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the best Active *Noise* Reduction technology existing

Discussion in 'Electronic Design' started by [email protected], Mar 7, 2006.

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  1. Guest


    I am trying to decide on an Active Noise Reduction (ANR) headset or
    earplugs. I have looked at the Sony, Bose, Shure and other models, but
    almost all reviews/comments mix two very different issues: noise
    reduction and sound quality.

    In my case, I have no interest whatsoever in sound quality, what I am
    looking for is the absolute best noise reduction possible. I have
    tried the Bose Quiet Comfort II, and they appear extremeny effective,
    but only in the middle-low frequency range. Also, I have a feeling
    that ANR earplugs might be more effective.

    Various manufacters claims various degrees of noise reduction. As for
    Bose, they do not even release their technical specs (although would it
    not be possible for any lab to measure them across a frequency range
    and publish the results?).

    Recently I met a guy who was working in the US Navy on the deck of
    aircraft carriers. He said that the headsets they had cancelled out
    everything, that nothing could be heared at all when he was wearing
    them. Since this hardly constitutes a secret technology, is it known
    who designs these headsets for the US Navy? Is this technology used
    for the civilian market by some manufacturer?

    Can anyone give me some pointers as to how I could go about finding the
    best ANR headset/earbuds out there?

    Many thanks,


  2. Depending on what you try to attenuate, they may not
    work as desired. While they are able to attenuate the
    lower frequencies, they produce higher frequencies
    that are making you more tired.
    In smaller airplanes I feel better with switching
    the active compensation off.

  3. Scott Dorsey

    Scott Dorsey Guest

    Why not use both at the same time? It is fairly standard practice to use
    foam plugs underneath active ear protection devices.
    For the most part, none of the active noise reduction systems work much
    above 1 KHz or so.

    My feeling is the Sennheiser pilot headsets are definitely better in an open
    cockpit plane (lots of repetitive impulse noise) than the Bose pilot ones.
    The consumer (open ear) ones are all pretty lousy.
    That's the way Bose is. Bose products are 90% marketing, and 10% technology.
    Some of them (like the headsets) work okay, just at much higher cost than
    they should. If you look at the over-ear Bose sets for pilots, though,
    they will have some minimal specifications on the data sheet.
    These are over-ear earcup devices. The high frequencies are blocked out
    mechanically, the low frequencies are blocked out with active noise
    cancellation. You can then use foam earplugs underneath them for additional
    high frequency cancellation.

    This is typical of the David Clark ground support sets:
    You'll notice that the David Clark website gives actual measured attenuation
    numbers across the band, rather than made-up crap or single-frequency
    marketing numbers like Bose shows.
    Your deck crew guy is probably issued a set of David Clarks. Sennheiser
    also makes some that are okay.

    The big question is what kind of noise you're trying to block out. If
    you're sitting in a 747 or a Stearman, the noise patterns are very different.
  4. hank alrich

    hank alrich Guest

    A few years ago I was on the tractor cleaering the road in a snowstorm.
    A tree branch that I overlooked whapped me on the side of the head and
    knocked the David Clark's off and onto the ground, where a tractor wheel
    rolled over one earpiece. I sent them to DC for repair and at no charge
    they sent them back fixed. Their quality and service are top notch.
  5. -hh

    -hh Guest

    Wearing both is what I do when I'm carrying my NC headphones. The net
    effect is quite good.

    Part of the challenge is that there's no clear "Underwriters
    Laboratory" standard (control) to benchmark test against.
    Truer words haven't been said about the $300 Bose headphones. There
    are some dramatically less expensive alternatives out there that are
    probably 90% the pragmatic performance potential, such as the "Plane
    Quiet NC6" headphones which sell for a bit over $50. Here's a review:

    FWIW, Sharper Image catalog has a NC headphone that looks suspiciously
    familiar to the NC6. It might be worth a visit to your local store to
    see a copy in person.

    CONTROL also made the famous 'Cone of Silence', which was extremely
    effective, but hard to use :)

    FWIW, a couple things about the practical use of a travel-oriented NC
    headset: some designs can be quite bulky, so do pay attention to how
    they fold (or not) for how much room they'll take up in your carry-on.
    Ditto for if they have a detachable cable (its just one more thing to
    lose). Also, many of them (including my NC6's) have smaller "on the
    ear" rather than large "over the ear" designs, which can have a
    tendency to pinch your ear onto your eyeglasses and become
    uncomfortable to wear after a few hours of continuous use. Afterall,
    it does you little good to have the "perfect" set if they end up being
    too big / too heavy to be worth carrying.

