DrWho Posted August 16, 2004 Share Posted August 16, 2004 ---------------- On 8/15/2004 10:20:51 AM dougdrake wrote: I guess it's too simple for me to understand. If the driven woofer is moving out, would that not decrease the pressure inside the box and cause the PR to move in, thereby having them out of phase? ---------------- No. The problem in your reasoning is that the volume in the box does not all change at the same time. Let's consider a simple example: Let's start with the driver diaphragm moving in while playing a test tone. Initially, when the diaphragm moves in, the air right behind it experiences a positive pressure (like you're thinking). This positive pressure then moves through the box at the speed of sound, causing a wave (just like how sound travels outside the box). Likewise, you have a negative pressure wave when the diaphragm moves out of the box. Now if you put a passive radiator on the speaker, this wave that travels inside the cabinet is going to cause the PR to move just a little bit...keep in mind that it takes time for the opposite wave to reach the PR. However, at the tuning frequency (which is dependent on the mass of the PR and the volume of the box), the PR is going to move a lot. The reason for this is that at the tuning frequency, the system is resonating...much like the standing waves you get in your room (aka, room nodes). When at resonation, the active driver barely has to move in order to maintain the "standing wave". As you deviate from the tuning frequency, the passive radiator will vibrate less, requiring the diaphragm to move more to achieve the same volume. To hopefully clear up some other wrong comments, the whole reason the PR is in phase with the active driver is because it takes time for the opposite pressure inside the cabinet to affect the PR; and I believe that it is only at the tuning frequency that the PR is exactly 180 degrees out of phase (someone correct me if I'm wrong). Anyways, the main idea is that the air in the cabinet is springy and it's bad to think of all the air inside the cabinet to be moving at once. Quote Link to comment Share on other sites More sharing options...
DrWho Posted August 16, 2004 Share Posted August 16, 2004 ---------------- On 8/16/2004 3:34:53 AM white_shadow wrote: I've half a$$ read Gils long reply, but I think you can understand. I just dont think it seems right. For instance "2) radiators smaller than half a wavelength don't work very well" Well, according to Gil, this qualifies 99.99999999% of all drivers in production. I tend to discredit this type of information. Its an incredible reply, but I'm looking for more information like Warren. ---------------- Well it's true, radiators smaller than half a wavelength do indeed suck. I'd suggest reading his whole post so that you realize that he's trying to talk in normal language so that those that only speak normal language can understand. I believe it was Einstein that mentioned that if you can't explain it to an 8 year old, then you don't have a good enough understanding and shouldn't be talking in the first place. Anyways, I also know that you're an electrical engineer undergraduate which means you like to see numbers (so in that light, your post doesn't seem that bad anymore). I'd recommend asking for numbers next time before discrediting someone (especially if you're oblivious to the topic) Ok, so why do small diaphgragms suck? The significant difference between a small and a big driver is the surface area. Let's say you have a speaker of surface area X. The lowest frequency that driver can reproduce is now dependent on the excursion of that driver (let's call this variable Y). Another driver with a surface area of 2X now requires 1/2 Y to obtain the same lowest frequency. For the record, excursion is very bad on a speaker...the more a diaphragm moves, the more doppler effect you get which we call "frequency modulation distortion." (higher notes being produced by a diaphgram moving back forth). You also get more 2nd and 3rd harmonics (and maybe even 4th and 5th if you're insane) as the diaphgram moves more as well. Anyways, the driver with SA=2X will sound better than the driver with SA=1X. Now to bring that concept back into the half wavelength thing...when a driver is smaller than one half wavelength, the excursion required to produce the larger wavelengths (lower frequencies) becomes exponentially larger the lower you try to go. Thus, a larger diaphgram driver will sound better. (for the record, the reason we don't see many 80" drivers is because the mass of the driver is another important factor that seems to become more of an issue after 18"...though there are other factors as well like build cost and size). Yes, there are tons of drivers out there that use small drivers and still play low notes, but the sacrifice is a ton of distortion (which for whatever reason, many people find enjoyable). Just look at the Philips WOOX technology (i forget what they call their new thing)...it's basically a 6" driver that hits very hard and very low. It's cool for the effect, but for critical listening you quickly realize how bad it sounds. Quote Link to comment Share on other sites More sharing options...
John Warren Posted August 16, 2004 Share Posted August 16, 2004 ---------------- On 8/16/2004 5:49:55 PM DrWho wrote: To hopefully clear up some other wrong comments, the whole reason the PR is in phase with the active driver is because it takes time for the opposite pressure inside the cabinet to affect the PR; and I believe that it is only at the tuning frequency that the PR is exactly 180 degrees out of phase (someone correct me if I'm wrong). --------------- Ok, I will correct you. Only at 0Hz is the woofer and the PR 180° out-of-phase. This is observed directly by moving the woofer cone (at the apex of course) "in and out" watching the PR move "out and in". At Fb (the box tuning frequency) the PR is in phase with the woofer but it really doesn't matter since the woofer is, for all intents and purposes, silent at Fb. In addition to 0Hz, the mechanical phase of the woofer relative to the PR approaches 180° out of phase at frequencies much higher than Fb also. This point I failed to mention in the Rev 0 post. Quote Link to comment Share on other sites More sharing options...
D-MAN Posted August 17, 2004 Share Posted August 17, 2004 ---------------- On 8/15/2004 4:11:15 AM white_shadow wrote: Why do we use passive drivers? It seems like there used specifically for bass. ---------------- The answer IMO is that it is easier to sell, i.e., it LOOKS like it has more woofers and COSTS less to produce. It doesn't sound better (or even as good) than a ducted port or a reflex enclosure, again IMO, but I am an old curmudgeon, anyway. DM Quote Link to comment Share on other sites More sharing options...
John Warren Posted August 17, 2004 Share Posted August 17, 2004 ---------------- On 8/15/2004 4:11:15 AM white_shadow wrote: Why do we use passive drivers? It seems like there used specifically for bass. ---------------- They are used only for bass. They are used when the specific woofer-enclosure combination requires the size of the port (or ducted port) be so large that it cannot be accommodated given the constraints of the cabinet dimensions. For example, say the design requires the ducted port to be 24" long but the enclosure is only 10" deep. Under these conditions, the PR allows for a much larger acoustic mass to be used in a much smaller space. As I've mentioned previously, the PR is identical to the ducted port (or vent). Quote Link to comment Share on other sites More sharing options...
PAR69 Posted August 17, 2004 Share Posted August 17, 2004 Wil, Awesome explanation. I think you made it very understandable. And, in my opinion, I think passive radiators work better than tuned ports or anything else, based on my KG 4's. Paul Quote Link to comment Share on other sites More sharing options...
WMcD Posted August 20, 2004 Share Posted August 20, 2004 I must say that John W. pointed out something which I glossed over. I agree entirely that in a port design, the port does take over the work. The main diaphragm does not move very much, and therefore does not radiate, because it is loaded by the box and port. I believe this may not quite the same with a passive. It would seem odd that the main diaphragm gets loaded so much that its displacement is reduced, only to have the passive of about the same side to move in as a replacement. It seems we don't gain anything. On the other hand, perhaps this will work if the passive has a much higher Q (almost certainly does), tending to larger excusions undamped by a motor circuit. Someday I'll take a tone generator to the Quartets or Forte II to check it out. In any event, ports tend to create pumping noise at high levels while the passive does not. And it has a large radiating surface. Still, the magic is the phase reversal (that is to getting them in phase). Also, just when system response is falling there are new factors introduced which allow a resonance in an additional source of radiation to prop up response. Best, Gil Quote Link to comment Share on other sites More sharing options...
white_shadow Posted August 21, 2004 Author Share Posted August 21, 2004 Thanks Will and Warren. I should have read Will's post fully, but I was tired. Regardless, I went over and read the replies and have a few thoughts. Tell me what you think. How does mounting (physical) the PR at 90 degrees relative to the active driver work. I've seen rear loaded PR's, no front or side loaders (that I know of.) What if the enclosure was a pyramid or wedge. The only connection between the PR and AD (active driver) is the air between them. Thus the PR would always have a 180 degree phase difference? In terms of driver mass, does the AD work harder to move the mass of the PR. Or is this a benefit of PR designs that allows an AD to work as a single driver but have qualities of mulitple drivers, such as larger surface area*excursion=volume. Does a PR design eliminate the characteristics of enclosures, like standing waves, resonance, port design, and in general the enclosure design. Has anyone designed their own PR sub. This topic came to mind because of the picture of the acrylic RSW15. I think those are PR designs. Quote Link to comment Share on other sites More sharing options...
John Warren Posted August 21, 2004 Share Posted August 21, 2004 ---------------- On 8/20/2004 10:53:43 PM William F. Gil McDermott wrote: I believe this may not quite the same with a passive. It would seem odd that the main diaphragm gets loaded so much that its displacement is reduced, only to have the passive of about the same side to move in as a replacement. It seems we don't gain anything. ---------------- Gil- Obvious differences aside, they are same relative to function. Compare the acoustic masses: Acoustic mass of port = mass of air in port / area of port squared = Ma/Sp2 where: Ma = density of air x port length x port area Sp = area of port Acoustic mass of PR = Moving mass of PR / surface area of radiating surface = Mms/Sd2 where: Mms = moving mass of PR diaphragm Sd = area of diaphragm The PR provides a means to substitute mechanical mass for the constraints associated with the density of air. From a mechanistic perspective, the system sees only the acoustic mass. Quote Link to comment Share on other sites More sharing options...
Ray Garrison Posted August 23, 2004 Share Posted August 23, 2004 One of the benefits of using a PR instead of a port that no one seems to have mentioned is that when the port is highly active ( at the resonant frequency ) you can get a lot of "chuffing" noises due to air moving rapidly through the port's mouth. Using a PR eliminates this. Quote Link to comment Share on other sites More sharing options...
D-MAN Posted August 23, 2004 Share Posted August 23, 2004 Some of the draw backs are the damping effect of the mechanical suspension, and the modulation distortion presented by another moving cone surface. I have not considered the passive radiator to be an effective alternative to a correctly sized reflex port for those two reasons. But that's just me. DM Quote Link to comment Share on other sites More sharing options...
John Warren Posted August 24, 2004 Share Posted August 24, 2004 ---------------- On 8/23/2004 5:41:35 PM D-MAN wrote: Some of the draw backs are the damping effect of the mechanical suspension, >>The suspension compliance is for all practical purposes negligible in good PRs. and the modulation distortion presented by another moving cone surface. >>modulation distortion with reference to what? As I've stated a too many times to mention, the PR is the only "moving cone" at Fb, the woofer is damn near silent. And besides, if we were to buy into your argument wouldn't a port produce the same IM distortion you claim exists in a PR? In the final analysis a volume of air is accelerated. I have not considered the passive radiator to be an effective alternative to a correctly sized reflex port for those two reasons. >>As mentioned earlier, the PR is not used unless it is the only option. It isn't an effective alternative, it is a design solution. If a vented port works and one goes to a PR, a cost penalty is incurred. But that's just me. DM ---------------- Quote Link to comment Share on other sites More sharing options...
D-MAN Posted August 24, 2004 Share Posted August 24, 2004 It seems to me that air pushing adiabatically on air is different than a cone surface pushing on the atmosphere. Cones being cone shaped are subject to various problems resulting from a moving surface of a differing angles moving in the same direction, such as surface distortions like rippling, and phase distortions from differing degrees of accelleration along the surface. The modulation distortion occurs in that the front of the cone is faced with the entire atmosphere and the back is not. There is no way to balance the resistance (reactance) equally and the cone will therefore not move equally in both directions, therefore the waveform is distorted. Modulation distortion is unavoidable in such a case. Ok, I'm done now and will let this go. DM Quote Link to comment Share on other sites More sharing options...
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