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The Audio Butterfly Effect

Issue 143

The best-fitting suit is a well-tailored one. You likely would never expect to try on a garment in a store and have it fit perfectly, if taken randomly from just any hangar displaying your size. You appreciate that the tailoring process adds value and yields a better fit and comfort. The other side of the coin is that you may be content with grabbing whatever garment is on offer, throwing it on and being satisfied with a one-size-fits-all experience. If you do, it’s reasonable that you would have to put up with the limitations of a tight-fitting seam or longer arms than you may prefer. The cool thing, though, is that tailors do exist who can measure us and perform a fantastic service.

Our listening rooms also benefit from tailoring, and yet, although this too may be obvious, how many of us have taken the measurements of our room’s frequency response and then made adaptive changes accordingly? (I’m sure some have, but not anywhere near a majority). In a previous article, “Standing Room Only” (Issue 139), we examined the reality that rooms have a “personality,” or sonic characteristics, especially in the bass frequencies, which are defined by their dimensions and their standing waves. These create room modes, areas in the room where frequencies are cancelled and reinforced. So, rather than getting frustrated with the limitations of our room, it can be good for us to look at the room’s personality as a foundation on which to build our subsequently tailored sound to our advantage.

All rooms have influence over the sound of our systems, so how can we achieve a “best-fit” scenario from our listening rooms? With careful measurements – with or without the aid of measurement software – and by investing some time in making adjustments, we can make vast improvements, and without hiring an installer or audio specialist.

Any room with four walls will be subject to three main sets of sound wave propagation (and reflection): front to back, left to right, and top to bottom. These are axial room modes, which involve standing waves that travel along two parallel surfaces. (There are also tangential room modes, which occur between two sets of parallel surfaces, and oblique modes, where all six room surfaces are involved, but these modes are weaker in their effect.)

 

Illustration of room modes. From the GIK Acoustics website.

Illustration of room modes. From the GIK Acoustics website.

 

To some extent, by moving the speakers further from the back wall by a few feet (or if you don’t have enough room, try a shorter distance), this can greatly diminish the effect of problematic standing waves which may be created along the front-to-back dimension. This can be particularly effective for managing the bass response in small rooms, especially from around 30 Hz to 150 Hz.

The smaller the room, the higher the frequency of its standing waves, and so, for the majority of us with small rooms, this means that we are subject to the effects of standing waves from around 150 Hz and below. Larger rooms may be affected from about 90 Hz and below. Eventually, once the room is sufficiently large enough, the low-end frequencies that are affected dwell within a range that is well below 20 Hz and of less concern because you can’t hear the effect of the standing waves. (In larger venues such as auditoriums and theaters, they occur at such low frequencies that we are never going to hear their impact.) Also, the bigger the room is, the closer the frequencies of the multiple standing waves that occur in the room, which will make less overall negative impact on the sound.

As we go up in frequency, it can become easier to hear differences in how the sound is being affected by the room. We can more readily identify where the areas of bass cancellation (nodes, or nulls) are in smaller rooms by listening for them, because their frequency range lies in a more discernible area of human hearing.

Room measurement software such as REW Room EQ Wizard (covered in “Subliminal or Sublime Bass?” in Issue 138) can be especially useful, but you can also use any number of online utilities to calculate where the nulls in your room will be located.

One way to overcome the sonic disadvantages of standing waves is to use multiple floorstanding loudspeakers, as discussed in a previous article, “Sub Missive,” (Issue 137). However, this is an impractical solution for most of us who use two stereo speakers. On the other hand, adding one or more subwoofers can be a practical solution for achieving extended bass response. Being able to specifically place one or more powered subwoofers where or near where the room’s null frequencies occur can be an effective way of overcoming the bass-robbing effects of the null. What we want to achieve is a result that sounds good to our ears, with a more-even bass response being enjoyed in multiple seats in our listening room.

 

Illustration of the concept of room modes. From the Sonible website.

Illustration of the concept of room modes. From the Sonible website.

 

(Subwoofers have an additional advantage – they lessen the low-frequency power demands placed on the main speakers. Many stereo speakers can even “bottom-out” – or reach the limit of their woofer excursion if driven hard – and at a higher frequency than their stated nominal value. By giving the responsibility of bass frequency reproduction to a dedicated sub and locating it very specifically in the room, this can radically start to transform the way your room behaves and is not to be underestimated.)

