Why worry about absorbing deep bass in side reflections? Check out this link

[gremmy]

Before I pose this question, I want to make clear that I am not talking about corner mounted bass traps. I understand the need to absorb very low frequencies in the corners.

My question is with regard to the panels that we use to absorb early reflections. I've heard it said that if one can spare the room and afford it, it's good to make the panels 4 inch instead of 2, or perhaps even mount with an air gap, so as to achieve better low frequency response.

However, the wall itself will absorb quite a bit of the very low stuff (or simply allow it to pass through), which makes me question the need for the panel to do it.

Check out this link: The Sound Absorption Properties of Walls

It shows a peak absorption for a standard wall centered around 80hz, which slowly descends in absorption in a gentle slope that ends at around 500hz. This absorption range just happens to overlap the low end of a standard OC703 2 inch panel quite nicely.

So again, if the wall is reflecting a lower amount of energy back into the room between 80hz and 500hz, why go for the extra absorption at 125hz and below that comes with a deeper side panel?

Inquiring minds want to know.

[SAC]

Ignoring for a moment, the Schroeder critical point of transition, its pretty simple really.

While all boundaries have an absorptive aspect, walls, relatively speaking, are still considered reflective surfaces rather than absorbers - all things considered. (What is perhaps lacking here is a discussion of the boundaries acoustical impedance, but I can imagine that being almost as interesting to most as an economics lecture...)

The net energy content of sound is greater, the lower the frequency.

But simply make an ETC measurement.

Are the 1st order early arriving reflections of interest damped sufficiently by the naturally occurring wall surfaces relative to the direct signals?

Not likely.

Hence in some cases there is a need to add additional absorption or diffusion to spatially and or temporally modify the reflection of energy from most wall surfaces sufficient to create the response desired.

Ironically, in other respects we simultaneously wish the walls were more reflective such that some of the finite amount of energy were better preserved in the room.

But, once we go back and consider the transition point provided by the Schroeder critical frequency - the region of transition where the high energy of wavelengths larger than the boundaries behave as a pressure wave, and not as specular reflections. For that, the small absorbers will do little, as we are dealing with the modal behavior of the sound waves - the pressure regions. As soon as we reach the transitional 'break point' or critical frequency (generally around ~250 - 300 Hz) where the size of the wavelength becomes equal to and shorter than the boundary dimensions, the sound begins to act as specular reflections. Remember, the low order 'early refelctions' we deal with are specular reflections. And the lower the frequency, the higher the energy content that must be absorbed - thus the potential for the need to absorb more energy. And the easiest way to determine this is via the ETC response, which indicates the amount of total energy absorbed/reflected.

[gremmy]

Originally Posted by SAC

Ignoring for a moment, the Schroeder critical point of transition, its pretty simple really.

While all boundaries have an absorptive aspect, walls, relatively speaking, are still considered reflective surfaces rather than absorbers - all things considered. (What is perhaps lacking here is a discussion of the boundaries acoustical impedance, but I can imagine that being almost as interesting to most as an economics lecture...)

The net energy content of sound is greater, the lower the frequency.

But simply make an ETC measurement.

Are the 1st order early arriving reflections of interest damped sufficiently by the naturally occurring wall surfaces relative to the direct signals?

Not likely.

Hence in some cases there is a need to add additional absorption or diffusion to spatially and or temporally modify the reflection of energy from most wall surfaces sufficient to create the response desired.

Ironically, in other respects we simultaneously wish the walls were more reflective such that some of the finite amount of energy were better preserved in the room.

But, once we go back and consider the transition point provided by the Schroeder critical frequency - the region of transition where the high energy of wavelengths larger than the boundaries behave as a pressure wave, and not as specular reflections. For that, the small absorbers will do little, as we are dealing with the modal behavior of the sound waves - the pressure regions. As soon as we reach the transitional 'break point' or critical frequency (generally around ~250 - 300 Hz) where the size of the wavelength becomes equal to and shorter than the boundary dimensions, the sound begins to act as specular reflections. Remember, the low order 'early refelctions' we deal with are specular reflections. And the lower the frequency, the higher the energy content that must be absorbed - thus the potential for the need to absorb more energy. And the easiest way to determine this is via the ETC response, which indicates the amount of total energy absorbed/reflected.

