Listening Rooms2021-01-12T13:59:28-08:00
  • ImagHugePer

It all begins with the speaker drivers, pushing in and out, creating expanding spheres of sound. The audiophile’s ear, located about 10’ away, samples about 1 square inch of the expanding wave-fronts: A tiny piece of musical heaven.

The audiophile carefully selects components and interconnects to build up a high-performance audio playback system. It begins with power conditioners which deliver clean AC power to the audio equipment which is made up of a source material player, preamp, amp, and loudspeakers. These components are connected together with balanced and impedance matched sets of interconnect cables that seamlessly convey a signal from one component to the next. This assembly is commonly known in HiFi circles as the audio chain. It processes, produces and delivers a nearly perfect replica of the audio signal in the form of a sound wave emanating away from the loudspeaker.

But the purpose of all this effort is to deliver the sonic form of the audio signal to the listener. There is a well-worn saying: A chain is only as strong as its weakest link. The whole audio chain includes the signal as it is passing through the expanse of air that surrounds the speaker and the listener. It is after all the air which enables the listener to hear the sound of the audio signal. The listener always hears the “direct signal” or that nearly perfect part of the sound wave that travels the straight path from the speaker to the listener. The last link in the audio chain is the acoustic link, the air through which the signal passes.

Unfortunately high-end audio systems are not played outside on a large field of grass. They are played in rooms, listening rooms. Once the listener has heard the near perfect wavefront, the direct part of the audio signal, the wavefront continues to expand away from the speaker until it begins to impact the walls, floor, and ceiling of the listening room. Because the human reaction time to sound is so slow, the reflections are blended with the direct signal into one total composite sound.

Often the reflections are so loud and numerous they collectively sound louder than the original direct signal and act to drown out the nearly perfect direct sound. And so the last link in the audio chain is the weakest link in the audio chain, the not so perfect, usually far from perfect ….. Listening Room.

Listening Room Acoustics

The listening room begins with the listening footprint which is then is located somewhere inside the listening room. The listening footprint varies with personal taste but generally is an isosceles triangle with the speakers located at the two corners of the short side of the triangle (b) and the listener set back, a further distance (h) at the vertex. The base of the triangle identifies the plane of the speakers. Typically speakers are 7 to 8 feet apart and the listening position is set back about 10 feet from the plane of the speakers.

The location of the listening footprint tends to be centered in the listening room with the speaker end of the footprint closer to the front end-wall of the listening room than the back end-wall, which is behind the listener. Speakers tend to be directed towards the back wall with a slight toe-in towards the listener, but not aimed directly at the listener. The distance between the side-wall and the speaker is usually less than the distance between the speakers and the front wall.

These dimensions, as well as their Room Ratios, are given a lot of attention, the goal being to position the loudspeakers and listener in the listening room such that the sound heard is the most faithful version of the actual audio signal, which can always be referenced by listening to the audio signal over headphones.

Once the listening footprint is sensibly located and oriented inside the listening room, sound absorbers, commonly known sound traps are added to various surfaces of the listening room. Typically a thick carpet is moved into the room and slid under the two speakers and as far back into the room, past the listening location. Bass traps, low-frequency sound absorbers are located in the corners of the room and treble range sound panels and sound diffusers are located along the walls of the room.

Considerable effort is applied to determining where each bass trap and each treble range panel should be best located. The overall goal is to minimize the plethora of room reflections so that the direct sound from the speakers can be clearly heard within the context of an ongoing and evolving ambience which is comprised of the reverberation and reflections of those sounds most recently played.

The process of converting the relatively empty sounding listening room into a high-performance listening room is very complicated, but the techniques which yield amazing listening performance have been developed. We can skip over this technology for now. Once the pattern of sound reflections has been cut back and sculpted into a quiet and complimentary running ambience, the last link of the audio chain has been sonically groomed to near perfection, a real compliment to the nearly perfect electronic part of the audio chain. The problem is that this quiet, faithfully performing sonic link in the audio chain, now reveals yet another problem. This problem had remained hidden from detection as long as the sonic reflections remained out of control.

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Listening Room Construction

As the room quieted down a new kind of sound began to be detected, a rolling thunder that seemed to accompany the bass power part of the sound system. The natural threshold noise floor of the room is due to air conditioning noise, equipment cooling noise and the general distant din of outside noise leaking into the room. This noise floor is typically in the range of 20 dBA. But when music is playing a rolling thunder seems to develop in the room growing as loud as 40 dBA during strong bass passages and drops away during light bass passages while the listener is listening to music in the range of 70 dBA.

