Ah yes. The flat frequency response. The holy grail for any studio user.
I’m sure you’ve heard someone say:
“Oh, this dude’s room is perfectly flat. Like, you can hear every little detail. And the bass. The bass! Whenever I check my mixes in his room, we make like 2 tweaks and boom, there it is.”
The promised land of near effortless decision making.
What you’re hearing is what’s happening.
Any decision you make translates perfectly outside your studio. No more thinking about sound, only intuition. No longer do you question your sanity every day you come into the studio.
Tracks essentially mixing themselves. Every. Single. Time.
As simple as this solution to all your worries seems to be, as elusive it remains. It’s always someone else who has it.
But you never seem to get it yourself!
And as it turns out, there are two simple reasons for this:
1. In practice, getting a flat frequency response is pretty much impossible.
As I’ll show you in this post, getting a measurement to show a flat frequency response is only possible under very particular conditions. You basically need to get rid of ALL reflections. And unfortunately, the only time you’ll get that is under lab-like conditions.
As soon as you put anything at all into that room it creates reflections and ruins that perfectly flat frequency response. The studio desk being one of the main culprits in any room.
2. A flat frequency response doesn’t actually give you what you want.
Even if you somehow managed to get a frequency response that was close to flat, it still wouldn’t allow you to suddenly completely forget about the work at hand, to bask in intuitive decision “flow” states that produce punchy, loud, emotional mixes and effortlessly translate to the outside world.
The music studio is to the sound engineer what the musical instrument is to the musician.
The better the instrument, the easier and potentially better the results will be. But it is still up the musician to make their instrument sing. Without experience, technique and skill, it won’t happen.
Let me show you why “getting a flat frequency response” actually has nothing to do with.. getting a flat frequency response.
How DO You Get A Flat Frequency Response?
Alright, I get it. You still want to know what it takes. So let’s have a look.
Under optimal conditions, price not being the deciding factor, what can be achieved?
As it turns out, it’s not that easy to answer. It’s surprisingly hard to find publicly available measurements of pro studios. (I wonder why.. 😉 )
I myself haven’t had the opportunity to work under those conditions, otherwise I would show you the results. But I will show you later what I’ve achieved so far.
One acoustician who agreed to let me show his results is Thomas Jouanjean of Northward Acoustics. If you haven’t come across his work, I highly recommend you check it out.
Here is the 20 – 200Hz amplitude frequency response smoothed to 1/24th Oct, measured at the listening position, at Sunny Side Studios in Brussels, Belgium.
Note the effect of the floor reflection creating a comb filter causing, in this case, a slight bump at 95Hz, followed by a dip at around 140Hz, and another slight bump around 180Hz.
Thomas says himself that this reflection is the last and final reflection that simply cannot be avoided, no matter how well designed a room is, carpet or no carpet. He mentions that it will be broken up, however, once the studio furniture is put in its way.
That comment is very interesting as it tells us that Thomas measured this frequency response without any furniture in the room. The room is completely empty for the measurement.
Also note the scaling of the y-axis, going from 0dB to 120dB. That means this frequency response shows an overall variation of +-3dB, (6dB peak-to-peak), and up to +-5dB (10dB peak-to-peak) at the frequencies affected by the floor bounce.
In a reference-class control room, no expenses spared, you can expect a frequency response variation of around +-3dB (6dB peak-to-peak).
Of course this measurement doesn’t tell us what is happening above 200Hz, but given soffit mounted speakers, and no other object in the room, we can expect the rest of the spectrum to have a similarly small variation.
How Does He Do It?
Thomas’ Front-To-Back (FTB) design philosophy is one of the most (if not THE most) advanced available today, and his impeccable track record and work standards allow him to guarantee results on his builds. Something that no other acoustician does that I know of. I cannot emphasize enough how remarkable this is.
Without going into too much detail, an FTB room is two rooms in one. The speakers play into a quasi free-field condition to maintain their flat frequency response. The engineer on the other hand hears their own self noises bouncing off of strategically placed diffusors. The idea is to give them a sense of the size and shape of the room to go along with what they see, and put their brain into a relaxed state to better judge what they hear.
The FTB design is the ultimate combination of acoustics and psychoacoustics, and yes, I am a fanboy. You can read more about it here.
