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Room Acoustics by Mark Kelly - Delineation. - 19-8-2017

The benefits.

Room acoustics was something that I have read about for some time but it all seemed a bit daunting and potentially expensive to implement.

When I finished my Triptych 3 way speakers I assembled some nice gear to feed them with music but there was something not right, especially over long term listening.

Fatigue set in over time and I just wasn't locking in to the groove.

I was asked to write this article because I went through an exercise to show how you can get reasonable results.

A small amount of treatment can improve things.

I'm sure there are gaps in my understanding with Room Acoustics but I think I've made a good start.

The “Before”.
 
In my listening room – which is also my office where I try to look like I am doing my job - it became more and more obvious that the bass was woolly and the treble was splashy and tiring.

Measuring my latest finished speakers using gated measurements (i.e. excluding the effects of the room) shows the Triptychs to be very smooth - I designed them that way and used drivers that would give me a good result without having resort to a complex crossover. 

My office / listening room is about the worse- case scenario.

4.1m long, 4.0wide and 3.9m high and with highly reflective surfaces backed with either brick or sandstone walls (it's a converted Bank building).  

I have a 650mm deep desk that runs around most of the room except at the point where the doors opens into the main office area. 

I have 2 windows in the room and the only absorbing surface is the floor which has carpet tiles.

There are no soft furnishing except my overweight body and a leather office chair.
Pretty bad for a listening room really!

What I did.

I bought two HOFA baby bass traps - HOFA Bass Traps Page.

I didn't get the large ones as they would simply take over the room and the babies fit under the desk running around my room's perimeter.
I then got hold of four of these - Sound Acoustic HA600-75 Hybrid Acoustic Absorber / Diffusers Page.

The beauty of these panels is that they double as either reflective or absorptive panels.

Keep the plywood or plastic curved panel in place and you have a convex reflector and remove the panel and you have a broad-spectrum absorber.

I settled on using 2 of the panels to the sides of the room to reduce the early reflections and then 2 on the wall behind my listening position to provide a diffused sound. The idea being to reduce "the room" and hear more of the speaker. 
As soon as I set them up in their respective positions, the listening experience simply improved. Bass was somewhat subjectively reduced but the over-hang and lumpiness was reduced.

Treble was tamed but imaging became sharper.

And I did feel that I was hearing the speakers more rather than the contribution of the room. 
Basically the whole thing allowed me to relax into the music more as I am guessing my subconscious mind doesn’t have to compensate for lumpy bass or slurred treble.
Some hard-core room treatment people will be thinking, you need more bass traps and panels.

I'm sure I could get more but I have to be practical here - I'm in an office, not a recording studio!
 
I have tried quite a few tweaks in my system much to the dismay of the DBT- objectivists but I also believe in measuring things where possible- that's the only way to make a set of speakers that measure decently after all. Measurements inform and guide and listening in the final arbiter in my opinion. So if you think something has improved then it should reasonably be possible to measure a change (assuming that our measurement regimes can actually measure the thing that changed!).

 
Before measurements:

I fired up my laptop that has Room EQ Wizard (REW) and got my reference microphone and mic' stand and set up the mic' facing down in a location that was the same equated to my ear height in my listening position.
Firstly, I set up my room with no treatments and measured the response of the speakers in my room.
 
Here are the response graphs. First, the full range:

This graph is smoothed somewhat (1/6th octave) and the speakers seem to measure at about 25hz-20,000hz =/- 6dB using gated (anechoic) measurements.

Things go astray at the lower frequencies in my room though due to room modes that either conspire to increase the intensity of the sound at some speakers or cancel out the sound to cause dips.

REW does not ignore the room contribution which sets it apart from the other speaker measurement software.

In most of those programs, they "gate" the sound so that the room’s response is removed after the first reflection.

This allows us to approximate a anechoic chamber.

Conversely, REW takes into consideration the primary sound and the reflections in the room after that.

Which is essential if you want to understand how the room interacts to the sound from your speakers.

 

Then the 20hz-700hz range in close-up.

The bass region is very lumpy.

If we assume 60dB in the measurement (relative values are what matters here not absolute) as the reference, we are looking at -10dB dips and +10dB peaks.

Or +/-20dB from reference. Yicks. And that's without looking at the effects of reverberation into the high frequencies. 

 

Guidelines:

 

I won’t go into great detail here as there are massive amounts of information out there for free on websites and Youtube for example.

