Just a thought...

Nightfall

I see many people using products such as dynamat on the stamped baskets of the vintage MW's and I got to thinking about something.

A section of Norez in the cabinet behind the MW's is used to accomplish preventing sound waves bouncing back at the speakers cone causing it to vibrate when it shouldn't be. Does applying this section of Norez accomplish 100% of sound waves bouncing back? I'm guessing that answer is no because they are all over inside the cabinet, not just bouncing directly back and forth behind the MW. What if instead of a piece of Dynamat back there, Norez was used instead, or even both. Instead of increasing the resonance what if something that could absorb as well as increase the resonance was used? If the sound wave can be absorbed before it even hits the basket, would that not be beneficial?

geoff727

No, there is nowhere near total absorption. Little, in fact, and mostly only at higher frequencies. In the bass, there is hardly any absorption at all of the driver's backwave. Why? Because the wavelengths are too large. Consider this example....in an anechoic chamber, to enable absorption of bass frequencies, the absorptive wedges they place on the wall have to be a minimum of 1/4 of the length of whatever wavelength is to be absorbed. A 40 Hz wavelength = 1130/40 = 28.25 feet. 1/4 of that = 7 feet. That's 7-foot wedges on all 6 surfaces, or 4-foot wedges with 3-feet of backing absorptive material, etc.

The foam layer of NoRez, Sonic Barrier, etc. assists in reducing standing waves and reflections inside the enclosure which can color the frequency response. How do they color the response? Imagine a reflection going from the back of the driver, reflecting off that hard particle board wall of the enclosure, and coming back at the driver as an acoustic pressure causing the cone to re-radiate that signal. Depending on the sonic transparency of the driver material, it re-radiates the signal as a phase-delayed version of what it sent out only milliseconds before. This is unwanted radiation of the driver cone. This is a very simple axial reflection adding distortion to the output of the cone. In reality, these backwave reflections number higher than we can count.

Say an axial reflection existed inside the enclosure that had a depth of one foot. A frequency of 1130Hz has a wavelength of one foot. (1130 feet per second [the speed of sound]/1 foot = 1130Hz). When that frequency hits the back wall after coming out the back of the driver, it will reflect in phase with subsequent 1130Hz signals. This will also be true of its half-and quarter- wavelengths, asociated with frequencies of 565Hz and 282.5Hz. Because of the in-phase nature of the signals, standing waves will develop inside the enclosures with areas of stationary ridges (pressure highes) and troughs pressure lows).

This is a simple axial mode. Tangential and oblique modes also exist, where sound pressure waves are literally all over the inner walls of the enclosure, from one wall to its adjacent wall, to the bottom, to another adjacent wall, etc.

Stuffing the enclosure reduces these effects, especially if the stuffing is more substantial in the middle of the enclosure between walls. This is where sound velocity will be highest between reflections, and thus the stuffing has the greatest effect. The foam wall treatments help in reducing reflections that color the midrange and treble frequencies.

Part of your question address damping vibration of the enclosure walls. A very good idea, something that I think isn't approached nearly enough by people. Those little 4.6's I just sent off, remember those? The vibration of the enclosure walls was a lot less because the walls were so much thicker. More mass = harder to vibrate. This is simplistic, but let's go with it for a second. At the end of several days of them playing with the new crossover components 'waking up', they were sounding, quite frankly, VERY dramatic compared to what they originally were. A lot of this is due to vibration control.

Imagine a speaker cone putting out sound, at an SPL that you can't even see the cone move. But you can hear it very well. Now put your hand on the side of the enclosure. It's vibrating, but not nearly as much as the cone. However, it's vibrating with perhaps 40 times the surface area of the cone! Top, bottom, sides, front, back, right? Know what all that surface area does when it vibrates? It makes sound. Unwanted sound.

For a simple demonstration, try playing a swept sine wave through your speakers, over and over. Listen to the surfaces of the enclosure with a stethoscope. You will probably be able to pick out frequencies which seem exaggerated in level. These are resonances of the enclosure, things which muddy the music.

Okay, I'm done. I need to go do laundry. Give me a holler if you're around during RMAF.

G~

halo71

Damn...does DK have a second account? Nice write up Geoff!

geoff727

Thanks halo; how's those -IIIa's treating you?

Here's a conceptual experiment of the absorptivity vs. frequency curve of these materials...

Cover all interior surfaces of your listening room with NoRez or S.B. or whatever. Walls, floor, and ceiling. You've just turned your room into the inside of your speaker enclosures. Now play your speakers and listen to the frequency response.

halo71

Geoff...the IIIa's are still singing beautifully. Glad I did not give up on rebuilding them. Rebuilding the XO's was the ticket. Thanks again for your help and guidance!

Although I am terrified of blowing an EMIT. Even worry more when I am not home and I know my wife is listening to music! :eek: