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Designing for sound is typically an afterthought. The purpose of this paper is to help design engineers plan for the best sound system in time to complete the
project. This should also help in the form of better power management and efficient determination of the form-factor.
Designing for Sound Quality in Portable Devices
Thinking about speakers at the initial stages of the
design process can yield big rewards in sound quality.
By: Sherman S. Bybee and Dr. William E. Kunz, Regal Electronics, Inc.
Updated May 12, 2010
There are several key points to think about before designing the sound system of your portable device. Choosing a
speaker is typically the last item covered — usually in the context of what will fit. Designing the speaker system at the
initial stages of the project can allow for various possibilities and ultimately, better sound quality.
Introduction
The purpose of this paper is to help engineers design in a speaker or speaker system to best meet the acoustical
requirements of a product. Designing for sound quality should be addressed as soon as possible in the process and
no later than the beginning of the enclosure design. Typically, choosing a sound component such as a speaker is an
afterthought, which can cause tension and even scheduling problems. Any preplanning that can be accomplished will help make the design process go much smoother.
The following are key areas to address in designing for sound quality.
What range of frequencies will the proposed device be generating?
Planning and designing for sound quality is based on the characteristics of sound needed for an application.
Frequency response is one of the major characteristics used in selecting a sound device for a particular application.
In most instances a speaker will be used. For example, alarms, beepers, buzzers or voice devices all require sound
with different, and in many cases, limited ranges of frequencies. Other applications might include sound in frequency
ranges with both music and voice. For instance, handheld electronic game devices and CD/DVD players require sound generation covering a much broader range of frequencies.
With speakers, frequency response is influenced by mechanical and electrical parameters such as frame
configuration, magnet size and type, and both the shape, and material used for, the cone diaphragm (paper, mylar, etc.).
The following are the two variables used to describe the frequency characteristics of a speaker:
- Frequency Response as measured in decibels over a frequency range in Hz : The accuracy of
reproduction of the applied signal usually referenced to the human ear range of 20 to 20,000 Hertz, or cycles per second
- Resonant Frequency (fo) as measured in Hz : The frequency, measured in free space, where the
speaker’s impedance increases dramatically. At the resonant frequency, with a constant applied voltage, the current through the voice coil is the smallest.
Typical frequency response parameters for common applications might look like this:
- Voice would require an approximate frequency response of 330 Hz to 3,300 Hz.
- Music would require an approximate frequency response of 20 Hz to 20,000 Hz.
- Specialized sound conditions, above human hearing, such as for animal training and control. This would
require various frequency responses, for some animals, as low as 16 Hz and for others, as high as 150,000 Hz.
Knowing the intended frequency range beforehand is an important first step in speaker selection.
What are the environmental considerations?
There are at least two areas of environmental concern:
- Production environment: If the speaker is to be board mounted and wave soldered in place, it has to be
able to withstand the high temperatures encountered in the process. The most frequent heat problems are” a
warped frame, voice coil alignment, and melted or contaminated speaker cones. The easiest solutions are to
specify (and verify through testing) a heat tolerant speaker, or, specify a speaker assembly that can be dropped in place after the board is wave-soldered.
- Operating environments Consider where the speaker will typically be operated. Three areas to keep in
mind when anticipating an operating enviornment are:
- Temperature: A very high or very low temperature operating enviornemt can have drastiic effects on speaker performance
- Humidity: Speakers with paper cones do not do well in high humidity environments. An easy solution is to specify mylar speaker cones
- Rugged: Speakers are often subjected to high mechanical stress. For example, portable devices
may be dropped, mobile devices may be subjected to constant jarring, etc. Speaker frames and cones can be specified to anticipate rugged operating environments.
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Figure 1. Which speaker would you choose? These three speakers are all the same 28mm diameter size. The one on the
left is designed for high ambient temperature environments. The center one is for typical applications and high humidity environments. The one on the
right for high-power applications with little or no humidity.
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What is the loudness or volume required?
The next major characteristic is the loudness required for the application. For example would the application be:
- Close to the ear for hand-held devices such as cell phones
- In the ear for personal mp3 players
- At arm’s length for PDAs, instruments or desktop systems
- Also, the loudness level required should be sufficient to overcome the ambient noise of a typical operating environment.
Loudness brings a couple of other issues into play as well:
- Efficiency — because loudness directly affects battery life
- Measurement standards — that is, deciding which measurement reference will be used to determine the sound output levels.
