The most important insight to good sound

Frequency response, Frequency response and Frequency response!!!

In real estate, the three most important things are Location, Location, and Location. You can re-build the house, update the garden, put a new roof on etc. but you can not change the most essential; the location. The view, the closeness to the forest, public schools, sport facilities or the nearest town.

The same goes with audio equipment, and here the most essential, no matter what sellers, hi-fi gurus, or your friends try to tell you. It’s not distortion, the number of watts, or even noise floor. All good sound starts and ends with a good frequency response.

This should be common knowledge, but the commercials and people making money in the audio industry, tend to confuse this universal insight. But maybe it’s also down to the fact that obtaining a good frequency response is quite physical and not that easy, as it ties very much to the room in which the audio equipment is placed.

Yesterday I got this fact confirmed again, and in ways that really showed me the importance.

My listening room is arranged as a small home theater. Quite a lot has been done to the acoustics. The ceiling is curved inward to help diffusion, likewise the front wall. There are absorbing 12″ cubes in each upper corner, there is further absorption in the front corners, and the first reflections are treated with diffusers. More than what is possible for most people to realize, as the living room mostly doubles as the listening room. Even with all this treatment I get a far from even frequency response from my fairly linear main speakers (3 way, 2×8″ vented + 4″+1″).

I have been using my surround receiver for some digital frequency correction, but yesterday I installed a MiniDSP for the main speakers instead. The MiniDSP is more versatile and gives a bit more control. I use REW, with a E-MU sound interface and a Behringer EMC8000 microphone for the measurements. The sound has been quite good and I have been satisfied overall… but it can always get better 🙂

Reading audio literature and brochures, you’ll see totally flat frequency responses, of course for CD players, amplifiers and other equipment, but also for speakers. Speakers are tested and measured in special anechoic rooms where the reflections from the walls are essentially zero. This means you’ll measure only the direct transmitted sound and not all the multitude of reflections you’ll have in a normal room. A large study and work were done by Floyd E. Toole when he worked at Harman, showing well-constructed speakers, with a relative constant directivity and a flat frequency response will be preferred when playing in a normal room link to book and article. What was shown was also that a speaker which measures mostly flat in an anechoic chamber will tend to have a steadily decreasing frequency response in a normally well-treated room.

The first measurement is the speaker in the room, at ca. 2.5 m distance at my normal listening position. Because the room is quite well treated the response from ca. 400 Hz and up is not unlike the response I got when I made the speakers many years ago at university, where I had access to a real anechoic room. It is clear that the region between ca 1 kHz and 6 kHz is running too hot. This gives the speakers a very forward sound, not bad but not really good either.

The very uneven response from ca. 40 to 300 is due to the room modes, which are present in all rooms and are caused by standing waves between the walls. Small rooms will have fewer modes than large rooms and the response will be more uneven. This causes the bass to sound bloated and boomy. Unfortunately, it is also quite difficult to really treat and reduce. When people say they like closed-boxed speakers better than vented, and that vented speakers sound boomy, I actually think it is because of the room acoustics rather than the speaker! It’s a fact that most closed-boxed will not go as deep as a similar-sized vented speaker. The closed box will therefore tend to activate less of the low modes which cause the boomy sound, but unfortunately, it also means you’ll have less bass or low end, missing out on an essential part of the music!

Using the MiniDSP, and not trying to iron out every wrinkle, I ended up with a fairly flat frequency response in the listening position. But here is one of the points in this article: it sounded awful!!!

The bass sounded weak and the mid and highs were quite pronounced and sharp.

So what to do?

Well this is where we go back to Floyd Toole and his extensive work. He presented what was called the “house curve” which is how the speaker should sound in the room to be preferred by most people.

You can find different definitions, interpretations and implementations, but in essence, you should aim for a bit increased low end of ca 6 db and the high mids and treble rolling off ca 1 db per octave starting from ca 2 kHz

Using the simple house curve:

20 6.0
50 6.0
160 0.0
2000 0.0
20000 -6.0

Giving a raise of 6 db between 20 and 50 Hz, flat between 160 Hz and 2 kHz and then rolling of 6 db between 2 and 20 kHz. This is the result:

Here with the house curve overlaid

The sound is now rich, and clear but not sharp. Quite engaging.

Maybe the bass is now a bit too much on some tracks, and I might modify it to be only a 4 or 5 db raise, but overall the sound is very pleasing.

Is this then the “correct” frequency response?

The answer is that there is not really one correct response, and the nirvana of a totally flat and correct response is not really wanted.

When I was young it was ok to have an equalizer in your system, tone controls were also ok, and all used some form of loudness control. Somehow this became a full No Go in the 80’es and 90’es, where it could not get puristic enough, with pure direct, and no frequency control at all. 2 decades of bad sound 😉

With DSPs and software like Dirac and Audyssey, it has again become ok to tune the sound. Good for us 🙂

So is equalization the end of it, and now we can all enjoy good sound?

Well, my point is, that the frequency response is the starting point, any other improvement is futile if a good frequency response is missing.

And again no matter how much DSP you put into the equation, the physical laws still stand:

  1. Treat the room acoustically as well as you are able to, and have good large speakers that can play deep bass with enough volume (a lot of the room information in the music is found in the low end)
  2. Use speakers with controlled dispersion (towards constant directivity)
  3. Do not try to raise the deep valleys (nulls) in the response, as you’ll just pour in a lot of energy without much effect

In my view room acoustics and the frequency response is at least 80% of the way to good sound. Then comes better speaker drivers with better-controlled dispersion and less distortion (I measured less than 0.25% THD at ca 90 db, which is not bad at all) (also makes you think why we try to get amplifiers with less than 0.00005 % distortion ….. !!!)

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