To get this quasi anechoic/free-field condition, where ALL the energy is absorbed as it travels down the room (hence front-to-back), Thomas layers different types of resonance (pressure) traps and porous (velocity) material around the entire room. There are absolutely no reflective surfaces left for the speakers to “see” apart from the floor. All the walls and ceiling make up a huge full spectrum absorber.
That way there are absolutely no reflections left to mess with the clean, flat frequency response played by the speakers (apart from the floor bounce).
This next example is from acoustician Bogić Petrović and his MyRoom design. His work is equally inspiring and much easier to adapt to home studios as it is developed specifically for small rooms. Well worth checking out!
Here is the 20Hz – 20kHz full spectrum amplitude frequency response, measured at the listening position for each speaker individually at a Studio in Zaječar, Serbia.
Note that this frequency response is smoothed to 1/3rd Oct and the y-axis is scaled to go from 40dB to 100dB. The response shows a variation of +-2dB (4dB peak-to-peak) between about 50Hz and 6kHz.
Looking at the unsmoothed response below 200Hz below, variation increases to about +-4dB (8dB peak-to-peak):
Since the low end won’t change much when smoothing to 1/24th Oct, it is safe to assume that this room’s frequency response shows a variation of around +-4dB (8dB peak-to-peak) across the entire spectrum.
Also note the similar dip at around 100Hz, followed by a peak at 180Hz, that we saw in Thomas’ measurement. The floor bounce strikes again.
The Two Remarkable Aspects About This Measurement
The first is that this build is much simpler in design, and certainly cost far less to execute, than Thomas’ build.
Essentially it uses a layer of porous (velocity) bass trapping behind a cover of amplitude grating diffusors across the entire room. These diffusors let low frequency energy pass through openings between the slats to be absorbed behind, while the mids and highs are broken up and reflected back into the room. You can read more about exactly how it works here.
The other remarkable aspect is that Bogić took his measurement with a studio desk of his own design in the room. A testament to his skill at wielding sound. Although even he mentions that he usually measures his rooms without any furniture.
Despite these “shortcomings”, the frequency response only shows an increase in variation of +-1dB (an extra 2dB peak-to-peak) across the entire spectrum, in comparison with the Northward Acoustics room.
What Does This Mean For Us Home Studio Engineers?
- First of all, even the best of the best don’t get perfectly flat frequency responses without equalization. If you manage to get a variation of 6dB between the dips and the peaks, that’s about as good as it gets. It’s also the allowed variation under EBU Tech 3276, which is used to specify listening conditions for “reference listening rooms” and “high–quality sound control rooms”.
A “flat” frequency response simply does not exist. Accept it.
- In order to get that kind of result across the entire spectrum, the room needs to be measured in near perfect condition. ALL specular reflections need to be removed or broken up. So not even the effect of the studio desk is allowed in the measurement . (Btw, both Thomas and Bogić have recently designed their own studio desks, because they were not satisfied with what was available. You can find them here and here.)
So don’t worry if you measure your room and you see dips and peaks up to 10dB. It’s normal. In practice, they are tricky to remove from the measurements. And it doesn’t mean that your room sounds like crap.
- The closer you want to get to that ideal of a flat response, the exponentially harder it becomes. As with just about anything, in studio acoustics the 80/20 Pareto principle applies. I actually think that’s a great relief!
It means that all you have to do to get great sound in your studio is to get the fundamentals right! Anything beyond that will cost exponentially more, for a marginal improvement.
The Best I’ve been Able To Achieve
Let me put the measurements above into a more realistic home studio context for you. What does a frequency response look like in comparison, when you just focus on getting the fundamentals right?
Here are three measurements of rooms that I’ve treated. Each are full spectrum measurements of left (red) and right (green) speakers at the listening position, smoothed to 24dB/Oct, and a y-axis span of 40dB to give useful resolution. In all three cases the desk was in place (as was everything else in the room).
Notice that the main aspects messing up the frequency response are the comb filters caused by the floor reflection and the desk reflection, neither of which we can do much about.
Ignoring the worst offenders of these two effects, the overall response shows a variation of about +-8dB (16dB peak-to-peak) in the low end, which gradually decreases down to about +-4dB (8dB peak-to-peak) in the high end.
This next measurement is of Ruede Hagelstein’s studio, the treatment of which you can read about here. It features full treatment of the early reflection points, and a basic combination of pressure and velocity bass trapping.
