3D Printing Microphone and Loudspeaker Holders for the Interferometer

When Better Apparatus Makes Better Science

Some physics experiments are difficult not because the underlying idea is especially complicated, but because the equipment makes the effect difficult to see.

Wave interference is a good example.

The theory can be stated quite simply: when two waves meet, they combine. In some places they reinforce one another, while in other places they partially or completely cancel. However, demonstrating this clearly with sound waves requires considerably more care than writing the explanation on a whiteboard.

The loudspeakers must remain in the correct positions. The microphone must be held at a consistent height and angle. Components must not move accidentally when students adjust the apparatus. Cables must not pull equipment out of alignment, and the arrangement must be sufficiently repeatable for different students to obtain similar results.

This is why we have been designing and 3D printing purpose-made holders for the microphones and loudspeakers used with our interferometer at Philip M Russell Ltd.

Good alignment can turn a frustrating experiment into one in which students can clearly detect the pattern, understand what is happening and connect their measurements to the physics.

What Is an Interferometer?

An interferometer is an instrument used to investigate what happens when two or more waves overlap.

Interferometers are most often associated with light. In an optical interferometer, a beam of light may be divided into two paths and then recombined. Tiny differences in the distance travelled by the two beams produce a pattern of bright and dark regions.

The same basic principle can be investigated using sound.

In a classroom sound-interference experiment, two loudspeakers can be connected to the same signal generator so that they produce sound waves of the same frequency. Because both speakers are receiving the same signal, their waves have a stable relationship to one another.

A microphone is then used to detect the sound at different positions.

At some locations, the waves from the two loudspeakers arrive in step. Their compressions arrive together and their rarefactions arrive together. The waves reinforce one another, producing a larger signal.

This is called constructive interference.

At other locations, the waves arrive out of step. A compression from one source may meet a rarefaction from the other. The waves then reduce one another, producing a much smaller signal.

This is called destructive interference.

By moving the microphone through the sound field, students can detect alternating regions of greater and smaller amplitude. These regions form an interference pattern.

Path Difference and Interference

The pattern occurs because the microphone is not always the same distance from both loudspeakers.

At one position, the microphone may be almost equally distant from the two sources. At another position, the sound from one loudspeaker may have travelled farther than the sound from the other.

This difference in distance is called the path difference.

Constructive interference occurs when the path difference is a whole number of wavelengths:

Path difference = nλ

where:

• n is 0, 1, 2, 3 and so on

• λ is the wavelength of the sound

Destructive interference occurs when the path difference is an odd number of half-wavelengths:

Path difference = (n + ½)λ

Students may be able to quote these conditions, but that does not necessarily mean that they understand them.

A practical demonstration gives those equations a physical meaning. Students can move the microphone, observe the signal increasing and decreasing, and recognise that changing position changes the path difference.

The mathematics is no longer simply something to remember for an examination. It becomes a description of something they have actually observed.

Why Sound Interference Can Be Difficult to Demonstrate

Sound-interference experiments can be surprisingly demanding.

Unlike a diagram in a textbook, a real classroom or laboratory contains walls, tables, cupboards, equipment and people. Sound reflects from these surfaces, creating additional waves that can interfere with the intended pattern.

The two loudspeakers may not produce exactly the same output. One may be angled slightly differently from the other. A microphone may point towards one speaker rather than remaining equally aligned with both.

Even a small movement can affect the result.

Students may also adjust the apparatus while attempting to take measurements. A loudspeaker can be knocked, a microphone stand can rotate or a cable can drag a sensor away from its intended position.

The result may still contain interference, but the pattern becomes much harder to interpret.

This can create an unfortunate teaching problem. The experiment intended to clarify the theory may instead convince students that interference is unpredictable.

The physics is not the problem. The experimental arrangement is.

The Importance of Holding the Loudspeakers Securely

For a useful demonstration, both loudspeakers need to remain stable.

Their separation should be known and should not change during the experiment. Their sound-producing surfaces should face in the intended directions, and ideally their centres should be at the same height.

A general-purpose clamp can sometimes hold a loudspeaker, but it may grip the casing awkwardly, obstruct part of the speaker or allow the unit to rotate.

There is also a risk of overtightening a clamp and damaging the loudspeaker enclosure.

