John
A new city and a new home mean a new listening lounge room for this hi-fi commentator and one of the most important considerations when putting together a new space for loudspeaker listening is the room’s reverberation (reverb) time. That’s the degree to which a sound hangs around after the loudspeaker is done with it. If that sounds like an echo to you, it’s not. School yourself on the differences here.
But what causes the room to extend the lifespan of a sound with reverb? For the short answer, we look at its 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. 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 robs the soundwave of energy to render it quieter – and the level of thievery is determined by the surface’s material. Wood absorbs more sound than concrete. Glass more than tiles. Dig into a wide range of materials and how their absorption coefficients vary with frequency here.
It goes further still. As a general rule, the lower the frequency, the greater the volume of material needed to absorb it. That low whomp-whomp sound found on your favorite drum ‘n’ bass cut will cut right through a concrete wall but the high hat won’t. It’s why we hear the lowest bass in the techno club parking lot. Back at home in our listening room, what isn’t absorbed by the walls, floor or ceiling is reflected back into the room.
Now comes a wrinkle: when listening to a loudspeaker system in that room, our brain will subconsciously attempt to separate the direct sound from the indirect sound in order to make better sense of the former. The more reverb in play, the more work our brain has to do. This is why conversing in a swimming pool can feel exhausting.
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. Dig into 2022’s vlog about minimalism being the enemy of good sound for even more real-world exemplification.
Not only can we hear room reverb but we can measure it: 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 there is such a thing as ‘too much life’. The reverb time in my Lisbon lounge room is a whopping one second, almost double the recommended dose.
I wondered to myself: would adding a sofa and a 2m x 3m rug to occupy one-third of the floor space help bring down that reverb time to a more reasonable level? Collective audiophile wisdom says yes but the blue line in the graph above has a far less conclusive answer. The room’s RT60 measurement is ever-so-slightly improved by the sofa and rug but only above 1.5kHz. But wait: that high-frequency-dominated improvement is no bigger than the worsening we see further down the frequency range at 800Hz – 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.
Professional acoustician Jesco Lohan’s video about using rugs and carpets to improve the sound of a home studio goes some way to explaining the above. Just before the 3-minute mark, he calls a carpet a “very badly performing high-frequency absorber” before showing us how a rug/carpet won’t reduce the floor reflection’s comb-filtering effect. He concludes, “a carpet isn’t a good strategy for treating a room if you’re serious about improving the sound of your speakers”.
Moreover, the above changes to the RT60 affected by the rug and sofa are no larger than those caused 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 smallest of changes to the listening position can impact what we hear just as much as adding a rug:
There’s little here to encourage me to cover more of the lounge room’s floorspace with a second (or third) rug. Readers are strongly encouraged to watch my video on this subject to hear how the room reverb is very much present in the lav mic feed, even with the ruga and sofa in place.
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 time in the midrange and the treble.
Today’s story is about reverb time but this graph is for anyone wondering about any frequency response changes caused by adding the rug and sofa to the empty room:
