15 Mar The Haas effect – revisited

Definition: The precedence, or Haas effect (Haas, 1951), although its recognition predates Haas.

Haas (1951). Helmut Haas. “The influence of a single echo on the audibility of speech,” Journal of the Audio Engineering Society, 20, 2 (1972). (This is an English translation from the German by Dr. Ingr. K.P.R. Ehrenberg of Haas’ original paper in Acustica, 1.2 (1951).

Note: [from Architectural Acoustics by Marshall Long] “Data taken by Seraphim (1961), which were reproduced by Kuttruff (1973), led to the belief that when a series of delayed reflections arrives at a listener, the so-called Haas zone, where only a single sound is perceived, could be extended in time. Figure 3.29 gives the results of Seraphim’s experiments using sounds coming only from the front in an anechoic environment. Here the threshold of perceptibility of the delayed sound is constant with delay time even when delays extend to 70 msec. Olive and Toole (1989) pointed out that this experiment, which was carried out in rather unrealistic conditions with all echoes having the same level, has been misapplied to reflections coming from many directions.”

The following is an opinion paper based on my years in the music business – having designed, consulted, and built literally hundreds of recording studios.


If you don’t know where you are going, it is highly probable that you will NEVER get ‘there’.

As with any endeavor, planning is critical to the success of that endeavor, no matter what sort it is. When we talk about recording studio design or concert hall design, we must first know WHAT we want as well as HOW to get what we want. And when it comes to buildings designed for the purpose of performance and/or recording; sound isolation, residual noise (both acoustic and electromagnetic), and room acoustics will become the major players in the FORM of those buildings.

I have seen a great many visually interesting designs, many of which actually follow the function of the space and then some that do not. The latter would be considered a ‘fail’, in my opinion. One must never forget the importance of placing priorities. A studio or performance hall can be designed with optimal acoustics and also be architecturally pleasing and inspirational. The room acoustics criteria and isolation criteria will dictate the procedure. But if you do not have design criteria, you are pretty much guaranteed failure.

Design criteria will follow a theory and theories are things that evolve as we research, test, learn, and experience. I’ve thought long and hard about what, if any, philosophical bias could skew my own research, and I’ll be damned if I can detect anything other than pure curiosity. Of course there’s always a desire to gain renown among your fellow designers & researchers, but that’s ambition, not philosophy. And anybody who fakes results to get famous, or prop up their pet ideas, is playing a very dangerous game. We do tend to fall in love with our own theories, and may be reluctant to abandon them, but that’s careerism and not philosophy. And if your theory gets sufficiently shot full of holes that it becomes untenable, hanging onto it doesn’t help your reputation.

As J. ROBERT OPPENHEIMER (1949) said, “There must be no barriers to freedom of inquiry. There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors. Our political life is also predicated on openness. We know that the only way to avoid error is to detect it and that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think, free to think what they will, freedom can never be lost, and science can never regress.”

So, I don’t think everyone will ever agree on all of these issues. LEDE vs RFZ vs NE vs Ambechoic vs … But, like all ideas, design philosophies should be continually discussed, argued, criticized, and even ridiculed. It is the only way that we can get better at it. If you don’t know that there is a problem, you are not going to fix it.

Sorry for being long-winded…

For this paper and to keep the discussion focused, I will only write about the criteria for a CONTROL / MASTERING ROOM.

Let’s review the purpose of the control / mastering room:

The purpose of a control room for sound (any format) is to provide a clear, accurate, detailed representation of the recorded material to allow critical mixing/mastering decisions to be made.

That’s it, in a nutshell.

A control room should NOT:

  1. Add any sound character.
  2. Introduce phase anomalies.
  3. Try and emulate domestic listening conditions.
  4. Have early reflections or image-enhancing diffusion/diffraction.
  5. Have room noise above NC-15 (ideal)
  6. Have inferior isolation as to permit external noise to enter and degrade the NC-15 rating.


The highest hurdle in the design of an optimal control room is low frequency control. This is the major acoustic design flaw with most control rooms. It is the ability to recognize and adjust the low frequency balance of a mix that will ultimately determine the QUALITY of the mix, which basically represents the quality of the room.

We define this control/mastering room quality as a room in which one can produce a mix that translates. Real world mix translation is the ‘holy grail’ of design goals, because if you can produce a mix that sounds good or great on any system – any where, you definitely have a hit.

Like Maghan Trainor is ‘all about that bass’, control room design is even more so. I have said many times, “If you get the bass right (controlled), you win. The rest is easy.”

Reverberation and reflections must be kept to a minimum. However, a sense of ambience and the perception that one is ‘in a room’ are important parameters for a comfortable and fatigue-free environment. This is one of the reasons that LEDE and RFZ designs are so widely used. These rooms still sound like a room. But my problem with reflection control is the small ‘sweet spot’. And if the engineer moves out of the ‘zone’, or moves about in one of these rooms, that optimal response is gone because there is far too much room sound (diffuse field) to really determine what’s going on in a particular mix.

Technically (and rationally) speaking, reflections (delayed, attenuated, or both) cause distortion of the source signal when they combine in our hearing apparatus.

We like what tubes do to the signal. Some instruments benefit from the hard floor surface. A grand piano doesn’t sound the same on carpet. – We love to sing in a tiled shower. – We like how live music sounds in a great performance space. With the exception of the performance space, the modifications made to the original signal constitute distortion.

Now I’m not saying that distortion doesn’t have a place in music, I’m just saying that distortion (of ANY kind) has no place in a critical listening environment.

The LEDE/RFZ criteria require a fairly high energy return to satisfy the ISD termination. – I see absolutely no purpose in any high energy return. Why would you want that? It will and does cause comb filtering. – Yes, the signals picked up by our hardware (ears) are processed at a very high level by our jelly-ware (brain programs), and subsequently filtered by the best processors. But this is added stress in an ‘already-stressful’ work environment. Why would you want more stress?

Comb filtering is not good – no matter what you call it, even if the brain does indeed have the ability to filter much of this distortion, it is still distortion. What if we remove the need for this brain process and allow the engineer to focus his or her energies on mixing without subconsciously compensating for the distortion heard in the room? How does room distortion help you hear more detail? How does it help you know where the reverb tail really ends? – Especially when most reverb tails will be buried -10 dB or more below the room noise / reflections?

My job, as a designer, is to create a space where you, the artist/composer/engineer/producer, can work. The spaces that I create are meant to make your job easier, faster, more accurate, and MORE FUN. Why not create a mix/mastering environment that shows all of the flaws in the program material so that the engineer is able to their job quickly, accurately, and enjoy doing it?

I believe, like Tom Hidley, that control rooms and mastering rooms should have no sound of their own and they should all be relatively similar in response. They should add absolutely nothing to the source material. This is especially important in today’s market, when work is done in many studios to complete a project. In each facility, the engineers should be able to hear exactly the same things so that there is good communication and understanding between producers and artists.

The problem with the non-environment approach, as experienced by many, is that it is unnatural. This unnatural quietness can be discomforting to the subconscious, which in turn creates fatigue. My approach and design criteria resolve these issues.

Non-Environment (NE) DESIGN:

–           From “A Study on Acoustics of Critical Audio Control Rooms” by Bruno Fazenda

This design has been introduced by Tom Hidley in the 80’s. Hidley and other designers still make use of it or a variant form based on the same principles.

This room was an attempt to simulate outdoor acoustic behavior in an indoor space, with the benefits of not having any environmental noise. This was achieved by controlling all sound within the room using very powerful absorption on all reflective surfaces. This way all reflections that would be introduced by the walls of the room were effectively removed leaving a condition very close to anechoic. Although they do not “simulate” normal domestic listening conditions, these rooms provide a very strong and precise stereo imaging when sitting at the focal point. A neutral condition is achieved given that no extra acoustic effects were added by the room boundaries. Therefore, different size control rooms using this design would generally “sound” the same.

