Does a room mode exist if its frequency isn't present??

[Sammas]

This is one of those "if a tree falls in the woods and a person isn't there to hear it..." type of questions.

If you are presented with a listening position that has a mode at 150Hz , and you only play a sine tone at 118Hz does the room mode have any effect on the audio present in the room?

I had an interesting conversation about this today with a mate. I would have assumed that for room modes to have any effect on a particular frequency, that frequency or a division/multiple of it would need to be present in the room??

Thoughts?

...and yes. Slow day. I have had too much time to think. Obviously sound is much more complex then just a simple tone. The conversation came about when we were talking about acoustic ringing within a room and it being "out of key" or dissonant to the music being played.

[The Tasmanian]

I think it would be starting to exite it at 118hz - it does not have to be the exact frequency.
Mr Basset would be a good man to chime in here.

[Ben M]

Would it depend on the width of the Q of the 150hz mode?

[The Tasmanian]

But once you get down there I cannot imagine it would be so narrow a Q - from my experience close frequencies in the lower realm start to excite the standing wave - where is Mark when you need him?
Interesting subject

[Chris H]

150 divided by 2 equals 75......there is one of your answers.

150 multiplied by two is 300.........oh that's a nasty one for your second answer.

[Sammas]

150 divided by 2 equals 75......there is one of your answers.

150 multiplied by two is 300.........oh that's a nasty one for your second answer.

These are octaves above and below 150hz. They will certainly be modes in themselves when 150hz is simulated in the room... but what happens if only 118hz is present?

[Ben M]

Would it depend on the width of the Q of the 150hz mode?

But once you get down there I cannot imagine it would be so narrow a Q - from my experience close frequencies in the lower realm start to excite the standing wave - where is Mark when you need him?
Interesting subject

That's what I was meaning Chris. Not such a sharp Q in that region therefore spilling over to 118hz...or close too. Thus exciting frequencies around the mode.

[The Tasmanian]

There is another thing to consider here
Sympathetic vibration must come into play
The 2 frequencies are nearly a 5th apart - so if you played a bass guitar in A# (116.54hz) then D# is 155.
Close enough I would imagine to exite 150hz

[Drumstruck]

These are octaves above and below 150hz. They will certainly be modes in themselves when 150hz is simulated in the room... but what happens if only 118hz is present?

You just get sad because that's the relative minor

[Sammas]

There is another thing to consider here
Sympathetic vibration must come into play
The 2 frequencies are nearly a 5th apart - so if you played a bass guitar in A# (116.54hz) then D# is 155.
Close enough I would imagine to exite 150hz

Which direction is it sympathetic in though? Are we hearing 150hz being excited by 118hz?

...or is it potentially the wide Q of the 150hz centered room mode exciting the 118hz note @ 118hz?? (or de-energising it for that matter, depending if it is constructive or deconstructive interference).

Can you see how this is confusing me? Haha! Modes, while centered at certain frequencies, ultimately effect a waveforms amplitude... Not there pitch... at least that is what I would have thought. A 150hz tone coming from 118hz requires the wavelength of the waveform to physically change.

[The Tasmanian]

I'm waiting for Mark Basset to chime in - its a good topic -I've PM'ed him.
More thoughts - the more lively the space (reverberant) the more these things come into play like sympathetic vibrations and close notes partially triggering standing waves .
I know this from the Mona tunnel installation that I have running - the resonant frequency is basically D (146 hz) but if I put A (110hz) into the space then the D note will become exited (sympathetically). But this is a extremely reverberant space - not like a dead control room.
Also - I dont have to play the exact frequency of 146hz to get a build up - it can be nearly a whole tone off and still start to exite the D
As far as sympathetic direction - it would be both ways - eg: play an A - this triggers a D which re exites the A and so on

[Mark Bassett]

If you are presented with a listening position that has a mode at 150Hz, and you only play a sine tone at 118Hz does the room mode have any effect on the audio present in the room?

The chances of being in a position within a room that ONLY has a mode (be it a peak or null) at one frequency is small.

