The other day I played with the effects of a quarter wavelength resonator at the 150 Hz area (pressure minimum).

I tuned the pipet to 150 Hz, 344/150/4=0,57 m and attached the open end at 120 cm from S1, no damping, no paddles inside the horn.

I don’t think this is the right spot to work with a quarter wave resonator but it can be interesting nevertheless.

https://dl.dropbox.com/u/1320184/hififorum/Traadar/15%20tum%20MLTL/periskop.jpg

https://dl.dropbox.com/u/1320184/hififorum/Traadar/15%20tum%20MLTL/SPL-med-och-utan-resontor-150-hz.gif

Green curve: Just the horn
Red curve: Horn with 1/4 wave resonator tuned to 150 Hz

As we can see it didn’t make any change at all.

Just to experiment and see how things work I retuned the pipe to treat the peak at 170 Hz by cutting it a couple of cm shorter and got this result:

https://dl.dropbox.com/u/1320184/hififorum/Traadar/15%20tum%20MLTL/SPL-med-kort-resonator-i-mynningen.gif

It is interesting to how narrow the bandwidth that the tube affects is. So far the tube is without any damping.

Let’s see what happens when we connect the tube as close to S2 as possible. In this case the closest I could get is 25 cm from S2.

https://dl.dropbox.com/u/1320184/hififorum/Traadar/15%20tum%20MLTL/ror%20i%20borjan.jpg

After some trials I found the right amount of stuffing, which turned out to be next to nothing. Just too much and the effect of the resonator was ruined, just too little and I got a sharp notch.

https://dl.dropbox.com/u/1320184/hififorum/Traadar/15%20tum%20MLTL/ganska-liite-dampfluff.jpg

https://dl.dropbox.com/u/1320184/hififorum/Traadar/15%20tum%20MLTL/I%20b%C3%B6rjan%20170%20Hz%20SPL%20fas%203%20dist.png

Green Curve: Original horn
Blue Curve: With ¼ wavelength and damping tuned to 170 Hz

SPL, phase and third tone distortion.

Isn’t this pretty promising? One could take the volume for resonators in account in design rather easy and get a more controlled behavior above the crossover frequency.

/Forsman

Continue reading

Experiment with reduction of Cross section of Thorn F1

I am still working at the pressure minimum for the 150 Hz dip, which might be all wrong.

Let’s take the out the damping and construct a couple of paddles to fit in the holes, by doing so I obstruct approximately half of the horn area. The bends seam to help create standing waves in the horn. By cutting the cross section by half I hoped to induce the same effect as a bend  tend to do.

Unfortunately my idea didn’t work at all.

Have I cut the cross section down too little? Or am I trying to address the problem at the wrong place in the horn, should I maybe aim for the pressure maximum at 150 Hz instead?

/Forsman


Experiments with damping in the Thorn F1

An idea to address the dip at approx. 150 Hz (last curve in this post)  is to damp the horn at the position where the pressure minimum is for the frequency. Looking at the internal measurment measurements showed in the .pdf document that would be at a position about 120 cm from S1. I figure that where the pressure is low the air speed is high and therefore the right place to apply some damping.

I did drill three holes in the back of the horn, placed 80 g of polyester fiber wool there and sealed it up properly.

The measurements are made in the middle of the mouth of the horn. I have zoomed in quite a bit to get better resolution of the SPL response graphically.

Green curve is the original horn without any damping and red curve is with 80 g och polyester wool damping.

Well that much damping eats a lot of the bass response and makes the dip at 150 Hz even worse!

Reducing the amount of damping by half and a new measurement.

Not much better. The peaks are somewhat lower, so are also the dips but not to the same extent and I am loosing bass.

I am a bit surprised of this result. I really thought that damping at the pressure minimum would have positive effects on the 150 Hz fip.

It does look much better when I zoom out and apply som smoothing… ;-)

Kind regards

/Fredrik


Pictures from the buildning of the Thorn F1

Here are some pitctures from the building of the Thorn F1. First som cutting of particle board in my back yard.

I found it pretty useful to do a 1:1 drawing of the horn on one of the horn sides. That made the rest of the build much easier when it comes to fit pieces and drill holes at the correct places.

