response looks flatter with the mids not in line with the tweeter anyway. Still plenty of room for generous radius on the sides.
I could put a port where the X is but think I prefer to port to be rear facing, as trying to get length and exit the front would be challenging.
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