Your question depends on modal bandwidth which depends on RT60 which depends on absorption at the frequency of concern. The less reverberant the room, the wider the bandwidth. 0.5s RT60 at a frequency will give you a 4.4Hz bandwidth as a guide. If you have overlapping modal peaks (in a dead room), when one mode is excited, as they overlap, the adjacent mode will also be excited. So it all comes down to modal bandwidth.

Let's assume this room has an even RT60 of 1s - it's modal bandwidth is 2.2Hz. The 118Hz and 150Hz peaks aren't going to overlap. A room with a RT60 of 0.2s however has a modal bandwidth of 11Hz, there may be overlap there. Then there's the phase which no one has mentioned yet. You can't talk about combining modes without considering their phase.

I would have assumed that for room modes to have any effect on a particular frequency, that frequency or a division/multiple of it would need to be present in the room??

Theoretically yes, but no. If you've got two walls that are producing a standing wave at 50Hz, then they must also, in theory produce 100Hz. But this is if they are 100% reflective at these frequencies. If the room has a fundamental resonant mode that is actually present at 118Hz in the room, then 2x this frequency is not guaranteed to exist at all. The reason is that the walls may actually absorb this frequency, removing/reducing the standing wave's energy at that frequency. But 3x the fundamental may be a real problem, but unlikely. Short answer is that plotting modes in Excel will give you the impression that multiples of fundamentals are all problems. The reality is that without considering room materials, you don't even know if a mode will have sufficient energy to practically exist.

Can you see how this is confusing me? Haha! Modes, while centered at certain frequencies, ultimately effect a waveforms amplitude... Not there pitch... at least that is what I would have thought. A 150hz tone coming from 118hz requires the wavelength of the waveform to physically change.

You'd be surprised. The pitch of a standing wave can change over time (as it decays in energy), I've seen it many times on waterfall plots for example when modes fuse and start to beat.

[Hookemeister]

You'd be surprised. The pitch of a standing wave can change over time (as it decays in energy),

Yes I'd be very surprised. So does does it go sharp or flat???

I don't see how time will alter pitch... time is a constant.... A440 = 440 cycles per second.

Unless you can find a way to compress or expand time then pitch will not change over time.

Think about it... standing waves exist everywhere, not only in rooms, inside a guitar, piano, cello etc. Play a chord on a piano and hold that chord until it's completely decayed.... does it go out of tune?

I've seen it many times on waterfall plots for example when modes fuse and start to beat.

Waterfall plots are for eyes! Sound is for ears.

[Mark Bassett]

It does happen and you can often hear it. Feel free to be surprised, it remains common occurrence. Time does not alter the frequency of the mode, the frequency changes over time - different things.

[Alistair]

If I've put up room mics at the back of a big hall I can often hear a subtle detuning compared to the close mics.

[Sammas]

Cheers for the reply Mark.

Could I clarify one thing? By "excite", you are implying the generation of a new frequency entirely... not a change in the existing adjacent frequency (IE: Wavelength expanding or contracting). Energy shared from the overlapping bandwidth of a mode results in an increase in amplitude at the centre of the mode.

In a nutshell... it is no different to mechanical resonance. Instead of absorbing energy in the form of mechanical vibrations, energy is being absorbed/redistributed at frequencies @ the modal centres. Seems obvious, as we are still talking about the vibration of physical matter at the end of the day...

Please correct me if I am flying in the wrong direction.

[wez]

I know this from the Mona tunnel installation that I have running - the resonant frequency is basically D (146 hz) but if I put A (110hz) into the space then the D note will become exited (sympathetically). But this is a extremely reverberant space - not like a dead control room.

Chris, I was standing in your tunnel recently and I nearly threw up.

Although in fairness I was nursing the mother of all hangovers. The moral... don't go to MONA with a hangover. For various reasons.

OK, sorry for the OT - carry on...

[The Tasmanian]

Wez - sorry mate - its not exactly built for drinkers after a big night out!
The modulating frequencies definately go through your body - freaks some people out as they have never experienced sound going inside their bodies, people expect sound as a thing for ears... and others really love the feeling.
Anyway - there is another new commission going in on December the 10th that is my "pretty chaos" version - fine for hangovers - and not so extreme!
Back to the programming.....

