One of the most important factors in putting together a listening space is the room’s reverberation time: the degree to which a sound hangs around after the loudspeaker has finished played it.
But what causes the room to extend the lifespan of a sound?
The short answer is the room’s reflective surfaces. The walls, the ceiling and the floor. A loudspeaker’s dispersion pattern will cause it to send sound forward to the listener but also to the sides, up and down. And the lower the frequency, the more omnidirectional its dispersion pattern.
The upshot is that some soundwaves will arrive immediately at our ears as direct sound but others will take a journey around the room – making split-second pit stops at one or more of the room’s reflective surfaces – to arrive late at our ears, distorted and lower in volume. (Each reflection point ‘pit-stop’ robs the soundwave of energy to render it quieter).
Now comes the wrinkle: when listening to a loudspeaker system in a room, our brain will automatically and subconsciously attempt to separate the direct sound from the indirect sound in an attempt to discard as much of the latter as possible. Our brain also helps us become accustomed to a room’s reverb over time.
How can you hear your room’s reverb without your brain running interference? A single handclap is one way: listen for the ringing or a ‘zing’ after our hands have collided. Another way is to place a smartphone in the listening position and have it record you talking from a short distance away. The longer that distance, the more of the room’s reverb will be captured by the recording. The phone’s microphone is a dumb device. It has no brain to separate direct sound from indirect and so captures everything. Listening back to the resulting video (or audio) clip through a pair of headphones will allow you to properly hear your room’s reverb.
My new Lisbon apartment’s lounge room measures 7m and 4m. Worse: it has a tiled floor — and you can clearly hear its long reverb in this video and this video.
We can also measure a room’s reverb: 1) place a calibrated microphone at the listening position; 2) connect the microphone to a computer running Room EQ Wizard (REW); 3) connect the computer via USB to the loudspeaker system; 4) have REW run a frequency sweep. The resulting RT60 graph will show us the amount of time taken by each frequency to drop by 60dB (which sits below the noise floor of most rooms).
I did exactly this with a UMIK 1 microphone, a MacBook Pro M1, a Resonessence Labs Concero HD USB-S/PDIF converter, a NAD M10 V2 and a pair of Zu Soul 6. The loudspeakers sit on the long wall. REW’s RT60 reading of the empty room can be seen in pink:
Professional acousticians generally agree that a good-sounding listening room should have a reverb time of 0.3 – 0.7 seconds in the midrange and treble, often defined as 300Hz – 4kHz. Notice how a small amount of reverb is considered a good thing. It adds some life to the music. But we can have too much life. The reverb time of my new space is a whopping ONE SECOND. That’s almost twice the recommended dose.
Would adding a sofa and a 2m x 3m rug to occupy one-third of the floor space help bring it down? The blue line in the graph above has the answer. The room’s RT60 is slightly improved by the sofa and rug above 1.5kHz but the delta is no bigger than the worsening between 800Hz and 1kHz.
Looking up the average rug’s absorption coefficient affords us a little more insight: a rug doesn’t begin to absorb more sound than it reflects until it greets frequencies above 1kHz. Think: the sound of a piccolo!
Moreover, the above RT60 deltas are no larger than those introduced by simply moving the measurement microphone forward six inches — as I did when taking an alternative look at the room’s RT60 with a different loudspeaker – Buchardt’s A500 active two-way – whilst keeping the sofa and rug in place. The listening position matters!
There’s little here to encourage the covering of more of the lounge room’s floorspace with a second (or third) rug.
Perhaps DSP-powered room correction software can give us a way out? In the next post, we’ll examine to what extent Dirac Live and Buchardt’s room correction smarts can improve reverb in the midrange and the treble.
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February 17, 2023 at 08:30PM
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Can a rug improve the ‘sound’ of a listening room? - Darko.Audio
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