What is a resonant frequency, and how do you locate it.

[larryclutch]

I asked this same question over at Gearslutz, but am still in the dark. I'm hoping someone here can help me with it.

I have read several posts/articles that discuss manipulating the "resonant frequency" in various ways when mixing. Firstly, what exactly is a resonant frequency, and what are some common ways these are found and addressed when mixing? I know one article alluded that they are "bad" sounding areas, while another lead me to believe the exact opposite.

What are some different methods I could use to find the resonant frequency of say, a DI'ed bass track both by ear, and on a frequency analyzer within a DAW? I have started reading several EQ tutorials that specifically say "start by locating the resonant frequencies," but I have not a clue how to do this.

Thanks!

 

[SouthSIDE Glen]

A resonant frequency is the frequency at which an object tends to naturally want to resonate, or vibrate. Just like when you tap a tuning fork or rub a soapy finger around the rim of a crystal glass. Most instruments however are not that pure, and may have several resonant frequencies at which all or part of the body tend to resonate.

Just like having children around, resonant frequencies can be pleasant or unpleasant depending upon what you`re up to at the time and whether they are cooperating with that or not.

One way to find resonances would be to attach a transducer such as a speaker coil to various parts of the instrument body and, using a tone generator, sweeping through the frequency range until you hear a tone coming from the vibrating instrument body itself. Another less scientifically accurate method, but probably far more practical and applicable to our purposes, would be to play a recording of the instrument being played, and use the parametric sweep method to find those frequencies which stick out.

G.

 

[Waffleness]

Well I would start by questioning whether you know what the fundamental frequency is? Each note has its own fundamental which, in essence, defines that notes pitch.
Even non-tone instruments (such as drums) have a fundamental. Kick drum is about ~110Hz.

This is different to a resonant freq. A resonant is the frequency that a room, or object, oscillates at with the most 'ease'. Its defined as a standing wave - so the distance between the source and the reflection surface is an exact number of wavelengths. This means it hits a wall, say, and because its hitting exactly in phase with itself, the amplitude is greater due to superposition. This can normally be seen, and heard, as a spike on a spectrum analyser.

Now, there are various ways to remove this - acoustic treatment of the room is your best bet. On top of that EQ's and filters can be used.

You'll find a resonant will oscillate at multiples of itself. For example if you find it oscillating at 440Hz, you'll also find peaks at 880Hz, 1320Hz, etc etc. This, I believe, is why room treatment is a much better solution than EQ.

EDIT: Glen beat me to it - listen to him not me

 

[Ethan Winer]

You already got some great answers, and maybe I can add a bit more.

What are some different methods I could use to find the resonant frequency of say, a DI'ed bass track

A DI'd bass track doesn't have a resonant frequency. A solid body bass or guitar serves as a "stable platform" for the strings to vibrate against. The ideal solid body instrument is perfectly rigid, so the only frequencies you get are those in the string's own vibration with no abnormal emphasis of some harmonics but not others. Of course the string has a fundamental frequency and harmonics, and the string's motion can be considered a resonance. But that's not the same as "resonance" when talking about acoustics and recording with microphones.

To my way of thinking, a resonance is the inherent frequency (multiple frequencies actually) in a hollow body guitar, or violin, or cello, etc. Rooms also resonate. This frequency has nothing to do with the vibration rate of the strings because it's a function of the body and cavity (or room) dimensions. For example, my cello has a strong resonance around 95 Hz. This is halfway between F# and G. I know the frequency well, because I once recorded a pop tune using 37 tracks of the same cello, and I needed to notch that out with EQ to avoid having the mix sound like a muddy mess.

Finally, in the context of EQ'ing DI bass, it's important to understand that the room you listen in has huge resonances that directly affect what you hear. This is why bass traps are so important. Otherwise, you think you're EQ'ing the bass to sound even at various notes, but you're really EQ'ing out the room resonances. So the EQ'd bass will sound totally different in another room.

--Ethan

 

[mixsit]

..I have read several posts/articles that discuss manipulating the "resonant frequency" in various ways when mixing. Firstly, what exactly is a resonant frequency, and what are some common ways these are found and addressed when mixing? I know one article alluded that they are "bad" sounding areas, while another lead me to believe the exact opposite.

So, we have the resonant frequency' that is indeed the note the string' or drum' is tuned to, the resonances of the systems that created and captured that 'note, plus the overtones within the systems, and the coincidence of the frequencies due to the build up of said instruments/voices, each with the hot spots of their individual systems, combining in the mix.

We're typically finding and attenuating these hot spots' whether by eq or mix level, and sometimes by time alignment ('happy or bad phase tones in your multi-mic situations for example-
And eq' doesn't of course address a time decay issue -for example how long said 'tom (or systems) ring.
That was fun.

 

[GarrenKeith]

Resonant frequencies are usually caused by too much mids. Listen to each instrument individually and if it doesn't sound natural, then that'll more than likely cause a problem.

 

[gecko zzed]

This frequency has nothing to do with the vibration rate of the strings because it's a function of the body and cavity (or room) dimensions.

When you think 'cavity' you should also think 'mouth', because this is also a cavity that has its own resonant frequency, and this is responsible for giving each voice its own particular timbral characteristic. I think formant is the correct description here, but someone will correct me if I'm wrong.

Some singers have a very nasally sound, and to get them sounding halfway decent requires a fair bit of EQ modification.

Ethan referred to his cello with its strong resonance at about 95hz. My 12-string has a nasty bump at about 160hz, which is a nuisance because that's near a D, a kinda useful note!

One way of testing for this is to use a parametric EQ and sweep through the frequency range with it set to boost. You will usually hear the problem point when the volume really goes ballistic.

 

[mixsit]

Resonant frequencies are usually caused by too much mids..

Not to nit pick but let's fix this. The 'too much x' (insert freq here) can be caused.. by resonances.

'Body cavity, box, mouth, drum, room, etc, all external systems and each capable of piling up from one or several sources.

Then again a guy could just happen to play some notes louder on a d/i'd bass.