Article: 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:
  http://i18.photobucket.com/albums/b1...bleexample.png
  

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

http://i18.photobucket.com/albums/b1...ld/Beaming.png

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:

http://i18.photobucket.com/albums/b1.../impedance.png

• 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.

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?
  
  http://i18.photobucket.com/albums/b1...Distortion.png
  
  
  
  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?
  
  http://i18.photobucket.com/albums/b1...Distortion.png
  
  
  
  
• 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.
  
  http://i18.photobucket.com/albums/b1...ents96dB1m.png

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.

 

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.

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’.