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Feb. 2018
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Harmonic distortion of loudspeakers at low frequencies

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  • Harmonic distortion of loudspeakers at low frequencies

    When I was deciding on my first pair of loudspeakers over 30 years ago I looked at the measured performance of those on my short list before I did any listening. As I would with any hi-fi electronic equipment I looked at frequency response extent and deviation from flatness, and distortion. The distortion figures for loudspeakers were far inferior. In particular at low frequencies, harmonic distortion (e.g. at < 100Hz, 80dB signal) could reach 5% - in contrast the rated power for an amplifier is normally quoted as the wattage at which distortion reaches 0.1%. I chose a pair of Celef MiniProfessionals partly because they had much lower harmonic distortion at low frequencies (~1%) than the Spendor BC1s I also looked at.

    In recent years you never see measurements of harmonic distortion quoted in speaker manufacturers product information or in reviews. Is this because the technology has improved, or because the measurements are difficult (and hence expensive) to do, or because the level of harmonic distortion is not thought relevant. As regards the latter my understanding is that while a moderate level of 2nd harmonic distortion can be tolerated this is not so for 3rd harmonic distortion.

    So my questions:
    1) Do you regard harmonic distortion as an important element of loudspeaker performance?
    2) Is a high level of harmonic distortion (~5%) at low frequencies (<100Hz) significant to the listener?
    3) Have you made any measurements of harmonic distortion for Harbeth speakers?

    Thanks in advance for any illumination on the subject.

  • #2
    Importance of LF distortion?

    Originally posted by davidlovel View Post
    So my questions:
    1) Do you regard harmonic distortion as an important element of loudspeaker performance?
    2) Is a high level of harmonic distortion (~5%) at low frequencies (<100Hz) significant to the listener?
    3) Have you made any measurements of harmonic distortion for Harbeth speakers?

    Thanks in advance for any illumination on the subject.
    I'd say that the ear has such poor acuity at detecting LF harmonic distortion, especially in the messy acoustic environment of a speaker in an untreated domestic room playing music, that LF distortion is not a good criteria to make a speaker selection on. So to answer your questions:

    1) Maybe, but not at low frequencies
    2) Probably inaudible, almost certainly if even-order harmonics
    3) Not recently, because there is nothing one can really do about LF distortion. It's directly linked to excursion, and that in turn relates to the type of music and how loud it is played, issues that the speaker designer has no control over

    We can tolerate astonishingly high (measurable) quantities of 'distortion' because musical instruments do not produce clear tones: they are enjoyable because of the rich mixture of harmonics, so we are used to hearing anything but pure tones.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

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    • #3
      How to objectively compare low frequency performance?

      Originally posted by davidlovel View Post
      When I was deciding on my first pair of loudspeakers over 30 years ago I looked at the measured performance of those on my short list before I did any listening. ... In recent years you never see measurements of harmonic distortion quoted in speaker manufacturers product information or in reviews. Is this because the technology has improved, or because the measurements are difficult (and hence expensive) to do, or because the level of harmonic distortion is not thought relevant. As regards the latter my understanding is that while a moderate level of 2nd harmonic distortion can be tolerated this is not so for 3rd harmonic distortion.
      Another well-known British speaker concern quotes distortion figures in its product literature. For example, one of its larger products - which is in the same price range as the M40.1 - has maximum distortion ratings for 2nd and 3rd harmonics of <0.5% down to 70Hz and <1% down to 40Hz at a 90dB SPL @ 1m output level/distance. That particular model uses a pair of 200mm low frequency drivers (made from a structural foam material) compared to a single 300mm driver (of unstated composition) in the M40.1.

      As a prospective customer who is cross-shopping both speakers, it would be interesting to know how the Harbeth solution - which appears to have a similar, if not sightly larger, effective radiating area - measures in comparison.

      Likewise, given the assertion by A.S. that "the ear has such poor acuity at detecting LF harmonic distortion," is there a better objective means for the customer/listener to compare/evaluate the low frequency performance of competing designs? Does frequency response tell the complete story or are there other factors that should be taken into consideration?

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      • #4
        Designing with a priority list for a certain customer

        Two 200mm drivers have a combined surface area some 15% greater than one 300mm driver, so each one of those 200mm units (assuming that they are operated in parallel) will be working a little less hard.

        As you know, there is no such thing as as the perfect loudspeaker. Every speaker designer and speaker brand carves out a niche in the market place in part by the use of styling and other brand identity features, and in part due to the sonic capabilities of the product and brand, which in turn is connected to the choice of materials used in the designs. At Harbeth, recognising that our customers' expectations are for a very open, natural, coloration free sound in the region where the ear's sensitivity peaks-up, we place much effort on getting that region absolutely convincing.

        Other brands, driven with other objectives, could well prioritise different areas of the sonic spectrum knowing that, for example, their customers played heavy duty rock and roll at high sound pressures, or whatever their marketing people uncover.

