What difference does room treatment make?

In-room measurements

In this article, the impact of acoustic treatment is demonstrated. In part 1, a video demonstrates the subjective difference with an instant switch comparing playback with and without treatment. Now in part 2, we compare the measurements of both rooms.

We can see a significant effect on frequency response in the listening position. This is an in-room farfield measurement, meaning the result is predictably ragged due to the many room effects that have been captured. This type of measurement doesn’t represent the loudspeaker performance accurately, as the speaker and room are combined together. We are primarily interested in the differences seen.

We note that the speaker has useful extension to 30 Hz in this room due to the solid double masonry construction and solid plaster walls. Below 200 Hz we see the expected peaks and dips due to room modes.

In the midrange we see a recessed region centred on 400 Hz and one in the highs around 3.5k.

Fig 8 – Untreated room far field frequency response. Each vertical division is 5 dB.

Fig 9 – Treated room far field frequency response. Each vertical division is 5 dB.

Fig 10 – Comparison of both rooms.

In the red shaded regions the treated room has more output. In black shaded regions, the untreated room has more output.

We can see the treatment has reduced the top octave considerably, but overall from 200 Hz – 7k the treatment has resolved the recessed regions and flattened the response.

The reverb plot illustrates the subjective difference to a greater extent.

Fig 11 – Reverb time plot.

This plot shows the “reverb time” of the system in the listening position, in both the untreated room (black) and the treated room (red).

It should be noted that RT60 reverb time is normally considered a measure of a large space with an omni directional loudspeaker. It is a measure normally intended to characterize the space. The measurement shown here is not a true indication of RT60. However, my experience in measuring and assessing systems and rooms is that this kind of reverb time plot has a strong correlation with subjective impressions of listening rooms.

The untreated room has a fairly high reverb time, averaging around 0.7 seconds, with a peak in the lower midrange nearly reaching one second and a marked reduction above 6 kHz. The treated room has a far more consistent reverb time overall, free from the boom around 200 Hz and the high frequencies are absorbed in with a similar proportion to everything else. The highs have been reduced further, but not as much as everything else. The average is just a little more than 0.3s. Some might consider this room to have a “dry” sound.

What do you think?

We’d like to hear what you thought about the difference between the two rooms. It’s best to listen with good quality headphones or at least near field monitors to avoid undue influence from another acoustic space.

• In looking at the frequency response, can you identify differences you hear in the video? In other words, how well does the plot match what you hear?
• In looking at the reverb time plot, do you find it easier to relate it to what you can hear?
• Have the measurements changed your view about either room?

We’ve created a discussion thread for your feedback.

About the Author

Paul Spencer is a StereoNET Technical Contributor. Paul is a long time StereoNET member, and owner of Red Spade Audio, specialising in Room Analysis and Custom Audio Design.
For more information visit http://www.redspade.com.au/audio/