The Answer Man

Written by Jim Smith

Your questions – My answers

Following up from Copper # 26, where readers were asked to pick from a list of popular topics, we will start off with our reply to reader LMH, who picked two:

How to know when you’ve gone too far with room treatments

If you have modified your room with acoustic treatments, listen to the sound of the voices of people that you know well. Have the person talk to you from a point as near to the speakers as is practical. You should be in the listening seat. You should test from every speaker, but especially the fronts in a multichannel installation. If a live voice that you know intimately sounds dead and lifeless, you’ve probably gone too far in your acoustic treatment regime.

Another test is to have someone sit in a nearby seat in the listening area. Run the same test. How does their voice sound? If it’s dead and lifeless, you definitely have gone too far.

Ask two or three people to speak from the speaker area at the same time (one left, one center, one right). Can you follow them easily?  If not, the room may be too lively.

I find that if a room has acoustics that make conversation feel unnatural (either too lively and reflective or too dead and lifeless), listening to music will not be as effective as it might be in the very same room.

If you’ve ever noticed the acoustics in theaters, you can recognize the effect. Theaters are designed to maximize vocal articulation. They almost always make unsatisfactory places to listen to music concerts. If you have a multichannel system, this is especially important, as some combo rooms end up with a poor environment for listening to music with maximum impact.

The Top Three most important places for room treatments

This observation comes from over three decades of experience and hundreds (if not thousands) of successful critical installations.

You definitely want to address the first horizontal reflection from each front speaker. I’m assuming that in most installations that there is a nearby wall or other object(s) that can reflect sound from the side of your speaker.

It’s not so much for correction of tonal balance (although it may be required if your speakers have uneven frequency response off axis), but it’s mostly to prevent smearing of the sound. The slightly late arrival of reflected sound will muddy your overall sound and affect your imaging.

Think of a stone dropped into a pool. The waves are like sound waves. If you drop a stone simultaneously near the edge of the pool, those waves will merge with and affect the original waves.

Some audiophiles are surprised to find that with proper room treatments, including absorption, recorded reverberation will be increased, not diminished! For example, the sound of a choir singing a cappella in a large space will sound more spacious when you absorb the unwanted speaker/room reflections than it would if played in a live room without treatment. That’s because unwanted room reflections are smearing and even covering the sound of the subtleties of recorded ambience, spaciousness, and acoustic delay.

So my top three are:

(1) The side walls where the sound reflects from the speaker and then arrives at your ear.

(2) The area behind the listening seat.

(3) As many corners as possible.

Note:  With panel & open baffle speakers, you will probably need to absorb some of the anti-phase wave-front before it can interfere with your sound.  Dispersing this reflected sound is a popular, though questionable practice.  Yes, you can ask why… 🙂

Regardless of the type of speaker, if you determine that a secondary reflection from the opposite channel speaker is capable of reflecting off the wall at your seat, I’d consider addressing it as well.  However, when that is possible, you are somewhat likely to be sitting too far from your speakers.

There are two other places where room treatments can help, but they may not be WAF friendly:

(1) The first reflection on the ceiling from each speaker to your ear.

(2) If you have bare floors, the first reflection on the floor from each speaker to your ear. Of course, carpet or area rugs can take care of the floor-bounce issue in the mid and high frequencies.

Reader Hoytamundo requested:

Why you shouldn’t consider speaker placement final until you’ve discovered the correct AC polarity for all components  

Incorrect AC polarity from just one component can make your system sound harsher than it should. Since I assume that some of you already know this, and that you probably know how to correct it, here is one thing you may not have considered: Components with incorrect AC polarity will of course look for a ground path. Where do they share the ground?

Usually, it’s on the shield side of connectors and interconnects. Since this shield is common to both channels, it can present a slightly “grungy” center fill that you may not notice until it’s been eradicated.

If you have carefully voiced your speakers for a precise stereo image, and then you correct the AC polarity, you may get a slight “hole in the middle” effect when the artificial center fill (mono) information is removed. With some systems, I’ve found that I needed to bring the speakers an

inch or two closer together to correct for what had been a false center fill.

Try it and see what you think.


AC polarity test

As mentioned above, if you have the AC polarity inverted in any leg of your system vs. another – or at any component – it will likely harm your overall sound.  (Note – this is about North American electrical systems.  I have no idea about European or Asian systems.)

Usually the AC polarity will be inverted when an AC line is pulled from the opposite side of the AC panel. But that’s pretty easily fixed by reversing the hot and return side of the outlet.

The issue that can make for more sonic issues occurs when, for various reasons, occasionally a component will appear to be wired with the hot side reversed.  A qualified electrician can accomplish this task rather quickly.

Once you’ve discovered what is incorrect, you can begin to address it. This process takes a while, because you have to completely disconnect all cables that are attached to any other components while you run the test. Plus, some have a delayed turn on which, for some reason, may occasionally appear to be a different AC polarity than in standby.

In a somewhat complicated system (say 6 or more AC-powered devices), chances are that at least one or more will turn out to be inverted.

You’ll need to have a 3–2 prong AC adaptor (aka “cheater plug”). You should get one that will let you reverse its orientation. Most adaptors have a larger negative-side probe that will prevent reversing it. So you either have to find an adaptor with the two prongs the same size, or you’ll need to file down the larger prong slightly, so the new adaptor can be reversed.

You will need a portable volt/ohm meter (aka multimeter) for this application.  You do not need an elaborate meter – an inexpensive unit will be fine.

One set of instructions that’s especially useful regarding the use of a multimeter for checking AC polarity is on Galen Carol’s website.


I should mention that I’ve occasionally done the AC polarity test by ear. But it’s tedious, and it’s definitely time consuming. Do yourself a favor and get some device that will quickly measure the difference in ground potential.


Note, these answers are edited versions of topics from Get Better Sound and the companion Quarter Notes newsletters.

If you would like to submit a topic for discussion, you can find the list from Copper # 26 here.   You can also read Jim’s work at his website.   

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