When one listens to a set of loudspeakers, over 99% of the sound wave emitted from the speaker expands in all directions other than toward the listener and is called “off-axis” sound. There is a very small portion of sound that does pass by the listener and it is called the “direct signal”. One might wonder what the direct signal sounds like. It’s a similar sound to what we hear with headphones, except the sound comes from the speakers in front of us, while headphone sound comes from either side of our head.
The direct signal is what we would hear if we could play the sound system in an area where there are no reflections. Set up the HiFi system outside on a huge lawn area far from any buildings and we’d have a reflection-free playback experience. More formally we might rent an anechoic chamber, an acoustic test room that is 100% sound-absorbing. But in the real world, a reflection-free environment does not exist.
In a Hifi listening room, the other 99% of the sound, the off-axis part of the sound, can’t escape, it is reflected back into the room over and over again by the walls, floor and ceiling. It is essentially captured by the room and remains captured by bouncing around the room until it dies out.
With continuous music, there exists an ongoing plethora of reflections being created, replacing those which have died out, in the listening room, many of which cross and recross the listener’s ears which augments our perception of the direct signal. The effect of the reflections in the listening room is so powerful compared to the direct signal that the listening room (not the speakers or the direct signal) is referred to as the “last link in the audio chain”.
The listening room is where the direct signal is acoustically mixed with an ever-changing variety of previously created off-axis reflections. It is this sonic assembly of sounds built out of acoustic space and time that is impinging on the listener’s head that produce the gestalt of the stereophonic hifi listening experience. The last link in the audio chain is no small and simple thing.
The Listening Room Effect
In the simple view of audio playback, the speaker/listener setup and resulting soundstage exists independent of the listening room. This presumption leads to misunderstandings in discussions with seasoned audiophiles, who have well developed high power listening rooms, which include considerations for sound reflection containment and management:
To the right is shown an impulse response, sound level dB vs time in milliseconds. If the “direct sound” is a distinct quick snap then all the reflections; ”early” and “late” reflections are also distinct quick snaps, as shown on the right. The “Room Effect” or Room Gain is the sum of all reflections which is usually 5 to 10 dB louder than the “Direct Sound” alone. “Room Gain” which is what makes sound seem louder inside a room compared to when setup outdoors. In a typical room, under steady state conditions, a speaker plays 10 dB louder than if set up outside. Since the reflections (room gain) in the room are significantly louder than the direct signal, the audiophile has acclimated to a different concept. they don’t imagine that they are really listening to the gear and the direct signal, they have accepted that they are, for the most part, listening to the room as it is being played by the speakers. In other words, the room is essentially an acoustic instrument that is being played, plucked and bowed, struck and blown by the loudspeakers.
Although each reflection is less loud than the direct, the sheer number of them add up to deliver 5 to 10dB more acoustic energy to the ear-brain system than the direct signal alone. Again, the reflections of the room overpower the strength of the direct signal and literally change what the direct signal sounds like.