In short
Perceived strength does not rise in a straight line with concentration: doubling the amount usually gives less than double the strength, and the curve differs between materials. To rate strength reliably, use a scale tied to reference samples. This turns a vague impression into something you can compare from week to week.
What you will learn
- Explain that intensity grows less than proportionally with concentration
- Build an anchored intensity scale
- Use a dilution series to see the curve
Before this lesson: Bloom: How a Fragrance Opens Up
The relationship
Psychophysics has long described sensory intensity using power functions, often called Stevens' power law: perceived intensity is roughly proportional to concentration raised to an exponent. For odors, that exponent is commonly reported as below 1, so intensity rises more slowly than concentration. The exact exponent varies by material and study, and the law is an approximation. Practically, that gives you three things.
- Steps should be ratios. That is the logic of your ratio ladders.
- Small reductions may go unnoticed. Trimming a strong line by 10% may be imperceptible. Trimming it by half may not halve the impression.
- Materials differ. Two materials equally strong at 10% may diverge at 1%.
A rough worked illustration: suppose an exponent of 0.5 applied, purely for arithmetic. Doubling concentration multiplies intensity by 20.5 = 1.41. To double the intensity, you would need four times the concentration. Real exponents will differ.
Odor value, revisited
You met odor value as concentration over threshold. It tells you how far above threshold a material may be. It does not tell you how strong that seems, because each material's intensity grows along its own curve. A material with a high odor value may still be weaker than another with a lower one. Use both ideas with care.
Building an anchored scale
Unanchored ratings drift. "3 out of 5" today may be "4" next week. Trained panels use reference samples, and you can make a simple version.
- Choose one stable, pleasant material, such as a musk-free mild one you have in quantity. Phenylethyl alcohol works.
- Make a series by weight, for example 10%, 3%, 1%, 0.3%, 0.1%.
- Smell them. Assign 1 to 5 from weakest to strongest. These are your anchors.
- Label them and store them as you do your stocks. Make fresh ones periodically.
- When rating anything, smell the anchors first, and rate against them. Write "anchored to PEA series".
Reading a rated dilution series
Rate one test material at several strengths against your anchors, and plot rating against dilution on a log scale. Invented example:
| Strength | Rating against anchors |
|---|---|
| 0.1% | 1 |
| 0.3% | 2 |
| 1% | 3 |
| 3% | 3.5 |
| 10% | 4 |
Here, at the higher strengths, a threefold increase changed the rating by only half a point. If that is real, then it suggests diminishing returns: a formula using more of this material may gain cost faster than strength. Character also matters, since you may notice it turn harsh at higher strength.
With one assessor, treat this as a rough guide. Trained panels with many replicates give firmer results.
Practical exercise: Build an anchored intensity scale
- Build your anchor series. Label it.
- Choose two test materials you know. Make dilutions at 10%, 3%, 1%, 0.3% and 0.1%.
- Rate them against the anchors, blind if a helper can code them, and in random order.
- Plot both curves on one chart, with dilution on a log scale.
- Write where they differ, and what it might mean for dosing.
You have done this well if your ratings were anchored and blind where possible, and you describe the shape of each curve rather than one number.
Common mistakes
- Comparing raw ratings across weeks. Use anchors.
- Expecting linear returns. Double the dose rarely doubles the effect.
- Letting the anchors age unnoticed. Remake them, and note when you did.
Key terms
- Anchored scale
- A rating scale tied to fixed reference samples, so that a number means the same thing from one session to the next.
- Stevens' power law
- A psychophysical relationship in which perceived intensity grows as a power of stimulus concentration. For odors the exponent is commonly below 1.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.