[Hookemeister]

Time does not alter the frequency of the mode, the frequency changes over time - different things.

Could you gives us a quick physics lesson on how frequency changes over time.

If the sound source position is fixed and the listening position is fixed and the temperature of the room is fixed then the frequency won't change no matter how much time goes by.

Of course if we swing the monitors around then the doppler effect comes into play and we'll here a change in frequency... but last time I checked there weren't any studios using Leslie speakers for monitors!

[Hookemeister]

If I've put up room mics at the back of a big hall I can often hear a subtle detuning compared to the close mics.

I'd put this down down to a couple of possibilities.

If it's a big hall there maybe a temperature variation between the back of the hall and the temperature at the stage, which would account for a very subtle variation in pitch.

It also could be a perceived pitch change due to a difference in level (sound intensity) or a phasing effect as a result of time delay and reflections.

Hey Alistair next time your in a situation like that generate an 440Hz tone thru an amp on the stage, then record the close mic on one track and the mic at the back of the hall on another track. Play the individual tracks into a tuner and see if there's any actual difference in pitch. If you do get to do this at some time I'd be interested to know the results.

[Alistair]

Sure mate, will do. I'm not nearly smart enough to know why, but I have heard it.

[Sammas]

Time does not alter the frequency of the mode, the frequency changes over time - different things.

Could you gives us a quick physics lesson on how frequency changes over time.

If the sound source position is fixed and the listening position is fixed and the temperature of the room is fixed then the frequency won't change no matter how much time goes by.

Of course if we swing the monitors around then the doppler effect comes into play and we'll here a change in frequency... but last time I checked there weren't any studios using Leslie speakers for monitors!

I'll take a stab at it. I ain't no expert by any means, so feel free to jump in anyone to correct this ramble.

You have to remember that sound in a room exists in a 3dimensional area. Take your piano string analogy, this is only 1 dimension. If we apply it in a 3 dimensional fashion though things start to make sense.

Imagine stringing up a piano string with 6 ends (yup, 6) arranged in a 3D cross shape that represents the X, Y & Z axis of a room. Tension them all the same (to represent a constant temperature and air pressure), but vary their lengths to 1m, 1.5m & 2m. This gives us a fundamental, a fifth and an octave.

What happens if you stimulate just one string?
Is energy distributed to the other strings?
Will the other strings vibrate in unison or maintain their tuning?

Sound and a vibrating piano string are both mechanical waveforms travelling through matter. It isn't an unobtrusive phenomenon when you compare it to electromagnetic radiation like light, that will happily continue on its path in a straight line until it hits particles that stop it. Sound radiates through the air via a chain reaction of particles bashing into each other. They propagate into the 3Dimensional room just like the 6 ended piano string. The frequency changes over time not because of a stretching and shrinking wavelength, but because sound energy is distributed to all axes of the room that then realign to modal frequencies (if it is close enough in frequency).

[Hookemeister]

Take your piano string analogy, this is only 1 dimension. If we apply it in a 3 dimensional fashion though things start to make sense.

Well Nath I wasn't actually talking about a piano string, I was making the point that if you play a chord on piano and let it decay over time the frequency of the notes played will not alter.

The sound emanating from a piano is not one dimensional. Place a grand piano in an anechoic chamber or a free field where there are no boundaries to create standing waves or reflections. Get someone to play that piano and then walk around it, get under it, get on top of it.... what you'll find is that the sound is radiating out in all directions.

Your example of the 6 strings stretched out across the length, width and height of the room is a good one, and I'm guessing that you're using these to show the standing waves/modes of the room.

What happens if you stimulate just one string?
Is energy distributed to the other strings?
Will the other strings vibrate in unison or maintain their tuning?

Yes energy is radiated to the other strings but the only string that can vibrate in unison is a string that is tuned to the same frequency. All the strings will maintain their tuning as that's a function of length and tension.

The frequency changes over time not because of a stretching and shrinking wavelength, but because sound energy is distributed to all axes of the room that then realign to modal frequencies (if it is close enough in frequency).

