Transmission line bass cabs

Discussion in 'Amps and Cabs [BG]' started by Bass Unique, Nov 3, 2011.


  1. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    Hoping to put sme carpet on the cab this week. I think I may give it a layer of varnish first - would protect it against standing on a damp floor when unloading from the car or also the occasional beer spill :) will put up some pics when it is done.

    For those interested, I have added a little more stuffing in the first 1/4 line length (but it is still not packed) and removed a little directly behind the driver. To my ears it is slightly smoother and faster sounding but does not really measure much different in terms of response.

    Still no joy with the weather and number 1 job is taking up most of my time at the moment so may not get back to this cab until Friday.

    I have also completed a few calcs to see where I could make this lighter and more compact. I think I could remove approximately 30mm (1 & 1/4 inches) from the height and stick with the same format or using a 6 mid instead, it could be compressed by 100mm in height and maintain the line length (but this becomes a more classic TL format with the driver part way down the line and there would be less of a chamber behind the driver).

    Next project will be an 8 due to popular demand! (plus I can use up my spare wood :) )
     
  2. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    well here it is - The infamous T-line speaker. I got carpet on it so it is now ready to gig. The front baffle is coated with 4 layers of rosewood stain varnish. Total 22 kgs. Will put some build pictures up soon.

    Very pleased with it. For the first time in my life I had to reduce the bass control on my Markbass tube as it was too much!

    I did place a plot here but since discovered a simple error - I had my good microphone connected but it was reading noise from the cheap mic (probably with bass boost). So, no plot here for now and it is too late to re-do it. It appears to extend well to 29 Hz though and rolls off a little faster from this point. (Ignore the plot... more to come.)
     

    Attached Files:

  3. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    1210T speaker.jpg

    Hope it came out this time!
     
  4. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    You need to use a standard logarithmic chart. I also suspect that you didn't measure it correctly. The mic must be at least two meters distant for an accurate result, and you can't use only one mic placement. First you measure ground plane, the mic a half-inch off the ground, which gives a half-space result below the baffle step frequency. Then you put the cab on its back facing up, suspending the mic above it to get the half-space result above the baffle step frequency. You splice the two together for a full bandwidth half-space result.
     
  5. For real test results you need an anechoic chamber, analyzing software, scope, noise/signal generator, a real testing microphone, and etc.
     
  6. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    ok ok - I know I do have a good B&K mic and analyser for testing etc, however, only have part of the kit with me today. Will do it at work so I can believe the result. I did test at 1m so this appears to close for accuracy. How far would you recommend Bill? My guess, now you have mentioned it, is Pi time the largest dimension of the baffle?

    An anechoic chamber is probably not the best place to do this type of testing as they are only anechoic down to a cut off frequency. It would have to be a pretty special chamber to get this low! The typical small ones are not really very good at all for anything below 150 Hz at best. Testing outdoors means you get a result which is not affected by any wall etc but as pointed out, my ripples (and some boost) are probably due to my technique. Was interested to hear how Bill suggested I did the test outdoors. Will follow his guidance on this so we get a result that works.
     
  7. Cirk

    Cirk

    Jan 16, 2011
    Panama City, Florida
    Bass Unique, just wanted to drop you a note of encouragement. This design is quite interesting.
    What kind of power handling and SPL ratings do you expect to achieve with this design?
     
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    As far as is practical, which usually limits you to maybe four meters. When you measure close to the cab you introduce the nearfield condition, which influences the result.

    That's not an issue. What is an issue is that you simply don't need a chamber when an open field works just as well and is considerably cheaper. Companies that use chambers do so because they tend to be located in noisy manufacturing zones where simply going outside isn't an option.
    BTW, you can do the measurement in one shot if you dig a pit deep enough for the cab to fit into with the baffle flush with the ground, then backfill around the cab. Taking the measurement with a suspended mic then gives a full bandwidth half-space result.
     
  9. If you take an outdoor reading you might be surprised how noisy an open field is. ;)
     
  10. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Washington, Utah
    John K Custom Basses
    i agree. even a slight breeze will skew the results. the reason that pro speaker designers and manufacturers use anechoic chambers is to have a completely controlled environment to get accurate, consistent and repeatable results 24/7.
     
  11. +1
    The only place better is outer space - no joking.
     
