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Thread: Article: The Essentials of Sound Quality: IMHO

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    The Essentials of Sound Quality: IMHO

    Foreword:

    For a while now Iíve been in this hobby, learning and falling on my face with failures in understanding and application. Over the past few years through competition, get togethers, talking with friends, research and testing drivers for my site (in my signature), I feel like this stuff has finally started to really click.

    First and foremost, I'm a fan of music. All sorts. I'm not in this hobby or a fan of posting data just because. Like many of us here, I've been jamming out since a toddler (I have the video of me dancing to Michael Jackson as a 3 yr old to prove it... and I had some awesome moves, lol). Mix that with a desire for learning technical things, it seems like a no-brainer that I'd really have a passion for this hobby because it really does force you to study if you want to excel in it.

    What I notice on the forums is that a lot of people starting off tend to get confused by speakers and tuningÖ and itís not easy to weed through all the mess or ignore the bad advice when you donít know the person itís coming from. There are a lot of threads where people tell you what to do but they donít really explain why. Typically, in my opinion, a lot of this is just restating what someone else told us. Iím no stranger to this. This thread is not to be considered an all encompassing summation on sound quality. I am not the foremost authority on sound quality and speaker reproduction and I don't pretend to be. That said, I've spent the majority of my free time over the past 5 years researching and studying acoustics and providing 3rd party data for various drivers and products. I'd like to consider myself as an educated nooB... because I'm always learning, I'll never be an expert by any stretch. Hopefully, this thread will shed some light on the why and howís of tuning and help you understand why speaker measurements matter.

    Sprinkled in with some technical info are my own trials and tribulations; what matters and what doesnít. What is a good midrange? Why do I need to cross my midrange at 3khz instead of 8khz? Why is time alignment so important? How do I use time alignment and how is it different from polarity? Now, of course, I canít speak for all situations and I wonít say that the things I advise against canít work.

    Simply put: the following is in my opinion best practice advice. If you talked to me in person, everything I say below would be said to you regardless of who you are or what gear you run. Therefore, this thread is to be considered an attempt by an enthusiast at listing the basics of speakers along with some tuning tips/methods based on years of experience and reading on various internet sites. Iíll cite sources where necessary but since this is built on years of reading I canít really give credit where itís due in every instance because I don't know the first place I read something.
    I can say various forums, Linkwitz, Zaph, Toole, JBL/Harman, Keele, Geddes, and my friends and mentors in this hobbyÖ all of them are my resources. And in many ways, I am my own resource through trial and error.


    As of this writing, this thread is incomplete. I will update as time permits but wanted to get a jump on it so people can start reading and possibly go ahead and try to apply some of the content herein as I go.


    Cliffs:
    If you need cliffs already then this thread isnít for you. Youíre going to have to do some reading so consider this a warm up.
    (bet you didnít see that coming)

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    Re: The Essentials of Sound Quality: IMHO

    Caution... SCIENCE!!!

    Stereo is rooted in science. So, whether you believe science has a place in furthering the hobby is a moot point. That said, the science referenced herein is more of an off-the-cuff attempt at explaining the things I are concerned with than it is a doctoral candidate paper. I have made every effort to back up information with links or real-world measurements. If something doesn’t look right, contact me.

    Some aspects are left out on purpose; some things I already know aren’t mentioned. This is done for the following reasons: a) I can’t tailor my attempts at helping the DIY audio community to those who only wish to nitpick each and every aspect of this presentation nor will I exhaust myself replying to potential comments in advance by providing rebuttals in said fashion, and b) I cannot possibly cover - in detail - all the things I’d like to without making this thread just plain overwhelming.

    Again, consider the purpose: to really make things as concise as possible without leaving out important information.

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    Re: The Essentials of Sound Quality: IMHO

    Basics of Drivers:
    • Response Types:
      • Frequency
      • Polar
      • Power
    • Beaming
    • Acoustic (Natural) Roll Off
    • Distortion:
      • Linear
      • Non-Linear



    Tuning:
    • Setting Gains
    • Crossover Frequency and Slope
    • Time Alignment
    • Phase/Polarity
    • Levels
    • EQ

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    Re: The Essentials of Sound Quality: IMHO

    Basics of Drivers:

    The following sections are intended to help you understand the important aspects of a raw driver and data which will then lead to helping determine nominal crossover points.




    Response Types:

    Frequency Response:
    • The measure of frequency (Hz) vs amplitude (dB) across a given range
    • Multiple frequency response measurements at varying axes are taken to show how the driver behaves in all directions. Those axes are comprised of the following:
    • On-axis response: Speaker/system response when the listener is directly facing the speaker (SINGLE POINT).
    • Off-axis response: Speaker/system response when the listener is anywhere EXCEPT directly on-axis (typical range is 15 to 90 degrees off-axis in car).
  6. Ideally, the frequency response will indicate no hot or cold spots in response as the listener/measurement mic moves around the speaker
  7. The driver or speaker shall maintain it’s general response, other than a decreasing output level as the frequency gets higher


  8. Below is the frequency response on-axis (0 degrees) and off-axis (30 and 60 degrees). Note the graph legend in addition to callouts.









    Polar Response:
    • Another way of relating frequency response using a particular frequency or frequencies, mapped out in a polar pattern, representing the directivity of a speaker at varying angles (ie; as you move from 0 degrees on-axis to any angle off-axis)


    Below is a polar response example of a speaker modeled in LEAP. The model was derived based on a simulated horizontal axis measurement ranging from 10hz to 1.28khz. You can see that as the frequency increases, the radiation changes from omnidirectional to more directional.







    Power Response:
    • Single measurement which is the sum total of both direct and reflected sounds and is a representative example of what the listener will hear at a given location
    • Typically an average of multiple measurements in the listener’s head area
    • An “ideal” power response is one with no significant peaks or dips caused by irregularities from any single axis of measure
    • The total response shall roll-off smoothly. The rate of roll-off is a matter of directivity index.
    • When you RTA and average the results, power response is what you are measuring.


    The picture below shows a speaker measured both on and off-axis in varying axes. The subsequent picture is an average of all these measurements, which results in the power response measurement.



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    Re: The Essentials of Sound Quality: IMHO

    Beaming:
    • Beaming is kin to the acoustic low-pass (discussed in the following post).
    • Beaming is a function of the effective driver size: the dispersion narrows as the wavelength (frequency) becomes smaller than the size of the drive unit.
      Note: Effective driver size is taken from 1/2 surround to 1/2 surround.
    • All drivers beam! Though, some drivers are built to extend further on a given axis than others, but often at a cost (sensitivity, cone breakup, etc).
    • Frequency response measurements illustrate beaming easily; look for the on and off-axis responses to diverge. This is your beaming point.
    • The formula to approximate the beaming point is: 0.5*(Speed of Sound)/(Driver Size)
    • Below is a table illustrating approximate beaming points for a round driver with a given cone diameter:





    In the example below of a 4” midrange with an effective diameter of about 3”, beaming occurs approximately at 2khz.


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    Re: The Essentials of Sound Quality: IMHO

    Acoustic Rolloff:
    • Acoustic high-pass determined by:
      • Fs – Free air resonance of a driver
      • Qts – Bandwidth of driver resonance


    Fs and Qts can be obtained by manufacturer Thiele-Small specs or derived by an impedance chart if available:




    • Acoustic low-pass:
      • Determined by things such as motor force, suspension, and (namely) inductance.
      • If the inductance (essentially resistance to current, for our purposes) is high, the output of the speaker is less, regardless of the axis.

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    Re: The Essentials of Sound Quality: IMHO

    Distortion:

    There are two types of distortion:
    • Linear:
      • Any divergence from flat in the frequency response would be considered a form of linear distortion.
      • No driver is completely flat. Though, typically below the beaming point, drivers are fairly flat. Outside of beaming is where breakup occurs and is typically the area where linearity in response is compromised.
      • Does not change with volume.
      • This is typically used to determine low-pass values as cone break-up is the worst offender after the beaming point.



      Below is an example of poor linear performance. Notice after the beaming point of this 4” fullrange driver how it’s on and off-axis measurements differ?





      Below is an example of good linear performance. Notice after the beaming point of this 4” driver how it’s on and off-axis measurements follow the same trend?





    • Non-Linear:
      • Distortion that changes with volume.
      • Referred to as harmonic distortion, THD, etc.
      • This is typically used to determine your high-pass values.
      • While argued as to it’s merit of audibility, a good rule of thumb is to avoid the 3% THD range (Note: 3% THD is 30dB down from the fundamental).



      Below is the measured distortion of a 4” fullrange driver. Note the THD has reached the 3% mark at about 150hz. In addition, high frequency breakup exceeds 3% THD at approximately 3khz.


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    Re: The Essentials of Sound Quality: IMHO

    Tuning Advice:

    This section covers some of my “lessons learned” with tips based on my experiences tuning my car and helping tune others. As with anything, I should caveat by saying your mileage may vary. Not everything that works for one will work for another, but maybe some of my failures can lead to your success.




    Resources:
    Here are a couple recourses I'll reference in the body of text below. Go ahead and check them out. And you may want to go ahead and burn the CD linked below.

    • Test CD:
      This disc was given out at my GTG in November. There's a lot of tracks on here but for the purpose of this post, we'll only focus on the pink noise tracks in the last half of the disc.
    • Time Alignment Site:
      Robert Mcintosh (Pockets5) helped me make this site. The goal was for someone to easily punch in measurements of each speaker to the listening position and get the numbers needed to delay each speaker so it arrives at the listener at approximately the same time. It's not perfect, but should cut about 80% of your time off the manual way.

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    Re: The Essentials of Sound Quality: IMHO

    Setting Gains:


    Intro:
    • Setting amplifier gains can be as complicated as using an oscilloscope with a dummy load or simply using a $10 digital multimeter (DMM) and Ohm’s Law. A scope will let you know when the output signal is clipping (distorting). The DMM method assumes you know at what voltage for a given load the output signal will clip. I will discuss the latter.
    • Since most (brand-name) amplifiers will do their rated, continuous power per spec, all you need is to connect the multimeter to the speaker output of the amplifier and raise the gain until you achieve the required output voltage while playing a tone through your cd player
      • Ohm’s law is used to determine amplifier output voltage. An online calculator can be found here: Ohm's Law Calculator
      • Use an attenuated tone, such as -10dB or -5dB as this will allow additional volume for ‘quiet’ tracks. 0dB tones may be used in lieu of attenuated tones IF you listen to heavily compressed music.
      • Tones can be downloaded here: Realm of Excursion
    • The tone should be in the bandpass of what you are setting the gain for. For example:
      • If you are setting subwoofer amp gains, use a 40hz tone.
      • If you are setting midrange amp gains, use a 1khz tone.


    Setting gains with a DMM:
    • Here is a quick guide to properly setting your system gains using nothing more than a DMM.
    • Reset all your cd player DSP features, balance, fader, etc to ‘default’.
    • Disconnect all speakers to keep from damaging them (and your hearing).
    • We will assume your cd player puts out clean, undistorted signal at near maximum. Play your tone (-10dB suggested) and increase the headunit volume to a click or two below max.
    • Knowing the amps’ rated power at a given load (ohm) obtained from manufacturer’s spec (assuming they are a reputable brand), use Ohm’s law to determine voltage needed.
    • For example, with a 4 ohm load and 100wRMS rating, the output voltage should be 20VRMS.
    • Set the DMM to “VAC”, which is AC Voltage.
    • Connect the leads of the DMM to the amp’s appropriate speaker output and increase the amp gain until you achieve this voltage
    • Repeat for each channel.


    You’ve now set up your system to maximize output and performance with a wide variety of music.

    Keep in mind, when level matching, you can use your decrease amp gains as opposed to decreasing output level on your DSP. Though, I choose to maximize the gain structure up the chain as much as possible and make adjustments at one place (such as a DSP) just to keep things simple.

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    Re: The Essentials of Sound Quality: IMHO

    DSP Basics:


    Intro:
    • Most DSPs have these same features:
      • Crossover
      • Polarity (0/180 deg phase)
      • Time Alignment – ability to delay signal so all channels reach the listener at the same time
      • Levels – essentially gain for each output channel
      • Equalizer


    • The difference in DSPs is usually small, in regards to features. The difference typically comes in the resolution of adjustment of these features or (most notably) the interface. Here are some examples*
      *Note: This is NOT intended to be a summary of all features of every DSP. I’ve just chosen specific examples.:
      • Resolution of adjustment:
        • Alpine PXA-H800 uses 0.5dB increments for levels; Helix DSP uses 1dB
        • Alpine PXA-H800 uses 1/3 octave frequencies for crossover points; Helix DSP has 1/12 octave (in other words, Helix has a whole lot more flexibility); ARC Audio PS8 allows you to type in any number you wish
        • There are some exceptions where a DSP has something different (Helix offers adjustable phase in 15 degree increments on the subwoofer channel)
      • Interface:
        • Most DSPs are now controlled via a computer interface, though some are still controlled directly through the DSP (ie; Pioneer P99 or Alpine H800)
        • This is user preference. Some prefer to have a laptop and some prefer to be able to tune ‘on the fly’.

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