Thanks for the response. Next time I break out REW, I'll run ETC (need to do some more reading about how to conduct that test properly within this particular tool).

[SAC]

Verify the following with the REW manual...

But, you will want to drive one speaker at a time and generate an impulse response. I believe REW uses a logarithmic swept sine wave to generate an impulse response.

Leave the mic in the identical spot for all impulse measurements. Moving it will result in results that display significant variance and will make comparisons difficult at best.

From the impulse measurement you shluld be able to select the ETC (which may actually be a 'log squared' calc) option which will then construct the ETC.
What it will show you is the energy content (gain) of each reflection as well as its arrival time.

One thing you might also want to investigate is whether there is a way to NOT have the arrival of the direct signal be automatically translated and set to zero. While this feature is very handy later on, it is useful to know the propagation delay in order to know the time it takes the direct sound to move from the speaker to the mic.

The reason this is useful is that this time is part of the travel time that must be known in order to calculate the path length/distance of travel of each of the individual reflections.

For instance. Say the direct signal (assumes to have aligned acoustic centers of the various drivers (in a coaxial configuration! ) for simplicity) requires 4.00 ms to travel from speaker to mic.

This means that the distance of travel is ~ 1.13ft/ms (the speed of sound) x 4.00 ms = 4.52 ft.

If a prominent reflection then arrives after the arrival of the direct signal (cal me if it arrives before!!! - BTW, it 'can', if the wrong driver is chosen as reference!, but I digress) at say 5.78 ms, it may be translated to 1.78 ms if the origin is equated to the direct signal arrival point and the direct signal arrival time is automatically translated to 0 ms.

Thus the actual time of travel for that reflection is 5.78 ms relative to the position of the speakers, incident boundaries and the mic. But you may only see an arrival time of 1.78 ms which is actually the arrival time relative to the arrival of the direct signal - the difference.

The importance here is the actual travel distance is 1.13 ft/ms x 5.78 ms = 6.53 ft; NOT 1.13 ft/ms x 1.78 ms = 2.01 feet!!!

The significance is easily discovered if you attempt to use the 'simple' (most basic) way to determine the reflection path by cutting a string longer than that, but marking that distance and holding one end at the acoustic center of the speaker, the other 'end' at the mic, and 'stretch' the string loop body to see where and what surface it contacts in order to determine its reflective 'point' of incidence.

Other programs allow this translation of the origin in order to quickly show the difference in arrival times relative to the direct signal arrival time, which is useful for other purposes. But they either also display both times and corresponding travel distances or allow for a toggling of the views depending upon your goal.

So, just be aware of this and do not be fooled into thinking that the reflections arrive so quickly. Some such as the arrival times of the signals of various component drivers , and diffractive sources such as cabinet edges and mounting shelves very will may - but you want this information in a form that allows you to make full use of it, depending upon your goal.


As far as the frequency response and room modes, you can do this two ways. Note, if you drive both speakers simultaneously you will be discovering what we already know - that spaced sources reproducing the same signal results in the sound physically experiencing spatial polar lobing which appears as comb filtering in the frequency domain. So nothing is gained there.

There are two primary ways to measure the room for modal response. (And I am assuming that you are not employing a 'dodec' stimulus test speaker.) (I also do not think REW supports the convolution of the cumulative spectral decay waterfall from the impulse response...but you might want to check that as well...)
So....

1. You can place a speaker in the lower corner of the room and the mic pointed into the diagonally opposite diagonal upper corner in order to get a good idea of the rooms total modal response.

2. But you may be more interested in the modal response at the seating position, and for this you will repeat the measurement much as you did for the impulse response - with the mic in the SAME position if you are concerned with the response in the actual listening position.

Done in either manner with the swept log pink noise, the S/N and resolution may be sufficient to capture a good response of a LF 'pressurized' room. If not, you may want to carefully use a pink noise signal (do not turn it up too loud!) for a measurement recording the results for say ~15 seconds. This will also tend to average out any short duration noises.

From this frequency response, you can then choose the waterfall plot which can be windowed (focused in on ) up to about 500 Hz with a linear scale.

I hope that has helped you a bit more than it has confused you...

