Room Equalization Technologies 101 – Part 1

 in Acoustics / Audio  tagged Analog / Audio / Digital / Equilization / Room Correction / Room Equalization by

Room Equalization Technologies 101 – Part 1

Room Equalization Technologies 101 - Part 1

Sub Response Before Equalization

Room Equalization Technologies 101 - Part 1

Sub Response After Equalization

Introduction

You have done your best at treating your rooms acoustics with all those broadband absorber panels, diffusers and bass absorbers so what’s next? Very few rooms even if professionally designed for commercial use provide a frequency and time response that can be considered ideal without the use of some form of electronic equalization. In fact many residential rooms are so poorly acoustically treated, due to their multi-function or even dedicated use, that electronic equalization (room correction) is the only way to achieve an optimal listening environment.

Let’s take a look at the technologies and techniques that are available to the average user that can optimize their listening environment without bringing in audio consultants and breaking the bank.

Techniques

There are several techniques and technologies available to equalize either just your subs, satellites or all your satellite speakers and subs.

Manual EQ

The original and simplest are manual equalizers. They are placed in line with your line level active speaker feeds or between your receiver/pre-amp/processor line level output and input to your power amplifiers. They come as either fixed frequency band with adjustable cut and boost or parametric equalizers that allow you to also adjust frequency and Q (bandwidth) of the center frequency over which the cut and boost is applied. They are typical designed around analog circuitry but some modern designs can use digital signal processing to achieve the same result. Unfortunately these equalizers, at best, only support two channels for cost effective mono or stereo operation. High quality analog equalizers such as those from White and Neve are quite costly.

Room Equalization Technologies 101 - Part 1

Rane ME30B – Single Channel 1/3rd Octave Analog Equalizer

Room Equalization Technologies 101 - Part 1

White 4000 Series – Single Channel 1/3rd Octave Analog Equalizer

Room Equalization Technologies 101 - Part 1

Neve 5033 Five Band Parametric Analog Equalizer

Analog equalizers can only modify signal levels at a limited number of frequencies and cannot correct any type of time associated effects. No matter how well the equalizers are designed they all introduce various levels of unwanted phase (time) and ringing effects to a signal. Also with fifteen channels or more of equalization now required for multi-channel sound, the cost and complexity of using analog equalizers becomes prohibitive. As a result, these manual analog equalizers have mostly fallen out of favor for anything other than stereo, or specialist studio work. I use two Rane ME30B (see above image) 1/3rd octave analog equalizers when running my system as purely analog balanced stereo. Or have the option of using three for front LCR equalization for movies.

All analog equalizers are not equal and some can impact the final sound in quite a negative way. The design of the analog filter circuits has a significant effect on the final equalized sound and the Q of the circuits also effects how the adjustment of adjacent frequency bands effect each other. With the advent of low-cost digital signal processing (DSP) the functionality of analog equalizers can be easily replicated with this new digital technology at a lower cost. However, they often sound quite different and analog equalization is often preferred within a studio environment.

Digital equalizers have the ability to better control and correct the above issues and can cost effectively support many channels of equalization. So with inexpensive high speed DSP, high quality analog to digital (ADC) and digital to analog conversion (DAC), and the need to support multi-channel audio and multi-band equalization, digital equalization is now the de-facto standard for home theater systems. Some receivers/pre-amps/processors still provide the option of manual EQ for each channel. This digital EQ is generated by the same DSP hardware as the automated EQ and rarely provides more than eight fixed frequency bands per channel. Just as with analog equalizers all digitally based equalizers do not sound the same. With digital EQ there are far more areas for the sound to be negatively impacted, including the conversions between analog and digital and their associated filters, and the mathematical algorithms/techniques used to create the individual filters.

Automated EQ

Digital equalizers have numerous signal processing advantages over ‘simple’ analog style equalizers. First and foremost they can be controlled and programmed by software that is used to analyze your rooms acoustics, but are frowned upon by some audio purists because of the additional ADC and DAC conversions and the filtering involved. Like the above manual equalizers, these external stand-alone automated equalizers need to be placed in line with your line level speaker feeds, in a line level loop within the receiver or between the receiver/pre-amplifier and power amplifiers.  If the DSP technology is integrated into your receiver, processor or pre-amplifier no additional conversion to and from digital is required. At bass frequencies any possible effects from these conversions will be barely measurable let alone audible. At mid and higher frequencies at least 24bit 48KHz conversion is required together with well-designed filtering both pre and post the ADC and DAC processes respectively. Remember that subs have been using DSP equalization and control for many years. Advantages over analog EQ include:

  • Low cost, especially if integrated into your receiver or pre-amp
  • Supports many channels for surround and immersive sound
  • Are fully parametric – adjustable frequency bands, cut/boost and bandwidth (Q)
  • Can provide dozens of equalization points per channel
  • Can support time alignment and associated processing to improve timing issues
  • Can support very complex digital signal processing mathematical algorithms to support sophisticated room analysis and associated signal correction
  • Can provide individual channel delay timing, level control and matching, together with individual channel crossover frequencies to you sub(s)
  • Can automaticallly adjust the signal processing to suit the type of encoding, I.E. surround sound /immersive formats etc. to suit the number of speakers available
  • The EQ and processing software algorithms can be updated as new techniques and developments are made
  • They are mostly fully automated and require little knowledge on behalf of the user

It needs to be made clear that it is NOT the DSP hardware that provides the room/speakers acoustical analysis and solution. That is achieved by the accompanying automated software that programs the DSP’s. It provides them with all the coefficients and data that they need in order to correct the various speaker, room and timing acoustical issues. In many cases, this data is created and programmed into the DSP’s with very little, intervention by the user.

My Home Theater: The Equipment and Cabling it all Up

Audyssey Two Channel Automated Sub EQ

Room Equalization Technologies 101 - Part 1

Velodyne SMS-1. Single Channel Automated Sub EQ

Room Equalization Technologies 101 - Part 1

Audyssey XT. Eight Channel Automated Equalizer

Room Equalization Technologies 101 - Part 1

miniDSP. Eight Channel Automated Equalizer

Clearly the advantages offered by this new DSP technology far outweighs any possible signal degradation due to the ADC/DAC process. However, all digital equalizers are NOT created equal, far from it. Some are truly outstanding and some a lot less than stellar. Creating equalization results that may not sound very good (even if the CLAIMED and displayed frequency response is flat). Also depending upon the cost of the equalization system there are limits as to what it can do (analyze). I.E. what room problems it can detect and correct in the time/frequency domain and how many subs it can correctly handle. (It is NOT my purpose here to explain the technical differences between these equalizers as most of them use highly proprietary signal processing techniques all of which are based upon complex mathematical algorithms.)

Vendors, Products and Models

As I mentioned there are many products out there to choose from. However, your receiver, pre-amplifier or processor manufacturer will have already decided which vendors’ EQ software they are going to install into their various models. You are rarely given an option to choose. The remaining ‘stand alone’ equalization products are really for enthusiasts who want to either ‘tweak’ their system to perfection, are trying to get the very best out of what they own or attempting to fix a specific acoustical problem. Some receivers/pre-amps will offer the option of using the automated equalization system and provide an option of using a fixed multi-band manual equalizer. Typically, eight fixed frequencies per channel. Some products will also allow the mixing of the two techniques often allowing the front L&R channels to also be equalized manually after all the channels have been equalized by the automated system.

These automated room equalization systems provide the following basic functions in a modern receivers, processors and pre-amps:

  1. Level setting for each speaker
  2. Time/distance delay for each speaker
  3. Sub-crossover frequency selection and optimization (sometimes different for each speaker pair)
  4. Individual speaker frequency equalization
  5. Room reflection and speaker timing anomaly correction

These systems can also provide a whole range of other audio processing options depending upon the version and product cost, to include:

  • Sub EQ
  • LF containment
  • Boundary compensation
  • Dynamic EQ
  • Dynamic volume
  • Dynamic sound expansion
  • EQ preference presets
  • Automatic speaker selection and processing for different types of multi-channel sound
  • Apps that give you the ability to ‘tweak’ the final EQ results to your preference

Remember that all room equalization requires an acoustic room measurement system. These software measurement systems, including the microphone, usually come with the product but there are several stand-alone measurement systems, listed below, that are either free or low cost. However, stand-alone measurement systems require additional hardware like calibrated microphones, pre-amplifiers, USB interfaces, DSP hardware and laptops to run the software on. These specialist software audio analysis packages, like Room Equalization Wizard (REW), are really for the audio enthusiast as reading, understanding, interpreting and implementing their measurements is not always clear to the average user. There are however several associated web-sites where users can get their questions answered and results analyzed. Companies like Audyssey (used to) provide additional professional hardware and software packages (Audyssey Pro Installer Kit) that enabled experienced users to improve and refine the measurements carried out by the basic software contained in the receiver or processor.

Room Equalization Technologies 101 - Part 1

Audyssey Pro Installers Kit

So what technology is out there? Lets take a look:

The above automated digital equalizer hardware either comes with the required calibrated microphone and software or can be purchased with it. Generally a laptop is required to run the software. Also some, like the SMS-1 eight band parametric equalizer, can also be used manually.

  • Acoustic Analysis Software – You need a desktop/laptop to run the analysis software, and if not included a calibrated microphone and possibly a USB interface. The data can be downloaded directly into various DSP equalizers and sound cards.

  • Built-in Equalization

The most common digital equalizers and associated software are those that are integrated into your receiver, pre-amp or processor and sometimes even your speakers, these include:

Some built-in equalization products will still require an external laptop for the associated software to run on.

NOTE: The above is not a definitive list of all equalization hardware and software but some of the most common and popular that are currently widely used.

As you can see there are a bewildering number of equalization systems and unfortunately there can be large differences between their equalized results and therefore the final room sound. In the second part of this post we will look at a small selection of this software and hardware, that I have personnel experience with, and review how they are used and if they actually can do what they claim to.

Click on any of the following links for more information:

Part two of this post.

Introduction to room acoustics and treatment.

The design of my A/V room.

My AV rooms final (mainly sub) equalization. 

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