The floor and desk reflection induced comb filters are not as obvious in this measurement, although they are still responsible for the majority of the visible variation.
Overall, the response shows a variation of about +-7dB (14dB peak-to-peak) in the low end, which decreases to about +-4dB (8dB peak-to-peak) in the high end.
Finally, here’s the measurement from my own room. It features full treatment of the early reflection points, as well as full velocity bass trapping in the available corners.
In this case, the most obvious variations are caused by a room mode dip at 38Hz, and the desk reflection induced comb filter. The main floor reflection dip doesn’t show up here, because of the desk getting in the way and the upper end of the subwoofer spectrum filling in any remaining lack of energy.
Again, the overall response shows a variation of about +-7dB (14dB peak-to-peak) in the low end, which decreases to about +-4dB (8dB peak-to-peak) in the high end.
Comparing The High End To The Home Studio Frequency Response
Now you might be thinking: “Well Jesco, that doesn’t look particularly great in comparison to the Northward and MyRoom rooms.” You are right, it doesn’t… at first glance.
Notice though that a large part of what messes up these home studio frequency responses is the floor reflection and the desk reflection, neither of which you can do much about.
But overall, the variation in the response has only doubled from around +-4dB in the pro rooms, to +-8dB in the home studio rooms! Yet the treatment effort was a fraction of what the pros did.
Plus you have to be careful with how you look at your data. Look what happens when I plot my room’s frequency response with the y-axis going from 0dB to 120dB, as in the Northward Acoustics example above.
Suddenly doesn’t look so bad anymore, does it?
The point is, getting a flat frequency response is neither entirely possible, nor necessary. It would mean trying to optimize your room under usual working conditions to a degree that is actually only possible through skillful measurement under lab-like conditions.
You Don’t Actually Want, Or Need, A Flat Frequency Response
Now there is no doubt that in a direct comparison, listening to a smooth frequency response with an extended low end is a more pleasurable experience than having a lumpy frequency response with “hot spots” and “suck outs”.
If you’ve ever successfully applied any of the available automatic speaker calibration systems to your setup, then you will know what I am talking about. The difference can be drastic, and there is plenty of research to back it up .
But there is one major issue with this fact: It doesn’t hold up if you try to apply it to music studios and the work of audio engineers.
The reason is that as an audio engineer, you are not purely passively listening to music as is the case in most studies on the subject. Instead you are interacting with what your hear. You are constantly actively reshaping your own listening experience.
And that has direct consequences on both the technical aspects of the sound reproduction system, i.e. your speakers and room, as well as the way you need to work with it if you want to reliably produce consistent results.
The Hunt For Your Prefered Tonal Balance
First, as you are shaping your music, you’ll automatically gravitate towards a tonal (frequency) balance that resembles your own personal taste.
Some people generally prefer more bass, some less. Some like more present highs and “forward” mids, some prefer a “neutral” representation.
If you’ve ever applied EQ to a speaker system for a client for tonal shaping purposes, you’ll know that a balance they like can be drastically different from what you like.
The point is that you have developed your very own preferred subjective frequency balance through years of listening to your favourite music on various speakers and headphones, at different volumes, throughout different periods of your life, etc.
That subjective frequency balance is personal to you, and you alone.
That’s why acoustician Bob Hodas says:
“I have yet to find an engineer or studio owner who actually wanted a “flat” room. Experience shows that a flat room has no personality and is no fun to work in. Equally important, working in a flat room does not necessarily ensure a recording that sounds good elsewhere.” – Source
The problem is that if the frequency response of your system does not mirror your personal taste, you’ll constantly be over-compensating for it.
For example, let’s say you personally prefer more low end in your balance than the frequency response of your system is set to have. Now as you set out to balance your mix, you’ll still follow your inherent taste. You’ll set the bass to where it feels right to you.
But in fact, you’ve now put too much low end in the mix! What you should have done instead is balanced according to the frequency response of your system, even though it would have felt as though you aren’t putting enough low end in.
The result is a mix that sounds “boomy”, “muddy” or “dark”, when you listen to it outside of your studio.
If you work from a studio whith a sound that you consider balanced, but you are still experiencing this kind of consistent imbalance in your mixes, then now you know why. The answer is to “tone” your speaker system to better match your own personal taste, no matter how “true” it already is. The shelving filters on the back of your speakers work wonders for this purpose.