OK you've lost me here...

Modal frequencies are a function of the rooms dimensions and they are fixed by those dimensions and room temperature.The frequencies of the piano strings are a function of length and tension of those strings. Those strings will not change their tuning no matter how close or prominent the room modes are.

If room modes changed the frequency of the instruments in the room, then you would never be able get anything in tune.

Now if one of the room modes is just below the frequency of the E string on a bass, and resonates in sympathy with it, then when the bass player mutes the string the decay of the room mode will be heard giving the listener the perception that the frequency has changed or gone flat... but it hasn't, it's just an illusion. Maybe this is what's making Mark think that frequency changes over time.

Think about it... if frequency changes over time, then anything that's sustained is going to go sharp or flat... I don't think so... well at least not in my world.

I'll do an experiment tomorrow... I'll generate tone at 220Hz then fire up my trusty tuner and walk around the room and see if it goes out of tune.

[Drumstruck]

It's more that frequency changes over distance subject to air density - amplitude will reduce over time and a change in amplitude will result in a perceived change of pitch - whereas frequency will change dependent upon medium (i.e. air) density. Altitude, temperature and humidity combine to produce this (see QNE - pilot meteorology 101) - http://en.wikipedia.org/wiki/Pressure_altitude

[a.d.a.m. baby]

It does happen and you can often hear it. Feel free to be surprised, it remains common occurrence. Time does not alter the frequency of the mode, the frequency changes over time - different things.

I wonder if this is in any way related to the phenomenon of apparent pitch change heard between the sound coming from headphones when worn as compared to when heard from a small distance away?

[Hookemeister]

It's more that frequency changes over distance subject to air density

But that's not going to happen in a controlled in environment like a studio... unless there's something very strange happening with the aircon system.

amplitude will reduce over time and a change in amplitude will result in a perceived change of pitch

Yes I agree with that but with an emphasis on "perceived change of pitch." This is a result of the way we hear. If measured, it is shown that the frequency doesn't change. It also requires a major change in amplitude for that perceived change in pitch to be noticeable.

whereas frequency will change dependent upon medium (i.e. air) density. Altitude, temperature and humidity combine to produce this

Yes, but in a studio all these things are reasonably stable, and any change in temperature and humidity would be over a longer period than the decay of a musical sound.

This does raise an interesting point Ian.

Over the years of working with session musicians, on various occasions, string players and guitarist have complained about certain studios where the tuning of their instruments starts to drift and maybe some members of the forum have experienced this situation.
I investigated this problem at one major studio, which will remain anonymous, and discovered that the thermostat in the air-conditioning system was allowing the studio to heat up over time, then it would kick in and there'd be a sudden rush of cold air into the room and within a very short period of time the temperature would drop by 7 - 8 degrees. They eventually updated the aircon system and the tuning problem went away.

So the lesson here is
1: Make sure the temperature in your studio remains constant.
2: If you're not running the aircon 24/7 then make sure you turn on the aircon at least an hour before the session so the room temperature can stabilize.
3: Try not to run the thermostat at extremes, the idea is set the temperature at a comfortable level then leave it alone. It's very tempting, on a hot day when you walk into the studio, to dial the temperature down.

Finally, nothing to do with tuning, if you're using a split system, as opposed to ducted aircon system, make sure you have a ducted fresh air supply and return, via plenums, otherwise after a few hours you'll find everyone getting tired.

But I digress... I'm still looking for an explanation of how frequency changes over time.

[Mark Bassett]

I'm still looking for an explanation of how frequency changes over time.

Room modes combine in interesting ways over time.

If a room mode is excited at 100Hz, there is no guarantee that the currently high amplitude at 100Hz will remain at 100Hz while its energy decays.

The high amount of energy in the bandwidth centred on 100Hz may cause an adjacent (dormant if you will) room mode to become excited and the energy of the 100Hz mode is transferred to the 'new' mode at say 125Hz. The pitch of the center frequency of the room mode has shifted from 100Hz to 125Hz, and there was no DeLorean required.

A room mode at 100Hz may initially exist, it may then combine with another mode slightly higher in pitch and decay at 125Hz.