However, such an environment can be disconcerting due to its unnaturalness and lack of reflections, which are important in providing us some information on the space that surrounds us. Some users would describe them as not being conductive to the mood for recording or mixing.

The front wall and floor were then made totally reflective to add some “naturalness” to the acoustics inside the room. This way, the speakers would radiate into an effectively anechoic room, whilst people speaking somewhere in the centre of the room would perceive some reflections from the floor and hard front wall making the ambient sound more natural. The speakers are usually flush mounted on the front wall removing the possibility of any comb filtering effects.

One drawback is the need for large physical space to accommodate large sized absorption material. This makes this type of design take up a considerable area of floor space and not very suitable for most projects where customers try to “squeeze” the control room into smaller spaces.

These rooms are designed to have very short reverberation times, which enable any reverberation or distinct delays in the original signal to be monitored.

This type of room often has a limited area where the sound is best. This is the corner of an equilateral triangle between the two speakers and the listener, for a stereo set-up.”


My design criteria is called The Balance, but not because control room design can be a balancing act, but because a balance is needed between the extremes of LEDE and NE – as well as the importance of a balanced frequency response in any critical monitoring environment.

Our solution is accomplished by positioning attenuating diffraction/diffusion arrays in a way that will keep energy in the room a bit longer WITHOUT comb-filtering the source signal. This does two things;

  1. Remove the foreign environment.
  2. Widen the sweet spot.

Most of our rooms boast an extremely large mixing zone and it has been said that the entire room is a sweet spot. The diffusion elements used do not return any sound higher in level than -20 dB and must be time delayed by at least 20 mS, relative to the source material.

The fronts of our rooms are designed to be reflective/absorptive whether or not the mains are flush-mounted. We offer two types of flush-mounting; hard and soft. Both have their advantages and disadvantages. I believe that near-field monitoring is critical to starting and balancing a mix, therefore these speakers must be able to perform optimally in our rooms along with the flush-mounts, if used.

Our revised design criteria – The Balance control/mastering room design criteria:

1) There is a low-frequency symmetrical outer shell, determined by ratio, prime numbers, and large enough to provide modal support of all the notes on a keyboard. (at least to 27.5Hz) The ideal volume being close to 120 cubic meters.

2) The low-frequency outer shell is complimented by a symmetrical inner shell which provides isolation MAM and the crossover frequency between the outer shell and the inner shell and is the square root of 2 (1.414) times the lowest modal frequency of the room.

3) Trapping and wave guides are integrated into the studio shell system and into the surface treatment to provide low-frequency damping to conform to AES, ITU, & EBU standards; RT-60 = 0.25*((Room volume)^(1/3))

4) There is an effectively anechoic path between the monitor loudspeakers and the mixer’s ears extending in time to at least 20 milliseconds after the source sound reaches the mixer’s ears.

5) The Critical Distance (Dc) of the room will be at least the size of its smallest dimension. A diffuse field is to be avoided.

6) No early early sound (EES) is present. This is sound that arrives at the mixer’s ears ahead of the direct sound traveling through the air.

7) The front surfaces of the control room can be reflective/diffusive/diffractive/absorptive as long as no diffraction or diffusion occurs from the source material. A soft or hard front baffle wall with flush-mounted speakers – similar to the non-environment approach.

8) All walls, ceiling, and work surfaces are so designed to provide a balanced and reflection free zone for all occupants of the room without measurable anomalies.

9) Diffusion / diffraction will be positioned to provide a room effect as one nears the boundaries. The diffusion/diffraction will not interfere with the source material and provide even response in all parts of the room. Diffusors are built as single units calculated to provide minimal lobbing with diffusion extending from the transition region of the room to 4kHz minimum.

10) A flat frequency response should be perceived in the largest possible area within the room.

11) Background noise levels should be low and follow published recommendations i.e.;  NC-15 to NC-25

12) The control room monitors should be able to reproduce the entire musical frequency range without any phase distortion and be able to do this at various levels from very low to very loud.

It is important to me that you are comfortable for the long hours that you will spend in one of my control rooms and that your efforts will be rewarded with a mix that translates.

– John H. Brandt

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