Speaker Stories

Standing Room Only

Issue 139

Wouldn’t you hate it if you went to the theatre and had purchased your higher-tier seats with guaranteed optimal views of everyone on stage, the sets and even a glimpse of the musicians in the pit below, only to find that your seat had been taken, or worse still, there were no seats in the house! When you pay for a good seat, you don’t want to be left hanging with standing room only.

Similarly, when you invest in high-quality musical equipment for your home, the last thing you want is a compromise in seating. Not because your listening chair or couch is uncomfortable, but because you have discovered that you can hear a better sound somewhere in the room other than in your main seating position. It can be frustrating to say the least. You could liken it to the awkwardness of asking that person who took your seat to move, in a polite yet firm way; “I’m sorry, I think you are sitting in my seat.” But what you’re really thinking is, “Just what do you think you are doing there? Get out of the way!”

Likewise, a sonic imposter is likely invading your listening room right now, except that this one is not brazen enough to impede your listening experience by making itself visibly known. Oh no, this interloper is invisible, but nonetheless is discernibly detracting from the enjoyment of your musical performance. We are talking about the presence of a standing wave (or waves).

Just what is a standing wave, what does it do to our sound, is it really that bad, and more important, what should you do about it?

In physics, a standing wave is one that varies in amplitude over time, but its peak amplitude profile doesn’t move in space. Standing waves are formed by the combination of two waves of the same amplitude and frequency moving in opposite directions.

Illustration of a standing wave. Courtesy of Wikimedia Commons/Lucas Vieira.

Illustration of a standing wave. Courtesy of Wikimedia Commons/Lucas Vieira.

 

Why does this matter in acoustics, and in our listening rooms? Standing waves may create areas of undesirable resonances within a room and so produce some notes that will be louder than others in those areas, especially in the bass. (Conversely, there may be areas of cancellation, where certain frequencies may be attenuated in specific locations in the room. We touched on this in “Subliminal or Sublime Bass?” in Issue 138.)

Standing waves happen because the dimensions of a room are the same or multiples of the wavelength of the sound at a particular frequency. The room dimensions, speaker position and listening position are what determine the amplitude of a sound waveform at a given frequency. Rather than decaying quickly, that sound wave is emphasised, and you hear it as an unwanted resonance or volume peak. Other frequencies are less prominent from where the same sound wave reflects back on itself over time, leaving a null or a part of the frequency response where there is little sound pressure, and a volume “suck out.”

What is the overall result? You have sound wave energy which is emphasized in some frequencies and nulled in others, which causes an in-room frequency response that is uneven and measurably imbalanced. If you think of a butterfly, the body part is where there is no change in movement is the null sound, and the wings represent the sweep of the amplitude of the sound wave, where there are changes in movement – and peaks in the standing wave.

If you are a bass guitar player like me, you may have heard such resonances when trying out different-sized speaker cabinets. Depending on the note you play (the sound waveform) and the size of the cabinet (the “room dimensions”), you may notice that at certain frequencies the sound is no longer even and smooth, but instead, uneven in volume. I have even seen some smaller bass cabinets (a 1 x 10-inch model, for example) actually move around the floor because the cabinet wasn’t sturdy enough to compete with the standing waves created by the cabinet and floor and the note being played.

Standing waves can really mess with the low-frequency response in our music. This is part of the reason why, as you move around your room, some areas sound fuller and richer in bass, while others are lacking. The practical problem is that for most of us with “smaller” rooms (i.e., up to hundreds of square feet and not concert halls or stadiums), the bass, which comprises about 30 percent of what people like in an audio system, sets the context for our midrange and treble – and that in most home listening rooms, the bass is adversely affected up to and including about 150 Hz – right where our low end “lives.” The range of frequencies which are impacted decrease as the room size increases. Bigger rooms are affected up to about 90 Hz, and much bigger rooms are affected up until about 60 Hz.

Standing wave propagation in a room.

Standing wave propagation in a room.

 

If there is any compensation in this, some of us can take consolation in the fact that we have listening rooms that are less than ideal in their shape and lack of symmetry. However, those “flawed” room shapes, like an “L”-shaped room, or a room which is open on one side or is asymmetrical, can help break up standing waves rather than reinforce them.

Standing waves are a fact of life. You can’t eliminate them, unless you plan to listen to music in an anechoic chamber, or outside, where there are no room boundaries that would create standing waves. (One interesting solution would be a listening room that opens out to the great outdoors on one side.) The bigger issue is how much control you have over them. In fact, to complain that you have standing waves is a bit like complaining that you have walls.

OK, rooms may not be perfect or even close, but the very fact that the room has such a massive influence over your sound means that you can do more to change your room’s behaviour, by addressing standing waves and correctly aligning your low frequencies, than swapping out your equipment or loudspeakers.

In a following article we will look into how we can tailor our room to taste by addressing standing waves, in our hunt for that sound – the sonic ideal we all strive for.

 

Header illustration of a standing wave courtesy of Wikimedia Commons/Vegar Ottesen.

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