Everyone knows about wall reflections and bass buildup in corners. Many even understand the influence of time delay and reflection level on sound stage and stereo imaging.
But what about the buildup of early reverberation in the plane of the speakers? What is this called and why do I care about it?
“Head End Ringing”
This phrase coined by Art Noxon succinctly describes a complicated phenomenon that destroys the articulation and intelligibility of your room.
The phenomenon raises the running noise floor to almost deafening levels.
What Happens First?
Some portion (up to 80%) of sound expelled from a loudspeaker is projected away from the listening position.
This is either projected upwards, downwards, backwards, or laterally.
This sound collects within the X-Z plane, and is momentarily “trapped”
Then What Happens?
This trapped sound slowly expands through the room, weakening as its volume increases
The weakened, time-delayed sound reaches the listening position well after the direct signal + early reflections. This is different than room RT60.
Let’s See it in Action
ASC Set up a test according to the following diagram. Balloon bursts proved to simulate an impulse response most effectively, so were used for the final testing.
A balloon was first popped outdoors in a grassy field to capture the response curve without room reflections and without the dreaded Head End Ringing. The audio was run through a strip chart and displayed as voltage versus time. Note the very fast and smooth decay.
The Close Microphone
A similar balloon was popped in the test room opposite the “dead end” with the microphone less than a foot away. Once again the audio was run through a strip chart and displayed as voltage versus time. Note the increased sound level because of all the early reflections, along with somewhat of a dog-leg slope, with a fairly long overall decay.
The Far Microphone
The other channel of the same balloon pop recording was fed by a microphone located at the “dead end.” Once again the audio was run through a strip chart and displayed as voltage versus time. Note the even higher sound level of the onset and then another huge bump in level around 35 milliseconds. This is the head end ringing washing over the mic after slowly marching its way down the room. Overall decay (RT60) is similar to the close mic.
My RT60 is Still Good & Frequency Response is Flat
In a room whose head end ringing is left uncontrolled, every sonic event launches another sound “bump” down the room that muddies and colors any and all direct signal reaching the listening or mixing position. In a real room, without the totally “dead end,” this bump continues wreaking havoc as it moves back and forth through the room like an evil fog.
How can you hear the details of the music if the sound level does not die down between notes?
Where does the low-level imaging and dynamic information come from if the running noise floor is a mere 10 dB below the direct signal?
How are you ever going to reduce the buildup of early reverberation in the front of your audio room?