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... but you want to design your own loudspeaker?

My conversations with Fitz

 

... your own loudspeaker?

 

... come into my parlor ...

1 - How we got started with this conversation
2 - An introduction to Acoustics and Hearing
3 - Acoustics and Mechanics
4 - Electronics and Sound
  1 - The Creation
2 - Me and the Planet
3 - Reality and Meaning

Association_Model_of_Perception.pdf

 

 

3 - Acoustics and Mechanics

3.0 - Introduction
3.1 -
Your woofer box
3.2 - Your woofer box at higher frequencies
3.3 - Wave diffraction at the cabinet edge
3.4 - The LXmini woofer
3.5 - The woofer & tweeter combination in Design #3
3.6 - The LXmini tweeter/fullrange

 

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3.4 - The LXmini woofer

Fitz:  Yes, my woofer design is different from the LXmini. And that is because my domestic partner does not like to have those two poles standing in our living room. They look too industrial, too much like plumbing.

SL:  What can I say. Taste is in the eye of the beholder. My philosophy is: 'Form follows Function'. The Bauhaus motto for design. A loudspeaker is a transducer to me: electrical in, acoustic out. It's a machine. Though to me there is a beauty in a well executed machine design.

         

Fitz:  So my box is conventional and and has a familiar look, except for the tweeter on top. I think it's cute. 

SL:  And like the majority of conventional box speakers you have a baffle step in the on-axis response. And after you have equalizes it to flat, then you have the off-axis roll-off.   

The LXmini woofer also has a baffle step, but it is in the response pointing to the ceiling. The woofer is mounted to point at the ceiling, but I listen at 90 degree off-axis, where the response is flat. You can see this behavior in the graph for a theoretical point source in the center of a cylinder. Since I operate the woofer driver only below 1 kHz, it is acoustically small. Acoustic measurements showed that I needed no eq to have a flat response. Only at the low end, where the woofer rolls off at 12 dB/octave rate and is 6 dB down at 60 Hz, did I apply some boost to extend the response to 45 Hz, -3 dB.    

        

Fitz: Yes, as I said earlier I will get more bass output from the 8" driver, than the LXmini can get from its 5" driver. For the same cone displacement I should get roughly (8"/5")2 = 2.56 times or 8.2 dB more output because of the larger cone area. I also chose a sealed box, which means that the low end rolls off at 12 dB/oct. The LXmini has its -3 dB corner at 45 Hz. I could equalize the 8" woofer to have its -3 dB corner at (45 Hz) / [sqrt(2.56)] = 28 Hz and have the same maximum SPL at 28 Hz as the LXmini has at 45 Hz. With DSP it is easy to extend and flatten the low frequency response.

SL:  Yes, and when you are done, you need a stand for your woofer box to bring the tweeter to ear level height. That stand could do more than just holding up the box. It could provide additional air volume for a lower box resonance and thus less equalization to extend the low end.

Fitz:  I do not necessarily put my speaker on a stand. It could sit on a bookshelf or a commode.

SL:  Well, now you are talking about a different animal. I have found that a speaker needs to be free-standing for best performance, ideally a meter from large reflecting surfaces. The LXmini, though, with its wide and spectrally neutral dispersion of sound in all directions, does quite well even when placed close to large reflecting surfaces. 

Fitz:  But your woofer is beaming to the ceiling and has a baffle step in the vertical direction.

SL:  Yes, in principle, and that is why the diameter of driver and mounting adapter are kept as small as possible. Rounding the ring edge has no effect, because the radius of curvature is too small compared to the shortest radiated wavelength from the woofer driver. Furthermore reflections from the ceiling are delayed relative to the direct sound from the woofer. But radiation in vertical and all other directions contributes to the reverberant sound in the room. And that sound should have the same timbre as the direct sound. 

The woofer pipe has a major benefit over your box, even though their internal volumes are not greatly different. The pipe gives me a conduit, an acoustic transmission line, for the wave from the rear of the woofer cone to travel along. For example, if a step is applied to the woofer, then the pressure wave travels from the cone to the end of tthe pipe. Since the pipe is sealed, the wave is reflected and travels back up to the cone. There it adds to the pressure, which the cone was radiating into free-space. You can see the process in the left picture below. The picture was derived from an electrical transmission line model.

   

SL:  Obviously you do not want any reflected wave from the inside of the pipe to add to the direct wave coming off the cone. That's why I put just the right amount of stuffing into the pipe to completely smooth the frequency response. I can see the effect of stuffing when I place a microphone within 1 cm of the driver's dust cap, or when I look at the driver's terminal impedance. Using tone-bursts I have measured the reflections from the end of the pipe in front of the woofer cone to be 40 dB lower than the direct signal from the cone. That is a 1% reflection factor or 40 dB return loss. Not bad for someone like me, who worked with microwave transmission lines.

SL:  There is one more big benefit from using a pipe to contain acoustic energy and structure borne vibrations: A pipe is extremely stiff and there is practically no spurious radiation coming off its surface. For a box speaker it is very difficult to keep the flat surfaces from not vibrating. You need lots of internal bracing. And then you have the air borne sound energy inside the box, which has not been turned into heat and which will in some form contribute through the thin cone to what you hear. No wonder there is a generic sound to the majority of box speakers. 

Fitz:  This is all good and fine, but there are realities of life, over which I have little control. There is a price for everything. I try to keep my priorities straight.

            

SL:  The pipe brings the woofer up and off the floor. That helps to reduce room mode excitation. Room modes, or standing waves, have maximum pressure on the large wall surfaces of a room, where the acoustic particle velocity has to be zero. So when a box woofer is placed on the floor it couples optimally to certain modes. It tends to couple optimally to all room modes when placed in a room corner. This results in a booming and spatially unevenly distributed bass in the room. For example, there could be a floor to ceiling mode when the distance between them corresponds to half a wavelength. For a 2.4 m ceiling height this would be at f = 344 / (2 x 2.4) = 72 Hz.  If you measured at 1.2 m height in this room the response from a loudspeaker it is likely to have a deep interference notch in its frequency response if the woofer was seated on the floor. But if the woofer was at 1.2 m height it would not sustain a 72 Hz floor to ceiling standing wave and not produce a corresponding notch in the room response. 

Fewer than 10% of LXmini builders add subwoofers to go lower in frequency or to get higher SPL. A pair of subs reduces distortion from the LXmini woofer drivers, which have to work pretty hard when by themselves in a stereo setup. A sub also gives an enhanced sense of space when the recording has it. The LXmini+2 system uses the small and unobtrusive LXsub2, which is a U-frame dipole woofer. A dipole excites the vertical and sideways room modes much less, because it does not radiate in those directions. Alternatively there is the LXsub4 with 10 dB higher output capability. It is intended for professional, mixing studio applications as part of the LXstudio system. Either sub uses the same driver. The LXsub4 has 6 dB more output because it uses 2 drivers, and adds another 4 dB more because the baffle is deeper. 

 

3.5 - The woofer & tweeter combination in Design #3

SL:  It occurred to me, Fitz, that I am spending valuable time of my life on explaining things to you, which I have already dealt with on my website some time ago. You may not have seen or studied the subjects or come to different conclusions. In any case you already have made up your mind for Design #3 and want to find out for yourself. 
Just be sure to have studied Acoustics and Hearing - 2.1.

Have fun with your project and hopefully reach full satisfaction for yourself and for those you should please! 


3.6 - The LXmini tweeter/fullrange

SL:  This has been discussed on the LXmini pages.

 

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What you hear is not the air pressure variation in itself 
but what has drawn your attention
in the streams of superimposed air pressure variations 
at your eardrums

An acoustic event has dimensions of Time, Tone, Loudness and Space
Have they been recorded and rendered sensibly?

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Last revised: 01/11/2017   -  1999-2017 LINKWITZ LAB, All Rights Reserved