        The issue that all speaker designers face is the certainty that real domestic listening rooms, sonically untreated, and where hifi is just one of many activities shared in the space, do terrible things with sound waves, especially in the bass. The only sure way to know if certain speakers match a certain room construction/room damping (likely to be relatively negligible) is to try them in that room. There is no practical substitute for that.
        Alan A. Shaw
        Designer, owner
        Harbeth Audio UK

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        • #5
          Bass distortion matters

          Revisiting this issue, I think there is something missing from the above explanations.

          The ear's effective frequency 'response' or sensitivity can be objectively described by what are termed "equal-loudness contours" (https://en.wikipedia.org/wiki/Equal-loudness_contour ) which show in particular how the threshold of hearing - the sound pressure required to be just audible - rises rapidly at low frequencies.

          The importance of low distortion at very low frequencies can be deduced from the form of these equal-loudness contours. The normal average threshold of hearing at 20 Hz is around 70 dB SPL (which is about the typical level of normal conversation). With increasing frequency this threshold drops rapidly.

          The contours (measured in loudness units of 'phon') can be seen to have an initial slope of 80 dB/decade or 24 dB/octave at low frequency, low volume levels. This means that if the 40 Hz 2nd harmonic of a 20 Hz fundamental tone is 24 dB lower in level, which corresponds to 6% 2nd harmonic distortion (not untypical of the average woofer), then it will sound equally as loud as the fundamental ! The 3rd harmonic distortion would have to be around 40 dB down, i.e. 1%, just to sound less loud than the 20 Hz fundamental.

          The curves become a little less steep at higher loudness levels but the same general analysis applies. It all points to low bass distortion being a significant factor. When the fundamental frequency sound pressure level needs to be above a certain threshold to even become audible it should not be masked by its higher frequency distortion products if accurate reproduction is the goal. Small loudspeakers in particular will almost entirely substitute surplus harmonic distortion products for actual bass notes, and the end result is never truly convincing.

          (The tighter bunching of the contour curves at the lowest frequencies also leads to something subwoofer owners will recognise: small changes in sub level adjustment lead to significant changes in perceived output loudness. )

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          • #6
            Lower harmainic distortion?

            Interesting analysis.

            While as AS said the harmonics are probably 'inaudible' I do wonder whether their removal would improve the perceived sound. It has been suggested that while equipment noise levels are typically inaudible in most electronic equipments, reducing the level of noise seems to aid our listening (subconsciously??).

            I've noted that the prime benefit quoted for adding a subwoofer to an audio system is an improvement in the perception of the ambience of recordings ('being there'), which appears counter-intuitive as most of the recording cues (apart from underground trains!) are in the treble frequency range. Could it be that the lower harmonic distortion at low frequencies provided by a subwoofer allows the background ambience to be heard more clearly?

            {Moderator's comment: Unlikely. There must be a better explanation.}

            Comment


            • #7
              A predisposition issue?

              Originally posted by davidlovel View Post
              . Could it be that the lower harmonic distortion at low frequencies provided by a subwoofer allows the background ambience to be heard more clearly?

              {Moderator's comment: Unlikely. There must be a better explanation.}
              I agree with the moderator here, especially when you consider that the "ambience" effect of adding a subwoofer seems to occur even when the main speakers are not high-pass filtered, meaning no actual reduction in harmonic distortion is occurring.

              I would hypothesize instead that our ear/brain system instinctively recognizes low bass frequencies as characteristic of large or open spaces, and long distances (also because bass frequencies are omnidirectional and travel a long way), and this causes an unconscious sense that the listening space has become larger.

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              • #8
                Expectation bias?

                Originally posted by davidlovel View Post
                I've noted that the prime benefit quoted for adding a subwoofer to an audio system is an improvement in the perception of the ambience of recordings ('being there'), which appears counter-intuitive as most of the recording cues (apart from underground trains!) are in the treble frequency range. Could it be that the lower harmonic distortion at low frequencies provided by a subwoofer allows the background ambience to be heard more clearly?
                Apart from expectation bias (which is always at work, when you change something to a system you already know... ) a possible reason could be increased "physical" feeling of lower bass as ambient or body vibrations. Take as an extreme example, the experience of feeling the floor and your seat shaking while listening live to lower pipes of a great organ or an orchestral fortissimo.

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                • #9
                  Warming-up the bottom end

                  Originally posted by EricW View Post
                  I agree with the moderator here, especially when you consider that the "ambience" effect of adding a subwoofer seems to occur even when the main speakers are not high-pass filtered, meaning no actual reduction in harmonic distortion is occurring.

                  I would hypothesize instead that our ear/brain system instinctively recognizes low bass frequencies as characteristic of large or open spaces, and long distances (also because bass frequencies are omnidirectional and travel a long way), and this causes an unconscious sense that the listening space has become larger.
                  Your hypothesis is rather correct, under some conditions You could get a more or less enhanced illusion about the environment and "size" of the instruments recorded. This also partly explains the "warm sound" of LPs, arguably due to non correlated noise generated during the disc playback (maybe an amount of similar noise is already present due to mastering process). As always, psychoacoustics play a huge role...

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