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Facing the challenge of real speakers in real rooms

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  • Facing the challenge of real speakers in real rooms

    Loudspeaker designers go to great trouble to design their speakers to perform as intended in a known acoustic. That could be an anechoic chamber, outdoors or in their own living or working listening space. Regardless of the efforts that they make, once those finely crafted speakers are sold to consumers, not one consumer is likely to have even similar listening environment acoustics; most will be far from optimal and the speakers will always behave below their design and intended potential.

    Real consumers make space for their loudspeakers amongst the paraphernalia of ordinary domestic life and there is a limit as to how much physical adaptation they can or will undertake to improve the acoustics of their listening space. The application of damping materials to 'dry out' the space and tame standing waves and echoes needs thick coverage of a significant proportion of the surface area, perhaps 50%, and that is not domestically acceptable.

    What can Digital Signal Processing contribute to taming the domestic listening environment? Is it a technology 'cheat'? Is DSP a devil, interfering with the music? Or is it a highly cost effective, adjustable solution that can work in most rooms most of the time and retain harmonious relationships with other members of the family that physical damping solutions may not?

    Should every home have a DSP room correction system available for really critical listening to get the very best performance from the hifi system? Should Harbeth positively encourage the use of reversible in/out DSP as a means of maximising the latent potential we know exists in our speakers?

    Crucially: should DSP be confined to improving the sound quality of the lowest (bass) frequencies only, where the room has a dramatic impact on the sound, almost always for the worse, or should DSP be applied as a wide-rage solution to correct not only the room itself but oddities in the speakers (or other elements of the home system) right across the audio band? That may be technically defensible, but is it morally right to use brute-force electronic technology to correct the designer's electro-mechanical shortcomings? Can DSP make a cheap low-end budget speaker sound like and behave like a high end speaker? How would our driving experience change if we disabled the Engine Management Computer that all contemporary cars depend on for modern-world performance or the Flight Management Computer in an modern aircraft.

    An interesting debate.
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

  • #2
    At LF only

    Not experienced, but I think DSP room correction is an attractive solution to deal with the low bass frequencies - to be more precise, given a certain room, below its "Schroeder frequency": in this way I suppose We get the advantages (no need to install ugly and generally not so efficient sound absorbers; DSP, i.e. signal pre-treatment outside - below "mid-band" where human ear sensitvity is higher).


    Originally posted by A.S. View Post
    Loudspeaker designers go to great trouble to design their speakers to perform as intended in a known acoustic. That could be an anechoic chamber, outdoors or in their own living or working listening space. Regardless of the efforts that they make, once those finely crafted speakers are sold to consumers, not one consumer is likely to have even similar listening environment acoustics; most will be far from optimal and the speakers will always behave below their design and intended potential.

    Real consumers make space for their loudspeakers amongst the paraphernalia of ordinary domestic life and there is a limit as to how much physical adaptation they can or will undertake to improve the acoustics of their listening space. The application of damping materials to 'dry out' the space and tame standing waves and echoes needs thick coverage of a significant proportion of the surface area, perhaps 50%, and that is not domestically acceptable.

    What can Digital Signal Processing contribute to taming the domestic listening environment? Is it a technology 'cheat'? Is DSP a devil, interfering with the music? Or is it a highly cost effective, adjustable solution that can work in most rooms most of the time and retain harmonious relationships with other members of the family that physical damping solutions may not?

    Should every home have a DSP room correction system available for really critical listening to get the very best performance from the hifi system? Should Harbeth positively encourage the use of reversible in/out DSP as a means of maximising the latent potential we know exists in our speakers?

    Crucially: should DSP be confined to improving the sound quality of the lowest (bass) frequencies only, where the room has a dramatic impact on the sound, almost always for the worse, or should DSP be applied as a wide-rage solution to correct not only the room itself but oddities in the speakers (or other elements of the home system) right across the audio band? That may be technically defensible, but is it morally right to use brute-force electronic technology to correct the designer's electro-mechanical shortcomings? Can DSP make a cheap low-end budget speaker sound like and behave like a high end speaker? How would our driving experience change if we disabled the Engine Management Computer that all contemporary cars depend on for modern-world performance or the Flight Management Computer in an modern aircraft.