Taste: A Magic Bullet in Disguise
Unfortunately, you’re still not done though. And this next point is what really makes all the difference.
The problem is that your taste isn’t actually a fixed reference point.
The reason is your extremely short auditory memory (aka echoic memory), which only lasts a few seconds.
In terms of remembering what you just heard, you are a bit like a fish in a glass bowl. You only remember the last few seconds.
Your momentary taste constantly adapts to what you just heard, making it your new temporary reality.
Taste is like a kite in the wind, tethered to a spot on the ground that represents your overall personal tonal preference. The kite gets blown left and right, up and down, constantly at the mercy of winds that are made up of the last few sounds that you heard.
And so, as you’re mixing your record, pushing around levels, EQing, soloing, turning up the volume, then turning it back down, your taste constantly drifts around, never really stopping to let you decide: “Ok, THIS is how I like it.”
I’m sure you know the feeling. It’s that moment when you suddenly realize: “Man, I really have no idea what I’m hearing. Is this too much or not enough..?”
Or maybe you’ve been in the situation where you mix a track one day, leaving the studio at night thinking that “this shit is the bomb. Best. Mix. Ever.”, only to come back the next day and to seriously question your sanity?
The whole mix sounds dull, the snare is WAY too loud, you have no idea why you put that delay on there, and the vocals are so harsh they are making your ears bleed?
Welcome to the world of “taste drift”.
Without some sort of auditory anchor, your point of reference constantly moved as you were mixing the record, leading you down a path that sounded great in the moment, but ultimately had little foundation in reality.
The answer of course is consistent and meticulous referencing, to actively nudge your taste back into the right direction and keep it from drifting off.
But the bigger point is that this “taste drift” makes any detailed tuning of your system’s frequency response largely irrelevant.
It’s a far stronger and immediate effect, and one way or the other, you have to deal with it. The great thing is: As you do, you inherently compensate for any imbalances your room’s frequency response has!
This is because as you listen to references to nudge your taste back into position, you’ll automatically include the room’s frequency response and tonal balance. You are listening to the reference with that frequency response imprinted on it after all!
And therein lies the advantage that you, as an audio engineer, have over the passive listener.
It’s not just that you can compensate for any imbalances in your rooms frequency response, you actually have to every time you sit down to work. There simply isn’t any other way.
And not because of the how the room sounds. But because of “taste drift” and the fact that you constantly have to work for a fixed reference point to be able to judge what you hear.
Getting To The Promised Land of the “Flat Frequency Response”
Of course, no matter how good you become at referencing and keeping your taste in line, it will not compensate for a serious lack of information. You cannot judge what you cannot hear. If the frequency response at your listening position is simply too crooked to start off with, no skill in the world can make up for it.
That’s why the basic requirement for your home studio is this:
Your frequency response should be balanced to match your taste, and nothing should be drastically missing.
Once you’ve got that, you’re good to go. The rest is up to you and navigating the choppy waters of “taste drift”. Interestingly, looking at the big picture, your room’s frequency response suddenly becomes a non-issue.
You’ll then realize that the really important acoustic aspects of your room are less about frequency, and much more about time.
Because the way that reflections mess with your perception of the sound stage and dynamics, and the way that resonances and excessive reverb cause masking, is not something you can cure with skill and technique.
“Contrary to popular belief the big problem with bass in hifi is not lumpy bass, standing waves, room modes, hot spots and suckouts. The big problem is sound masking.” – Art Noxon
And solving those issues, more than anything, will help you get what you want. A sound that you can trust, a sound that lets you reliably make decisions, one mix after the other.
Sure, you won’t be able to say: “My room is perfectly flat”, but then why would you want that anyway.
Instead you’ll be able to say: “I know and trust my room. I can hear every little detail. And I know that the decisions I make translate perfectly and exactly how I expect them to.”
 Olive Sean E., Jackson J., Devantier A., Hunt D., Hess S.M., ”The Subjective and Objective Evaluation of Room Correction Products,” 127rd Convention, Audio Eng. Soc., Preprint 7960, (2009)
 Toole, F.E., 2008. “Sound Reproduction: Loudspeakers and Rooms”. Focal Press. pp. 518
 EBU document Tech. 3282: “Listening conditions for the assessment of sound programme material: monophonic and two–channel stereophonic”