Most of the information is fairly consistent but remember that it will most likely be given from the point of a recording studio perspective so make sure you adopt the advice accordingly.

Mixing / monitoring scenarios involve near field listening so that affects the types of treatments required.

Also the terminology needs to be adapted from the professional studio to the domestic listening environment.

 

This is an excellent resource to both give you some guidelines on what acoustic treatment is needed to get a satisfactory result.

Make sure you choose a domestic environment rather the default professional studio in the settings - HOFA Acoustics Wizard.

 

For the more engineering minded folk – the Siegfried Linkwitz Website makes good reading.

He has some very strong views on room acoustics so treat all his advice within the context of the bigger picture.

Mr Linkwitz has a particularly strong preference for dipole speakers.

 

In terms of my room, the first thing that I discovered through measurements was that placing the 2 bass traps at the back of room had the greatest effect – as close to the corners as possible.

This is totally in line with the advice that I read and reflects the physics or how room nodes work.

 

The other thing that I have read time and again is that there needs to be absorbing panels on the side walls at a point that sits perpendicular to the front face of the speakers - this is shown by the dot-dash line coming from out parallel from the speakers front faces.

 

The next topic is one of the correct method and logic to adding reflective surfaces.

There are conflicting pieces of advice on this topic.

Some people state that there should be reflective surfaces behind the speakers and absorptive behind the listener. Others state the opposite.

I’d advise using a combination of listening tests and measurements to figure out which works best and which you prefer.

In my case, I placed 2 reflective panels on the wall behind my listening position.

I found that this gave a pleasant softening of reflections and seems to make the room less obvious - that is after all the point of the whole exercise.

In future testing I will be adding panels behind the speakers – and I’ll try both reflective and absorptive panels to see which works best.

After adding the bass traps I took some measurements.

Here is the "untreated" bass region (red) versus having bass traps directly behind the speakers (blue):

 

 

 

 

Well it seems the laws of physics are still at work but the peaks and dips are now +/-7dB from reference.

There is also a slight smoothing of the bass region - a function of the reduction in the modes.

I use Roon and feed my music into a PS Audio Directstream Junior using ethernet.

Using REW, I can create a filter to reduce the peaks and even EQ up the dips slightly to flatten this out (dips are less audible though and EQ'ing out dips can tax your amplifier at low frequencies).

 

I then moved to the higher frequencies and measured with the side absorption panels in place (as well as the bass traps) and finally with the bass traps, the side absorbers and some reflective panels to the rear of the room (behind my listening position that is).

Longer RT (reverberation times) are what causes music to sound sloppy in low frequencies and splashy in the higher freq's. The smaller the RT60 the better. I believe that a RT60 of 0.5 or below is regarded as highly desirable.

The absorber / reflectors I used were these - Soundacoustics HA600 Product Page.

As I said, they are multi-functional so you can insert the panel and it becomes a reflector or remove it to turn the panel into an absorber.

I haven’t included a graph here but there was a definite rolling off of the upper midrange and treble as the higher frequency room treatment was added. It was probably about 0.5dB at 20,00khz to it might be slightly noticeable – in subjective terms it took some sting out of the upper regions that was very welcome.

 

There are a series of metrics that are used quantify the performance of a room.

Waterfalls show the rate of the fall off of sound across the spectrum (more later) and RT graphs show the amount of energy lost after designated time periods.

RT60 is for example the time that it takes for the sound to decay by 60dB in seconds.

A really good room will have an RT60 of no more than 0.6 seconds.

Here is the RT60 graph showing the “before” in red and the fully treated room in purple.

Assuming the red graph for the untreated room is correct (I'll do some more measurements as a sanity check), then the RT60 with no bass traps or treatment jumps up to 4.5 in the bass region and spikes at 3.5 at 500hz. It drops down to a more reasonable figure at 900hz-8000hz though.

The fully treated room is dramatically better. It peaks at about 0.7 at 240hz - which is a bit more than ideal but not terrible. From 500hz on though, it hovers around a very respectable 0.5 or thereabouts which is all the better when you consider that I am only using 4 strategically placed panels and 2 smallish bass traps.

 

Lastly, I did some waterfall plots that show the decay of sounds across the audible spectrum over time. A shorter falloff is better than a longer one. The Green trace – “after” shows a better decay at all frequencies and the mode at around 40-50hz seems to be significantly reduced which is satisfying.

