Tim's Site on Various Things

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====== Use of Absorptive Materials in Loudspeakers ======

1. The Case of Loading a Dome Midrange (and maybe some woofers too) into a Large Waveguide.

I have been recently thinking about and experimenting a little with this project. One problem I immediately encountered is that the acoustic center of a dome is up toward the center of the dome. If the dome is placed in a waveguide so that the top of the dome is protruding inside the waveguide to some degree, as is typically seen in many studio monitors with waveguides, the sound radiating from the center of the dome will cause reflections off the sides of the waveguide at higher frequencies, which destroys the desired constant directivity that the waveguide is meant to provide. This means that the dome must be moved back so that the sound waves that enter the throat are all perpendicular to the sides of the waveguide. But how do studio monitors get away with it and still show smooth dispersion? The answer is somewhat complex, but generally they use very shallow waveguides, and sometimes some complex shaped items in front of the tweeter to better facilitate spherical wave entry into the waveguide. They may also use other tricks with the precise shape of the dome. 

The reason the dome acts more as a point source at it's center is that the dome moves like a piston in the air, and the surface of the dome at it's center is most perpendicular to the piston motion, so the center produces most of the sound. If we move this point source back behind the throat opening of the waveguide it can produce smooth dispersion from the waveguide without creating internal reflections. However, sound energy not directed into the throat will escape to the sides. If the sides are enclosed to create a back chamber, the higher frequencies will bounce around the chamber and enter the waveguide late, causing poor frequency response and resonances. To solve this, absorptive material can be added to the chamber. The compromise that's been made is efficiency loss due to absorption in exchange for a more constant directivity output.

Porous or fibrous absorptive material will tend to become less effective at lower frequencies for any given thickness. So high frequencies are more readily absorbed. This can be a problem sometimes, helpful in other situations. In this situation the high frequency effectiveness works as an advantage. As the frequency goes down the waves will get longer and eventually not be able to bounce around in the chamber, but will instead emerge mostly in-phase with the direct sound from the speaker dome, complementing it and increasing output from the waveguide. This is helpful because EQ can be used to flatten the response, and the driver will now be more efficient and produce reduced distortion, and possibly allow it to be crossed over at lower frequencies.

Another advantage to having a chamber with absorptive material behind the waveguide throat is that it provides an opportunity to load more low frequency drivers into the chamber from the sides. This is a version of Danley's Multi Entry Horn concept. It's potentially better in that it doesn't require holes to be made in the sides of the waveguide to accommodate the low frequency drivers, which can be a source of diffraction of higher frequencies inside the waveguide. Note that nobody ever puts holes along the sides of their waveguides if they don't have to.

My first experiment with this idea involved the use of a conical horn with a 60 degree flare, a 1" throat, and about 18" at the mouth. The result of loading the driver into this horn with an absorptive back chamber was a reduction in on axis treble energy at 13 k Hz, but no reduction in off axis energy at that frequency. This meant that 30 degrees off axis now had the same energy as on axis. At 400 Hz there was a 6 dB gain in out put both on and off axis. This meant that the on axis and 30 degree off axis frequency response were now the same, so constant directivity was successfully achieved albeit with some loss of total energy output in the higher frequencies, and a gain in output in the lower frequencies. There was some roughness in on-axis response in the middle frequencies due to mouth reflections from the conical horn, which does not have an exit flare. It may be possible to reduce these by using absorption around the mouth of the horn, which could be smaller and easier to construct than an adequately sized exit flare.