About The Rooms In Which Loudspeakers Work
Last month I ranted about the state of loudspeaker design and performance, and promised you that I’d devote several columns to this worthy topic. I noted that loudspeakers are the most important devices in the audio chain, that we tend to be in denial about their performance limitations and variability, that they are in fact highly and wildly variable in their performance, and that numerous difficulties exist regarding their use and abuse. Not the least of these difficulties is their engagement with the rooms in which they are used.
I noted that a loudspeaker’s performance is inextricably bound up with its room (indeed, from a conceptual standpoint, the loudspeaker includes the room). There are several implications to this. First, the size, shape and composition of the surfaces of the room have a profound effect on loudspeaker performance.
Second, the position of the loudspeaker in its room also greatly affects its performance, particularly at low frequencies. This effect is sufficiently strong that it is virtually impossible to meaningfully compare two loudspeakers unless they occupy the same point in space.
Harman International, for example, has constructed an immensely complex and expensive listening room where the speakers under test ride around on little motorized pallets under computer control, just to reduce this one variable. Even then comparison of loudspeakers can be tough, because, due to their variability, loudspeakers may interact with the room differently at that single point. It can be really hard to get useful results.
To make matters worse, the relationship of a loudspeaker to its room has several different aspects, based on frequency and directionality. Let’s discuss frequency first.
LOW FREQUENCIES? IN ROOMS?
At low frequencies, room modes (aka "eigentones" and "standing waves") are excited by the loudspeaker. These are resonances in the room that occur at specific frequencies determined by the room dimensions. They can be more or less complex, depending on the room’s shape.
We do the vast majority of our listening in what are known as "small" rooms – rooms whose dimensions are smaller than the wavelengths the loudspeakers are generating. In such cases, room modes are significant and problematic in the bottom two or three octaves of the audio spectrum.
There are several features to these room modes that are of interest. First, they represent "standing" conditions of amplitude at specific frequencies (so that nodes and antinodes of vibrating energy remain at fixed points in the room). This means that, at a resonant frequency, the perceived amplitude of the frequency is different at different points in the room. It is not possible, in such conditions, for the loudspeaker to have "flat frequency response," because at these frequencies the response changes as a function of position in the room.
Second, the distribution of these resonant frequencies can be a serious problem. If, for instance, two dimensions of a rectangular room are very similar or are multiples of each other (e.g., 8 feet high and either 8 feet or 16 feet wide), the variability in amplitudes at the corresponding resonant frequency (approximately 70 Hz, in this case) increases.
Further, there may be wide gaps in frequency between closely grouped bunches of such frequencies, leading to a particular "tonal" signature to the sound that is dependent on the room, not the loudspeaker. A given room may support B minor, for instance, but not C major.
Fortunately, by the time we get up to 125 Hz, the distribution of these modes is usually so dense that such gaps are no longer a problem.
At high frequencies, we are faced with a different but related conundrum. Loudspeakers have variable frequency response as a function of direction (this is the measurement called polar response). We usually measure the loudspeaker’s frequency response on one axis, and try to make it "flat" on that axis. This means that in all other directions the response is generally not going to be flat.
At the same time, this deficient high-frequency content is also retained within the room and reflected to the listener via multiple paths. It turns out that such reflections affect the timbre of the sound profoundly.
There is another issue regarding playback rooms that also needs to be considered. For some very complex reasons having to do with the way our hearing works, it is easier for us to extract an illusion of the "space" of the recorded room if the playback room is laterally symmetrical. This is particularly true (actually, true with a vengeance) when we work in stereo or surround and need to maintain a median plane or point.
WHERE THE RUBBER MEETS THE ROAD
What all this means, in a simple sense, is that we need to make some serious architectural accommodations if we wish to have reasonable loudspeaker performance.
• We need to account for low-frequency behavior. This is done by management of room dimensions and by "trapping" specific low-frequency bands of energy. It is also done by careful choice of position of loudspeakers in the room.
• We need to account for high-frequency behavior. Reflected energy may have deficient response, resulting in a loudspeaker that sounds bright on-axis but muddy otherwise. The addition of "room absorption," such as Sonex foam, is a popular but often counterproductive measure that can actually exacerbate the problem.
• We need lateral symmetry. Unfortunately, this includes furniture, fixtures, walls and wall hangings, doors, etc. Such an accommodation turns out to be surprisingly difficult.
Now, this is all simple, physical stuff. We have not yet begun to consider a whole bunch of higher-level concerns. There is the obvious question of how humans hear and how we can optimize the illusions inherent in audio. This means extending our system design to include the listener – an obvious but difficult and seldom taken step.
Further, we need to consider why we are doing this. In our case, it’s not just for fun. The fruits of our labors are going to be heard by many people in many rooms over many different loudspeakers. This little fact has some big implications.
Next month, I’ll discuss in a little more detail the criteria and the schools of thought regarding small control rooms and their accommodations of loudspeakers.
Thanks for listening.
Dave Moulton is an audio person living in Groton, Mass. You can complain to him about anything at his Website,moultonlabs.com.
Get the TV Tech Newsletter
The professional video industry's #1 source for news, trends and product and tech information. Sign up below.