Join us this week to read this presentation from the 79th AES Convention by ASC founder, president and TubeTrap inventor, Art Noxon, PE. The main topic is one of our favorites: corner loaded bass traps. Art explains why they are needed, how they can help your room, and what they are really doing. Enjoy!
We’re going to talk tonight about the musical tone burst and its transients, and about small rooms and their corners. We’ll blend these topics together into the problem area of low frequency room articulation and then show how bass traps help the situation. Finally, we’ll discuss the new generation of bass traps we’ve developed over the last two years.
Traditional testing of rooms utilizes both pink noise and slow sine sweeps to evaluate the suitability of the room for listening. Music is neither noise nor steady state tone. The ability of the room to articulate music is closely related to its ability to track the details of each discrete tone burst.
The typical listening room is frequently without proper low frequency decay constants. Instead of actively tracking the tone burst, the room distorts both of the burst transient: attack and decay.
Let’s start by looking at the tone burst decay. In a furnished room without bass traps the low end tone burst decay will vary between two extreme characteristics, first there can be the prolonged decay — that boomy sound — because the frequency of the tone burst matches one of the room’s resonant mode frequencies.
The second decay extreme occurs when the room is driven at a non-resonant frequency. This so-called anti-resonant frequency decay is characterized by an initial very rapid decay rate, followed by a resurgence of sound to within 10 dB of the original level. Detailed observation shows that the resurgent sound has changed frequency of a nearby resonant mode — that is some components of a musical chord actually change frequency during the decay, resulting in ‘room coloration’ of the music.
The location of bass traps in a room needs to facilitate the damping of all resonant modes. There are eight places in each rectangular room where high sound levels exist for all from resonance modes. There are the tri corners-for example, the intersection of 2 walls and the floor. Each tri corner is part of each of the three sets of parallel walls that determine the room’s resonance mode. Properly designed bass traps can be installed in the tri corners to dampen all resonance.
Rooms sound better when bass trapping is added. Prolonged resonant frequency decay times are reduced; non-resonant frequency rapid decay time is increased, and frequency shifted resonant boom is eliminated. Clearly, bass trapping in the listening room does equalize the tone burst decay constants, in that both the mean and the deviation of decay constants are reduced frequency to frequency.
Pink noise tests are typically used to EQ a room. Curiously, only a minimal 1-2 dB readjustment towards equalization in the mid bass is noticed after the transient features of the burst have been suitably controlled by trapping. The slow sine sweeps tests of a trapped room will show a slight 1-2 dB reduction in peaks and similar increase in levels of the valleys of the response curve. The curve’s fine structure however, is obviously cleaned up and sharpness of the variations is softened. This change means the ‘q’ of the room has been reduced, and typically measured to be a factor of 4.
We’ve been discussing the decay transient of the tone burst. Now we move onto the second significant feature of the tone burst, it’s leading edge, the attack. The critical element in the tone burst attack is phase alignment. It’s been long established that the phase shifting of components of a complex musical tone is not discernable for the steady state condition. But phase alignment is easily noticed in the attack transient.
If we analyze the case of a speaker near a corner, we see that two wave trains are simultaneously heard at the listener’s position. The direct signal from the speaker is laced with the weaker signal reflected off the nearby corner. If we compare the phase of the reflected wave train with that of the direct wave train, we see that the reflected wave runs through a series of relative phase shifts with frequency due to its turn-around path distance and subsequent time delay.
Read Art’s entire article to learn even more. Thanks for reading and have a great weekend!