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In Reply to: Re: Speaker design philosophy posted by DavidLD on February 21, 2007 at 11:04:22:
David,Your observations bring up a couple of points. There was a very interesting post by Ken Kantor on the MadBoard regarding Acoustic Suspension speakers, and by comparison, ported ones. That was in response to my son-in-law's asking why there are so few AS drivers around. I thought Ken's response very informative. He commented that it is much more difficult and expensive to build As woofers, because of their low resonance and floppy suspensions. He also commented that foam suspensions don't suffer from audible resonances like rubber does. And in fact, you do see a little bump and dip, indicative of a resonance, in the response plots on most rubber surround models. Usually up around 1 kHz. Ken also commented that since ported systems make use of a higher Q resonance to extend the woofer response, and once the forcing fuction is removed, they will ring at that resonant frequency. Ported systems do store energy.
A lot of the doubling isn't objectionable because of a psychoacoustic effect known as "synthetic bass" If you play a bass note with the harmonics in tact, but the fundamental removed, it is perceived as lower than a note one octave higher, even though the lowest reproduced tone in both cases is the same. (2nd harmonic of the first tone is one octave above the fundamental) The ear/brain processing fills in the fundamental from the interval of the harmonics. So if a speaker can't reproduce 32 Hz, but can reproduce the harmonics (64, 96, 128, 160, 192, etc) the brain will believe it's there. However, synthetic bass and real bass do not sound the same if you compare them.
Another effect is the ear's preference for certain frequencies. In recent years, two very expensive speakers tested by Stereophile had bass resonances at about 48 Hz, with rather large peaks (8-10 dB)at 55 Hz. In both cases the reviewers commented on how deep and powerful the bass was. I sincerely doubt that the designers of those speakers didn't peak the bass on purpose. If you put in a similar peak with an EQ, you'll see the effect; it's quite nice. My JBL's have a resonant frequency of 48 Hz and the bass seems satisfying and deep, that is, until I compare them to the Advents which have resonances down around 40 Hz. (those are measured figures) Then it's clear the JBL's don't go all that low, nor with the same authority.
I very much suspect that the 6.5" drivers in the ported boxes benfit from both the effects I described. In other words, if they double a bit (or a lot) and can produce a lot of energy between 50 and 60 Hz, then they will be perceived as having good bass. I think part of it is the measuring technique used by the DIY folks. It's all computer based and done in-room with a time gated system, so that response below 200 Hz is not measured. Thus, they reply on listening for the bass response. You can do close miked measurements with sweep tones or pink noise, but their systems don't do that in most cases, so they would need additional equipment besides learning a new technique. Plus, ported systems are a lot harder to do for the close miking technique and the results much less reliable. Even John Atkinson seems to have problems getting good results in his Stereophile measurements.
If you add up those three factors, I can see why a lot of modern systems don't have the bass of the older systems. I think doubling is still very much with us, it just isn't being talked about. It might be the elephant in the living room that no one wants to talk about.
Jerry
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- Re: Speaker design philosophy - Bold Eagle 12:38:24 02/21/07 (0)
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