Axiom Audio - Home Theaters Axiom AudioFile Newsletter
Published Monthly Since 2002
Issue 86 | May 2009 (almost!)
Axiom's Anechoic Chamber

In This Month's Newsletter

Financing At Axiom!

The Archives

Selecting Your Home Theater:

Home Theater Basics

What to Look For When Buying a Receiver

Spotlight on: Home Theater Buying Tips

Choosing a TV

Ten Tips to Getting a Big Screen TV 

Buying a DVD Player


Do I Need Two Subs?

What's in a Cable?

Cable Quandary: Composite, S-Video, Component Video,
DVI, and HDMI Connectors

Choosing A Home Theater: Ten Mistakes to Avoid

Why Wall Units are the Enemy of Loudspeakers

Going the Separates Route

Beginners' Guide to Home Theater

Budgeting and Building a Dedicated Home Theater Room

Home Theater Setup Guides:

An Essential Guide to Home Theater Layout

Stereo Setup Guide

Subwoofer Connections

How to Manage Video Connections

Five Totally Simple Ways to Get a Better TV Picture

Subwoofer Placement Tips

Running Multiple Sets of Speakers in Other Rooms

What is Impedance

Choosing Surround Sound Modes

The Forgotten Component: Getting Room Acoustics Right

Basement Home Theater

AV Connections

5(.1) Ways to Improve Your Home Theater Experience This Weekend

Five Steps to Beautiful A/V Installations

Soundproofing your Home Theater: The Basics Part 1

Soundproofing your Home Theater: The Basics Part 2

5.1 Symptoms That Your TV Display Needs Proper Setup

Bringing Sound Outdoors

The Tech Talk:

Axiom Speakers and the NRC

Bass Management

Understanding Frequency Response

Secrets of Amplifier and Speaker Power Requirements Revealed

Soft to Loud: The Nature of Power and Dynamic Headroom

Excavating Real Deep

How to Judge Loudspeaker Sound and Accuracy

Describing Speaker Sound

The Inside Dope on Surround Speakers

DVD -Audio vs Super Audio CD (SACD)

Stereo's Intrinsic Flaw

Dolby Pro Logic II

Standard or High Definition--It's All in the Pixel Count!

Analog to Digital TV:
How to “Get” HDTV

What Defines a Reference Loudspeaker?




















Axiom AudioFile Newsletter Continues

You could win a pair of outdoor speakers!After several impassioned pleas from customers who forgot to review their secondary systems, we've decided to extend the contest until June 21st. It's not too late to enter for your chance to win!

Submit a review of your Axiom speakers on our website and you'll be entered in a random draw to win a pair of Axiom Algonquin outdoor speakers!

To enter, just select any product you own, and click on "log in to write a review", to the right of the product. Thanks in advance to all who enter!

Void where prohibited by law; in the case of an international winner, prize includes any customs or duty.

Axiom AudioFile Newsletter Continues

Featured Review
Ian with the Epic Accent Home Theater System"After some Beta testing in the Axiom Owners Club and repeated listening tests we’re pleased to announce the launch of the Axiom Accent system on our web site. The Accent system comprises a décor-friendly group of five identical satellite mini-speakers plus a subwoofer that costs less than $1,000.00 USD, free shipping included. The Accent is a very small, inconspicuous and attractive wall-mountable mini-speaker that does not make significant compromises in Axiom’s well-known neutral, detailed and musical sound quality."

"The idea of designing a micro system that could overcome all the inherent problems created by small size has been a challenge I have wanted to address for quite some time. The reality is that many people do not have the budget, space, or ‘decorator’s’ approval to install a full-sized theater system. We felt we should address this with a really compact and affordable system that would please anyone concerned about the physical presence of six speakers in a room. And with Axiom’s wide selection of custom real-wood and stock finishes, the Accent systems can be configured to blend in with any room décor. We did a lot of work to get this system to meet our rigorous criteria for musical, detailed and transparent sound quality with strong dynamics that have been established by our larger bookshelf and tower speakers. We’re confident that the Accent system will bring the undeniable pleasures and thrills of great surround sound and music to a new audience of customers.”

Axiom AudioFile Newsletter Continues

Featured Article

Alan LofftComb Filtering—Popular Misconceptions
by Alan Lofft

Perhaps it seems odd to discuss the teeth of a comb in connection with loudspeaker sound reproduction or the propagation of real sound waves, but it is relevant.

Comb filtering is a catchy audio phrase that’s used in audio discussions on forums, in articles, and often in the context of critical comments about the specifics of a particular speaker design. The fact is that comb filtering is simply a measurement artifact and does not detract from the listening experience. The research shows that comb filtering is not detrimental to accurate loudspeaker sound reproduction; at worst, it’s irrelevant, at best it actually adds a pleasurable element of spaciousness to stereo and surround sound.

That said, you might ask if it’s a measurement artifact, and careful measurements are instrumental to the scientific approach to acoustics and loudspeaker design that Axiom espouses, then why don’t we hear comb filtering with music and speech?

Let’s break it down.

A Microphone Is Not Two Ears
It must be pointed out that a measurement microphone—even a very expensive lab-calibrated model like the one Axiom uses (a B & K)—is like a single ear with no brain. As human beings, we hear with two ears and a brain, the latter being an incredibly sophisticated audio processing unit that is constantly comparing signals received from our two ears and sorting out not only directional cues and amplitude (loudness) differences but also ignoring or disregarding information that might be confusing or detrimental to our sound localization, spatial perception and tonal identification abilities.

What Is Comb Filtering?
Simply stated, comb filtering is two signals arriving at the same location at different times. Because of the differences in the arrival times, the sound waves will have additions when they perfectly overlap and reinforce each other, and also have cancellations or nulls where they cancel each other out (the latter is called destructive interference). This occurs in virtually all speaker systems whose musical ranges overlap, where both drivers are reproducing the same sounds, as in stereo or surround sound, and because of multiple drivers with different physical locations used to cover the same frequency range.

To illustrate how a single measurement microphone “hears” or identifies comb filtering, we set up an interesting experiment in Axiom’s anechoic chamber. Two M2v2 bookshelf speakers were placed in the chamber 6 feet apart. The calibrated B & K microphone was placed 6.5 feet away and directly in the center in the sweet spot between the two speakers. A standard frequency sweep from 20 Hz to 20 kHz was played back over the two M2 speakers and we recorded the test sweep with the measurement microphone. The purple curve in Figure 1 shows the frequency response with the microphone exactly centered in the sweet spot between the two M2 speakers.

Figure 1
Figure 1

Then we moved the measuring microphone ½-inch to the side, off center from the sweet spot, and recorded another frequency-response curve. The green curve in Figure 1 shows the first comb cancellation effect at 15 kHz.

Then we moved the microphone 1 inch off center and ran another curve. In Figure 2, the green curve shows the next comb filter cancellation at 5.5 kHz. In Figure 3, the measurement microphone was moved 8 inches off center from the sweet spot. The dark greenish curve shows the pronounced comb-filtering cancellations beginning just below 1.5 kHz and extending all the way up to 18 kHz. The dips in response resemble the downward teeth of a comb, hence the name “comb filtering”.

Figure 1
Figure 2

Figure 1
Figure 3

The cancellations (dips) are what the single measurement microphone “hears” and measures using a full-frequency test sweep when the signals from the two M2 speakers don’t perfectly overlap. This seems like an acoustic effect that may be potentially nasty in nature and should be avoided. These are pronounced cancellations, yet when we play music or speech over a pair of M2 speakers, we don’t hear these comb filtering effects. Why is that?

How Does the Brain Deal With Comb Effects?
The precedence effect (previously known as the Haas Effect) dictates that our brain and ears pick out the location of a sound source that reaches our ears in the first few milliseconds of a sound’s arrival. The first sound to arrive at the ears enables you to determine the direction of the source. After hearing an initial signal, the brain will suppress any later-arriving signals, up to about 30 milliseconds. These later-arriving signals that show up with steady-state pink noise (within the 30-millisecond window) do not disrupt the brain’s precise localization mechanism. What occurs is that you do not “hear” the contributions of the later-arriving sounds from the adjacent drivers that are responsible for the measurement artifact of comb filtering. Or rather, your brain hears and processes them but disregards them lest they confuse our directional acuity; in fact all they do in the listener is create a sense of added spaciousness. Numerous scientific researchers, including definitive experiments conducted by Dr. Floyd Toole and Dr. Sean Olive, have verified this. Even in a room having lots of reflections, our brains correctly determine the direction of sounds. (By the way, sounds arriving at our ears after a delay of more than 30 milliseconds are perceived as a second sound or echo.)

Critics of comb filtering who believe it to be a big issue in speaker design have the option of listening in mono to avoid the comb filtering. But we all much prefer listening to music and vocalists in stereo—it’s far more spacious and realistic--and the reason is that our brains and two ears simply ignore those canceling signals that on paper show up with a test signal and a single microphone.

So put on a great recording, even in 5.1 channels or SACD/DVD-Audio multi-channel, pour a nice glass of wine or open a beer, and thrill to the realism and spaciousness of great musical reproduction. Stop obsessing over comb filtering; it doesn’t matter! – A.L.

(Enthusiasts who would like to read further about comb filtering and psycho-acoustics should explore Sound Reproduction: Loudspeakers and Rooms, by Floyd E. Toole, Focal Press. Available from

Axiom AudioFile Newsletter Continues

Question of the Month: My Blu-ray Outputs Only Stereo

Q. I have a P-965 preamp that lacks HDMI and a multi-channel power amplifier. My Sony Blu-ray BDP-S300 player can output true surround audio only through the HDMI output. When it is connected through the optical cable, it mixes down everything into 2 channels. Since my P-965 preamp does not have HDMI, how do I get true surround sound? – D.M.

A. There are two ways to get surround sound from your Sony Blu-ray player.

All Blu-ray discs must contain a "legacy" standard Dolby Digital 5.1-channel soundtrack mix that you can output from the Sony's digital optical or coaxial output to the P-965 preamp. If all you are getting is a 2-channel stereo mix, then you may have inadvertently clicked on "PCM" (Pulse Code Modulation) in the setup menu for the Sony Blu-ray player.

You will have to go into the Sony's audio setup menu and turn off the Dolby TrueHD soundtrack via HDMI. Then you'll need to activate the Dolby Digital 5.1 data stream via the optical/coaxial digital output. It might be called "Raw" or "Bitstream". Carefully check the Sony player's owner's manual. Now connect the digital optical or coaxial output to your P-965 and you should get Dolby Digital 5.1. The P-965 should recognize that when you have it set correctly and will automatically switch to DD 5.1, and display that on the P-965 front panel or on-screen display.

The other way to get surround sound from the Sony BDP-S300 Blu-ray player is to use the Sony’s internal Dolby TrueHD decoder (it may or may not have this feature) and a set of analog 5.1 or 7.1 multi-channel RCA output jacks. If it has this feature, connect the 7.1 channel analog outputs to the multi-channel analog input set of jacks on the P-965 preamp. - A.L.

All contents © Colquhoun Audio Laboratories 1999-2009