Aerodynamic Assessments of Voice (Low-Tech Measures)

Laryngeal Function Studies (LFS) are a type of assessments used to objectively and quantitatively assess voice production. LFS involves two main components: (1) acoustic assessments of voice; and (2) aerodynamic assessments of voice. In the United States, both acoustic and aerodynamic assessments need to be completed in order to bill for LFS. If only one component is completed (acoustic or aerodynamic), then a -59 modifier has to be included with billing to indicate reduced services (reimbursable, but at a lower rate).

A variety of equipment are available which can be used to complete acoustic and aerodynamic equipment. These equipment range significantly in price, from free to > $10,000 USD.  Typically, free/low-cost versions of acoustic assessments of voice are more than sufficient for clinical and research practices. To learn about more about acoustic assessments of voice using low-cost equipment, see the "Acoustic Assessments of Voice" tutorial in the tutorials section of this website. 

In this section, we will review a few lost-cost aerodynamic assessments of voice measures that you can begin integrate into clinical practice. Low-cost aerodynamic assessments  of voice are a bit more limited in terms of what can be measured, when compared to more expensive equipment. However, current research supports the validity of several low-tech measures, which will be outlined below. One measure we will not discuss is the s/z ratio. The validity and utility of the s/z ratio is currently unclear (unless I'm unaware of any critical studies demonstrating it's importance). Therefore, in this tutorial, I will cover: vital capacity, maximum phonation time, phonation quotient, and mean estimated airflow rate.

Vital Capacity

Vital capacity involves assessing the maximum amount of air a person can exhale following maximum inhalation. This measure is discussed within the context of aerodynamic assessments of voice because it is used to ultimately calculate Phonation Quotient and Estimated Mean Airflow Rate.

The following pieces of equipment are needed to measure vital capacity in a relatively affordable manner:

To measure vital capacity using the above equipment, instruct a person to: 

Repeat the above for a total of 3 times, then record the maximum in mL of air. There is a learning curve with this, with the first and second trials often times being lower than the third. Therefore, it is important to do all three trials in order to identify the patients true maximum vital capacity.

The norms I currently use for clinical documentation are outlined below:

Maximum Phonation Time

Maximum phonation time is measured by instructing a person to inhale as much air as possible (i.e., to the top of a person's vital capacity), then sustain an /a/ at a comfortable loudness and pitch for as long as possible. They should ideally continue until they are completely out of air (i.e., at the bottom of their vital capacity). The duration, in seconds, is then recorded as their maximum phonation time. 

Maximum phonation time should exclude any time at the end of the task where the patient may transition to vocal fry. The idea is that we want to evaluate the maximum amount of time a person can sustain typical/habitual phonation.

Maximum phonation time will be longer if someone is using phonating into a facemask (e.g., if/when using the PAS system), and will be shorter if completing the task without any sort of covering over the mouth. Therefore, the norms used to determine if someone has a long vs short MPT relative to their age and sex should take into account the conditions within which the MPT was completed. 

The norms I currently use for clinical documentation are outlined below:

Phonation Quotient and Estimated Mean Airflow Rate

After having measured vital capacity (mL of air) and maximum phonation time (seconds), two additional low-tech aerodynamic measures of voice can be obtained.

Phonation Quotient (PQ)

Phonation Quotient is calculated by dividing vital capacity by maximum phonation time. For example, if a person's vital capacity is 4700 mL of air, and their maximum phonation time is 14 seconds, then the PQ = 4700/14 = 335 mL/second. Phonation Quotient is clinically relevant because several studies have identified a strong correlation between Phonation Quotient and mean airflow rate using gold standard equipment. While Phonation Quotient tends to have a strong correlation with mean airflow rate during voicing, phonation quotient shouldn't be used as a substitute for mean airflow rate during voicing (which tends to be a bit lower) . 

The norms I currently use for clinical documentation are outlined below:

Estimated Mean Airflow Rate  

Estimated mean airflow rate is a measure originally published by Rau and Beckett (1984). The purpose of this measure is to more closely approximate (true) mean airflow rate during voicing. Estimated mean airflow rate is calculated using a person's Phonation Quotient and the following formula: 

Estimated Mean Airflow Rate: 77 + .236 * PQ

For example, if a person's vital capacity is 4700 mL of air, and their maximum phonation time is 14 seconds, then their PQ is 4700/14 = 335 mL/second. Subsequently, their Estimated Mean Airflow Rate would be 77 + .236 * 335 = 156.06 mL air/sec.

Given the paucity in norms for Estimated Mean Airflow Rate (aside from a sample of ~20 healthy young adults in the original publication) I currently use norms observed for true mean airflow rate for clinical documentation. These norms include: