Technical Article: Listening Room "Modes" (Frequency Response Changes)

[Cosmik]

Psychoacoustics and room correction algorithms are far outside the scope of this thread.

This was just to show the effect, which is quite real and measurable as well as audible, to give folk a visual reference. I am not sure what you mean by "typical experimenter" but this was in no way meant to delve into complex sound fields and such. It was just meant to provide a pictorial example of comb filtering. I have had to deal with this many times, partly because I lived with dipole speakers for many years, but also in applications such as hanging choir mics to provide the best frequency response for the congregation. You can clearly hear the dips and valleys in the response when the choir is singing and the impact of moving the mics or adding treatment to minimize the effect.

I am not sure where you want to go with this but it sounds like a great topic for another thread exploring the interaction with musical or more complex sources. Like most of these threads, they are meant to be introductory and help beginners understand various concepts. More advanced readers will find them boring and very limited in scope and variables (too simplified).

Yes, sorry for maybe straying off topic. But on the other hand, the absolute, indisputable physical, measurable effects of room modes may lead some people to think that they are audibly as bad as 'advertised' - where they may just not be.

 

[DonH56]

Yes, sorry for maybe straying off topic. But on the other hand, the absolute, indisputable physical, measurable effects of room modes may lead some people to think that they are audibly as bad as 'advertised' - where they may just not be.

That is a very good point, especially since my plot is very much a worst-case scenario you will not find in the real world. Real-world reflections are lower in magnitude and much more diffuse. That said, actual (true) room modes can cause very deep nulls; I have measured 30-40 dB fairly often. SBIR and comb reflections, not so much , especially as frequency goes above the deep bass regions. So many surfaces, and as you said in music they tend to be masked by the complexity of the signal. I notice them most with solos, and again have been sensitized by all those years listening to my Maggies.

 

[Krunok]

But here's a thing: an algorithm could be designed to work back to the source even if it knew nothing about the room. Physically moving an array of two microphones around would help to, effectively, infer a model the room, and allow it to be removed from the signal with DSP. The room sound could be kept as a separate stream.

While theoretically possible it would require sooo many measurements I doubt any room EQ alghorithm would rely on such strategy.

 

[Krunok]

Yes, sorry for maybe straying off topic. But on the other hand, the absolute, indisputable physical, measurable effects of room modes may lead some people to think that they are audibly as bad as 'advertised' - where they may just not be.

The same goes for reflections - some of them are desirable.

 

[LTig]

[..]
A clue that this idea is true: record a concert with a cassette recorder from the back of the room and play it back when you get home. It will sound nothing like the concert (much more hollow, echoey) because you no longer have the complex sound fields available that are letting you separate the source from the room. It's why most recordings are made close-mic'ed, and why an experiment that applies comb filtering to the sound of a speaker is not a simulation of a real reflection.

Use binaural microphones located in your own ears and listen with headphones and you may experience a very realistic representation of the original recording venue.

Somewhere end of the 70ies I used a Sennheiser headphone as microphones (sitting on my head) and recorded the sound in my room when the door bell rang and a friend came to visit me. I had him seated where I sat when I did the recording and played it. After a minute he said: Well, this is nothing special, but why don't you open the door, the bell just rang ...

 

[Hipper]

Psychoacoustics and room correction algorithms are far outside the scope of this thread.

This was just to show the effect, which is quite real and measurable as well as audible, to give folk a visual reference. I am not sure what you mean by "typical experimenter" but this was in no way meant to delve into complex sound fields and such. It was just meant to provide a pictorial example of comb filtering. I have had to deal with this many times, partly because I lived with dipole speakers for many years, but also in applications such as hanging choir mics to provide the best frequency response for the congregation. You can clearly hear the dips and valleys in the response when the choir is singing and the impact of moving the mics or adding treatment to minimize the effect.

I am not sure where you want to go with this but it sounds like a great topic for another thread exploring the interaction with musical or more complex sources. Like most of these threads, they are meant to be introductory and help beginners understand various concepts. More advanced readers will find them boring and very limited in scope and variables (too simplified).

I don't want to be argumentative but we have already strayed off the title of the thread - 'Listening Room Modes', which I understand usually refers to lower frequencies (those below the Schroeder, or Transition, Frequency) - into the area of reflections - the behaviour of the higher frequencies. Psychoacoustics certainly plays a part in the latter and if we are going to discuss reflections I think it would be useful to also talk about how we deal with them because otherwise it might be thought that the comb filtering you describe is all there is to it.

In fact psychoacoustics - how the ear and brain interpret what we hear - is a very important part of how we listen. Early and later reflections can be distinguished and are compared by the brain with the direct sound in order to interpret the source and loudness of the signal.

Anyway, back to modes and below the transition frequency, why are these lower frequencies omni directional? In other words the lower frequencies seem to lack some if not all directionality unlike the higher frequencies. We can't tell where the source is.

 

[Krunok]

Anyway, back to modes and below the transition frequency, why are these lower frequencies omni directional? In other words the lower frequencies seem to lack some if not all directionality unlike the higher frequencies. We can't tell where the source is.

Fromt he Wiki:

"Directivity of any source corresponds to the size of the source compared to the wavelengths it is generating: The larger the source is compared to the wavelength of the sound waves, the more directional beam results."

P.S. Just to clear it out: it's not that LF are omnidirectional by itself - it hapens only because drivers that are producing them are small relative to their wavelength.

 

[DonH56]

I don't want to be argumentative but we have already strayed off the title of the thread - 'Listening Room Modes', which I understand usually refers to lower frequencies (those below the Schroeder, or Transition, Frequency) - into the area of reflections - the behaviour of the higher frequencies. Psychoacoustics certainly plays a part in the latter and if we are going to discuss reflections I think it would be useful to also talk about how we deal with them because otherwise it might be thought that the comb filtering you describe is all there is to it.

In fact psychoacoustics - how the ear and brain interpret what we hear - is a very important part of how we listen. Early and later reflections can be distinguished and are compared by the brain with the direct sound in order to interpret the source and loudness of the signal.

Anyway, back to modes and below the transition frequency, why are these lower frequencies omni directional? In other words the lower frequencies seem to lack some if not all directionality unlike the higher frequencies. We can't tell where the source is.

There are several other threads on psychoacoustics. I don't generally care about "thread wander" but the idea of the threads in the technical area was to stay more focused so later readers would be able to quickly find the relevant data. That said it is not my call.

As Krunok's linked article says, wavelengths are very long at low bass frequencies so they become omnidirectional. At 100 Hz the wavelength is about 11.27 feet, much larger than a typical woofer or subwoofer, or the space between your ears for that matter. This has been known for decades.

I'll bow out of this since it is moving into areas I do not know much about (psychoacoustics).

 

[Krunok]

At 100 Hz the wavelength is about 11.27 feet, much larger than a typical woofer or subwoofer, or the space between your ears for that matter.

Are you sure that space between the ears is problem? I recall I didn't have trouble identifying direction of some very deep sounds in open space (nature).

 

[DonH56]

Are you sure that space between the ears is problem? I recall I didn't have trouble identifying direction of some very deep sounds in open space (nature).

I don't think the space between the ears matters for such low frequencies. I thought that was mentioned earlier so I threw it in there. Nor do I know if, when we localize something very low infrequency "in the open", we are able discern the direction of the very LF sound itself, or if it it is higher-frequency content or other clues that allow us to localize say a tree falling or explosion off in the distance. Not my field (no pun intended ).

 

[Jack B]

I don't think the space between the ears matters for such low frequencies. I thought that was mentioned earlier so I threw it in there. Nor do I know if, when we localize something very low infrequency "in the open", we are able discern the direction of the very LF sound itself, or if it it is higher-frequency content or other clues that allow us to localize say a tree falling or explosion off in the distance. Not my field (no pun intended ).

This topic should encourage everyone to do a thorough reading (at least twice!) of Dr. Floyd Toole's book (Third Ed.). If it is not the end-all of this topic, it is a helluva great beginning! Crammed full of the related physics as well as perceptual data, and references.