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Old 21st June 2012, 10:52 AM   #1
cs is offline cs  United Kingdom
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Default Tapered TL vs MLTL

I'm interested in building a slim floor-standing column speaker using either a tapered TL or an MLTL configuration. After carrying out a few simulations using the MJK worksheets, I found that for a given driver, the response of the two types is quite similar. The main differences are that the TL has a small amount (~2dB pk-pk) of ripple between 100Hz and 1kHz when using a 5:1 line taper, whereas the MLTL has much higher port velocity (about 5 times higher).

Are there any benefits in terms of sound quality for one type of cabinet versus the other ? The TL needs a slightly bigger cabinet, maybe +5% in each dimension.

Out of interest, I also put the 'coupling chamber + line' configuration, as used in IPL Acoustic's TL designs, into the MJK worksheet, and this gave a terrible response however much I fiddled with the line damping.
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Old 21st June 2012, 01:45 PM   #2
pkitt is offline pkitt  United States
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I've modeled many, many TLs and built quite a few for personal use. The main advantage of a tapered TL is its lower terminus air velocity, and the main advantages of an ML-TL are it's simpler to build and will have a smaller volume for the same F3. What I've found is that is takes 20-25% more volume in a tapered TL to achieve the same F3 as in a truly equivalent ML-TL.

The higher port air velocity in an ML-TL isn't necessarily a "show stopper". If you can keep its peak velocity to no more than 3%-5% of the speed of sound with an input power to the driver that causes it to reach Xmax+15%, while the system output SPL reaches whatever you want it to, the audible results should be satisfactory. You also need to pay attention to the frequency at which the port air velocity peaks; if it occurs at 40 Hz it will be more likely to be excited and audible than if at 20 Hz, for instance, just due to musical content considerations. This is just one of the compromises one has to deal with.

But, to answer your main question if the two types of TL designs are as similar as you've described, there will be no audible differences in their sound, at least IMO and from my experience.
Paul

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Originally Posted by cs View Post
I'm interested in building a slim floor-standing column speaker using either a tapered TL or an MLTL configuration. After carrying out a few simulations using the MJK worksheets, I found that for a given driver, the response of the two types is quite similar. The main differences are that the TL has a small amount (~2dB pk-pk) of ripple between 100Hz and 1kHz when using a 5:1 line taper, whereas the MLTL has much higher port velocity (about 5 times higher).

Are there any benefits in terms of sound quality for one type of cabinet versus the other ? The TL needs a slightly bigger cabinet, maybe +5% in each dimension.

Out of interest, I also put the 'coupling chamber + line' configuration, as used in IPL Acoustic's TL designs, into the MJK worksheet, and this gave a terrible response however much I fiddled with the line damping.
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Old 21st June 2012, 03:11 PM   #3
cs is offline cs  United Kingdom
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Paul,

Thanks for the info.
The MJK worksheets indicate that the MLTL peak port velocity is indeed at around 40Hz, and at the driver Xmax it will be reaching about 7% of the speed of sound.
The cabinet dimensions are coming out at 19 x 24 x 90cm for the MLTL and 21 x 31 x 95cm for the TL, so there isn't a huge size penalty.

Regards,
Chris.
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Old 21st June 2012, 04:53 PM   #4
pkitt is offline pkitt  United States
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Chris, you didn't mention what size of port tube you modeled. Can you increase its diameter to lower the air velocity (meaning, of course, its length will need to be increased, also, to maintain the system tuning frequency)? If you did that and were also able/willing to tune the system a bit lower, you could move the peak down in frequency some. One more thing, you are aware that the Y-axis on the air velocity graph has units of mm RMS? If you're using the driver's Xmax in mm Peak directly on the graph, instead of converting to mm RMS by multiplying the rated Xmax by 0.707, you're actually pushing the driver to 41.4% over its rated Xmax (just a thought).
Paul

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Originally Posted by cs View Post
Paul,

Thanks for the info.
The MJK worksheets indicate that the MLTL peak port velocity is indeed at around 40Hz, and at the driver Xmax it will be reaching about 7% of the speed of sound.
The cabinet dimensions are coming out at 19 x 24 x 90cm for the MLTL and 21 x 31 x 95cm for the TL, so there isn't a huge size penalty.

Regards,
Chris.
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Old 21st June 2012, 05:24 PM   #5
cs is offline cs  United Kingdom
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Quote:
Originally Posted by pkitt View Post
Chris, you didn't mention what size of port tube you modeled. Can you increase its diameter to lower the air velocity (meaning, of course, its length will need to be increased, also, to maintain the system tuning frequency)? If you did that and were also able/willing to tune the system a bit lower, you could move the peak down in frequency some. One more thing, you are aware that the Y-axis on the air velocity graph has units of mm RMS? If you're using the driver's Xmax in mm Peak directly on the graph, instead of converting to mm RMS by multiplying the rated Xmax by 0.707, you're actually pushing the driver to 41.4% over its rated Xmax (just a thought).
Paul
Paul,

My MLTL simulation used a port dia of 5.5cm and length of 13cm. This just happened to be a readily available port with a flared flange, which tuned correctly. However, I could use a bigger dia (and longer) port as you suggest, although the pipe resonance in the port itself might then be noticeable.

