Author Topic: My next speaker build (again...)  (Read 6791 times)

Offline JimGore

Re: My next speaker build (again...)
« Reply #90 on: January 26, 2018, 12:05:11 PM »
Apologies for total off-topic question, but have you ever considered building a turntable? With your knowledge, skills and attention to detail it should be 'easy' for you.  :)

I haven't really given it much thought to be honest  :thinking:  I don't currently have a TT either so not something that's drawing my attention on a continual basis.  Perhaps at some point I will give it a go  ;)


This is a very beautiful build and I like the ply wood, but I do have a question after I see all those holes in the braising panels. When I used to build a lot a speakers I was always wondering if they don't mess with the internal acoustic resistance the same way the ports is acting on Fs, and that the original design in simulation ( software ) with box volume and the port size will the response still the same with all these tiny " ports, resonators, filters ... "  inside the box between the driver and the actual port. ?


This question is fascinating to me because answering it can be done very simply and very complex.

The simple answer is that yes, the bracing will have an effect on the way the speaker behaves.


The more complex answer can be a PHD thesis, but for the sake of keeping things at a reasonable level, let's try and discuss some of the things that happen.

I can identify at least 3 major aspects that need consideration:
1.  Chamber / air volume resonance.
2.  Tube resonance.
3.  Standing waves.

So for the sake of discussion, let's start looking at chamber / air volume resonance for a bass driver in a reflex enclosure: Let's imagine we have a floor standing speaker with a 50 liter enclosure and no bracing in it.  Being a floorstander, the cabinet is tall and narrow.  The driver is mounted on the front, towards the top of the cabinet.  There is a port on baffle towards the bottom of the cabinet.  Pretty standard thing.

So what's happening here is that the dimensions of the port is such that it allows the internal air volume of the cabinet to resonate at a certain frequency.  This is called the port tuning frequency, and for argument let's call that 40Hz.

Now what's real interesting is that the resonance created between the air volume and the port has nothing to do with the driver.  This happens independently from the driver.  When the driver excites the air inside the cabinet to the same frequency as the port tuning frequency, we get an increase in SPL because now the port is creating sound together with the driver.

Right, so let's now add a single shelf brace to our cabinet.  We use 16mm MDF and place it exactly in the middle of the cabinet, and it has one single hole in it of 100mm diameter.  In reality we now have 2 separate air volumes which are separated by a "port" 16mm long and 100mm diameter.

So now the top chamber where the driver is mounted has 25 liters internal volume.  A "port" as described above being 16mm long and 100m diameter in an air volume of 25 liters creates a tuned system with Fb being around 103Hz which is very different from our original 40Hz target.  This has a rather high Q factor so the gain is huge and causes quite an ugly looking peak in the speaker's frequency response.  Meanwhile the bottom air chamber is being "fed" by the top air chamber via that "port" in the shelf brace.  The bottom chamber is also now only 25 liters (rather than the original 50), so the port that used to give us the beautiful Fb of 40Hz has now changed to an Fb of around 56 Hz.

Rather than having a single air volume resonating at 40Hz we have now created 2 resonances at 103Hz and 56Hz respectively.  In essence it's some weird kind of a N-th order bandpass system now and not at all what we had intended initially.  The original 40Hz port tuning is still in the mix because ultimately we still do have an air volume of 50 liters, but this is now much constrained because of the new secondary resonances we have created. 

It comes down to how much we are restricting the airflow and pressure flow between the two chambers.  Because there is a single 100mm hole there is significant restriction between the two air chambers, thus the secondary resonances become dominant.  The more we open up that hole in the shelf brace, the less we are restricting the air and pressure flow between the two chambers, and therefore the smaller these secondary resonances become.  They will always be there, but we can lessen them to the point where they are insignificant.

Now I could go on for hours and days about this - there is actually a huge amount more discussion points floating around in my head, but I don't want to kill the topic by getting too heavy with the discussion just yet.

Hope that makes sense and is at least somewhat interesting?  Let me know if you want to continue the discussion.


Kind regards,
Ian.

Offline CAD

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Re: My next speaker build (again...)
« Reply #91 on: January 26, 2018, 12:36:26 PM »
Excellent and fairly easy to understand explanation Ian  :thumbs:
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Offline Mongoose

Re: My next speaker build (again...)
« Reply #92 on: January 27, 2018, 10:49:01 AM »
I cant bare to look at these damn images!

Nice going Jim

Offline Dubya Jay Dee

Re: My next speaker build (again...)
« Reply #93 on: January 27, 2018, 10:58:43 AM »
I cant bare to look at these damn images!



I don't think he would want you too either! :giggle:

Offline Mongoose

Re: My next speaker build (again...)
« Reply #94 on: January 29, 2018, 10:23:26 PM »
Quote
I don't think he would want you too either! :giggle:

 :ROFLMAO:

Offline Mongoose

Re: My next speaker build (again...)
« Reply #95 on: January 29, 2018, 10:28:20 PM »


WTF!!!!

Offline JimGore

Re: My next speaker build (again...)
« Reply #96 on: January 30, 2018, 08:53:24 AM »
WTF!!!!

 :giggle:   That's not related to the speaker build.  It's 10mm aluminium plate - the top plates for the prototype monoblock amplifiers I am building.




I have been making progress with the build, but nothing interesting enough to take pictures of.  I am sealing up the side panels to get them ready for paint.  It's a bit of a slow process because i am doing all sides evenly, but should be ready to continue in a couple of days.

Kind regards,
Ian.

Offline Rhino

Re: My next speaker build (again...)
« Reply #97 on: February 10, 2018, 06:13:11 PM »
Hope it all is going well with the projects.
Polk Audio Tsi500 - Fronts, Tsi200 - Bookshelf, Tsi100 - Front Height,  Cs10 - Centre, Integra DTR 50.5 AV Receiver,Sony BDP-S370 Blu-Ray, LG 3D Blu-Ray, Energy Veritas 10" Sub, Clavel 3 Way speaker

Offline LouisF

Re: My next speaker build (again...)
« Reply #98 on: June 10, 2018, 09:05:36 PM »

Standing waves will develop in an enclosed space, no matter what the shape of the space is.
No, that is not right. Read this:
https://www.crutchfield.com/Learn/learningcenter/home/speakers_roomacoustics.html
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how you treat people ultimately tells all."
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I have learned more from people who have differed from me than from those who have agreed with me.