---
title: Application - The Outer Ear
---

# Application: The Outer Ear



## Background

The outer ear or pinna consists of little folds and various cavities in the external ear (Fig. {ref}`fig_outerear`A). Everyone has a differently-shaped pinna, and so in principle one could identify people on the basis of the folds in their earflaps. 
Importantly, these folds and cavities form an asymmetric acoustic aperture into which sound waves diffract, resonate, and reflect in a location-dependent way. Knowing how the outer ear changes the incoming sound signals helps us in understanding how we can localize sounds, and is a necessity to create virtual reality with realistic acoustic scenes (\href{https://www.youtube.com/watch?v=IUDTlvagjJA}{Barbershop Illusion}).

```{figure} images/outerear.jpg
:name: fig_outerear
A. The outer ear or pinna with its many folds. B. Simple model for the spatially-dependent filtering properties of the outer ear.
```

We could model this, and apply physical principles (Fig. {ref}`fig_outerear`B). For example, suppose there's a sound coming from a particular speaker location from down as indicated by the arrow of path 1. The sound will directly enter the ear canal, but that sound will also travel along the same path, hitting the ridges of the outer ear, bouncing through the folds and cavities and ultimately may be reflected back to the ear canal. After bouncing in a particular pattern, the sound inside the ear canal will consist of the direct input, and a slightly delayed, reflected sound. Sounds that have the same exact wave form, but are coming from a different origin, follow a different path. The sound in the ear canal will again consist of the direct and indirect reflected input, but now the delay will be slightly different. This is actually a simple model on how the pinna might function as a direction-sensitive acoustic antenna. The total sound pressure in the ear canal is given by the sum of the direct input pressure and the delayed input pressure.

```{math}
p_{sum}(t) = p_{direct}(t) + p_{indirect}(t-\Delta t)
```

Actually, modelling this system is quite hard computationally because of the complex structure of the ear (and the continuous nature of frequencies and locations). So, in practice we measure this. 

## Problem Statement

Your team is a group of auditory neuroscientists and hearing researchers studying sound localization. The overall study aims to understand how differences in ears lead to differences in the ability to localize sounds. Your task is to design an experiment to characterize the spatially dependent filtering properties of the outer ear.

## Data and Resources

You have access to our labs, which includes a setup with 120 speakers. The speakers can play any arbitrary signal. 

Two miniature microphones (Etymotic) are available to measure pressure in the ear canal.

## Task

Teams should consider:
- Hypothesis: What are you expecting to find about the outer ear's filtering properties? Is it a linear or nonlinear system?
- Experimental Design: How can you characterize  a system?
- Stimulus: Which stimulus will you present? What are the ups and downs of using that signal?
- Analysis: Given your measurements, what is the system's response?

## Reporting

Each team will present their findings in a brief presentation (slides). Please use visual and auditory aids like graphs or charts to illustrate your findings.

## Follow-up

After the presentation, we will discuss how this application session went. In the next modules, you will learn more about the Head-Related Transfer Function or Impulse Response and its role in (human) sound localization, and you will participate in an experiment, similar to the one you just designed.