Timbre (II) - Basics of Acoustics: Timbre (II)

In part I we gave a “physical” definition for timbre. We stressed its “plural” aspect and discussed the elements which make it up and define it. Above all we discussed the infinite number of possible combinations between these spectral components.

We’re going to further this analysis by focusing on how different types of parameters such as intensity, duration, and space influence our perception of timbre. In fact, these variables are not isolated from each other: by changing one, we effect another either in a real, tangible way (which means that it can be physically measured) or subjectively ( perceived by our ears and our brains). Perceiving timbre as a whole is the result of our brains assimilating and evaluating all psycho-acoustics parameters.

The Stevens Effect

This experience concerns simple sounds (sine waves). On a given and constant frequency, for example 5000 Hz, intensity will be increased by about 40 to 90 dB. The listener honestly thinks that the sound has become higher, about 40 Savarts, which means around a whole step (a “tempered” whole-step = 50 Savarts). This is due to a virtual frequency variation, which only effects the human ear. For frequencies below 1000 Hz, it’s the opposite. You’d have the impression that the sound gets lower when it’s intensity increases (See Fig. 1). This is called the Stevens effect.

Consequences of the Stevens Effect

A real change in the intensity of the spectrum’s components creates a subjective variation in timbre (tone).

In the case of a complex signal, in other words the majority of sounds that surround us, when there’s a progressive variation in the intensity of a sound, each component will undergo its own Stevens effect. You can see how our ears erroneously perceive the components of a sound when there’s a trumpet crescendo for example. Can we still speak about harmonic spectrums if certain frequencies make us believe that they are higher or lower than they really are? For our ears there’s in fact a sensation of a real timbral change when intensity increases.

It can therefore be deduced that all complex signals subject to a variation in intensity will be perceived as having had it’s timbre altered.
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The Role of Formants

In part I, we saw that a formant is a frequency of the spectrum which is particularly strong in terms of energy. Some wind instrument players and classical singers use this fact to gain power (or at least to give you that impression) without really producing more energy. Trying to do so would be really difficult on their lungs.

How does this illusion work? Well, it’s a little bit like the trick-question about which is heavier; a ton of feathers or a ton of lead? The instrumentalist works on his sound (timbre) to give the impression that they’re increasing intensity. Thanks to his/her technique, he/she will change the energy in the sound’s spectrum, concentrating it more specifically around 3000 Hz, there where our ears are more sensitive and react to the most weak intensities. The sound is perceived as being stronger by the listener, while on a console, we’ll just see a small change on the vu-meter.

Real Modification of Timbre and the Sensation of Frequency Variation

When one part of the spectrum is filtered, which can happen on stage because of an obstacle, you can a get the unpleasant impression that the musician is out of tune: a little sharp or a little flat. This phenomenon is due to the absorption of a frequency band of the spectrum by the obstacle. This “hole” in the timbre can be enough to make us believe that the frequency has changed.

This effect is noticed in the case of separated sounds, which is the case of music in general. It will be more flagrant if the spectrum of the instrument isn’t very, or not at all, harmonic: for example bell sounds or a xylophone.

This phenomenon doesn’t happen when there’s a continuous harmonic sound. Our memories remember the spaces between the harmonics and the in-tune aspect is kept. Only changes in timbre, due to filtering, will be perceived.

Timbre and Duration: The Role of Transients

Attack

At a later time we will deal with duration, where envelope curves will be discussed in detail. But for the present it’s difficult not to speak about the notion of the evolution of timbre in time and not at least stress the importance of the nature of the attack in the determining of the spectrum that follows.

Everyone knows that the tool used to generate a sound releases a certain number of components. A cymbal “attacked” with a drumstick or brushes won’t sound the same. Both liberate deferent elements of the spectrum.

Sustain

Only synthesizers are able deliver a signal that’s perfectly stable in its timbre for the duration of the sound. This is exactly what people don’t like about “samples” that tend to lack contrast. When dealing with acoustic instruments, timbre is constantly evolving, with a certain amount of unpredictability due to physical and human aspects of playing technique.

Release

The place in which sound is captured, as well as the listening space, effects timbre. We’ll dedicate some articles to architectural acoustics at a later time. For the moment, we’ll mention that a room’s (or place’s) acoustics (or reverberation) modifies “release” and delays or shortens the time it takes for components to disappear. Timbre is thus either “dilated” in time or “shortened”. Timbre from the same source can be altered depending on the listening space, due to the fact that attenuation isn’t linear in frequency.

Law of loss of timbre according to distance

If you hear an orchestra outside in the distance, you will first perceive the bass. Then as you get closer, you’ll hear the mids and once your close enough you’ll hear the highs. This phenomenon comes from the fact that each frequency travels at a different speed depending on the speed of sound and the wavelengths.


Depending on how far you are form the sound source, timbre is altered. The fact of getting further away is associated more with a « lack » of highs, than a decrease in the sound’s signal. A distant sound can thus be recognized by its timbre.

Distorsion

When you measure an amplifier’s efficiency, one of the things that is measured is Total Harmonic Distortion (THD). When a signal passes through a non-linear device, additional content is added at the harmonics of the original frequencies. THD is a measurement of the extent of that distortion. This value is expressed as a percentage (%) and represents the quantity of undesirable content (harmonic frequencies of the signal, noise, parasites, etc.) that are added to the signal at the device‘s output. The higher the value, the lower the quality of the device. Yet, musicians sometimes seek distortion out: many guitarists tend to love distorted sounds and distortion pedals. If you pump a sine wave into a device at a greater level than it can take, you’ll saturate the input and create a type of musical distortion: the energy that’s lost in amplitude will transform itself into harmonic components and will enrich the timbre (Fig. 4). Fig. 4 shows the result, with an incoming sine wave. If we apply a complex sound wave, the increase in richness is much greater.

The impact of timbre on the signal’s level

We’ve already seen that if we move a signal’s energy zones towards the zone to which our ears are sensitive, we get the impression that the signal has intensified. But…when you EQ you add or take away real electrical energy that has a real effect on the signal’s level. You must therefore keep an eye on the input gain level, which you might have to adjust in order to avoid clipping.

But, you could also bring a certain sound in a mix closer by simply increasing one of the EQ levels slightly (more likely in the mid-highs…).

Changes in timbre perception due to distance

As we’ve just said, you can make a signal more present in a mix by adding highs to it. This is, as we’ve said before, because high frequency harmonics die out sooner. We can then deduce that a distant source will have a more muted sound then the same source which is closer. This is one way of mixing that favors realistic parameters by altering timbre instead of levels: the foreground will be more brilliant while the background will be less brilliant. This mix’s realism will benefit from keeping a homogenous sound that would not have been achieved by just adjusting faders .