So the objective measurements tally up quite well with the subjective listening tests. Who would have thought? 

 

EQ – Evil or Good ?

 

Some people consider EQ a great tool.

Others think it is the path of slacker’s who just think that adding some attenuation and boost will fix everything.

I personally think (I assume like many reasonable people) that it is best to be used in collaboration with acoustical treatment otherwise it will be a band aid approach that will never reach your rooms full potential.

The fact is that EQ is usually applied to the lower frequencies so it can’t do anything of value at the midrange or treble regions which can greatly affect our enjoyment of music.

The other thing is that except in the case of very sophisticated software, EQ cannot generally change the decay characteristics (i.e. time characteristics) of your room.

For example if I attenuate a peak in my bass region and the RT60 at that frequency in 1second then by adjusting the bass down, I will reduce the decay in its relative level to the other frequencies but I will not change the actual decay time.

So I may make the output flatter but I may not subjectively make the bass response any nicer.

Another issue with EQ is that in order to reduce dips from destructive node interactions, you need to use boost and at low frequencies adding boost can require large increases in the power requirements of your amplifier.

Remembering that a 3dB boost at 20hz equals a doubling of required power, theoretically it might push the amplifier into clipping if it does not have enough headroom to handle it.

At best it will slightly increase distortion in the amplifier and also increase distortion in the speaker which is now being asked to provide twice the excursion at the frequency that is being boosted.

EQ can be applied in hardware using something like a Behringer DEQ2496 which daisy chains into your digital or analogue system or via proprietary hardware that might be found in an AV amplifier under different brand names.

I prefer software solutions such as convolution filters.

You can make a convolution filter in REW and save it is a WAV or text file. Roon, Jriver and most other paid software allows you to add convolution filters into the playback and depending on the software these can be transparent in operation.

I tend not to use excessive EQ in my room for purely subjective reasons. Maybe I just like fat bass? Anyway, by using REW to set up a detailed EQ filter, here is the predicted bass response:

There is no boost applied to the EQ above.

I've only attempted to reduce peaks.

At first when you listen to the setup with this EQ in place, the bass seems to be missing.

It's a bit the same as when you add the higher frequency absorbers though.

You are hearing what the speakers would sound like if they were in a much larger room which is a more accurate reflection of how they were designed to operate.

A bit like removing grunge from your setup with a mains conditioner and feeling that things sound dull even though in reality, you are hearing less pollution and more music.

 

Lastly, the predicted response in room with the same reductions as above but now with a bit of boost (bot no more than 3dB and no lower than 45hz):

 

 

 

 

 

 

 

 

 

 

 

I accept that the above graph is still not totally flat but this isn't a big concern.

Our brain and ear have ways of filtering out anomalies in frequency responses of speakers (and rooms).

In general terms, our brain picks out broad-band issues better than narrow band (also called high Q I think) issues.

If that were not the case then we wouldn't be able to listen to any speakers in practical rooms and we know we have been listening to our room and other rooms for years that are less than optimal.

So our brain has been actively filtering out a lot of sub-optimal conditions and if these exceed certain levels then we experience fatigue. You know that feeling when you get half way through a song and you want to press skip.

Or you fiddle with the volume or (heaven forbid) the tone controls (if you have then on your gear).

Those may be signs that something in your chain or you room is not right.

At least by trying some treatment, be it as humble as my attempts here, you can start to see what the room really is doing and perhaps start to address this. Then you give your gear a fighting chance!

 

Balance:

 

One more point, applying excessive acoustical treatment and perhaps excessive EQ in order to flatten out the frequency response could conceivably end up making your room dull.

Siegfried Linkwitz takes the position that no room treatment is required – just soft furnishings, curtains and so forth.

I think that might be correct IF you have a massive listening space and lots of soft surfaces but this may not reflect the reality of the types of spaces that we have.

In small, badly proportioned and hard surfaced spaces we need to use some treatment but listening to music an anechoic chamber might seem ideal but it might just drive you nuts ;)

 

Conclusions:


Even in a bad scenario, such as my office space, a small amount of acoustical treatment when applied with some thought can offer both subjective and measurable improvements in the output of your speakers.

Room treatment does not have to be ridiculously expensive and can have a positive effect even is small applications.

I am planning on getting more reflective / absorptive panels in the near future in order to see what further results can be wrought.

Mark Kelly.