A purpose-designed 3D-printed holder can support the loudspeaker at several carefully chosen points. It can be shaped to match the dimensions of the particular unit and can include:

• A stable base

• A cradle fitted to the loudspeaker casing

• A fixed centre height

• A controlled angle

• Cable clearance

• Attachment points for a rail or laboratory stand

• Rounded edges to make it safer for students to handle

The holder should secure the loudspeaker without covering its cone or interfering with the production of sound.

Once both speakers are mounted in matching holders, it becomes much easier to place them accurately and reproduce the same arrangement in future lessons.

Designing a Better Microphone Holder

The microphone presents a different set of problems.

It may need to move through the interference pattern while remaining at a constant height. Its angle should not change as it is repositioned, and students should be able to move it without holding the microphone directly.

Holding the microphone by hand is rarely satisfactory.

The student’s hand and body can affect the sound field. The microphone will move slightly while a reading is being taken, and its height may change from one measurement to the next.

A 3D-printed microphone holder can solve these problems.

The design can include a close-fitting clip or cradle that supports the microphone without pressing against sensitive controls. The holder can then be attached to a sliding carriage, measuring track or conventional retort stand.

A good microphone holder should allow the microphone to be:

• Positioned at the same height as the centres of the loudspeakers

• Moved smoothly along a measured line

• Kept at a constant angle

• Removed easily when needed

• Replaced in exactly the same position

• Protected from being dropped or knocked

Cable management is also important. A microphone cable that hangs loosely can pull on the sensor and change its position. A small guide or strain-relief feature can keep the cable under control without gripping it tightly.

From Measurement to 3D Model

The first stage of the design process is careful measurement.

The loudspeaker and microphone dimensions need to be checked with callipers. It is important to identify where the equipment can safely be supported and where the holder must avoid switches, connectors, sound openings or moving parts.

The holder can then be designed using computer-aided design software.

This involves more than drawing a box around the component. The design needs to consider:

• The tolerances of the 3D printer

• The thickness and strength of the printed walls

• The direction in which the part will be printed

• The stresses placed on clips and joints

• Whether the component needs to slide in or snap into position

• How easily the apparatus can be assembled by students

• Whether the design can be printed without excessive support material

The first printed version is rarely perfect.

A clip may be slightly too tight. A base may need to be wider. A mounting hole may be in the wrong position, or a cable guide may need more clearance.

That is not a failure. It is part of the engineering process.

Designing scientific apparatus usually involves producing a prototype, testing it, identifying weaknesses and improving the next version.

Prototyping, Testing and Improving

Once the first holders have been printed, they can be tested with the actual equipment.

Several questions need to be answered.

Does the loudspeaker fit securely without being difficult to remove? Does the microphone remain stable when the carriage is moved? Are the sound-producing surfaces obstructed in any way? Can students adjust the equipment without loosening the holders?

The complete experiment must then be tested.

The loudspeakers can be connected to a signal generator and the microphone linked to an oscilloscope, computer interface or data-logging system. The microphone can be moved through the sound field while the detected amplitude is recorded.

If the design is working well, the maxima and minima should be easier to locate and repeat.

It may then become clear that the holder needs another small improvement. Perhaps a reference mark is needed so that the microphone position can be read more accurately. The base may benefit from a groove that fits a particular track, or the loudspeaker holders may need an alignment guide.

One of the major advantages of 3D printing is that these alterations can be made to the digital model and a revised part can be produced without starting the entire manufacturing process again.

Improving Repeatability

Repeatability is one of the most important reasons for producing dedicated holders.

In a scientific investigation, students should be able to repeat a measurement under the same conditions and obtain a similar result.

If the loudspeakers move between readings, or if the microphone angle changes, it becomes difficult to know whether differences in the results are caused by the physics or by the apparatus.

Stable holders allow the main variables to be controlled.

The speaker separation remains fixed. The microphone height remains constant. The direction of the components is preserved, and the apparatus can be returned to a known starting position.

This makes it easier for students to investigate questions such as:

• How does changing the sound frequency alter the spacing of the interference pattern?

• What happens when the loudspeakers are moved farther apart?

• How can the wavelength be estimated from the positions of maxima and minima?