[avare]

A simpler explanation is to discuss meeting the AES/EBU/ITU/NARAS/Tonmeister recommendation reflections within the first 15 ms be at least 10 dB lower than the initial signal for frequencies greater than 500 Hz. Absorbency figures usally published and discussed are for diffuse field absorption. This means sound coming from all directions. When sound comes directly at a porous absorber, the frequency at which the absorbency starts to diminish is approximately one octave higher than diffuse field sound. Using 4" thick 703, the diffuse filed absoprtion starts to reduce around 250 Hz. Go up an octave for estimating direct sound, and the lower limit is 500 Hz. The recommendation.

Absorbingly,
Andre

[SAC]

Andrei, thanks...as always!

But those standards are most appropriate for LARGE acoustical spaces with a truly reverberant sound field where the probability of a reflection arriving from one direction is equally probable to every other direction. The placement of absorption is a bit less critical!

Those standards are a bit more difficult to achieve in small critical listening rooms where the natural ISD is often excessively short and where the use of large porous non-frequency selective bass traps tend to eliminate much of the precious finite reflective energy. Thus a roadmap telling us what we have and the effects of the various treatments is worthwhile. that is, unless you simply Want a dead room...in which case, many roads get you there.

More difficult in a small acoustical space where one must track down the specular reflections that have finite paths. Hence the use of the ETC to identify and isolate specific problem paths while NOT eliminating the remaining desired reflective energy. And this information is helpful in effectively damping specific, but not necesarily all, early arriving energy.

Also, the ISD one creates should be ~2-5ms longer in duration than the natural ISD in a small space. Thus a bit of 'magic' need be done to accurately identify those specular reflections.

In any event, some means of identifying which reflections exist as well as their amplitude relative to the direct signal Ld within the period of the ISD are to be mitigated, as we do NOT want to simply eliminate all reflections. Especially as the value of the ISD is enhanced by its termination with a significant reflection thus 'invoking' the Henry Precedence effect and locking the locational cues to the direct signal.

[DanDan]

+1 to all of the above. To reinforce the deeper is better view, I have another reason. I refer in particular to the small room scenario. A 100mm panel mounted over a 100mm airgap is likely to have significant absorption in the modal frequency band. This is particularly true of the width and height modes, usually higher in frequency than the length. This 100mm +100 system is also entirely effective for the usual RFZ/Cloud purposes. The little extra cost involved makes it a no brainer to go thicker IMHO.
DD

[avare]

Originally Posted by SAC

Andrei, thanks...as always!

But those standards are most appropriate for LARGE acoustical spaces with a truly reverberant sound field where the probability of a reflection arriving from one direction is equally probable to every other direction. The placement of absorption is a bit less critical

You are welcome. I think you are confusing standards in the first sentence of the second paragraph quoted. Standards for testing absorption coefficients are as you described in the rest of the paragraph.

The recommendations I am refering to are for control rooms, with a nominal 100 m3 (3,500 ft3). No true reverberant field even has a chance to exist. Of course that makes it somewhat inconsistent in these recommendations when refering to reverb time when they really mean decay rate.

Discretely spacious,
Andre

[SAC]

Thanks! Yes, you are absolutely correct!

I mistakenly associated the standards with the studies resulting from Schroeder and others regarding the the ISD of concert halls and your reference to "sound coming from all directions" which I interpreted as a statistically reverberant field - which as you refreshingly noted do NOT exist in a small acoustic space. (It is so nice to see references to RT60s in small rooms decreasing!)

In any event, I still think that basic measurements provide details that are very useful - especially in accomplishing exactly what you recommended in terms of an effective ISD. And while I too can look at a room and quickly do napkin analysis based upon pattern recognition, we are often advising folks who, respectfully, may not know the difference between a dB, and absorber, and a diffuser!

I prefer one having the benefit of just a bit more real and objective information, as well as some of the tools and being able to see exactly what the effect of their actions are, is this is always a plus. And while some rooms are similar, few are as cookie cutter as some are want to make them. I don't subscribe to the school of 'just do this and don't worry why'. I rather fancy that those treating their rooms learn a bit of the why they did what they did in the process.

But then, I read labels on food too...which may explain allot - including why I don't buy much of that which I read! (Besides, the healthiest stuff is that (whole food) which doesn't require a label!)