REGISTER NEW USERLOST PASSWORD? WELCOME, Logout

Is your music system hypo allergenic?

Paul's Posts — 16 June 2012

By

Is your music system hypo allergenic?

Just heading out the door, went to fire up the dishwasher and the marketing pitch on the soap caught my eye.

“Now, Hypo Allergenic!”  What the hell does that mean?

It was the main selling feature of the dishwashing soap and for years I have taken it for granted.  In fact, I might choose one soap or another because one advertised Hypo Allergenic and one did not – never knowing what the heck it meant.  It’s gotta be good or they wouldn’t advertise it.  Right?

The term has been around for years and simply means it is “relatively unlikely to cause an allergic reaction.”  It came into use many years ago by the cosmetic industry because some goos people put on their face might cause an allergic reaction in a tiny fraction of users.  Fair enough, but dishwashing soap?

I am reminded of these marketing “truths” that we just accept without thinking.  Like “time aligned” loudspeakers.  Ever think about that?  The practice of slanting the speaker baffle back to time align the tweeter with the woofer or midrange.  Sounds good right?  I would choose one speaker over the other if one was aligned and the other not – after all, who wants an out of time speaker?

While the tweeter’s upper frequency wavelength are inches, a woofer’s wavelength varies from many feet to many inches – far more than any “time alignment” could possibly allow for.  So no speaker is truly “time aligned” by a sloping baffle – it’s just better aligned.

So the marketing term more correctly is “closer to time aligned” but then that’s not very encouraging is it?

email Is your music system hypo allergenic? Forward to a friend and help us engage more readers

Get new and fresh stories like this each morning by joining the folks reading Paul's Posts. Click here

Related Articles

Serving it up Rolling it off What’s all the fuss about? Young turks

Share

About Author

Paul McGowan

Paul McGowan is the CEO and co-founder of PS Audio Inc. a Boulder Colorado design and manufacturing company of high-end audio products and services. McGowan has been designing and building high-end products for nearly 40 years. Hobbies include skiing, music, hiking, artisan bread baking, kick boxing and cooking. He lives in Boulder Colorado with his wife Terri and his 4 sons.

(10) Readers Comments

  1. emfoods

    June 16, 2012 at 5:32 am

    You struck a nerve for once! I truly need hypoallergenic products. Would you believe I put my hand into a bag of Beggin Strips for my dog and my whole hand broke out. Soaps can be even worse (you should see what non-hypoallergenic bubble bath can do). The only audio I’m allergic to is Bose. Sure there’s worse audio out there but not at that absurd pricing.

    The only sloped baffle speakers I ever owned were Thiel’s. They were quite nice speakers but tough on my amp.

    • June 16, 2012 at 7:23 am

      The Thiels are indeed nice speakers and with their sloped baffle get closer to aligning than not, but “aligned” as in correct from woofer to tweeter they are not.

  2. Soundminded

    June 16, 2012 at 8:05 am

    Having rethought the whole problem through from scratch many years ago I have a high index of refraction…to manufacturing claims and advertising.Buzz words and catch phrases come and go but the physics of sound and psychoacoustics of hearing remains the same.

    About time alignment, another catch phrase along with absolute phase (among many others), this whole idea must have been thought up by an electrical engineer whose concept of acoustics (an application of mechanical engineering in three dimensions) is based on knowledge restricted to electrical circuit theory which has zero dimensions. Does tilting a speaker backwards change its sound? Yes, you are then off the tweeter’s axis where its high frequency response is greatest and there will be more reflections especially at high frequencies from the ceiling.Is this an improvement? I don’t know, maybe but that’s not the same as “time alignment” also known as “phase coherence.”

    What does time alignment or phase coherence really mean? Here’s an illustrative example in two dimensions that might help explain it. Throw a rock into a still pond and when it hits the water you will see concentric ripples traveling outwards from the point where it lands. That’s the goal. But now you have two rocks (analagous to a woofer and tweeter) and you are going to throw both of them in more or less together. If one lands before the other then their ripples won’t coincide, they won’t start traveling at the same time. To correct for this “time alignment gets them to land at the same time by one way or another delaying the one thrown first. With speakers this can be done by physically placing the sooner one at a greater distance from you than the one that is slower to start. What does it mean when you say one speaker is “faster” than another? Maybe not what you think. Take any two speakers of the same size, play the same frequencies at the same loudness and at any point in their travel they will be moving at exactly the same speed. The difference is the time it takes between the application of voltage to the voice coil and the mechanical response of the cone. This is called the group delay. If they are different for a woofer and tweeter then when creating sound in their crossover region they will not move in unison, one, invariably the tweeter will move sooner than the other, the woofer.

    Can correcting for this time gap difference result in a phase coherent wavefront? Not necessarily. Imagine the two rocks landing at the same time in the pond but not in the same place. As the waves from each travel outwards they create interference patterns where they intersect. They can add in phase creating a 3 db peak or negate each other creating an infinitely deep trough in response, that is the water remains undisturbed at some spots. What happens depends on the spacing of the drivers, where you are located in relation to them, and what frequency is being produced. This is classical wave mechanics. Can it be fixed? Yes, if the two rocks land not just at the same time but in the same place. This means that the speakers must be coaxial as well. You can place the tweeter at the center of the woofer where the would is or use a delay line with biamplification to compensate for the difference in their group delay.

    So does all this trouble get you anwhere? Nobody really knows because it would require careful testing to see where time misallignment becomes audible. As this is only one aspect of a speaker’s performance it’s no magic bullet, there are a lot of other things that can go very wrong and always do.

  3. lmbwave

    June 16, 2012 at 9:40 am

    Hi Paul, isn’t time-alignment more about phase alignment and when the wave front arrives at the listening position since the speed of sound is constant regarless of frequency? I know the marketing with my Dunlavys specified a 12 foot listening postion from the drivers which were then “time-aligned” at that position. Thanks.

  4. lmbwave

    June 16, 2012 at 9:51 am

    From Soundminded’s perspective, an orchestra would suffer many of the complex wave interactions but I think the idea is the sound originates at apprixmately the same time and arrives at the same time. Translating this to a few microsphones and speaker drivers makes for some interesting analysis. His analogy is a insightful and conveys the concept clearly.

    Regarding the whole marketing thing – I wish more people would simply trust their ears. Purchases based on appearance and rhetoric are compelling. It also differentiates designers from each other. Beyond that it’s all pretty much noise distracting from the real goal of the sound. I guess without the interest we may not show up at all…

  5. June 16, 2012 at 2:33 pm

    Perhaps an easier way to understand it is to simply look at a woofer vs. a tweeter. Note the woofer is a cone (usually) and its center is perhaps a few inches or more back from its front. Now look at a tweeter, which can either be a dome which comes out forward, or a flat surface, or even an inverted dome. The trick with “time alignment” is to try and get the center of each driver in the same plane as the other because at the point where the two drivers produce the same frequency (the crossover point) it is important that they start at the same place. In many cases, the center of the woofer is a few inches back from the center of the tweeter and this is where a stepped or sloped baffle makes sense.

    But the speaker can’t ever be “time aligned” by physically sloping it – it can only get closer. To have a situation where all frequencies hit the listener at the same time I would think you’d need a DSP solution that handled group delay, driver differences, diaphragm mass differences etc.

  6. hahax

    June 16, 2012 at 8:03 pm

    I guess some of my favorites in the hypo allergenic venue a the totally inert speaker cabinet and the driver that has no break up modes.

  7. oliver T. Finch

    June 16, 2012 at 9:26 pm

    Thanks for drawing attention to the industries attention grabbing terms. They do sound quite impressive but it behooves us not to take them for granted. Instead one should think whether they really do mean something or not.Thank you once again for the reminder. Regard.

  8. David Bevin

    June 17, 2012 at 1:51 am

    I dare you to send this blog on to Richard Vandersteen in Hanford, CA.

  9. June 19, 2012 at 10:06 pm

    Time Alignment and Phase Coherency are two completely different aspects of “waveform fidelity.” In order to properly reproduce an accurate waveform, such as a square wave for test purposes, a speaker system has to be engineered to project all of the frequencies so that they arrive at your ear completely synchronized. The purpose of waveform fidelity is to recreate the musical note as closely as possible, so that the timbre of each instrument is not altered. In addition, if a multi-way, multi-driver speaker system cannot reproduce a “coherent” wave form, such as a square wave, the stereo image will be diluted.
    Since most dynamic (cone) systems use more than one driver, it is necessary to find a way for the drivers to radiate sound waves that are “coherent,” i.e. they are synchronized so that there is only a single wave form that can be heard and/or measured. The fellow who described the coaxial speaker as being the best solution is correct, but it is quite difficult to construct a truly great coaxial driver that can cover the full range from 20Hz to 20kHz and beyond. For this reason, most designers use a multiple of different sized drivers that can reproduce a certain bandwidth with wide dispersion and low distortion. This driver array is then designed by physical placement on the baffle, using horizontal mounting offsets to enable the acoustic centers of the drivers to be aligned – thus the name Time Alignment – since the end goal is to have all of the various frequency waves reach the ear or mic at the same time. This is what is meant by a ‘coherent’ wave. Now, since crossover filters must be employed to enable the drive units to work in their best range, it is extremely important to design the filters so that the drivers can all be wired in absolute polarity. Many brands of speakers are wired with the woofers and tweeters in positive phase, while the midrange driver is wired “out of phase” since the designer has chosen to use second order filters with a corresponding 12dB/octave roll-off, with a resulting 180-degrees of phase shift. Since the phase shift will cause a large cancellation “dip” in the response due to destructive interference, the midrange can only blend with the woofers and tweeters in the “out of phase” connection, since the 180-degree shift can be compensated for with reverse-phase operation. Of course, this is ancient technology which invalidates the premise of ‘waveform fidelity’ since this type of design cannot reproduce a square wave or any type of musical note without damaging the timbral response and image focus. Yet you see these type of designs in almost every issue of Stereophile, where John Atkinson states that the drivers are not wired with in-phase polarity – yet he does not render judgement on this topic, most likely since it is a wide spread disease that most of the advertisers use in their designs. In my 36 years of experience, using FFT measurement equipment, a speaker system is “defective by definition” if the drivers are not wired in correct polarity. It is a sign of ineptitude or a total lack of hearing acuity. If one bothers to read any of the literature, such as the excellent book called Psychoacoustics by Fastl and Zwicker, two German researchers, it is quite clear that phase coherency is extremely important, as important as flat amplitude response. There is simply no scientific counter-argument against Time Alignment and Phase Coherency! Note that I am not endorsing first order filters, since they also have many problems related to off-axis abberations in the summation of the driver pressure responses – I use filters of my own design that have steep slopes that work as well off-axis as they do in the on-axis plane. However, unlike 3rd or 4th order filters, my design does not have high Q ringing and does not have parasitic absorption of the low level detail. For more information, please visit my website if you’re interested in the math. Happy Listening, Albert Von Schweikert

Leave a Reply