A couple of years ago I wrote for myself a twenty six page paper on measuring with REW my dedicated listening room of 14' x 13' x 8' (4.2m x 3.86 x 2.4m) as I made various changes, adding a third piece of glazing, lots of room treatment, and finally some EQ. Here's a summary.
The theory I followed for improving the sound of music in my room is to:
1. Reduce Ambient Noise.
2. Position the speakers and listening chair as best I could.
3. Add room treatments particularly to control the frequency response (mostly to tame the bass but also to control reflections) and control the decay times (resonances). Both are important but I wonder if the time domain is actually more essential.
4. Carefully use an equaliser to tidy up, mostly in the bass region.
5. Use sophisticated measuring software to determine the best solutions - Room EQ Wizard (REW).
Ambient Noise - Triple Glazing
I live on a busy High Street about thirty metres from my flat, with buses, lorries and the rest. I have double glazing installed and this does muffle the sound but in my listening room I installed a third set of glass specifically to reduce this noise.
Firstly therefore, with no Soffit Traps in the room, and the third set of glass panels removed, I measured the ambient noise. I did this by using the measuring system on REW with the volume on my DAC turned off so no signal was emitted. I then replaced the glazing and measured noise again.
Here is the frequency response graph (FR) of these two measurements:
Red is without the glazing, green with the glazing.
You can see that, firstly, at 44Hz there is a large peak and the glazing makes no difference to that.
The addition of glazing does seem to knock the 100 - 1000Hz section down by around 6dB. Indeed the glazing makes the room sound much quieter and I hardly notice the noise except some heavy vehicles and occasional motorbike or boy racer.
Add Room Treatment
I then proceeded to add bass traps (GIK Soffit Traps) in wall-wall, and ceiling-wall corners. I measured each time I added a set but here are the results at the beginning and end.
No smoothing. Clearly a large improvement in smoothness (yes, the full treatment measure - purple - is really with no smoothing!), reduction of nulls and peaks. However all is not perfect, in particular the dip centred around 50Hz has stubbornly remained.
Again a remarkable improvement, particularly in the time for sound to die away. The lump at 40Hz I still assume is from traffic. Spectograms confirm this - the noise appears before the signal from REW is made.
Conclusions
The Soffit Traps have clearly had a major impact but they don’t complete the job. Apart from the major dip at 50Hz there are some other problems. In addition they don’t do anything for reflections off side, front and rear walls.
As it turns out, using an equaliser helped solved the dip and generally make the frequency response smoother, whilst the use of acoustic panels - GIK’s 242s and Monster panels - dealt with reflections.
EQ to Complete the Job
For 20-200Hz I used REW to measure and then generate some filters to get a flat response. I then applied these filters manually to my Behringer DEQ2496 Parametric Equaliser (PEQ). This worked extremely well. Having only ten PEQ filters meant I couldn't solve the dip at 200Hz. I could do this with the graphic equaliser (GEQ) but I like what I hear and haven't felt the need, yet.