I’ve seen two schools of thought regarding whether to place a subwoofer right in the middle of a null, or at either side of it. Placing a sub in the middle of a null can result in achieving greater linearity – a smoother overall in-room low-frequency response – but at the expense of losing output (volume). On the other hand, placing the subwoofer on either side of the null effectively “breaks it up,” and also creates a more even bass response. Then, if you  place additional subs to break up the nodes of additional frequency null points in the room, it is possible to further improve linearity without necessarily losing as much bass volume.

It should be noted that placing a single subwoofer in a corner of the room is a common technique, because it’s a convenient locations for many people, and also because the sub will massively excite the room as a result of the boundary gain/reflections from the wall, which will maximize the subwoofer’s output. However, this may not give the smoothest in-room frequency response, so complimenting it with an additional sub for eradicating 50 Hz nulls can be effective in achieving greater low-frequency linearity.

All this is well and good, but how can you find where these nulls are in the first place, and then, what do you do about them?

It is possible to identify where nulls may be occurring without software, a microphone or room analysis programs. Take a test CD that can play a continuous 50 Hz tone, for example, or play or download a 50 Hz tone at this link. 50 Hz is a typical null frequency that many rooms are dogged by. Play the tone while moving your head from left to right at different distances back from the speaker, and you will notice the locations where the volume of that frequency drops. The places where this occurs are where the nulls occur.

Look at the butterfly wings as in our illustration below. The red line represents the waveform of the sound coming from our speakers. The butterfly’s body represents the null point where there is the least change in the audio signal. The butterfly’s wings represent the areas where there is the greatest change in audio signal happens. These are on opposite sides of the null and are represented one positive and one negative side of the audio waveform.

Illustration of the "Audio Butterfly Effect" with caption, "No movement at this point."

Placing the sub on either the positive or negative side of the null allows the sub to gain a better “grip” on that 50 Hz frequency and the null is all but vanquished. This method alters the amount of pressure on either side of the null, disrupting the standing wave reinforcement and preventing it from manifesting. It’s almost like creating more of an up and down pistonic action at that point, thereby reducing the audibly-noticeable fixed-point nodal effects. The room will have less “control” over that null point.

This is why using up to four (or more) subwoofers in multiple positions is so effective in eradicating other nulls which also occur but at different frequencies – and at different distances from the speakers within the room. If you listen to where these different frequency nulls are occurring, and place a second sub near these locations on either the positive or negative side of the nulls, you crush the null out. This greatly contributes to making more of the room respond with improved seat-to-seat listening position consistency.

Ideally, the key to best identifying the nulls is in using measurement software and a room mode calculator app, but if you don’t have access to or the ability to use these, you can still achieve great audible results by listening carefully to where the nulls occur with just your ears and placing a sub to the positive side of one frequency null, and, if you have multiple subs, by placing another along the negative side of another frequency’s null.

In the next installment we’ll cover some specifics of subwoofer placement.

2 comments on “The Audio Butterfly Effect”

  1. Great article. I have gotten great results using a corner sub playing stereo aimed diagonally across the room together with two side wall subs playing left and right bass channels aimed at a slight diagonal.

    Using the Octave RMA app on my iPad with a microphone plugged into the headphone jack shows a very even response as I wander about the room (and the house as well).

    I am fortunate that my room is fairly large (540 sq. ft.) and has a peaked ceiling which opens into the rest of the house. Bass levels are even in other parts of the house.

    The corner stereo sub is a REL Carbon Limited and the side-firing subs are REL S-5 SHOs, all crossed off very low and played at relatively low levels. Images of bass instruments are focused at the front of the room where they belong and the soundstage is huge and deep. The speakers (Usher Audio Dancer Mini Twos) are run full range…

  2. My Klipschorns and my room are now suffering from bass bloat after I got rid of a cloth covered 3 seater sofa and replaced it with a leather one.
    It’s the only change. I suspect the leather is more reflective.
    I am going to move the K’horns out of the corners a tad, 6 inches or so, and see how it goes.
    Otherwise I’ll probably use a MiniDSP and the REW.

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