About measurement standards: Loudness, or volume as it is sometimes referred to, is measured in terms of dB
Sound Pressure Level (SPL). However, it is important to be aware that SPL specifications are not all the same and
are based on different reference standards by a speaker manufacturer to present it’s product in the best light for an
application. The most recognized standard is referencing 1.0 watt (w) of RMS power driving the speaker and
measuring the SPL level at one meter away from the speaker face (Referred to as 1w/1m). There are times where a
speaker is designed to operate at power levels of less than a watt. In this situation, in order to measure the SPL, the
1w/1m distance reference is modified by the square of the RMS power. For example, when driving a speaker with 0
.1w, the equivalent distance as referenced to one watt would be 0.316 m.
Another sound device similar to a speaker is a receiver. A receiver is used in applications where the listener is in
very close proximity to the device, such as, a cell phone, head phone or ear phone. The receiver is a low power
device, with power specified in the range of 10 to 100 milliwatts (mw) and typically has a much broader frequency
range. There is a different standard for measuring the SPL of the receiver— typically measured with 1.0 mw RMS
driving power and installed in a fixture called an Ear Simulator or Artificial Ear.
Additional parameters to keep in mind when designing for sound:
- Efficiency: The loudness or volume desired for a given input power, i.e., the highest SPL for a given input power level to fit the space.
- Distance to the listener: The loudness at a typically used distance for the device
- Measurement Standard: The normal standard goes out the window and usually gives way to what the marketing people hear and expect.
General guidelines for SPL specifications, depending on the requirements, may include:
- Portable multimedia devices, such as laptops, CD/DVD players or PDA’s where the distance from the
sound source to the listener is about one to three feet, the speakers’ SPL would be specified in the range of 78dB to 86 dB.
- Cellphones, or other devices held directly or within six inches to a listener’s ear may use both speakers
and receivers. Receivers are specified at a much higher SPL of approximately 90 to 120 dBas measured in an Ear Simulator.
- In-Ear receivers are specified the same as the cell phones above
What is the desired sound quality?
Once the frequency and loudness characteristics have been determined, physical requirements can be worked on.
Physical requirements, such as, size, depth and enclosure dimensions are affected greatly by the desired acoustical
and electrical characteristics. In addition, to help with the acoustics of the device, a chambering scheme or sound wave-guide may also be integrated into the enclosure design.
Sound quality can also be affected by nonlinear distortion, particularly where a speaker is designed to be operated
near, or even above, its specified power handling capability. Linear distortion changes over a speaker’s frequency
range and should be evaluated in the context of a specific application. The best way to address the nonlinear
distortion issue is to work with a speaker manufacturer and look at specific speaker response/distortion
measurements for your particular application, including the enclosure in which the speaker will be mounted.
The measurement for distortion is given as a percentage, comparing the perfect sine wave signal to the distortion
generated by the acoustical output of the speaker. As a reference, a typically specified distortion of 5% is about the
maximum percentage allowable to be able to discern the original sound detail. Typical distortion is about 1% to 3% for
general purpose speakers, even though a manufacturer may have specified 5%.
Distortion measurements for common applications would be:
- Voice applications: may have acceptable distortion levels from 1% to 3%.
- High fidelity sound applications: require a distortion level of less than 1%.
- Alarms, beepers and buzzers: can tolerate a fairly high distortion under most applications and is typically not specified.
What is the required impedance?
Specifying a speaker impedance must also take into account the amplifier circuitry requirements, the DC supply
voltage, the amplifier output signal voltage (measured in volts RMS) and the desired loudness (measured in dBSPL).
For maximum transfer of energy —electrical to acoustical—the goal is to have the speaker impedance match the
output impedance of the amplifier. The amplifier impedance can be matched to the speaker impedance by specifying a
particular audio IC Chip and in some cases, modifying surrounding circuitry to reach a desired impedance. The
dynamic impedance of a speaker is dependent on frequency and is at its highest at the resonant frequency (Fo).
The basic speaker impedance is specified as DC resistance and measured in Ohms.
The following are impedance related parameters :
- Speaker impedance: should take into account the DC supply voltage available, the signal output of the
amplifier and the loudness required to make an efficient, quality device. Generally, the lower the DC supply voltage, the lower the speaker impedance
- Speaker/Amplifier efficiency: is highest when the speaker impedance matches the amplifier output impedance
- Available speaker impedance: are typically designated by DC resistance of 4, 8, 16 and 32 ohms, but can
be custom made to any impedance up to about 150 Ohms
- Amplifier impedance: is designed by choosing from a wide variety of standard output impedance IC Chips,
or modified circuitry to match the required speaker impedance
The effects of mismatched impedances are seen in the areas of efficiency and sound quality. A financial impact
could result by having to redesign and fabricate new circuitry.
