The BBC Dip

May 14, 2021
 by Paul McGowan

15 comments on “The BBC Dip”

  1. This is an in treating subject. It arose when the BBC started experimenting with 8″ midrange drivers, with varying levels of Bextrene coating to increase its stiffness. You can read the science on the BBC R&D Research website, the papers from the late 1960s to 1977, but issues include off-axis response relative to the tweeter given the 3khz crossover and the wavelength relative to the midrange driver size (which had been increased).

    Within a year or so of the last of these papers, Dudley Harwood left the BBC after 30 years when head of research and started making loudspeakers under the brand name Harbeth.

    If you read the research papers, whilst the BBC runs a science-based research centre, this approach arose from subjective listening tests using experts from the music side of the BBC rather than the research engineers and their measuring devices.

    Bextrene degrades, but Harbeth have always stuck to the 8″ driver because the company was formed to sell this sound quality. The Bextrene was replaced with a polymer with the same acoustic properties that does not degrade and will last a lifetime.

    The 5″ drivers of the P3ESR (LS3/5a) use the same material, but to different effect. The BBC designed them for a different purpose – portable monitors for outside broadcast. I prefer a slightly brighter speaker for near field listening.

    1. One of my regrets was selling my ‘Harbeth HL Compact 7ES’ with the very 8″ driver
      that you describe above, back in 1999…”Don’t it always seem to go…”

    2. The layers on bextrene cones were clear coats of PVA, poly viny lacetate, used to dampen the bextrene cone which had significant surface breakup. By the way I recall Elmer’s Glue being essentially PVA. Of course all cones break up some ways including the classic material paper. And after the BBC used PVA on bextrene it started appearing on paper cones.

  2. As far as I know sound engineers prefer studio monitors with a flat frequency response to be listened in near-field set-ups. And modern most sophisticated active studio monitors as from Genelec or the Kii Threes and Grimms follow this ideal – a most logical ideal when the goal is to get highest fidelity for the reproduction and to reproduce the overtones of each instrument’s tone correctly. Could it be that when listening to these correct speakers in far field conditions as mostly found at home the highest frequencies are more dampened by the air than the lower ones? Thus the BBC dip could be a necessary! Aren’t the frequency response measurements for the upper frequencies normally made having the microphone in 1 m distance? My actual 2-way speakers have a dip in the crossover frequency range which requires a correction by my digital equalizer and the result is that I can hear more fine details now!

  3. It’s what I love about my NHT 2.9 speaker’s. They are flat throughout the mid and highs without being forward or bright sounding. People try and compensate for poor speakers that lack detail and transparency by boosting the treble control on their preamp or EQ. Good speaker’s are transparent and detailed without doing that. Moreover a speaker that is flat in frequency response does not have to sound forward and bright if its designed properely. Theres no need to implement the BBC dip on properly designed speaker’s. When Ken Kantor designed the NHT 3.3 and 2.9 he captured lightning in a bottle.

    1. Those are two of the best sounding truly full range speaker’s that you will hear regardless of price in a sealed cabinet. No ports to possibly screw up the sound. They sound like electrostatic speakers that have a true low end and can play loud and clean too. Total and complete lack of any grain and they retrieve every nuance of the recording. What more can you ask for at the price they sold for? Way underpriced for the performance they offer.

      1. My speaker design hero’s that brought performance and affordability to audiophiles on a budget are Henry Kloss who started Acoustic research then went onto Advent and Cambridge Audio, Winslow Burhoe who started EPI, and Ken Kantor who started up NHT. If Chris follows in their footprints he will do fine in producing great affordable speaker’s. Those companies made great speaker’s for the money while those three giants were with those companies. Things changed when they left and they were not as good. Other notable companies, PSB, Epos, Kef, B&W, Spendor, Dunlavy, Vandersteen, Thiel, Acoustic Energy, Snell, DefinitIve, ProAc. There are more I didn’t mention.

        1. No doubt Infinity’s flagship models were great but I didn’t care for their lower priced models and that’s why I didn’t include them on my list.

    2. On conventional forward firing speakers flat on axis response tends to be bright. This is because as dispersion reduces the off axis energy is received in a much narrower angle concentrating it. So the power response at upper frequencies sounds bright. One solution is flat on axis but with the on axis response slopping slightly as frequency increases.

      Also one of the reasons for the BBC dip was to change the apparent row the listener was at in the listening environment. This was especially true for near field monitoring as was usually true for the LS3/5a. Sitting so close made the sound seem brighter than for a more normal seat and making it difficult to get the correct octave to octave balance for the listener at home.

  4. Interesting explanations. So it seems it all depends whether you’re listening in the near of far field, what speaker design quality you’ve got, and also the material the mids are made of, regarding cone breakup and directivity.

  5. Quad had a marvelous tilt adjustment on their preamp years ago which allowed the user to slightly depress the frequencies around 1000 Hz and achieve the “BBC dip.” It was well engineered but used cheap parts; yet it served it’s purpose. The Cello Palette had the same capability, with much better parts, but was way too complicated.

    I found a modern tube unit in the pro market, made by Tube Tech, which is well made, sounds great, and is easy to implement. Plus it offers a few other useful features.

    1. Good morning Jim!
      Two questions for you.
      Is that preamp still being made and sold?
      I just made a down payment on a pare of vintage Pioneer IVP 3-way towers.
      Each tower, has a 12inch woofer, 6inch midrange, and 3 2.5inch horn loaded tweeters.
      They go as low as 35HZ, and as high as 20000KHZ.
      They were made, circa 1960-1961.
      The way the specs read on these speakers, they sound like they can do HD audio.
      Have you ever heard these speakers before?

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