After some investigation we find that this rolling thunder effect is due to sympathetic vibration of walls, floors, ceilings, windows and doors. And so, the next problem facing the development of high-end audio listening rooms is to calm down the tympanic, freely vibrating thunder plate walls and along with that is the soundproofing issues where other people in the house and even neighbors are bothered by the bass part of music being played, often late at night. Unfortunately adding acoustics to the room does not change the response of walls or ceiling to the deep bass sound pressures.

In general, listening rooms are constructed as other rooms, with an inside wall surface or leaf and an outside wall leaf. In residential construction, a single 2×4 stud has a single leaf or layer of sheetrock on each side of the wall. Knock on these walls with your knuckle as if looking to find the stud and you can hear the hollow wall tone, which definitely is not an agreeable resounding to have to listen to during loud music passages. Thumping these walls with the tight heel of your fist amply demonstrates the thunder wall effect.

One of the interesting things observed in listening rooms with well-developed acoustics is that how the other side of a wall behaves is audible inside the listening room. That means both sides of the listening room wall needs focused attention. The bare minimum remodel work to convert a residential constructed room into a listening room is to dampen the free vibration of the walls and ceiling.

To do this, structural damping materials need to be applied. ASC WallDamp is one such material and it is supplied with adhesive on both sides of a thin sheet which is protected by release paper. The sheetrock is removed from both side of the wall, WallDamp strips are attached to the faces of the wood studs, blocking and upper and lower plates. The cavity is laid in with 2” mineral wool. Sheetrock is applied as usual, sheets oriented vertically, and lightly screwed. Then WallDamp squares are applied on 12” centers to the back side of the second layer of sheetrock, release paper removed and the second layer is also tipped into position, sheets laid vertical and the vertical seams are offset. This is done to both sides of the wall.

Now the wall remains flexible but it absorbs energy when displaced, pushed and pulled back and forth by the deep bass pedal notes. The wall no longer thunders and the entire wall has become a deep bass membrane bass trap. A similar remodel is done for the ceiling overhead although it’s more work because everything is overhead and also because we can’t remove the floor off the floor joists.

The point here is that we need to calm down any vibrating surface in the room; walls, floor, ceiling, doors, windows. The powerful sounds developed by high-end audio systems stimulate most any nearby surface into sympathetic vibration and each of these vibrating surfaces push air around and makes sound waves which become audible in acoustically developed high-performance audio listening rooms.

In more serious listening rooms the inside wall is more lightly held in position with resilient channels instead of wood studs. The outside wall is as before or even more heavily built to achieve soundproofing. In both cases, the sheetrock application remains the same. Notice here that when the bass pushes the inside wall back and forth, the springy quality of the resilient channel allows the inside wall to move while the outside stud/wall assembly is not directly pushed on. This assembly is much more soundproof and more importantly for the audiophile, much calmer.

Listening Room Conclusions

The audiophile listening room is a room dedicated to playing high-quality high power music. The room is not sonically dead but calm and comfortable to be in and listen to audio in. In rooms with acoustic treatment, the vibration of the structure of the room can be minimized by keeping the sound levels to a low sound level. Audiophiles learn how loud they can play in their room before it begins to “break up” or go into uncontrolled structural vibration. It’s similar to driving a car with worn out shocks. Keep it slow and gentle and you can get around. But start to put the car through its paces, and watch out…

What is amazing about remodeled damped construction listening rooms is that there seems to be no upper sound level limit where the room begins to break up. You can play the room as loud as you want and it still hangs together as a sound (which is to say … quality) listening environment. What also happens in the well-developed room is that not only does the acoustic conditioning develop the sonic stage and crystal clear imaging but the structural damping develops its ability to handle and deliver explosive dynamics.

When the sound inside of the listening room is properly damped and the structure surrounding the listening room is properly damped there is no after-shudder following explosive or dynamic pulses of sound, particularly important and meaningful in the first octave of sound and below. With no after-shudder in the room, only the clean and fast rise and fall of sound pressure in the original audio signal is heard. The high-end listening room becomes a fast-tracking listening room, able to track and deliver sonic dynamics, rapid sound level changes. Of course, you have to have an audio chain capable of delivering fast dynamic level changes to hear fast dynamic level changes. And finally, in quality built and outfitted listening rooms, the running sonic noise floor is so low that electronic upgrades in your system become readily appreciated.

By Art Noxon, 2018

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