B&W Aluminum Phase Plug

 

Background:

The New 800 Series has adopted a new phase plug made of aluminum. This phase plug only applies when the protective grill is taken off. In order to examine the mystique behind the alleged improvement upon switching to the new aluminum phase plug, I ordered some duplicates from a friend who was able to consult an engineer working on a computer-aided metal lathe. The story began as I read from some audiophile magazines about a kind of copper (or bronze I am not sure) phase plugs being sold at a high price in Japan for improving clarity of the Nautilus 800 series. The B&W engineers admitted the improvement and further announced that aluminum is the best material they found in their research lab. Soon after the new 800 D series is launched, I found an individual seller was trying to sell his pair of aluminum phase plugs on a second-hand audiophile product market website in Taiwan. He claimed that the new genuine aluminum phase plugs of the 800 D series, which are apparently different in shape, have an even better sound than his old ones. Then I found some other B&W fans were discussing these new aluminum phase plugs in an audiophile website in South Africa. I was full of both interest and doubt having read through a lot of threads about the improved clarity.

 

Experiment:

Now I would like to report my own experience in some simple technical measurements rather than emotional words. The solid core aluminum plugs and the original hollow plastic phase plugs were switched for comparison. A real time spectrum analyzer (RTA) was implemented with an omni-directional microphone placing at the exact position of the listener who is in the equilateral triangle configuration 3.1 meters away from the two N801 speakers. With the RTA spectral window being set at 300Hz-3kHz for the FST mid-range of my N801, the background noise was sampled first, followed by the plastic plugs and then the aluminum plug set on the mid-range unit. The sound source was a full spectrum computer-generated white noise. The RTA was adjusted with the following parameters:

Flat-weighting

48kHz sampling rate

FFT size: 65536

Time window: Hanning

Frequency Response Curve smoothing with average of 8 second.

 

Background nosie consisted of the air-conditioner, computer fans and my breath.

The original frequency response curve showed a “flat” response above 500Hz in my listening room.

Now the aluminum phase plugs were switched on. An overall impression of “no change” was noted.

 

In order to take a fine analysis, I merged the 3 curves together in one. The frequency response curve of the aluminum phase plugs now overlaps exactly onto the curve of the original plastic phase plugs. It is clearly shown that the aluminum phase plug has incurred some changes on the frequency response but in a subtle way.

 

Discussion:

Now you can see clearly what the aluminum phase plug has done to the frequency response of the FST mid-range unit. There is no obvious frequency response change in shape (I did expect some changes towards the higher frequencies though). Instead, the aluminum phase plugs has a very subtle change on the “spikes” of the frequency response curve. My interpretation on these larger “spikes” is that some of the frequencies in a microscopic view are in fact altered while the overall response curve maintains the original shape. These subtle changes along the frequency curve in term of higher sound pressure level fluctuation (spikes) within various small regions are obviously the key to the improved clarity. My overall audition impression to the improved clarity is very closed to the applying of BBE Sonic Maximizer over the sound source. For people who do not have any idea about BBE Sonic Maximizer, I would like to explain briefly here. Like many other sound enhancement PA products for live performance, BBE Sonic Maximizer does not simply equalize the frequency response curve in order to bring out the low bass or the high crisp, it acutally alters the phase distribution of the sound source in order to correct the muddy sound inherited from the electronic circuit during a live performance recording, no equalization is done. My speculation is that the randomly generated white noise when being transduced and coupled to the air from the aluminum-phase-plugged FST mid-range unit has a more coherent distribution or say more “in phase” sound elements within the white noise. Exactly how a solid aluminum phase plug has done to the sound distribution is unknown to me. But there has been a lot of arguments over the theory of smoother magnetic flux distribution change. This theory has a weak standpoint to be picked out. As aluminum or copper both are inert to any magnetic flux and thus should not contribute any change in the magnetic flux around the moving coil. But it is interesting to note that the name of the device in question has already implied that it is strongly related to the sound wave phase: it is called “phase plug” or “phase cone” (not like the “dust cap” in the center of a woofer). So, a phase plug within the middle of a mid-range speaker has a phase distribution function rather than being a simple ornament. The B&W research team should stand up and answer this question from lots of supporting fans. However, I have done my job to show you people what happens when you switch the original plastic phase plugs to these aluminum phase plugs.

 

April 24, 2005

Paul Ng