  6. are those active or just the same type we use on the shooting range
    during arms drill?
    Honestly, I have been through the Bose (the new one), the Sennheiser
    250 and 350, the AKG 28 and the JVC. They all sort of work with Bose
    taking the lead (well, you'd expect that for twice/three times the
    price), but they will only cancel out some of the engine noise. The
    difference is noticable and you'll have a nicer flight. You will still
    hear some humming from the engine and you will hear everything that is
    above a certain frequency (voice included).

    However, these headphones are only worthwhile if you need to actually
    use them as headphones to watch a movie or listen to the radio/MP3.

    If you just want a good sleep, go to Walmart or your local drugstore
    and pickup some of the in-ear noise blockers. They cost 10 cent or
    something in that range and will block out *everything* resulting in
    much better noise suppression than any headphone.
    (if they are made by 3M, they are the ones we use for military
    exercises - they are pretty good and comfortable)

    If you want a headphone that blocks all the noise (and, well, works as
    a headphone) I would pick one of those higher price SHURE headphones at
    99,- or 199,-. Just by itself, they block noise as efficiently as those
    Walmart thingies. If you pick the right headphone, they will be
    comfortable enough to wear for the duration of a red eye flight.
  7. Scott Dorsey

    Scott Dorsey Guest

    They might be the DC passive ones, but I know the Air Force guys are
    being issued the DC active ones. The actives are really amazing... low
    frequency cancellation from the active hardware, good padding for the high
    frequency rejection.

    Most shooting range hearing protectors are piston-type protectors that
    are only effective against impulse noises. They have a diaphragm that
    seals and unseals the chamber. This way you can hear people talk, but
    when there is a short shockwave from a gunshot, the chamber seals and
    most of the gunshot noise is kept out. Really ingenious idea. I think
    it was a German idea in WWII that we adopted.
    The Sennheiser PXC 250 and 350 have really very poor high frequency rejection
    because they don't have a good ear seal. Skip those and take a look at
    the HMEC250 as an entry-level unit and the HMEC450 as one with better
    sealing. Yes, they are more money, but they work much, much better.

    Which Bose units did you try?

    Honestly, you cannot expect any of the "over-ear" units to be effective
    at all at higher frequencies. And if you are on jets, it's the higher
    frequencies that are a major problem. With a piston engine craft, the
    active noise cancellation can deal with a lot of the noise, but on a jet
    you're relying a lot on the ear cup design to eliminate the noise.
    I recommend using these WITH the noise-cancelling headsets. This gives
    you much better high frequency rejection than either one alone, and it
    also helps a lot of you have problems with pressure equalization and ear
    popping as well.

    I will say that the noise cancelling systems are more effective than
    plugs at low frequencies, and generally less effective than plugs at
    high frequencies. And it's the low frequency stuff that tends to give
    you the headaches after ten hours in the air on a C-130.
    I did not know Shure made any headphones... I know they make some IEM
    earpiece for monitoring, but no actual sealed phones.
  8. etc. snip.

    HEY! This is GREAT!. Let's keep up the dialog on ANR headsets and
    hopefully soon earplugs as well. The public true knowledge extant so far
    has been woefully inadequate. The topic needs a good wringing out here.

    1-What Units are available?

    2- What are the low frequency quantiative noise reductions feasible?

    3- What are the maximum exterior amplitude vs octave band limits (10Hz -
    1kHz) where clipping and degradation might occur?

    4- What ANC circuit prinicples apply. electronic design hints and kinks.
    It is high time this no longer be a trade or personal secret, INHO. I
    saw two types at Oshkosh over a year ago (David Clarks?); a chepo
    'analog' that allowed a tad of engine noise to purvey, and a 'digtal'
    that pertty much wiped out everything up to midband, but had a whisper
    of digutal noise in its place.

    5- Ways to meld speech sound from the local environment, narrowband and
    just enought to get a high AI, into our ear.

    6- Anytihing else we can tout as a plus.

    Angelo Campanella
  9. Scott Dorsey

    Scott Dorsey Guest

    Well, that's the problem. First of all, we have millions of Chinese
    factories making cheap ANR headsets. Philips is selling one for $19 at
    K-Mart. Sporty's and Aircraft Spruce sell similarly cheap ones for
    under $50. There is an over-the-ear unit sold at Lowe's, rebadged by
    John Deere. Most of these units don't make good ear seals and none of
    them come with real specifications or any OSHA approval.

    Then, on the other side of the line, we have Sennheiser, Telex, David Clark,
    and a couple others. These units come with real specifications for noise
    reduction although it's sometimes not clear how much of the reduction is
    from the ANR and how much is from the padding and baffling. Yes, there is
    a cutover frequency at which one becomes more effective than the other, but
    I'm not sure I know how sharp it is.

    What is interesting is that Bose is trying to play both sides of the line.
    Well, the David Clark H3530 claims 14.3 dB of attenuation at 125 Hz,
    21.5 dB at 250 Hz. Now, that is the combination of the ANR and the
    passive stuff, but I am willing to bet at 125 it's entirely due to ANR.
    I have seen a little bit better numbers on some Sennheiser sets, but
    I am not sure where the theoretical limit is. Be interesting to do some
    That's an interesting question. In the case of the David Clark units,
    they use seperate speakers for the ANR system and for the actual comms,
    so the headroom of the ANR system is not degraded by the comm usage.
    But the actual measurement of where the system bottoms out... that isn't
    published at all. So I am making the assumption that it is well above
    any level that would be of interest, but it might be worth making a call
    to DC to find out.
    I don't think any of the DC units use any digital stuff. I know that the
    Chinese cheapies don't, although I haven't taken them apart. I should
    probably get a $20 K-Mart special just to disassemble and see what's inside.
    I'm assuming it's probably one does-everything-IC.

    My experience with all of the things is that they work pretty well on random
    low frequency noise, and okay on repeititive impulse noise. This goes for
    the $20 specials as well. But the high grade ones actually have decent high
    frequency rejection from passive stuff, and the crappy ones do not.

    I was actually surprised how effective the $50 special from Sporty's was
    on our Convair at work, but on the B-200 they really weren't worth a damn.
    In the case of the cheapies, they don't have any high frequency rejection
    at all. By 1 KHz, it's all gone. So there is no need to do this, because
    speech comes through audibly without any problems. In the case of the higher
    end units, they're intended to plug into intercom systems anyway. Much
    better than shouting through that rubber hose...
    No more headaches after 12 hours in a jumpseat. Lots of other things still
    hurt, though.
  10. Tony

    Tony Guest

    I have done some indicative tests using a few types of ANR headphones on an
    acoustic manikin. (Manikin measurement is not the right way to do these
    tests for several reasons - but it is easy and convenient for a rough
    indication of performance.)

    Normal ear muffs have quite significant attenuation in the 125 Hz band -
    good ones could do around 15 dB. However, one thing I found, is that on
    the manikin, at least some circumaural headphones show an *amplification* of
    ambient sound at low frequencies. I presume this is because they cannot
    have the damping material that is fitted in standard ear muffs. I've done
    these manikin tests only on ANR headphones, and of course the amplification
    changes to attenuation when the ANR is switched on. I've not had the
    chance to investigate this amplification further, but there is a real effect
    present as I could hear it when I wore the headphones myself. At a guess I
    would say it could be a resonance of the mass of the shell assembly and the
    springiness of the enclosed air and seals, but I haven't measured the mass
    or tried to do any calculations. Anyone else?

    An example - with Sennheiser Noiseguard headphones, in the 125 Hz 1/3 octave
    band, the amplification with ANR off was around 10 dB (rather variable
    between different trials when the headphones were re-fitted to the manikin),
    and the attenuation with ANR on was about 17 dB, so the ANR is certainly
    doing something, but the net effect is not much better than a good pair of
    passive muffs. If the effective attenuation is really 27 dB, that is way
    above what muffs could do by themselves.

    Incidentally, the crossover frequency between active and passive attenuation
    was about 500 Hz on those headphones. The 500 or 630 Hz band seems fairly
  11. Quite so. I've known and experienced that for decades. It's not touted
    commercially because it undermines a huge amount of lore (that earmuffs
    are the 'answer').

    Its source is the mass-spring resonance that always occurs when a mass
    (cup) encloses a volume of air (that captured inside the cup). The
    resonance frequncy so occuring is approximately via the panel resonance
    formula; 170*SQRT(WD) wher W is the area mass of the panel in pounds per
    square foot and D is the thickness of the air under that panel. This is
    around 100+-Hz for earmuffs. Its maximum amplitude idepends on the
    damping available. But it is likely about 10 dB.

    When ANC is introduced, 10dB of its attenuation capability is used to
    counter this effect. My experience is that common ANC capabilities are
    suffiient that there results a welcome reduction in the amount of such
    "drumming" heard.
    IMHO, it is not productive to try to remove this via damping. One such
    measure is to increase the clamping force of the headband. All this does
    is reduce the time to where a headache results. In this day and age, the
    effort is ptter spent in improving the ANC low frequency attenuation
    That agrees with my experiences.
    Affirmative. And when speech sound is introduced via a secondary
    speaker in the earcup volume, whatever below 600Hz is cancelled is of no
    consequence since speech intelligibilty contribution is greatest above
    1kHz, peaking at 2kHz. It's a win-win situation. It accounts for the
    universal acceptance of ANC technology by pilots and other militar

    A word of advice:

    The subsequent universal ANC acceptance by commerce and industry is
    right around the corner. This is a good time for any of you to get into
    the ANC marketing (headsets, NOT fixed equipment) business.