  12. wcriley

    wcriley

    Apr 5, 2010
    Western PA
    Nope. No air = No air pressure changes = No sound = No reading.
     
  13. Oops you’re right, no medium no sound waves - I stand corrected. Anyway, an open field isn’t good above 80Hz.

    EDIT:
    PS - and thank you too, thats what I like about this forum - everyone helps to keep the information accurate.
     
  14. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Tell that to Danley Sound Labs. :meh:
    Slight breezes and the occasional passing bird do not skew the results, even at the 10 meter mic distance that Danley uses, as the measuring software filters that out. Anechoic chambers pre-date modern measurement software.
     
    speyer likes this.
  15. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Washington, Utah
    John K Custom Basses
    i'm sure that there are exceptions, but IMO, most pro companys wouldn't want to have to wait for a clear day with the right conditions (and the same ones) to test their speakers. plus it appears that Danley does alot of outdoor stadium installations.
     
  16. John Meyer’s of Meyer Sound says an anechoic chamber is an essential tool for designing new loudspeakers.

    by John Meyer
    In November of 1994, at Meyer Sound we embarked on the design and construction of our own anechoic chamber. After careful evaluation of the alternatives, we decided that such a room was an essential tool for designing new loudspeaker systems that would perform precisely as specified in critical sound reinforcement applications.

    And he say’s more - check it out.
    Meyer Sound : The Anechoic Chamber
     
  17. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    If you read the link to Meyers anechoic chamber, it re-inforces what I said. You cant do bass in a typical chamber. Thats because the walls only absorb reflections down to a cut off frequency - Meyer says 80 Hz for his which is quite good and would suggest it is reasonably large, has thicke wedges on the walls and non-parrallel walls to break up standing waves. Below the cut off, it behaves just like a normal room of the same size. He goes outdors too below 80 Hz....
     
  18. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Chambers make sense when you're taking measurements manually, measuring each frequency individually and then plotting them. That procedure takes about ten seconds per frequency. With bare-bones 1/6 octave resolution there are 64 points to plot. And then you have to repeat for off-axis at each desired angle, usually 15, 30 and 45 degrees. Measuring one speaker can be an all day affair. :crying:
    With measurement software a complete plot takes only 5 seconds, and that's with 1/64 octave resolution. :hyper:
     
  19. Bass Unique

    Bass Unique

    Nov 3, 2011
    Wiltshire
    When I worked for GLL, we had a semi anechoic chamber for measuring loudspeaker drive units. This had Acoustic wedges on the walls, floor and ceiling except for the one wall with the drive unit in it which was flush with the wall (kind of like burying your speaker in the ground to measure the response). I am not sure of the system used but this was automated and took about 1 minute per loudspeaker.

    I have also worked in the Dyson Semi-anechoic chamber which was pretty crude but sufficed for some sound power testing. We used non-parrallel walls in that chamber to minimise standing waves (although the floor and ceiling were still parallel so not ideal).

    I have since designed a number of anechoic and reveberrent chambers. You can use a reverberrent chamber to measure response as well but it is wise to have non-parrallel walls, curves reflectors/diffusers and multiple microphones. You also have to be positioned in the reverberrent field (further away than near and far field). You lose any useful off axis information though...

    The chambers are repeatable, have low background noise and ideal for higher frequencies in midrange or HF. Unless you have a lot of space and an unlimited budget, there is no point in using one for very low frequencies (below 100 Hz) for occasional measurements as an outdoor space at ground level is also typically semi-anechoic. Thats how they usually test the sound power of JCB's and other large plant without the need for a very large NASA style chamber.

    I built a small tweeter anechoic chamber a few years back which was not big at all - pretty much the same size as my speaker (less than 1m cubed). It was useful in determining the absolute response of the high frequency drive units. We also used something similar at GLL for quality testing which allowed a quick test to be performed which ensured the drive units were within set limits and reported a "fail" if it was not.
     
  20. Yes I read the whole article and many others too (and about measurement software not being really accurate). Oh and btw, the reason why you’re getting ripples is because of ground reflections - maybe if you can get the speaker about 50ft up in the air and test it you might get better test results. Also you need to test above 80Hz too. I was just thinking - maybe Meyer Sound would let you test your loudspeaker at their facility for free or a small fee - if you are serious about your loudspeaker deigns you might want to contact them and ask. Have fun and good luck.
     
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Aug 4, 2021

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