I was reading articles on the preparation of shellac-based composites when I encountered the following statement (in ref. 1):

Wood flour and fibres are interesting because of their low cost, good specific strength, low density per unit volume, along with their renewable and degradable features. The drawbacks of natural organic fillers include poor adhesion between the filler and the polymer matrix, and poor thermal and water resistance (Saba et al. 2014).

Note the same phrase is used in the reference cited in the passage quoted above (Saba et al. 2014) but then the trail goes dead.

Is "density per unit volume" a typo or other error that's been propagated during citation or a real concept of concern for material scientists? If it is, what is the meaning of this concept?


  1. Obradovic et al. (2017). “Cellulose-shellac composites,”BioResources12(1), 1943-1959.
  • 2
    $\begingroup$ I suppose it is a common error, when a writer thinks about 2 related things or sentence variants, ending up somehow with their hybrid, that does not make sense. I do such errors quite often.:-) $\endgroup$
    – Poutnik
    Commented Sep 30, 2019 at 10:35
  • 5
    $\begingroup$ Unless it is a measure of density variability across the bulk object volume. $\endgroup$
    – Poutnik
    Commented Sep 30, 2019 at 10:37
  • $\begingroup$ @Poutnik I don't know either way. I make such mistakes myself often enough, but this article is very clearly written otherwise, so it makes me wonder... $\endgroup$
    – Buck Thorn
    Commented Sep 30, 2019 at 11:58
  • $\begingroup$ If you can, you should call the vendor and ask them to connect you with a technical support person who can answer this question. I assume the vendor literature is correct and ask them to clarify. Vendor literature is not always written to be easily understood by people outside their market. $\endgroup$
    – J. Ari
    Commented Sep 30, 2019 at 12:10
  • 1
    $\begingroup$ @J.Ari Thanks? The article I reference is academic, but I suppose I could contact one of the authors. $\endgroup$
    – Buck Thorn
    Commented Sep 30, 2019 at 13:51

1 Answer 1


Here, "density per unit volume" implicitly indicates that the densities of the substances being compared have been normalized.

It is redundant but safe - it rules out the possibility of having compared the density of one pound of something with that of a kilogram of another, for example.

The authors probably should have used the phrase "specific density" instead, although, as seen below, "specific density" is equated with "density."

From the Grundfos company website:

The specific density of a material or liquid is the term used to describe the unit mass of a material or liquid.

The technical term for specific density, also referred to as density, is $\rho$ and is measured in $\mathrm{kg\cdot m^{-3}}$.

The specific heat capacity is dependent on the temperature of the medium. The specific density of water in a heating system at a temperature of between 20–90$^\circ$C is 1,000 $\mathrm{kg\cdot m^{-3}}$.

In the case above, specific density appears to always reference the mass (in kilograms) of one cubic meter of a substance (here, water in a certain temperature range).

In the literature, the phrase "density per unit volume" appears to be used when describing bulk properties of materials which consist of a wide range of particle sizes (as in your case, wood flour and fiber).

I note that the widely-used phrase "energy density per unit volume" and other variants such as "energy density of states per unit volume" are not included in my explanation here, but do apply to the field of materials science and many others when describing systems of interest.

See here, in reference to different particle sizes of wood contributing to "density per unit volume" when looking at bulk properties of adsorbates; and here for dry-matter "density per unit volume" of forest biomass.

  • 1
    $\begingroup$ Have you seen usage of this specific term "density per unit volume" anywhere else? Any references? Your explanation is reasonable otherwise. $\endgroup$
    – Buck Thorn
    Commented Sep 30, 2019 at 19:46
  • $\begingroup$ It is a kind of average density, is this that you mean? Still I don't see why it should be called something else that simply density. I am not convinced, but it is just an opinion. I see density per unit volume as redundant while in some cases, I see the use proposed by @Buck Thorn as common, though mass density require a very special contest to be meaningfull, eg the amount of mass ablate off a thin layer on a surface. $\endgroup$
    – Alchimista
    Commented Oct 1, 2019 at 11:53
  • 2
    $\begingroup$ Some years later.... I believe a common alternative term is "bulk density": en.wikipedia.org/wiki/Bulk_density You correctly refer to bulk properties in your post. $\endgroup$
    – Buck Thorn
    Commented Jun 25, 2023 at 9:11
  • $\begingroup$ I'm unconvinced. If "density per unit volume" has some special meaning, that is different from "density" than what is it? Density is density; it definition is mass per unit volume. And what units you measure it in is irrelevant. The density of a kilo of lead is the same as the density of a pound of lead if measured in the same units. $\endgroup$
    – matt_black
    Commented Jun 25, 2023 at 10:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.