    Angelo Campanella
  12. Tony

    Tony Guest

    Yet many passive muffs do have useful attenuation in the 125 Hz band,
    without excessive headband force. I think there is a difference between
    active and passive in this resonance. There are a couple of factors I can
    think of, acting in different directions on the resonance frequency: the
    transducer has mass, and it takes up space. It is also a resonant system
    itself but I don't think this is a controlling factor as the transducer
    resonance will probably be higher. I did a brief test that showed there was
    no significant difference in attenuation whether the transducer was open or
    short circuit.
  13. The problem is that ther are many headphone systems that resonate so.

    Consider that earmuff sustems, often headphones, must be worn where it
    is also noisy; the aim of the headphone being to improve the hearing
    capability in such noisy environments.
    Consider also that in hte headphone design, it is often reasonable to
    enhance low frequency sound (modern music listening). A poor noise
    isolation results. This is not to say that they are "improper" noise
    reduction devices, since it was not the original intent of the design to
    be so, or if it were, the design is indeed "poor". ANC applied there
    reamarkably improves matters. I think this has been the r\drift of
    recent comments.
    The mass is defineiteely a problem since it shifts the resonance to
    lower frequencies.
    Agreed, the transducer being activated, or not, presents a minor
    effect. It is the physical mass added to the headphone, combined perhaps
    with a lightweight earcup for comfort, that presesents a mechanically
    resonant system.

    My experiences come from some early aircraft cabin headsets, with boom
    mic (making the unit special, if not necessary for flight), that makes
    this resonance amplification a nuisance.

    If one wants the advantage of a head-mounted boom mic and earphones for
    better speech clarity, they have had to put up with the nuisance of the
    bass resonance.

    My final solution in this special case was to buy earcup ANC modules
    from Headsets, Inc in Amarillo TX, and fit them into my old boom mic
    headset (with co-pilot intercom integrated). Now I have all four
    functions in one unit; earphones, boom mic, ANC and intercom. The ANC is
    moderate but quite comfortable. Can't buy this system anywhere.

    Angelo Campanella
  14. glenn P

    glenn P Guest

    I think perhaps the "amplification" at certain frequencies is a function of
    the enclosure's mechanical resonant frequency.
  15. Yes, but be aware of two tpes of "mechanical" resonance:

    1- The bending resonances of the earcup material. In noise-reducing
    earmuffs, this is hardly ever a problem since the materils and shapes
    are specifically chosen so as to be stiff or to have very much damping.
    In common earphones, this also not be a problem since sound quality
    would suffer if any ringing resonances were in a manufactured product.

    2-Cup air/clamp-spring resonance, where the earcups are the mass and the
    clamp spring in combination with the spring caused by the enclosed air
    volume between the ear & head and enclosed by the cup. This is the
    resonance I believe to be most prominent in any earmuff (aka earphone,
    headset, etc.). It is usually active in the 100 Hz range but can be at a
    much lower or higher frequency. In the best of situations, theactual
    sound atenuation afforded at this frequewncy is but a few decibels.
    Under the wordst conditions, this could actually be resonant
    amplification of several dB.

    In practice, the higher frequencies are passively attenuated by the
    material of the earcup, as planned, and the sound level at lower
    frequency range remains. The resulting overall timbre of sounds heard is
    that of a very boomy and dull soundscape.

    When virtually and ANC system is instlled in any earcup, the quality of
    the net sound heard immediately improves, being very much less in
    amplitude over the effective ANC frequency range, which often extends to
    100 Hz and less because the speaker need drive only the tiny volume of a
    few cubic inches. It is not extremey quiet unless the ANC system is very

    But any speech sound introduced, say by an auxiliary loudspeaker will he
    heard and understood with ease. True, some speech sounds will be
    actively attenuated by the ANC system. But human speech has such a wide
    diversity of frequency components, and especially at high freuencies, 2
    kHz and above, where earcup ANC cannot work because of min/speaker/ear
    spacings, that the surviving speech is "crystal clear".

    Hence we see Communicatons headsets to be invading the aircraft and
    industrail markets, and with good cause.

    Angelo Campanella
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