Starts at 100Hz, finishes at 125Hz = pitch change of mode.

Multiple modes often combine to form a new mode, in this scenario there are 2 changes in pitch of 2 separate room modes.

Maybe this is what's making Mark think that frequency changes over time.

The center frequency of room modes can change when they combine with other modes, and shift in pitch up or down.

I don't just think room modes can change pitch, it is a well established fact that they do. Don't assume I don't know what I'm talking about because you don't understand it or don't want to believe it. Don't know why I bothered in the first place. I'm out.

[The Tasmanian]

That all makes a lot of sense, and is put quite simply and really answers the original question.
I would like to thank Mark for his input here - as he has dedicated a massive portion of his life to this field.
rather than opinions...

[Hookemeister]

Room modes combine in interesting ways over time.

Hi Mark
I’m sorry I've upset you, and I'm certainly not being disrespectful, but you haven’t answered my question:
“an explanation of how frequency changes over time” and that’s frequency (singular) and not pitch.

Pitch is not the same as frequency as it is perceptual and subjective. It is the way we hear frequency.
It has a different unit of measurement and can vary with sound pressure.

Pitch can change with a variation in amplitude. Frequency does not change with a variation in amplitude.

Usually technical discussions about room modes refer to modal frequencies and not pitch.

If a room mode is excited at 100Hz, there is no guarantee that the currently high amplitude at 100Hz will remain at 100Hz while its energy decays.
The high amount of energy in the bandwidth centred on 100Hz may cause an adjacent (dormant if you will) room mode to become excited and the energy of the 100Hz mode is transferred to the 'new' mode at say 125Hz. The pitch of the center frequency of the room mode has shifted from 100Hz to 125Hz, and there was no DeLorean required.

No… the frequency of the 100hz mode hasn’t changed or shifted. A new mode, 125Hz, has become active. You said it yourself in the previous sentence “and the energy of the 100Hz mode is transferred to the 'new' mode at say 125Hz” This is not an example of frequency changing over time.

A room mode at 100Hz may initially exist, it may then combine with another mode slightly higher in pitch and decay at 125Hz.

You are talking about 2 modes: 100Hz & 125Hz… 2 individual frequencies.

Starts at 100Hz, finishes at 125Hz = pitch change of mode.

No… just a change of mode. One mode has become dormant and the other has become active. The respective frequencies of the 2 modes have not changed.

The center frequency of room modes can change when they combine with other modes, and shift in pitch up or down.

What we hear is a perception of a change in frequency as one mode fades and another becomes active but the frequency of the initial mode has not changed.

This is no different to playing G on a piano then as it’s amplitude dissipates we play the adjacent A note.
As the first note decays the last note we hear is A.
The note G hasn’t changed in frequency, we just don’t hear it anymore.

The modes of a room are defined by the dimensions of that room, the speed of sound at a given temperature and a set of integers. The modal frequencies will not change unless we alter one or both of those parameters.

I don't just think room modes can change pitch, it is a well established fact that they do. Don't assume I don't know what I'm talking about because you don't understand it or don't want to believe it. Don't know why I bothered in the first place. I'm out.

I’m not assuming anything here. If you can explain it clearly, I’m sure I have the ability to understand it.
Please don’t expect me to believe something, which goes against my understanding of physics, without satisfactory proof.

You keep talking about pitch, which doesn’t address my question.

I just asked you to explain you're statement: “frequency changes over time”

Don't know why I bothered in the first place.

If we all had that attitude we wouldn't have much of a forum.

Looking forward to your reply.

Thanks for your time.

Greg.

[Sammas]

No… the frequency of the 100hz mode hasn’t changed or shifted. A new mode, 125Hz, has become active. You said it yourself in the previous sentence “and the energy of the 100Hz mode is transferred to the 'new' mode at say 125Hz” This is not an example of frequency changing over time.

I dunno Greg... If I were to put the sound in the room on a graph of frequency against time, the statement "the frequency changed over time" would be 100% correct. Perhaps things a heading to far towards semantics... but the statement would be correct unless you walk out of the room with just the 100Hz sound you walked in with...