Cutting the hole for the driver

Starting off with the internal structure.

When you have drawn where all the pieces are going to be, it is very easy to pre drill holes for the screws. I don’t think it is necessary to use screws when you glue it together but the assembly gets a lot easier and doesn’t require as many clamps.

Some bracing and “baffle” in place

A simple and easy way (Q&D) to fasten the driver. I don’t think I would recommend this sulotion for permanent use. At least not without proper washer’s…

More bracing.

Time to put the last side on.

I used Polyutherane based glue which I think is excellent for this kind of wood working. It expands and creates nice foam that seals any cracks or bad fitting between the panels. One should be very careful with getting it on the skin because I stains the skin and is impossible to get rid of.

/Forsman


Internal measurements on the Thorn F1

Measurements

I do would like to be able to set the crossover at about 150 Hz. Unfortunately there is a valley in the SPL-response there. I was thinking that if I can find the pressure minimum at 150 Hz that would be the place to put some damping in the pipe. Where the pressure is lowest the speed is highest why that theoretically would be a good place to start to work with the damping.

Let’s drill some holes and do some measuring.

My microphone i 8 mm in diameter and thus I bought a bach of wooden dowels in the same size.

Bild

I sealed evry hole with filler and not glue in case I needed to redo some measurements. Then I have to pull them out again and glue them propperly if am going to keep this box.

Bild

The measurement rig.

Bild

Bild

Closing up on the mouth of the horn.

Bild

All measurements done

Bild

The first SPL-curve at position 10 cm from S1 (beginning of the horn).

Bild

All measurements in a pdf-document (1 Mb).

For you who want to do your own interpretation of the measurements i have put all the measurement files in a RAR-archive for easy downloading. The software I’ve used is the Holm Acoustics.

All measurement Files from Holm Acoustics in a RAR-archive (13 Mb).

/Forsman


Thorn F1, my learning experiance with a Tapped Horn

Thorn F1, a DIY Open Source Project

This is a thread about my attempts to build and better understand Tapped Horn loudspeakers. I started out with a simple build and want to share my ideas and hopefully get some feedback on my thoughts.

Anders Martinsson often emphasis the strength in collaboration and the open source atmosphere.   What I have learned so far about DIY hifi is all thanks to that attitude and the people who share that belief. I hope to start a thread that can contribute to that DIY collaboration thought and mutual learning.

OK, loudspeakers for bass!

I want bass in my home and I want it to be clean, loud and accurate. I have looked at different options to reproduce bass mostly variations on Transmission Lines and Mass Loaded Transmission Lines but finally decided to go with Tapper Horn, for now.

My design goals are:

  • Bass with high quality and ability to play loud
  • Extension downwards to be able to reproduce the lowest note of a normal electric bass guitar (about 41 Hz I think).
  • Cabin depth of max 50 cm
  • I do want as much “kick” as possible without sacrificing the bass around 40-50 Hz.
  • Possibility to cross over as high as 150 Hz
  • High efficiency with max SPL at X-max of at least 125 db at 1m in 1 pi
  • Manageable size for domestic use.

I am a great fan of Anders Martinsson and his THAM designs which have been a significant source of inspiration in this build. I have also got a lot of help and feedback on this design on HiFiForum.nu from many, espessially Circlomanen, petterpersson and Martinsson just to mention a few.

I am very appealed of the robustness of PA elements and I really do like Eighteensounds products. When I did get the opportunity to buy a new pair to a heavy discount I couldn’t resist the temptation. The element of choice is Eighteensound 12LW800. I think it is very suitable for tapped horns with high BL, high fs, low Qts and fairly long excursion. Other elements that is suitable in this design without any adjustments are Beyma 12LX60, EighteenSound 12LW1400. Thanks to their longer X-max (9 mm) they will provide even higher max SPL.

Speaking of robustness; I think one can use the 12LW800 to support a car when changing tires without damaging it. The basket is built like a tank.

Internal with: 42 cm inclusive bracing.

The cabinet is just for trying out the concept. I have therefore used the cheapest particle board available and not gone for Baltic Birch Plywood or something similar at this stage.

Maybe I didn’t meet all my goals but quite a few I think. :)

/Forsman


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