How and where will the speaker be mounted?
How and where a speaker is mounted affects several important issues, such as, frequency response, loudness and
quality of the sound emanating from the enclosure.
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Figure 2. Most speakers cannot handle the temperatures of wave soldering. This unique PC Board mount speaker
snaps into a PC board and is soldered in a separate assembly step.
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Speaker mounting configuration brings into play a variety of variables, such as:
- Volume of air behind the speaker
- Where the speaker is mounted in an enclosure
- How the speaker is mounted, and
- Grill size and configuration.
If speaker mounting is done properly, any distortion present will be the result of the speaker or amplifier
design and not the mounting process. A properly mounted speaker would be:
- Mounted flush with the enclosure surface
- Mechanically secured (even if a “snap-in”), and
- Have no sound leaks around the mounting surfaces
Speaker mounting alternatives include:
- Screw mounting: physical mounting to the enclosure using screws through mounting holes in the speaker
frame. This method usually requires a separate soldering operation to attach speaker leads or a cable
assembly to the speaker which are then connected to the PC board. Typically a cable assembly would be
installed on the speaker by the manufacturer. Sometimes a connector is used on the PC board so that the enclosure can be separated from the PC board for servicing.
- Gluing:
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Figure 3. This assembly (On left: speaker, plastic collar and pre-soldered leads) was designed by the speaker
manufacturer to snap into an injection molded receptacle (center, right).
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to either the printed circuit board or to the enclosure requires careful consideration. If the speaker is glued to the PC Board, the location should ensure that maximum
radiation of sound comes out of the enclosure. When glued to an enclosure, particularly for smaller speakers, the mounting surface is usually designed with an
injection molded cup shape tooled in, with dimensions specific to the speaker being used. When glued to the enclosure, the mounting surface must be flat, because any
separation between the speaker and enclosure will cause an air leak and efficiency will be decreased.
After the speaker is glued or snapped in the enclosure, speaker leads are then connected to the PC board as a separate step.
- Spring mounting: is accomplished by gluing a speaker, with attached spring contacts, to the enclosure in
the exact position where the trace pattern on the PC board will make connection, when the enclosure and PC
board are sandwiched together. An alternate approach is to solder a spring contact mounting clip to the PC
board and then later insert a speaker into the clip, similar to a PC board mounted battery holder. In either
situation, the speaker is mounted on the PC board, facing in the direction of an opening in the enclosure or, in
some cases, just in the electronics bay where high volume sound is not required.
Other location considerations:
- Air volume: The area behind the speaker should be large enough so as not to restrict the speaker cone
movement, due to back pressure, thereby affecting frequency response and efficiency.
- Venting: Venting refers to the air column movement in front as well as behind the speaker.
Most designs insure for a minimally restricted flow in front of the speaker, but many times the flow behind the speaker is overlooked or neglected.
If there is insufficient porting on the back side, an unbalance of air pressure can occur, resulting in a non
sinusoidal response of the speaker cone, and, sound response. For a small speaker, of nominally one inch
(25 millimeters) or less in diameter, a rear port the size of a soda straw should suffice. This is what we use in our small bass reflex housings (Figure 5 below) .
- Acoustical coupling: Acoustical coupling refers to the mechanical connection between the speaker frame and
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Figure 4. Testing the effect of acoustic coupling
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the transference of acoustical energy to a radiating surface, or sounding board.
When the speaker cone is energized, some force is also coupled from the voice coil via the magnet to the
speaker frame. This force, if properly coupled to the radiating structure, will cause an increase in the sound level to the listener.
A small speaker hanging in free space by its two wires will have a minimum sound response. Rigidly mounted
on a baffle board, the normal testing method, the same unit sounds much louder, with a wider frequency response.
For example, take a common table fork (Figure 4), hold it above a table top (left) and “twang” two of the tines,
noting the sound quality and level. Then, repeat doing this with the tip of the handle touching a table top (right). Notice the increase in the sound!
- Acoustical wave guide: Acoustical wave guide ducting or tuned ports:
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Figure 5. An enclosure area can actually be “tuned” to enhance frequency response, as it is with this
miniature “bass reflex” design above.
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will enhance the low frequency response (a la the Bose Wave Radio). When the speaker can be placed in a closed ported box, perhaps within the main
enclosure, the result will be a better low end response and a higher volume or loudness (the same effect as high fidelity bass reflex enclosures).
- Sound apertures and grills: are typically used to minimize dust and moisture from getting onto the face of the speaker. The
grill also keeps the face of the speaker from being damaged by handling. Also, the apertures in the grill must be sufficiently
sized not to restrict airflow in the front of the speaker. A restricted airflow would adversely affect the frequency response
and loudness. As a general guideline, a grill aperture opening should be equal to about 50% of the speaker cone size. Grill louvers
are a good alternative as they can be designed to maximize air flow yet still provide a great deal of physical protection.
Other considerations
Speaker shape: By an overwhelming majority,
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Figure 6. If space is available horizontally, an oval speaker in place of a round one will yield better response and
loudness.
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round is the shape of choice for most design engineers. Designing for a round speaker is easier as they are familiar, and plentiful. However, in situations where speaker
size might be constrained in one dimension and not the other, an oval shaped speaker could offer advantages such as a better range and more loudness.
Heat: Heat is not a friend of speakers. That’s why mounting techniques are such an issue. While there
are speakers that are certified for “high temperature” environments, they are custom made and in some cases can go through the wave soldering operation.
In the majority of applications where fidelity is even moderately important, speakers are integrated into assemblies last, usually as a separate step in order
to avoid problems with heat.
RoHS compliance: Many of Regal’s miniature speakers are RoHS compliant (the European Union’s Restrictions on
Hazardous Substances). To become compliant, lead free solders and cadmium free metal plating on the frame were
incorporated into the speaker manufacturing process. However, being lead free does not mean that the speakers are
suitable for high temperature reflow soldering processes, such as used on PC boards with chip capacitors, resistors,
etc. The water resistant mylar cones will either melt or be distorted at these elevated temperatures. A secondary
assembly operation is preferable to insure safe installation. These can include using a carefully controlled (no
preheat) wave solder process for PC mount speakers, or incorporating two wires and a small connector onto the speaker.
Size matters. Speakers generate sound by moving air and this means, for low frequency sounds in particular, the
speaker cone, which is attached to the voice coil, must be large, and/or, the excursion distance must increase. The
excursion is the distance the speaker cone and voice coil moves in and out of the magnetic gap. The magnetic field
that is generated in the voice coil gap is a major factor in the excursion distances and is a factor of both the voice coil
design and the magnetic material used in conjunction
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Figure 7. An actual SPL vs. Frequency test curve for Regal’s RSH-36-E-8 speaker. The inset shows part of
Regal’s sound testing facility at its headquarters in Sunnyvale, California.
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with the voice coil.
Speaker magnets: Magnets used in speakers are of two general types: rare earth and ferrite. Rare earth magnets have
a stronger magnetic field than a ferrite and are used to make more efficient speakers, particularly in mini and micro sized speakers. Ferrite magnets are generally
used where there are no size constraints and the magnet can be made larger. Ferrite material is much less expensive than rare
earth material and is used in speakers with much higher power requirements.
Testing:The vast majority of speakers are not manufactured in the United States. Consequently, stateside testing of
speakers before delivery can save time and money by helping ensure speakers meet requested quality and sound specifications. :.
Speaker assemblies
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Figure 8. A speaker as part of a complete “drop in” assembly
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Another potential time saver and production consideration, especially
where heat in the production process is a problem, is to have a speaker manufacturer deliver a speaker as part of a complete “drop in”
assembly. For example speakers could have leads pre-soldered as shown in Figure 3 (above), have leads pre-soldered with a connector
as in Figure 8 (on right), or come as part of a complete, injection molded plastic snap-in assembly. A “drop in” assembly also avoids
problems with re-flow soldering issues where heat from the soldering process can warp speaker frames.
When evaluating manufacturing options, it is always a good idea to
involve your speaker manufacturer early in the design process.
Final recommendations
The best way to design for sound quality is to involve your speaker manufacturer early in the design process. This
approach has many benefits, such as:
- Identifying, and testing, speakers that meet your specific sound quality needs.
- Testing sound alternatives using your actual PC board or enclosure prototypes.
- Help with the actual design of your PC board layout or enclosure to enhance sound quality.
- Development of a value added speaker subassembly for your specific application. Sub assemblies might be
as simple as a speaker with lead wires and PB board connector already attached. Or, could be as involved
as an injected molded speaker subassembly specifically designed to snap into your device.
Good sound quality in portable devices is readily attainable — if you plan ahead.
If you have questions about how to design for sound quality in your portable device, please:
- Phone Grant Murray or Dr. William E. Kunz
at Regal Electronics, Inc., 408/988-2288, or
- E-mail: hal@regalusa.com
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