• Does the measured wavelength agree with the value calculated using wave speed ÷ frequency?

• How does the detected amplitude change as the microphone moves through the pattern?

The experiment can therefore move beyond a simple demonstration and become a meaningful quantitative investigation.

Making the Pattern Easier for Students to See

Sound waves are invisible, which creates an additional teaching challenge.

Students can see water waves in a ripple tank and can observe light and dark regions in an optical interference pattern. They cannot directly see compressions and rarefactions moving through the air.

The microphone and data-logging system act as a way of making the invisible pattern visible.

As the microphone is moved, the changing amplitude can be shown as a trace, a graph or a numerical reading. Students can mark the positions of maxima and minima and relate these positions to the geometry of the apparatus.

Well-designed holders make this process much clearer because the microphone can be moved in controlled steps.

Instead of watching a fluctuating reading while somebody waves a microphone through the air, students can collect an organised set of measurements.

Position can be plotted against amplitude. The interference pattern begins to appear on the graph, giving students a visual representation of the sound field.

Adapting the Apparatus for Different Students

Purpose-made apparatus is especially valuable because it can be designed around the students who will use it.

A holder can include larger adjustment handles for students who find small screws difficult to operate. Clear reference marks can help students position the microphone correctly. Components can be colour coded or labelled to show where each part belongs.

The apparatus can also be arranged so that students spend less time struggling with clamps and more time thinking about the science.

This does not mean removing all practical challenge. Students still need to make decisions, measure carefully and evaluate their results.

However, unnecessary mechanical difficulty should not obscure the concept being taught.

A student investigating wave interference should be concentrating on frequency, wavelength, phase and path difference—not trying to stop a loudspeaker from falling over.

3D Printing as Part of Laboratory R&D

The microphone and loudspeaker holders are part of a wider approach to laboratory research and development at Philip M Russell Ltd.

Not every useful piece of educational apparatus can be purchased from a catalogue. Commercial equipment may be too expensive, designed for a different experiment or insufficiently flexible for a particular group of students.

3D printing allows us to create parts that match the equipment we already own and the experiments we want to teach.

It also brings several disciplines together:

• Physics identifies the measurements that need to be made.

• Engineering determines how the components should be supported.

• Computer-aided design turns the idea into a model.

• Materials science influences the choice of printing material and structure.

• Testing reveals how the design performs in practice.

• Teaching experience determines whether the finished apparatus actually helps students learn.

This is one of the most satisfying aspects of developing apparatus in-house. A small printed component can improve not only the appearance of an experiment but also its reliability, safety and educational value.

Personal Reflections: Small Improvements Can Make a Large Difference

It is easy to focus on the most impressive parts of a laboratory—the signal generators, oscilloscopes, sensors and computers.

However, the success of an experiment often depends on something much simpler.

A holder that keeps a microphone at the correct height may not look as sophisticated as the electronic equipment connected to it, but it can make the difference between a clear result and a confusing collection of readings.

Over many years of teaching physics, I have found that students are much more likely to understand a difficult idea when the apparatus behaves consistently.

When the experiment works, discussion naturally moves towards the science:

Why did the signal become smaller here?

Why are the minima separated by this distance?

What would happen if we increased the frequency?

When the apparatus is unstable, the discussion instead becomes:

Has the speaker moved?

Is the microphone pointing the right way?

Why is the reading different from the previous one?

Good apparatus does not replace good teaching, but it creates the conditions in which good teaching becomes much more effective.

Conclusion: Designing Apparatus Around the Learning

The purpose of 3D printing microphone and loudspeaker holders is not simply to make the interferometer look neat.

The holders improve alignment, stability and repeatability. They make the sound-interference pattern easier to detect and allow students to collect more reliable measurements.

Most importantly, they help students connect the theory of superposition, path difference and phase with a real experimental result.

This project is a useful reminder that innovation in science education does not always require an entirely new experiment. Sometimes it means looking carefully at an existing experiment, identifying what prevents students from understanding it and designing a practical solution.

A microphone holder may be a relatively small component, but when it keeps the apparatus aligned and the experiment repeatable, it can make a difficult wave concept considerably easier to understand.

That is the real value of laboratory R&D: not simply making equipment, but making science clearer.