Well spotted on the cone displacement plot. I must admit I did not notice that it was rms displacement !

The port velocity in my worksheet (an old version from 2006) is scaled in velocity relative to the speed of sound.

Chris.
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Old 21st June 2012, 05:39 PM   #6
pkitt is offline pkitt  United States
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Chris,
Okay, a couple more comments. Martin's software is only valid for a non-flared port, which means that whatever length you modeled will need to be increased for the flared port in order to achieve the desired tuning frequency. Typically increasing the modeled length by 1/2" for each flared end will do the trick, but that's not a guarantee. Yes, if you use a larger diameter port that also has a longer length, a pipe resonance might rear its ugly head, but you can usually mitigate that effect at least partially if not completely by moving the port's location a bit which forces that resonance higher up in frequency and possibly better attenuated by the crossover. The graph for air velocity has always been scaled to a percentage of the speed of sound AFAIK and still is for later versions of Martin's software. So, a peak velocity of 3% of the SOS is equal to ~10 m/s, and at 5% of the SOS, it's ~17 m/s.

When I test a model to see what happens to the port/terminus air velocity, I increase the Reference input power (with Rref set to the actual nominal impedance of the driver as seen on the impedance graph) until the driver's excursion hits Xmax+15%, in RMS units, of course. For any specific air velocity magnitude, the lower in frequency of its peak, the better, and the lower in frequency where the driver hits Xmax+15%, also the better.
Paul

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Originally Posted by cs View Post
Paul,

My MLTL simulation used a port dia of 5.5cm and length of 13cm. This just happened to be a readily available port with a flared flange, which tuned correctly. However, I could use a bigger dia (and longer) port as you suggest, although the pipe resonance in the port itself might then be noticeable.

Well spotted on the cone displacement plot. I must admit I did not notice that it was rms displacement !

The port velocity in my worksheet (an old version from 2006) is scaled in velocity relative to the speed of sound.

Chris.
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Old 21st June 2012, 05:53 PM   #7
cs is offline cs  United Kingdom
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Paul,

Thanks for the additional comments.
I guess these potential port velocity issues tend to favour the TL. My offset tapered TL worksheet predicts a port velocity of 0.002*c at the default 1W input power, so even at 100W input, it is still going to be below the recommended 0.03*c

Chris.
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Old 21st June 2012, 06:03 PM   #8
pkitt is offline pkitt  United States
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Yep, that's a big advantage of a tapered TL. I've built several tapered TLs for personal use, but a few more ML-TLs for their advantages. If I'm really trying to minimize overall size, I'm willing to live with the higher port air velocity of an ML-TL as long as it doesn't appear there will be objectionable port noise. Sometimes, too, a particular driver just doesn't perform as good as one wants in a tapered TL. All speaker designs have compromises unfortunately.
Paul

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Originally Posted by cs View Post
Paul,

Thanks for the additional comments.
I guess these potential port velocity issues tend to favour the TL. My offset tapered TL worksheet predicts a port velocity of 0.002*c at the default 1W input power, so even at 100W input, it is still going to be below the recommended 0.03*c

Chris.
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Old 21st June 2012, 07:48 PM   #9
sreten is offline sreten  United Kingdom
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Hi,

One thing you can do with a MLTL you can't with a TL is tinker with the tuning.
Sometimes for vented boxes in some rooms lower tuning simply works better.

rgds, sreten.

FWIW MJK reports he has modeled Fried type TL cabinets and found they
simply don't work as described or very well given the total box volume.
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Old 21st June 2012, 07:56 PM   #10
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Quote:
After carrying out a few simulations using the MJK worksheets, I found that for a given driver, the response of the two types is quite similar.
Hmm? While ther are different ways to tune, as far as I can tell a Tapered TL should have an almost entirely damped lower impedance peak:

Click the image to open in full size.

resulting in a shallow knee response profile like this:

Click the image to open in full size.

An ML-TL will have a dual impedance peak
Click the image to open in full size.

resulting in a more efficient, but sharper knee response profile like this:

Click the image to open in full size.

The different between the two alignments results in a different amount of overlap between the terminus' and driver. The Tapered TL will have an excursion profile like this, where the line controls the driver lower in frequency:

Click the image to open in full size.

while the ML-TL will have an excursion profile like this, where the port starts to act against the driver:

Click the image to open in full size.

While I don't want to make claims of what's "right" and "wrong", i think that if the tapered TL (quarter wave) is getting the same response profile as the ML-TL (bass reflex) then you're probably modelling the Tapered TL... unusually.

Last edited by RockLeeEV; 21st June 2012 at 08:18 PM.
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