    An interesting debate.

    Comment


    • #3
      Why not?

      I think Alan raises two different issues here, and I think they ought to be distinguished.

      The first is the use of DSP for room correction. I think this is an excellent idea, and I have been more than pleased with my own experiences in this area (using a DSpeaker Antimode 8033 unit to equalize a subwoofer). From what I understand, this is most effective at lower frequencies.

      Not advocating this for higher frequencies is not, for me, therefore a moral issue or whatever, but simply a practical one: it works best at lower frequencies. This also means that correcting small speakers in small rooms with room modes at higher frequencies is likely to be less effective (and less necessary) than correcting large speakers (let alone subwoofers) in larger rooms with room modes at mostly lower frequencies.

      That said, and in those circumstances, it is a very useful tool, particularly if you want to reproduce just that little bit more bass than your room itself can really handle. And that happens sooner than you may think: it certainly was the case in my 60sq m listening room. The success can easily be seen in any measurement of in-room response.

      Remember, these room modes also have harmonics, so equalizing not only cleans up deep bass, but also higher frequencies (which always was one of the reasons to avoid speakers that put out too much bass).

      The second issue is that of using dsp to improve the properties of a speaker, even in a perfect room. Or, as Alan put it, making a budget speaker behave as a high end one. It is what some active subwoofers like my B&W PV1d already do (and maybe some active full range speakers as well, I don't know). It is also what Devialet seems to try with their system. My answer is a simple one: use any tool in your tool box (but let it be the speaker designer's toolbox). I have no idea how useful it would be as a tool, but I see no reason in principle.

      The underpinning of much of Alan's design work is now computer simulations. Turning those results into a physical reality by means of dsp rather than a passive network of capacitors and resistors is just another and maybe not even so different technology to reach the same end. The real achievement was and remains to overcome the physical limitations of an electromechanical system.

      Comment


      • #4
        DSP cannot replace Room Treatment

        This was discussed a bit on this thread:

        http://www.harbeth.co.uk/usergroup/s...ight=behringer

        I posted of my experience on post #21.

        Some speaker manufacturers have used electronic (active) crossovers using equipment that has DSP. Salk speakers and using a DEQX, and VMPS (no longer made) with a Behringer DCX2496. VMPS would ask information about your room, notably dimensions, and then but in a suitable frequency curve.

        I'm not convinced, in my limited experience, that DSP alone can deal adequately with bass problems caused by the room, although if that's all you can do it will help.

        Comment


        • #5
          The digital tool

          A very interesting debate, indeed: kudos to Alan for putting forth this argument!

          In my opinion active digital signal processing must be the path to follow by researchers to improve overall system performances, instead of trying to improve what is already beyond hearing threshold.

          First of all must be noticed that DSP is not a magic wand to solve every possible problem and assure perfect performances from a mediocre design. As a matter of fact, when adding a signal processing module to a system in order to improve its performances against a known reference, it constitutes a control system, with a lot of implications in term of added complexity and basically no control system can reliably push the controlled one beyond its physical limits. In other words a DSP can lower bass distortion within its specs, but cannot make a P3ESR play a 30Hz tone at a useful SPL, not without breaking it at least.

          Nevertheless I think that a DSP inserted in a closed loop system (i.e. a feedback control system) can improve a lot speakers performances, as they are the most non linear and distortion generating component of the whole chain and if a speaker designer cooperates with a control designer and maybe designs with this systematic vision in mind the results will be even better (for example, I've always wondered why they don't make, to my knowledge at least, woofers with an accelerometer in the cone: this simple device could generate in real time a very helpful signal to syntesize a correction action).

          Comment


          • #6
            Devialet - not authorised by Harbeth

            It looks as though the Devialet SAM system already has a downloadable calibration file for the Harbeth M30.1:
            http://en.devialet.com/speakers

            This is a DSP correction (can I use that term without offending present company?!) system (i.e. sophisticated EQ system) with a database of files for various commercial speakers, so no measurements by the user are necessary.

            http://en.devialet.com/technology/devialet-sam-en

            Not sure whether the effects are bass only, or whether it extends higher up.

            {Moderator's comment: How and where Devialet created a so called calibration file for our M30.1 is a mystery. We have never been contacted by them and we most certainly have not supplied them with any technical information nor sample speakers. Any such so-called 'calibration curve' cannot in any way be considered as authorised by Harbeth. Consumers are entirely on their own. Beware, and pass this on.}

            Comment


            • #7
              Full range correction

              For me, using D.S.P. throughout the frequency range makes sense. In the bass most necessary, for the reasons AS points out. But I listen mainly to classical music, including much orchestral music, and am faced with less than wonderful - closely miked - recordings. Generally more treble energy than I hear in a hall live. So I pull down the treble on most recordings.

              Well designed concert halls reinforce the bass and attenuate the treble, and my aim is to replicate as closely as possible what I hear live in good halls (I do attend concerts regularly).

              Comment


              • #8
                Real science that works

                Many years ago Philips developed Motional Feedback speakers, that had precisely such motion detection. At this moment the top of the line Velodyne subwoofers have something similar.

                To be sure, such real time feedback is different from using dsp for room equalization, where the system emits a series of test tones, evaluates how the in-room response in your specific room deviates from the ideal, and then sets about to correct that by in future equalizing the signal that will be fed to the speaker, applying a pretty large number of narrow filters.

                Of such a system I can only say that it works, and very well indeed. Unlike all the voodoo science that is thrown at us, this piece of real physics and real engineering does the job, and the difference is not minor, and future iterations are likely to only get better. Room interaction is the elephant in the room.

                Comment


                • #9
                  EQ v. tone control

                  Probably a stupid question, but what is the difference with an equalizer or tone control ?

                  Comment


                  • #10
                    Fantasy (mis)correction

                    Originally posted by G Spiggott View Post
                    It looks as though the Devialet SAM system already has a downloadable calibration file for the Harbeth M30.1:
                    http://en.devialet.com/speakers
                    Looking at the schematic, it's a quite simple open loop feedforward control system. It is based on a mathematical model of the target speaker, made up from some basic parameters, known or measured, like frequency response, impedance modulus and phase etc... starting from this a unique control law is synthesized. This law is then applied to the input signal to (hopefully) correct for the (supposed) deviation of the speaker from ideal behavior.

                    Those kind of systems are rather easy to build, but have worse performances than closed loop ones because they have not any actual knowledge of the real system they are controlling neither they monitor real time output to measure and correct for actual errors.

                    It is a step back also from those systems which synthesize the feedforward control law by measuring the specific speakers and their in room response with test signals (the ones that are normally known as room equalizer DSPs).

                    Comment


                    • #11
                      Overview article

                      Here's an interesting article on the subject of DSP {from a trusted source- Mod}.

                      http://sound.westhost.com/articles/dsp.htm

                      Comment


                      • #12
                        Overcomplicated solution

                        Originally posted by Ned Mast View Post
                        So I pull down the treble on most recordings.
                        Does this work more effectively than using a treble cut via tone controls on an amp?
                        DSP used for bass response adjustment does more than just a bass cut - the good ones measure room responses to below 200hz energy, and attempt to smooth the peaks in user room responses to improve bass SQ. I would think that for treble a simpler solution may be good enough, hence the question.

                        Comment


                        • #13
                          No sound degradation

                          DSP can be used in several ways, including:

                          1. Compensate for frequency response deviations in a speaker ("EQ") as measured in an anechoic chamber.
                          2. Compensate for phase deviations and delays in the speaker (achieve zero phase shift) - which some people say is pointless, while others disagree.
                          3. Replace conventional crossover filters with digital filters (active speakers)
                          4. Compensate for power compression (where drivers lose sensitivity due to voice coil heating when driven extremely hard)
                          5. Protect the speaker against overload and overheating
                          6. Compensate for the effects of the room
                          7. Synthesise surround sound etc.

                          Mostly it is done 'feedforward' i.e. modifying the signal in advance to reverse an effect that has been predicted, or found previously through measurements. The Devialet SAM system mentioned earlier is certainly doing 1, 2 and 5.

                          The DEQX system can do 1, 2, 3 and 6.

                          As far as I know, the Beolab 5 speaker does 1 - 6.

                          Personally, I find the whole subject fascinating but, (like many people, I suspect) before dabbling in it I somehow couldn't imagine that the results wouldn't sound artificial and obviously 'digital'. Surely a signal that has passed through coils and capacitors will still sound 'musical' while a signal that has been digitised and then had actual calculations performed on it, must always sound artificial. But it isn't the case: the DSP can improve the sound in subtle ways, and in my experience never sounds 'digital' at all.

                          Comment


                          • #14
                            Using DSP potential to the max

                            Originally posted by willem View Post
                            Many years ago Philips developed Motional Feedback speakers, that had precisely such motion detection. At this moment the top of the line Velodyne subwoofers have something similar.
                            After posting I realized I had some remembrance of such systems. I remembered Infinity Servo Statik bass module (late '60) and some B&W subwoofers. Anyway those are all active speakers where the signal from the accelerometer is used as feedback signal to close the loop with the internal amplifier and the control law is made with analogic proportional, integrative or derivative networks, no DSP involved.

                            To be sure, such real time feedback is different from using dsp for room equalization, where the system emits a series of test tones, evaluates how the in-room response in your specific room deviates from the ideal, and then sets about to correct that by in future equalizing the signal that will be fed to the speaker, applying a pretty large number of narrow filters.
                            To be clear: in scientific and engineering literature, DSP means Digital Signal Processing (http://en.wikipedia.org/wiki/Digital_signal_processing) or secondarily, Digital Signal Processor (http://en.wikipedia.org/wiki/Digital_signal_processor). To digitally process signals, nowadays are often used general purpose CPUs instead of DSProcessors, as they have all the processing power required even for real time applications (i.e. a new full calculation of processing law during every sampling period, to correct the next sample to be sent to DAC).

                            Room equalisation is just an application of digital signal processing, not the smartest possible. What I'd like to see as hi-end industry next move is a more extensive use of digital signal processing to realize closed loop real time control systems and improve the overall performances of speakers, which are still the only electro-mechanical devices in the replay chain and the main source of non linearities and distortion. By improving performances I mean minimizing distortion as sound source in the first place, then adapting to room characteristics.

                            It surely is much more challenging (not rocket science anyway) but much more effective than faking improvements by pushing more bits and faster sample rates!

                            The Smyth Realizer (http://www.smyth-research.com/products.html) is an example of such DSP use, together with head motion trackers and microphones in the loop to simulate in real time ambient speaker listening on headphones. It can simulate different venues, even owner's room after having measured with test signals his speakers performing at home.

                            Comment


                            • #15
                              Adjust before or adjust after speaker

                              Originally posted by hendrik View Post
                              Probably a stupid question, but what is the difference with an equalizer or tone control ?
                              The ears and brain behind. To use an equaliser the user first listens and then move the sliders to apply the correction he feels adequate, a room correction module do all this by himself.

                              By the way, as normally a human makes modifications to the sliders by ears while listening, he approximates a 'closed loop feedback system' {adjust while hearing) while a room corrector is a simpler 'open loop feedforward system' (pre-adjust signal sent to speaker, no further evaluation while playing)! ;)

                              Edit: if the human actually don't go by ear while listening but uses a microphone and a spectrum analyzer to calculate the attenuation needed at different frequencies and adjusts the slider positions accordingly, once and forever, then the two approaches coincide.

                              Comment

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